Emerging Plant Growth Regulators in Agriculture: Roles in Stress Tolerance

Emerging Plant Growth Regulators in Agriculture: Roles in Stress Tolerance presents current PGR discoveries and advances for agricultural applications, providing a comprehensive reference for those seeking to apply these tools for improved plant health and crop yield. As demand for agricultural crops and improved nutritional requirement continue to escalate in response to increasing population, plant researchers have focused on identifying scientific approaches to minimize the negative impacts of climate change on agriculture crops. Among the various applied approaches, the application of plant growth regulators (PGRs) have gained significant attention for their ability to enhance stress tolerance mechanisms.

This book was developed to provide foundational and emerging information to advance the discovery of novel, cost-competitive, specific and effective PGRs for applications in agriculture.

• Highlights the latest developments in stress signaling, cross-talk and PGR mechanisms as applied to agriculture and agronomy
• Includes case studies and examples to provide real-world insights
• Presents resources for future research and field application

• Language: English
• Publisher: Academic Press
• Release date: Nov 13, 2021
• ISBN: 9780323910064

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Chapter 1

Emerging roles of plant growth regulators for plants adaptation to abiotic stress–induced oxidative stress

Akbar Hossaina, Biswajit Pamanickb, Visha Kumari Venugopalanc, Ulkar Ibrahimovad, Md. Atikur Rahmane, Ayaz Latif Siyalf, Sagar Maitrag, Subhrajyoti Chatterjeeh, Tariq Aftabi

aBangladesh Wheat and Maize Research Institute, Dinajpur, Bangladesh

bDepartment of Agronomy, Dr. Rajendra Prasad Central Agricultural University, Samastipur, Bihar, India

cDepartment of Agronomy, Bidhan Chandra KrishiVishwavidalya, Mohanpur, West Bengal, India

dInstitute of Molecular Biology and Biotechnologies, Azerbaijan National Academy of Sciences, Baku, Azerbaijan

eSpices Research Center, Bangladesh Agricultural Research Institute (BARI), Bogura, Bangladesh

fDepartment of Plant Breeding and Genetics, Sindh Agriculture University, Tandojam, Sindh, Pakistan

gDepartment of Agronomy, Centurion University of Technology and Management, Odisha, India

hDepartment of Horticulture, Centurion University of Technology and Management, Odisha, India

iPlant Physiology Section, Department of Botany, Aligarh Muslim University, Aligarh, Uttar Pradesh, India

Abstract

Major abiotic stresses such as heat, drought, salinity, heavy metal, light, pesticide, and cold are considered the great threat for the food and environmental security of the increasing population. During abiotic stresses, reactive oxygen species (ROS) is produced in the plants’ cell that leads to inhabit physio-biochemical process of affected plants, which ultimately hampers the usual growth and development of plants. To avert the abiotic stresses–induced oxidative stress by hindering the production of harmful ROS (i.e., hydroxyl ions, superoxide ions, hydrogen peroxide, and other free radicals), tolerant plants generally enhance/accumulate various growth regulators (i.e., jasmonates, salicylates, brassinosteroids (BRs), nitric oxide, hydrogen sulfide, polyamines, glycine-betaine, oligosaccharides, strigolactones (SLs), melatonin, karrikins, sugars, serotonin, turgorins, system in myo-inositol, etc.) in plant cells. Among them, proline, glycine-betaine, polyamines, and sugars (i.e., mannitol, sorbitol, galactinol, trehalose, etc.) are known as osmolytes that have significant role for plant adaptation against abiotic stresses. Earlier studies revealed that during abiotic stresses several phytohormones (i.e., abscisic acid (ABA), BRs, cytokinins, ethylene, jasmonates, salicylic acid and SLs, melatonin, karrikins, etc.) encourage to enhance the accumulation of osmolytes in plant cells. Several genes involved signaling pathway also play an important role for the biosynthesis of these growth regulators for enhancing survival ability against abiotic stresses–induced oxidative stress in transgenic plants. As both osmolytes and plant hormones have been known to play most important roles during adverse ecological condition; therefore, it is crucial to understand the regulatory mechanisms of phytohormone-mediation for the accumulation of osmolytes in plants during abiotic stress. The chapter deliberated the fundamental mechanisms of growth regulators for abiotic-induced oxidative stress tolerance in plants.

Keywords

Osmolytes; Phytohormones; Plants; Abiotic Stress; Oxidative Stress

1.1 Introduction

For crop plants, abiotic factors are the most significant yield-limiting factors (Canter, 2018; Zörb et al., 2019). Temperature extremes, drought, flooding, salinity, and heavy metal stress, to name a few, all have an effect on crop plant growth and yield creation (Waqas et al., 2017; Zafar et al., 2018). Approximately 90% of arable lands are susceptible to one or more of the aforementioned stresses (dos Reis et al., 2012). According to estimates based on the integration of climate change and crop yield models, the productivity of major crops such as rice, wheat, and maize will continue to decline, potentially posing a serious threat to food security (Tigchelaar et al., 2018). According to recent meta-analysis study, global average temperature will rise by 2.0–4.9 °C by 2100 (Raftery et al., 2017). Drought stress has become more common and severe due to changes in precipitation patterns and a rise in evapotranspiration caused by global warming (Dai, 2011). Also, the extent of land salinity has increased to a larger extend. Warming, droughts, floods, and storm events are expected to become more frequent and extreme as a result of climate change, reducing crop yields even further, especially in the tropics and subtropics.

Abiotic stresses, such as low or high temperature, inadequate or unnecessary water, high saltiness, weighty metals, and radiation, are threatening to plant development and improvement, prompting incredible harvest yield reduction around the world (Shao et al., 2015) (Fig. 1.1 and Table 1.1). Crop plants are encounter by various abiotic pressures that frontier their growth and development. Stresses such as drought, heat, pathogen attack, heavy metal, salinity, and radiations impose negative effect on plants (Wada and Murata, 2007). The decrease in crop profitability in the current period of environmental change is bargaining the endeavors/techniques utilized for economical farming practices. Along these lines, plant pressure physiologists are designing plants with reasonable exogenous flagging elicitors to design resilience to different anxieties. The current idea may prompt the improvement of methodologies for unwinding the fundamental instruments of plant chemical intervened abiotic stress resilience in crop plants. Yield plants are presented to heaps of abiotic stress conditions and go through a critical impediment in development and advancement, in this manner lessening crop profitability (Wang et al., 2016).

Fig. 1.1 Plants response to abiotic stresses

Table 1.1

1.1.1 Heat

Temperature rises around the world have become a major concern, affecting not only plant growth but also plant productivity, especially in agricultural crops. When plants are subjected to heat stress, their seed germination rate, photosynthetic ability, and yield all suffer. Temperature is an important ecological factor that influences the physiological behavior and distribution of living organisms, as well as plant growth and development. It is always believed that, optimum temperature helps the plants to perform better in terms of physiological morphological, biochemical, and cellular processes under stressful environment (Basbouss-Serhal et al., 2016; Raza et al., 2020). Heat stress affects plant growth, production, physiological processes, and yield in a variety of ways, most of which are negative (Hasanuzzaman et al., 2012). Excessive production of reactive oxygen species (ROS), which contributes to oxidative stress, is one of the most serious consequences of HT (high temperature) stress (Hasanuzzaman et al., 2013). Heat stress alters the expression of genes involved in direct defense from HT stress at the molecular level that control the expression of osmoprotectants, detoxifying enzymes, transporters, and regulatory proteins are among them (Chinnusamy et al., 2007). One of the most heat-sensitive physiological processes in plants is photosynthesis (Crafts-Brandner and Salvucci, 2002). High temperatures have a greater impact on plant photosynthetic capability, especially in C3 plants, compared to C4 plants (Yang et al., 2006). The primary sites of damage at HTs in chloroplast are carbon metabolism in the stroma and photochemical reactions in thylakoid lamella (Wang et al., 2009). While heat stress affects all plant tissues at almost all growth and developmental stages, reproductive tissues are the most vulnerable, and even a few degrees of temperature increase during flowering time can result in the loss of entire grain crop cycles. A short duration of heat stress during reproduction can cause significant reductions in floral buds and flower abortion, despite wide variations in sensitivity within and among plant species and varieties (Sato et al., 2006). Even in the midst of a heat wave, a plant’s reproductive developmental stages can result in no flowers, or flowers that do not produce fruit or seed (Maheswari et al., 2012). It has such a devastating effect that even a slight rise in temperature (1.5 °C) has a substantial negative impact on crop yields (Warland et al., 2006). Grain yield is influenced by higher temperatures primarily by phenological production processes. Many cultivated crops, including cereals (e.g., rice, wheat, barley, sorghum, and maize), pulses (e.g., chickpea, cowpea), and oil seeds (mustard) have shown a drastic yield reduction (Tubiello et al., 2007; Kalra et al., 2008; Hatfield et al., 2011; Ahamed et al., 2010).

Warmth stress can unfairly affect basically all pieces of improvement and headway of plants, rising temperature achieves cell destruction inferable from water lack to plants (Priya et al., 2019). Like warmth stress, heat pressure which has actually emerged as the most trading off test and one of the enunciated impacts of ecological change wonder and a hazardous climatic deviation may be taken care of with PGR application under various agrocharacteristic conditions (Ray et al., 2015). Warmth stress is characterized as the ascent in soil and air temperature past a limit level for a base measure of time to such an extent that perpetual damage to plant development and advancement happens. A point-by-point multiarea study features the effect of temperature impacts on the yields (Easterling et al., 2007). These unfavorable conditions are adding to the improvement of dry spell inclined zones and thus on the plant development and harvest profitability of significant yields. Under the heat and humidity, unnecessary radiations and raised temperatures are another significant restricting component to plant development and improvement (Kumar et al., 2016). High temperatures may cause the burning of the twigs and leaves alongside visual manifestations of burn from the sun, leaves senescence, development hindrance, and staining of leafy foods (Lesk et al., 2016). High-temperature stress diminished the number of spikes and the number of florets per plant in rice and seed-set in sorghum was likewise contrarily influenced under comparable conditions. Plant water status is of prime significance under changing temperature conditions. The plants attempt to settle their tissue water content independent of temperature changes when the abundant amount of dampness is accessible; not withstanding, the temperature increment demonstrates deadly under the restricted inventory of water (Lobell and Field 2007; Lobell et al., 2011).

Temperature stress lessens the plant’s ideal biochemical and physiological working by regulating subatomic systems. Cold pressure is huge abiotic stress that influences the development and improvement of yields, prompting loss of solidarity and sores on a superficial level. Outrageous temperatures are one of the excellent reasons for various abiotic stresses such as dry spell. Increments or diminishes in temperature, both unfortunately influence the plant’s development, advancement, and yield. Cold pressure happens when plants are exposed to extremely low temperatures. Cold pressure is a significant abiotic stresses that decrease profitability of yields by influencing quality and life after gather (Hussain et al., 2019). Crops are subjected to very elevated temperatures, heat stress happens, for an adequate moment to cause permanent injury to functioning or development of the plant. In any case, seed preparation improved cold resilience by diminishing leaf senescence and keeping up the overall water content in plants developed at 4 °C. Low-temperature stress diminishes grain yield, as well as influences, crop grain quality (Zhang et al., 2016).

1.1.2 Drought

Dry season is likewise a major danger in horticulture, especially in dry or semiparched locales where precipitation is scant and accessible water is restricted. It is the best destroying pressure and could cause a decrease in crop efficiency (Hussain et al., 2019). Quickly changing environmental elements convey dry season a genuine intimidation to the manageability of food creation frameworks all through the world. Plants adjust in different manners because of dry season pressure, for example, changes in development design, plant morphology, and safeguard components. Dry spell pressure happens when soil and barometrical stickiness is low and the encompassing air temperature is high (Javid et al., 2011). This condition is the consequence of lop-sidedness between the evapotranspiration transition and water consumption from the dirt (Alamri et al., 2020). Water lack and soil saltiness obviously address significant difficulties confronting profitability as they trigger oxidative, osmotic, and temperature stresses. Under these water testing conditions, plants see stresses through different sensors engaged with reaction flagging. These are transduced by different pathways in which many flagging and transcriptional factors play significant and explicit capacities (Siyal et al., 2021). Water transport inside a plant happens under strain as controlled by soil water accessibility and the climatic fume pressure shortfall, making turgor pressure inside cells. Physiological changes that keep up turgor pressure are significant under changing ecological conditions (Zhang et al., 2016). Water transport in roots is influenced by different segments, for example, root life structures, water accessibility, and salts in the dirt. These elements are impacted by the action of aquaporins, which are necessary layer proteins that work as channels to move select little solutes and water (Hussain et al., 2019). The underlying impact of dry spell on the plants is the helpless germination and weakened seedling foundation. Different examinations have announced the adverse consequences of dry spell weight on germination and seedling development. Plant development is basically cultivated by cell division, augmentation, and separation. Dry spell hinders mitosis and cell stretching which brings about helpless development (Hussain et al., 2019).

Plants under drought then synthesize protective compounds by mobilizing metabolites needed for osmotic adjustment. Drought is a major challenge to the sustainability of food production systems around the world due to rapidly evolving climate patterns (Kogan et al., 2019). Drought stress causes plants to adapt in a variety of ways, including changes in growth pattern, plant morphology, and defensive mechanisms (Zandalinas et al., 2018). Plants react to drought in a number of ways (Anjum et al., 2016). Plant growth is affected by changes in individual architecture, which result in lower height, smaller leaf size, fewer leaves, less fruit yield, and changes in the reproductive process (Table 1.2). Osmoregulatory processes protect membrane integrity and sustain water inflow to the cell, as well as the aggregation of organic solutes such as sugars, quaternary ammonium compounds (glycine betaine and alanine betaine) (Sakamoto and Murata, 2002; Ashraf and Foolad, 2007) hydrophilic proteins (late embryogenesis abundant proteins) (Chaves et al., 2003), soluble proteins, and amino acids (proline) (Sakamoto and Murata, 2002). Drought-induced chlorophyll depletion has long been thought to be a result of pigment photooxidation and chlorophyll degradation. Drought stress causes a decrease in chlorophyll content, which varies depending on the length and severity of the drought (Zhang and Kirkham, 1996). Drought has a negative impact on crop production in agricultural ecosystems, affecting the rate of growth and development of economically valuable plant parts including fruits, grains, and leaves. In dry years, production of crops such as coffee can be reduced by up to 80% without irrigation (DaMatta and Ramalho, 2006). The 2008/2009 soybean production in the Brazilian state of Paraná was decreased by 80% in areas without dry cover during a 45-day drought (Franchini et al., 2009). Sugarcane, corn, wheat, and a variety of other crops can all be estimated in the same way.

Table 1.2

1.1.3 Salinity

The magnitude of agricultural land affected by high salinity is increasing globally as a result of both natural and agricultural occurrence such as irrigation schemes. Soil salinity is a global threat to world agriculture because it reduces crop yields and, as a result, crop production in salt-affected areas. Around 45 million hectares of irrigated land are under saline stress worldwide, and more than half of arable land may be salt-affected by the year 2050, making salinity a significant limiting factor for food security (Munns and Tester, 2008). Salt stress affects crop growth and yield in a variety of ways. Salt stress has two main effects on crop plants: osmotic stress and ion toxicity. As there is more salt in the soil solution under salinity stress, the osmotic pressure in plant cells exceeds the osmotic pressure in plant cells, limiting the capacity of plants to take up water and minerals such as K+ and Ca²+. These primary effects of salinity stress result in secondary effects such as reduced assimilate production, cell expansion and membrane function, and cytosomatic function. Salt stress increases the development of ROS, which destroys biomolecules (such as lipids, proteins, and nucleic acids) and disrupts redox homeostasis (Kundu et al., 2018). Plants use a variety of mechanisms to detect, react to, and adapt to changes in the saline environment, including morphophysiological traits as well as ionic, biochemical, and molecular metabolisms.

In plant growth, salinity presents two primary concerns: osmotic stress and ionic stress (EL Sabagh et al., 2020). The unfavorable impacts of saltiness change distinctive physiological and metabolic cycles of plants. Regularly, the responses to these changes are joined by different manifestations, for example, diminished leaf region, expanded leaf thickness and deliciousness, leaf abscission, root and shoot corruption, and diminished internode lengths. Among the abiotic stresses, soil pungency is another abiotic limitation that has sabotaged the cultivating effectiveness all throughout the planet, fantastically in completely dry and semidry regions. It is evaluated that around 800 million hectares of arable territories across the globe are affected by soil pungency (Zhang et al., 2016). Saltiness is progressively turning into a significant restricting element to food security, as around 45 million hectares of watered land are under saline pressure around the world, and over half of arable land could be salt-influenced constantly by 2050. Most horticultural grounds influenced by saltiness are situated in semiparched or dry districts (Hussain et al., 2019). Consequently, the harm is increased by the synchronized activity of xerothermic angles, such as aridity and high temperature (Alamri et al., 2020).

1.1.4 Heavy metal stress

Mining, current agricultural methods, and industrialization are only a few of the anthropogenic processes that have a long-term negative impact on our climate. Heavy metal concentrations in soil, water, and air rise as a result of both of these factors. Heavy metal contamination of soil causes a variety of environmental issues and has a harmful impact on both plants and animals. By accumulating heavy metals, today’s widespread industrialization has negative effects on soil and crop productivity (Shahid et al., 2015). Heavy metals such as zinc, copper, molybdenum, manganese, cobalt, and nickel are needed for vital biological processes and developmental pathways (Salla et al., 2011). When their concentrations exceed, these metals, together with four other highly toxic heavy metals, such as arsenic (As), lead (Pb), cadmium (Cd), mercury (Hg), Cr, Al, and Be, can significantly reduce crop productivity (Pierart et al., 2015). These toxic elements cause morphological and metabolic defects in plants, resulting in lower yields. These abnormalities also result in the formation of ROS, such as superoxide anion radical (O2−), hydrogen peroxide (H2O2), and hydroxyl radical (OH−), disrupting cell redox homeostasis (Gill and Tuteja, 2010; Pourrut et al., 2011).

1.1.5 Water logging

Floods were responsible for nearly two-thirds of all crop damage and loss worldwide between 2006 and 2016, resulting in billions of dollars in losses (Food and Agriculture Organization of the United Nations [FAO], 2017). The transport of oxygen from the air to plant tissues is disrupted when soil is flooded (Lee et al., 2011), resulting in hypoxia (21% O2) (Sasidharan et al., 2017). According to (Wollmer et al., 2018), a 14-day waterlogging in rapeseed (Brassica napus L.) reduced seed mass by 75%–85%, while waterlogging for 5 days at the start of flowering in field pea reduced seed production by 38% (Pampana et al., 2016a). Waterlogging for 20 days at the 3- to 4-leaf stage in wheat resulted in yield reductions of 90%–95% (Pampana et al., 2016a), while waterlogging for 20 days at the 3- to 4-leaf stage resulted in yield reductions of 85%–90% in barley (Pampana et al., 2016b) (Masoni et al., 2016).

During abiotic stresses, ROS produced in the plants’ cell leads to inhabit physio-biochemical process of affected plants ultimately hampering the usual growth and development of plants. To prevent the abiotic stresses–induced oxidative stress, tolerant plants enhance/accumulate numerous growth regulators (i.e., jasmonates, salicylates, brassinosteroids (BRs), nitric oxide (NO), hydrogen sulfide, polyamines, glycine-betaine, oligosaccharides, strigolactones (SLs), melatonin, karrikins, sugars, serotonin, turgorins, system in myo-inositol, etc.) in plant cells. Besides theses growth regulators, several osmolytes including proline, glycine-betaine, polyamines, and several sugars (i.e., mannitol, sorbitol, galactinol, trehalose etc.) have significant role for plant adaptation to abiotic stresses. Researchers already recognized that phytohormones and osmolytes have interrelation to survive plants against abiotic stresses through encouraging their accumulation in plant cells under abiotic stress–induced oxidative stress. As both osmolytes and plant hormones (PHs) have important roles during abiotic stress–induced oxidative stress; therefore the chapter aimed to recognize and understand the regulatory mechanisms of phytohormone-mediated accumulation of osmolytes or vice-versa in plants during abiotic stress-induced oxidative stress.

Stresses are seen, and balance in the declaration of qualities encoding different proteins is initiated in the sign transduction pathways in plants (Shi et al., 2018). The utilization of phytohormones (PGRs) in the current part showed that the cross discuss phytohormones with different flagging specialists under various pressure conditions in assorted yield plants grant pressure resilience under pressure pressures. Interest for agrarian harvest is on rise in light of expanding populace and harm of prime cropland for development (Zhang et al., 2016). Exploration interest is redirected to use soils with minor plant creation. Dampness stress contrarily affects crop development and profitability. The plant development advancing rhizobacteria (PGPR) and plant development controllers (PGR) are essential for plant formative cycle under dampness stress (Javid et al., 2011). The integrative utilization of PGPR and PGRs is a promising technique and eco-accommodating procedure for expanding dry season resistance in crop plants. Plants have adjusted powerful reactions to deal with abiotic stresses at the morphological, physiological, and biochemical levels, permitting them to make due under factor ecological conditions. Plant physiological reactions to dry spell and warmth stresses can be ordered into two unmistakable systems. Evasion components are mostly morphological and physiological changes that give a getaway to the water or warmth stress, including expanded root framework, diminished stomatal number and conductance, diminished leaf region, expanded leaf thickness, and leaf rolling or collapsing to reduce evapotranspiration.

Therefore, it is confirmed that PGRs play most important roles during adverse ecological condition; and it is essential to recognize the regulatory mechanisms of phytohormone-mediation for the accumulation of osmolytes in plants during abiotic stress. The chapter deliberated the fundamental mechanisms of PGRs for abiotic-induced oxidative stress tolerance in plants. A detailed discussion of the PGRs and their mechanisms has been presented in following subsections.

1.2 PHs in abiotic stress tolerance

Plants grown in natural environment may face various abiotic factors that influence normal life processes, growth, and productivity of crops (Mokrani et al., 2020; Erofeeva, 2021; Lüttge U, Buckeridge, 2020; Doley, 2017). Abiotic stresses are extreme environmental conditions that impose adversity in obtaining growth and productivity of crops (Bhatt et al., 2020). In the recent consequence of climatic aberrations, abiotic stresses are very common phenomena. Crops grown in marginal areas of especially tropical and subtropical regions expose extremity because of abiotic stresses, such as temperature extremes, salinity, flood and drought, pollution, and heavy metal toxicity. In the course of evolution, plants try to cope up with the extremity caused due to abiotic factors and develop multifaceted range of complex strategies. Under the adverse situations developed because of abiotic stresses, plants evolve tolerance mechanisms and hormonal signaling for adaptation (Roychoudhury and Banerjee, 2017). PHs are compounds biosynthesized in the plants influencing physiological and metabolic activities related to plant growth even in abiotic stress situations.

1.2.1 Function of classical PHs for adaptation of plants against abiotic stress

In general, plant growth and developmental stages are influenced by different hormones (Bhattacharya, 2019; Vissenberg et al., 2020). Different stress situations transduce a distinct role in leading plant responses through hormone signaling pathways (Zhu, 2016). Classical PHs, namely, gibberellins (GAs), auxins, cytokinins (CKs), and ethylene (ET) are known to send basic growth signals in plant defense and mitigation of stresses. Plants deal with the abiotic stresses through signaling crosstalk with various hormones. The generation and scavenging of ROS grips a prime stage intricate in mitigation abiotic stresses (Das and Roychoudhury, 2014) and hormonal crosstalk prevails in the processes. Moreover, PHs help in improving crop growth and productivity (Goody et al., 2021; Wilkinson et al., 2012).

1.2.1.1 Gibberellins

Gibberellins (GAs) perform multifaceted roles in growth processes of plants. The biosynthesis of GA is controlled by developmental and environmental stimuli (Hauvermale et al., 2012; Hedden, 2020). Some classical phytohormones, such as ET and auxin influence the level of gibberellic acid in plants (Yamaguchi, 2008). Earlier Vettakkorumakankav et al. (1999) mentioned the role of GAs in easing of abiotic stresses in plants, but studies on the relationship between GAs and abiotic stress alleviation are meager (Upreti and Sharma, 2016). Under stress conditions, GA levels in plants are fluctuated (Liu et al., 2021). GA is known to partially alleviate ill effects of salinity (Hisamatsu et al., 2000) as some research evidences have showed that under salinity stress GA has increased germination and plant growth of different crops, namely, rice (Misratia et al., 2015), barley (Abdul-Baki and Anderson, 1970), oats (Chauhan et al., 2019; Zhang et al., 2013), wheat (Ashraf et al., 2002), olive (Shekafandeh et al., 2017), castor (Rajput et al., 2015), sugarcane (Shomeili et al., 2011), linseed (Nasri et al., 2017), celery (Aloni and Pressman, 1980), tomato (Maggio et al., 2010), and sugar beet (Jamil and Rha, 2007). In the presence of GAs, some physiological and metabolic activities in plants take place that reduce the ill effects of salinity. In rice, NaCl-persuaded growth inhibition is checked in the presence of gibberellic acid (Wen et al., 2010); however, more uptake of phosphorus and calcium was noted in salinity-stressed beans compared to sodium by plants when treated with GA3 (Starck and Kozinska, 1980). In Brassica, salinity stress was alleviated by the application of GA with nitrogen (Siddiqui et al., 2008). Enzyme activation induced synthesized RNA and protein were responsible in salinity alleviation in soybean by GA3 application as stated by Bejaoui (1985).

There is a negative regulator of GAs, known as DELLA protein and GAs degrade it. GAs endorse destabilization of DELLA protein and it is moderated by salt and light inclusive of crosstalk with auxin and ET (Achard et al., 2006). GAs are known to mitigate stress due to heavy metal toxicity (Sabagh et al., 2021). Scientists noted that GAs can decrease ill effects of cadmium in plants (Meng et al., 2009; Zhu et al., 2012), nickel (Siddiqui et al., 2011), chromium (Gangwar et al., 2011), lead and zinc (Atici et al., 2005). Moreover, GAs are reported to minimize stress due cadmium and molybdenum in legumes (Sharaf et al., 2009). Overexpressing DREB1 gene of American cotton (Gossypium hirsutum) in tobacco increases tolerance to cold. Shan et al. (2007) demonstrated that biologically GA level in transgenic plants is only half of the amount present in wild-type species. Exogenous use of GA3 represses GhDREB1 expression. These data indicate that DREB1 and GA signaling pathway crosstalk via a not well understood mechanism (Shan et al., 2007). In 2008, Achard et al. (2008) stated that the constitutive expression of CBF1/DREB1b gene in Arabidopsis increases lenience against stress due to low temperature. The overexpression of CBF1 results in increase RGA (DELLA) accumulation, which in turn results in stunted growth. This is accomplished by GA deactivation as CBF1 overexpressed plants showed enhanced expression of the GA 2-oxidase gene. In the case of tolerance to submergence, GAs play a vital role by regulating ET and ABA by enhancing growth (Colebrook et al., 2014). Further, GAs signaling in facilitating growth and stress tolerance to flood situation was narrated by Bailey-Serres and Voesenek (2010). Stomatal opening is checked by GA in submergence condition (Bashar et al., 2019). Transgenic tomato with reduced bioactive GA level by overexpression of Arabidopsis thaliana GA methyl transferase 1 (AtGAMT1) gene showed tolerance to drought due to smaller stomata and reduced pore size, resulting in reduced whole plant transpiration (Nir et al., 2014). It was stated that GA deficiency confers drought tolerance in small cereals (Plaza-Wüthrich et al., 2016). (Zawaski and Busov, 2014) mentioned that GA catabolism and repressive signaling arbitrates growth retardation in response to drought (Plaza-Wüthrich et al., 2016).

1.2.1.2 Cytokinins

CKs endorse cell division and different developmental processes of plants, such as chloroplast biogenesis, shoot differentiation, vascular differentiation, regulating senescence, and pigment creation (Bhatt et al., 2020; Kunikowska et al., 2013; Wang et al., 2021). CKs are formed in the roots and seeds, and translocated to shoots; and further, control physiological processes (Terceros et al., 2020; Zahir et al., 2001). Other than these, CKs take part in mitigation of ill effects caused by abiotic stresses (Barciszewski et al., 2000). Seed priming with CKs is known to alleviate abiotic stress in different crops (Rhaman et al., 2021). CKs initiate crosstalk signaling with other classical hormones (auxins and ABA) and act antagonistically or synergistically in influencing abiotic stress response (Pospíšilová 2003). Seed priming with CKs alone or combinedly with different phytohormones is known to alleviate abiotic stresses in different plants (Rahaman et al., 2021). Plants suffer from abiotic stress incline toward reduction of concentration of CK (Pospisilova et al., 2000) and after stress relief plants regain CKs’ concentration. Earlier studies evidenced that there was change in CKs’ concentration under abiotic stress in different plants, namely, rice (Peleg et al., 2011; Reguera et al., 2013), wheat (Joshi et al., 2019), apple (Zhou et al., 2004), tomato (Pillay and Beyl, 1990), French bean (Upreti et al., 1998), onion (Upreti and Murti, 2004), and grape (Satisha et al., 2005). Higher yield was recorded in rice when exogenously CK was applied than control in a research conducted by Zahir et al. (2001). Lowering of CK level was related to greater abiotic stress tolerance and Arabidopsis histidine kinase mutants showed increased tolerance to dehydration (Kang et al., 2012). CKs control stomatal opening by modifying ABA (Veselova et al., 2006). CKs and ABA share partly the common biosynthetic pathway and antagonistic relationship between them are observed. Therefore, under stress conditions, CK concentration is reduced and ABA concentration is enhanced (Upreti and Sharma, 2016). Recent studies established the crosstalk of CKs with other PHs, such as ET, ABA, SA, and JA (Artner and Benkova, 2019; Hai et al., 2020; Thu et al., 2017; Verma et al., 2016).

1.2.1.3 Auxins

Auxins, also known as indole acetic acid (IAA), are PHs commonly known for root growth, auxiliary bud formation, vascular tissue differentiation, apical dominance, and flowering (Jiang et al., 2017; Qiu et al., 2019) and respond to alleviation of abiotic stress (Goody et al., 2021; Bielach et al., 2017). Under stress conditions, IAA concentration in plants is changed either by expression of auxin polar transporter gene or inhibitions in polar transport (Korver et al., 2018). In stress, oxidative degradation of IAA takes place inclusive of ROS generation (Kovtun et al., 2000; Pasternak et al., 2005). Under salinity stress, IAA level is reduced in rice (Prakash and Prathapasenan 1990), tomato (Dunlop and Binzel, 1996), and wheat (Sakhabutdinova et al., 2003). However, seed germination of wheat was improved by auxins application in salinity stress as mentioned by Afzal et al. (2005) and Akbari et al. (2007). Application of IAA reduced ill effects of heat in capsicum (Upreti et al., 2012). Auxins are effective to combat cold stress too (Wyatt et al., 2002; Du et al., 2012; 2013) as auxin transport is modified in plants in low-temperature conditions (Shibasaki et al., 2009). Further, auxin-responsive genes become active in low-temperature stress (Chen et al., 2019). PIN3, an auxin efflux regulator, is subdued by cold conditions (Shibasaki et al., 2009; Armengot et al., 2016) and thus low-temperature stress affects auxin transport (Aslam et al., 2020). PIN proteins also impact on auxin transport in soil moisture stress as they regulate root and shoot development (Adamowski and Friml, 2015). Under drought, IAR3 (IAA-Ala Resistant3) mRNA is involved that produces auxins (Kinoshita et al., 2012). Flooding reduces root development and plants show a tendency of development of adventitious root in which auxins make a crosstalk with ET inducing adventitious root formation from stem base (Guan et al., 2019; Muday et al., 2012; Wei et al., 2019). Additionally, auxin positively moderated the expression of various genes, such as, RAB18, DREB2A, DREB2B, RD22, and RD29A having relation with abiotic stresses and ROS metabolism and antioxidant enzyme actions (Upreti and Sharma, 2016). Therefore, the studies revealed that the growth hormone auxin is also vital in modifying various physiological and metabolic activities and developmental genes under abiotic stress conditions.

1.2.1.4 Ethylene

ET is a gaseous phytohormone that adjusts a various physiological process of plants such as seed germination, root formation, flowering and pollination, abscission and senescence and fruit ripening (Dubois et al., 2018). ET is biosynthesized through the conversion of methionine to ET by involvement of two enzymes, namely, ACC synthase and ACC oxidase (Yang and Hoffman, 1984). It is also known as a stress responsive hormone (Khan et al., 2017). Under stress conditions, ET concentration is increased which enhances growth retardation and senescence of leaves and fruits and thus, water loss from plants is reduced (Upreti and Sharma, 2016). Earlier experimental results revealed that ET was increased under soil moisture stress conditions in orange (Ben-Yehoshua and Aloni 1974), faba bean (El-Beltagy and Hall 1974), French bean (Upreti et al., 1998), and other crops (Narayana et al., 1991; Irigoyen et al., 1992).

Waterlogging-induced hypoxia is a common stress to the plants grown in lowland situation and ET brings a morphological change by forming aerenchyma. In rice, plants form aerenchyma and that is a constitutive feature (Kawai et al., 1998). But scientists showed ET-induced aerenchyma formation in other plants under hypoxia, such as, barley (Larsen et al., 1986), soybean (Bacanamwo and Purcell, 1999), pea (Gladish et al., 2006), maize (He et al., 1994), wheat (Watkin et al., 1998; Yamauchi et al., 2014) and sponge gourd (Shimamura et al., 2007). ET produced adventitious root of different crops also in waterlogged situation (Pistelli et al., 2012; Sairam et al., 2008; Vidoz et al., 2010). ET is known to elongate shoots (Pierik et al., 2007). Salinity impacts biosynthesis of ET as it increases Na/K homeostasis (Tao et al., 2015). But salinity tolerance responses greatly depend on the plant species (Kazan 2013; Riyazuddin et al., 2020). Low-temperature stress increases ET level in plants and enhances tolerance (Zhao et al., 2009; Zhao et al., 2014). ET level was increased in capsicum in heat stress by gathering in ACC and initiation in ACC-oxidase activity resulting in abscission of flowers (Upreti et al., 2012).

1.2.2 Role of specialized stress-responsive hormones for adaptation of plants against abiotic stress

In the era of climate change, crop plants have been encountering a series of abiotic stresses (heat, drought, salinity, light, heavy metal, pesticide, radiations, pest attack, and cold) while producing food, fiber, shelter, fuel, and ecological balance to the ever-increasing population of the world. These abiotic stresses have been threatening continuously upon crops to the sustainable supply of the ecological services for the mankind (Alhaithloul et al., 2020). To make crop plants more adaptable to the harsh abiotic stresses, plant physiologists have been engineering on economic crops by increasing endogenous hormones of plants body by inducing exogenous signaling elicitors and applying exogenous growth and development accelerating hormones such as ABA, BRs, SLs, jasmonic acid (JA), salicylic acid (SA), polyamines (PA), etc. for increasing adaptability of crops as abiotic stresses have been recorded to reduce crops yield up to 70% (Alhaithloul et al., 2020).

1.2.2.1 Abscisic acid (ABA)

As an isoprenoid phytohormone ABA is essential for plant growth and development which regulates various physiological processes such as stomatal opening and protein storage and permits changes to several stresses such as salt, drought, and cold (Verslues et al., 2006). Glyceraldehyde 3-phosphate, isopentenyl diphosphate, and carotenoids develop ABA mainly in roots and slightly matured leaves in response to water stress caused by a lack of water. The amount of available ABA is determined by a complex equilibrium between its biosynthesis and degradation which is influenced by developmental and environmental factors (Cutler et al., 1999). ABA is treated as the central regulatory hormone in plants harmonizing a series of functions (Sah et al., 2016). Regulating stomata, embryo morphogenesis, leaf senescence, and synthesis of stored proteins and lipids are among its most important functions (Roychoudhury et al., 2012). To confer stress tolerance, high cellular ABA facilitates changes in stomatal activity, root hydraulic conductivity, photosynthesis, biomass allocation between roots and shoots, plant water relations, osmolyte processing and the synthesis of stress-responsive proteins and genes (Finkelstein et al., 2002, 2008; Hetherington, 2001; Kim et al., 2010; Hoth et al., 2002; Seki et al., 2002). Drought, low temperature, and salinity adaptations are mainly regulated by two distinct yet overlapping signaling pathways: ABA-dependent and ABA-independent signaling pathways. Several proteins known to control ABA signaling downstream include stress-inducible transcription factors such as DREB2A/2B, AREB1, RD22BP1, and MYC/MYB, which have been shown to play a role in stress acclimation (Roychoudhury et al., 2013). Selected studies also point out that both ABA-dependent and ABA-independent pathways are exclusively correlated only during cold acclimation excluding salinity and drought stress (Mauch-Mani et al., 2005). There are several ABA transporter genes in ATP-binding cassette (ABC) family under vascular tissues of A. thaliana has been reported. One of these genes encodes ABA transporting protein (AtABCG25) which transfers ABA from the inside to the outside of the cell. Moreover, another transporter (AtABCG40) expressed in guard cells introduced ABA from the outside to the inside of the cells where both genes are busy in ABA signaling. Numerous studies have been conducted to understand about the core components and mechanisms of ABA signaling under several stress conditions. Pyrabactin resistance (PYR)/PYR1-like regulatory component of ABA receptor, protein phosphatase 2C (PP2C; a negative regulator), and (sucrose nonfermenting) SNF1-related protein kinase 2 (SnRK2, a positive regulator) are three key ABA signaling components. These three signaling component known as ABA signalosome act as a double negative regulatory system (Mehrotra et al., 2014). Plants under stress display increased activity of enzymes involved in ABA biosynthesis, as well as a relative increase in mRNA resulting in ABA accumulation. Mediated ABA accumulation is influenced by a number of factors including the inhibition of ABA catabolism under stress (Jia et al., 1997). Plants control ABA levels by degrading it irreversibly into hydroxylated products such as phaseic acid and dihydrophaseic acid (DPA) (Zhou et al., 2004) or reversibly into a physiologically inactive derivative of glucose ester by glucosidase (Zhou et al., 2004; Boyer et al., 1982). ABA plays a key role in physiological and biochemical processes of plants such as biosynthesis of natural acids, phytohormones, and flavonoids as well as acts as an antioxidant enzyme to eliminate ROS (Debolt et al., 2007). Exogenous ABA has ability to safeguard flowers from oxidative strain and reserve the firmness of photosynthetic role. Overexpression of endogenous ascorbic acid content molecules in transgenic rice at high temperature was observed resulting in retarded GLDH (L-galactono-1, 4, -lactone dehydrogenase) enzyme activity and high ROS by endogenous ABA (Zhang et al., 2018). ABA levels are said to fluctuate continuously depending on external responses with a low ABA level assisting in seed germination by releasing seed dormancy. A high level of this hormone, on the other hand, is retained mainly under abiotic stress halting development before normal conditions are restored (Roychoudhury et al., 2009a, 2009b). In any sort of stress, osmotic imbalance and membrane damage are common phenomena to make plants unfit to serve and remain alive in stress condition but exogenous application or increase of endogenous ABA by expressing stress-related genes results in enhancement of cold, drought, and salinity stress in a separate or combined form of stresses (Tuteja, 2007). As calcium has been shown to play a role as a secondary messenger in the response to a variety of environmental stresses, calcium-mediated signal transduction stands out as a promising candidate for mediating these intermediate signals. Increased calcium levels in plants as a result of ABA, drought, cold, and high-salt treatment have all been linked in research (Tuteja, 2007). Further research revealed that several known stress markers as well as key transcription factors are upregulated under stress and by ABA application in a similar way. A drought-responsive (RD29A) gene’s transcript levels were found to be regulated in both ABA-dependent and ABA-independent ways (Basu et al., 2014a, 2014b). Another form of stress marker, proline, is accumulated in stressed plants and has previously been shown to be affected by both ABA-dependent and ABA-independent signaling pathways (Mahajan et al., 2005). Based on the findings, it can be inferred that a variety of stress signals, in conjunction with the stress hormone ABA, sustain cellular homeostasis through overlapping components of signaling pathways that are highly interconnected. Under stressful circumstances, the contents of PA and DPA increase in lockstep with ABA. Even after the ABA content hits a plateau, their levels rise under stress. When plants are rehydrated, the ABA level decreases but the PA or DPA levels either increase or remain unchanged (Zhou et al., 2004). Many plant species accumulate ABA in their leaves as a result of drought and salinity (Benson et al., 1988). The stress relief reduces ABA levels and restores them to prestressed levels. Increases in ABA allow plants to reduce water loss through transpiration after stomata are closed as well as improve plant water status due to increased root hydraulic conductivity (Thompson et al., 2007). ABA also plays a role in root-to-aboveground contact whether by stomatal closure, metabolic changes, or gene expression (Zhang et al., 2006; Sobeih et al., 2004). However, this regulatory mechanism is more closely related to soil moisture content than to leaf water status implying that ABA is a chemical signal generated by stressed roots (Davies et al., 1991). Stomata sensitivity to ABA varies between plant species and cultivars and is influenced by leaf age, climatic factors such as temperature and relative humidity, plant nutritional status, ionic status of xylem sap and leaf water status (Dodd et al., 1996). Variations in ABA for stomatal response may be due to differences in the magnitude of ABA transport to the active site at guard cells. The xylem ABA concentration and stomatal conductance are linearly inversely related, according to Tardieu and Simonneau (1998) with the slope of the relationship changing with the time of day. Exogenous ABA application improves plant’s adaptive response to a variety of stresses (Marcinska et al., 2013; Javid et al., 2011). Exogenous ABA application did not improve stress tolerance in some cases (Chen et al., 1983; Robertson et al., 1987) and this nonresponsiveness to ABA is due to ABA uptake or degradation by microbes. The turgor potential of the surrounding cells controls the stomata aperture. To control CO2 efflux for photosynthesis and transpirational water loss, the guard cell volume is actively sensitive to signals generated under stress. By encouraging stomatal closure, the increase in ABA in guard cells decreases plant water loss by transpiration (Harris et al., 1991). Guard cell volume is determined by the influx or efflux of K+ which is balanced by the flux of anions and this process is ABA regulated (Hetherington et al., 1991). Externally applied ABA elicits the efflux of K+ and anion from guard cells, according to (MacRobbie, 1992). Efforts are also being made to decode the electrical responses caused by ABA in the guard cells’ plasmalemma (Blatt et al., 1993; MacRobbie, 1992). The depolarization effect, which represents a net influx of cations, causes the cellular electrical changes induced by ABA (Thiel et al., 1992). The driving force for K+ efflux through the outward K+ channel is depolarization. Ca²+ is also involved in the closure of stomata mediated by ABA. Ca²+ is an intracellular secondary messenger that mediates the effects of ABA on the stomatal aperture and/or the plasma membrane channel. Irving et al. (1992) found that ABA causes alkalization of guard cell cytoplasm, which is needed for ABA-induced K+ channel activation (Blatt and Armstrong 1993). Wilkinson et al. (1997) demonstrated that ABA causes pH reduction, which sensitizes stomata for closure because guard cells absorb ABA more efficiently at acidic pH (Anderson et al., 1994). According to recent research, ABA closes stomata by involving signal transduction molecules such as H2O2 (Luan, 2002). Similarly, ABA stimulates the synthesis of NO in guard cells which inhibits ABA signaling in these cells (Neill et al., 2008). Furthermore, when plants are stressed, the ABA is involved in root-to-shoot contact. Abiotic stresses affect the ratio of root to shoot growth in plants and there is coordination between them through long-distance transport of substrates or signaling (Munns et al., 1996). Under stress, according to Passioura et al. (1993), the ABA acts as a feed-forward signal from the roots to the aerial plant sections. Jackson (1993) found evidence for the roots’ impact on shoot development through hormone transport in the xylem. Furthermore, according to Saab et al. (1990), the relationship between ABA and root growth differs significantly from that of shoots, as higher ABA levels in roots encourage root growth at low water capacity (Biddington et al., 1982; Watts et al., 1981). Exogenous ABA applications, on the other hand, have been shown to inhibit root expansion in some studies (Cramer et al., 1996). Increased ABA in roots stimulates water flow by increasing root hydraulic conductivity and ion uptake resulting in a higher water potential gradient between soil and roots (Glinka et al., 1971). It also helps plants transport more water and nutrients while they are stressed by increasing the water absorbing region of their roots. Stress causes metabolic changes in plants resulting in the synthesis and/or aggregation of a wide variety of proteins (Yokota et al., 2002). Drought and salinity tension as well as ABA have been shown to activate certain proteins in studies. This knowledge aided in the description of ABA’s role in cellular signaling processes in plant-stress interactions (Chandler et al., 1994). LEA proteins, dehydrin, lipid transfer proteins, desaturase, responsive to ABA saturase enzymes and other proteins are correlated with cellular structure defense. Nonenzymatic LEA proteins and hydrophilic globular proteins have been extensively studied and have defensive roles (Ingram et al., 1996; Cushman et al., 2000). Overexpression of GmbZIP1 transgenic plants causes stomatal closure under stress resulting in increased stress tolerance. ABP9 and other members of the bZIP family have been linked to increased photosynthetic capability in plants under drought and heat stress (Zhang et al., 2008a). OsABF1 is also active in abiotic stress responses and ABA signaling in roots (Hossain et al., 2010). SIAREB1, a bZIP transcription factor found in tomato, regulates oxidative stress–related proteins, LEA proteins and lipid transfer proteins in response to abiotic stress (Orellana et al., 2010). Thus, ABA is a vital PH to protect plant from various abiotic stresses and helping another PHs and even molecules to work properly in numerous physiological and biochemical processes of plants.

1.2.2.2 Brassinosteroids

A special type of polyhydroxylated steroidal hormones of plants playing fundamental roles in plant growth and development regulation processes having capacity to effect activities of other PHs are known as BRs. BRs are the sixth discovered PHs after well-known auxin, GAs, CK, ABA, and ET which are analogous to animal steroid hormones in structure (Taiz et al., 2010; Clouse, 2011). These novel group of steroidal endogenous hormones have been identified from pollen grains of different plant species (Steffens, 1991) those have ability to promote plant growth which have been confirmed after nearly 30 years effort and when this unidentified active compound was extracted from Brassica napus then it was named as brassin (Mitchell et al., 1970) afterwards, brassins as a new family of PHs was titled BRs. BRs regulate metabolism of plants oxidation radicals, ET synthesis, tolerance of plants responses to stressors (freezing, drought, salinity, disease, heat and nutrient deficiency), and root gravitropic response (Krishna, 2003; Ashraf et al., 2010; Bajguz et al., 2009). Based on various alkyl-substitution patterns of the side chains, BRs are classified as C27, C28, or C29 (Taiz et al., 2010) and the chemical structure of brassinolides (derivatives of BRs (BLs)) are complex ((22R, 23R, 24S)-2a, 3a, 22, 23-tetrahydroxy-24-methyl-B-homo-7-oxa-5a-cholestan-6-one) (Grove et al., 1979). BRs are needed for normal growth and development (Clouse and Sasse, 1998; Sasse, 2003), shoot and root growth (Nemhauser et al., 2004), vascular differentiation (Ashraf et al., 2010; Caño-Delgado et al., 2004), cell elongation (Catterou et al., 2001), xylem formation in epicotyls (Zurek et al., 1994), and fertility and germination of seed (Taiz and Zeiger, 2004). BRs govern a wide range of physiological processes such as leaf development, male fertility, senescence timing, cellular expansion and proliferation, expression of hundreds of genes, activity of numerous metabolic pathways and overall processes of morphogenesis which have been clearly proven by working on BRs biosynthetic or signaling mutants. Any deficiency of BRs in plants brings physiological impediments, decreased seed germination, dwarfism, delayed flowering, late senescence, decreased male fertility, de-etiolation in the dark (Clouse, 2015) and affects phenotypic expression but the mechanisms of BRs induced stress tolerance are still obscured. BRs have role in enhancing photosynthetic capacity and eventually increasing accumulation of biomass though the effect of exogenous application of BRs on plants largely depends on concentrations, plant species, stages of growth, conditions of growth (with or without stress), stress kinds, stress duration, interaction with other hormones, growth regulators, signaling molecules (Nolan et al., 2019; Yin et al., 2019) which is revealed by the genetic studies (Ahammed et al., 2020). BRs have ability to improve resistance of plants against several biotic and abiotic stresses such as drought (Fariduddin et al., 2009b), salinity (Hayat et al., 2010), temperature extremes (Fariduddin et al., 2011; Gomes, 2011), and heavy metals (Bajguz et al., 2009; Fariduddin et al., 2009a; Yusuf et al., 2011; Yusuf et al., 2012). Exogenous treatment of BRs increases the biogenesis of endogenous hormones such as ABA, indole-3-acetic acid, JA, zeatin riboside, BRs, isopentenyl adenosine, and GAs as well as regulates signal transduction pathways to stimulate stress tolerance (Anower et al., 2018). BRs act like animal steroid hormones having effect from embryonic development to adult homeostasis (Bergonci et al., 2014) by a complex signal transduction pathway. Radish seedlings when treated with BRs showed enhancing activity of plant antioxidant enzymes and reduced drought stress effect on plants (Mahesh et al., 2013). BRs can enhance chlorophyll accumulation, total protein contents, amylase activity, photosynthesis, stomatal conductance, and membrane stability which have been observed in the study on sorghum, maize (Talaat et al., 2016), Robinia (Li et al., 2008), and tomato (Yuan et al., 2010; Behnamnia, 2015). So, BRs not only mitigate the effect of drought stress but also enhance plant growth and yield. BRs level are increased in plants and even mutants (genes for biosynthesis and signaling of BRs) endogenously under drought stress which was first reported by Jäger et al. (2008) who observed that elevated castasterone levels in leaves of pea plants. Similar result was found by Gruszka et al. (2016) in barley. The overexpression of BRs receptor gene BRL3 was responsible for enhancing survival rate of plant during drought without reducing growth in BRs mutants such as loss of function mutant bri1 (Fàbregas et al., 2018). Inducing expression of glutathione S-transferase1 in wheat by superoxide scavenging promotes drought response after maintaining homology in TaBZR2, a BES1, and BZR1 (Cui et al., 2019). Studies on tomato (Solanum lycopersicum) observed tRs increase tolerance against drought but the overexpression of BRs receptor gene SlBRI1 had an opposite effect (Nie et al., 2019). Mitigation of drought stress therefore depends on some factors such as responsive gene expression, modulation of ABA levels, H2O2 manufacture, antioxidants production, and osmoprotectant compounds (Planas-Riverola et al., 2019) but the cumulative effect depends on survival of plant, species, manipulation method, and spatiotemporal context. As an aerobic organism, plants have own metabolic pathway to assimilate its energy in the presence of oxygen (Navrot et al., 2007) where baleful creation of ROS during ordinary photosynthesis, respiration, and nitrogen fixation (Mittler et al., 2011) is common but degenerative changes of nucleic acids, proteins, lipids (Prasad, 2004), and redox homeostasis (Gille and Sigler, 1995) may happen in oxidative stress condition. A number of ROS such as superoxide radical, hydroxyl radical, and hydrogen peroxide are generated when plants undergo drought or any other stress. This ROS can subject a series of oxidation/reduction reactions by following the Halliwell–Asada pathway (Gratao et al., 2006). In the stress condition, plants can produce antioxidant enzymes such as catalase (CAT), superoxide dismutase (SOD), guaiacol peroxidase (POX), monodehydroascorbate reductase (MDHAR), ascorbate peroxidase (APX), glutathione reductase (GR), and glutathione peroxidase (Ruley et al., 2004; Simonovicova et al., 2004) and non-enzymatic antioxidants such as ascorbate, α-tocopherol, gluthathione, and carotenoids (Vardhini and Rao, 2003; Ozdemir et al., 2004; Sharma et al., 2005) for defending themselves. Maize treated with BL (brassinolide, derivatives of BRs) at seedling stage under drought stress increases ascorbic acid and carotenoid content as well as increases the activities of APX, CAT, and SOD (Li et al., 1996). Drought tolerance of plants is largely associated with the accumulation of ABA which can be enhanced by the exogeneous application of BRs resulting in mitigation of adverse effect of drought on crops (Wang et al., 2019). EBR application in tomato improved photosynthetic capacity, leaf water status, and antioxidant defense under drought stress resulting in improved tolerance capacity (Yuan et al., 2012). Similarly, exogenous application of BRs (0.02 μM) can uplift light utilization efficiency and the indulgence of exciting energy in the photosystem II (PS II) antennae under drought in pepper leaves (Hu et al., 2013; Li et al., 2016). Likewise, drought tolerance is increased by exogenous treatment of BRs (0.1 μM EBR) in Chorispora bungeana caused by polyethylene glycol (PEG) application (Li et al., 2012a). Two major drought responsive genes namely BnCBF5 and BnDREB can increase their transcript levels upon EBR inducement resulting in enhancement of partly drought tolerance in B. napus seedlings (Kagale et al., 2007). Brassica juncea showed diminished growth and photosynthetic rate after 60 days if experienced weeklong drought at early growth stage though postdrought application of HBL (28 homobrassinolide) (0.01 μM) at 30 days after sowing (DAS) sharply increased both growth and photosynthesis after 60 days of sowing (Fariduddin et al., 2009b) but BRs application can sharply curtail the levels of ROS and lipid peroxidation under stress of drought (Yuan et al., 2010). Elevation of endogenous BRs content in tomato plants but not increasing BRs signaling intensity improves drought tolerance (Nie et al., 2019). By applying microarray analysis of BRs-deficient or BRs-treated plants, drought and cold stress response genes COR-47 and COR-78 were identified (Mussig et al., 2002). An increased zeatin content, nitrogenase activity, and root nodulation in Phaseolus vulgaris prior to drought stress by applying EBL or HBL will ensure protection from reducing trend of these parameters under drought stress though EBL is comparatively more effective than HBL (Upreti et al., 2004). Seedlings of A. thaliana and B. napus raised in 1 μM EBL solution and transplanting to sand bed and undergoing a drought stress by withholding water for 96 h (A. thaliana) or 60 h (B. napus) improved drought stress tolerance in all catered species (Kagale et al., 2007). Moreover, Li et al. (2012b) observed that EBL can progress plant growth under drought stress by shifting antioxidative enzyme activities and antioxidant content where untreated control showed leaf wilting, growth reduction and complete drying of some seedlings. Production of PS II, activity of ribulose-1, 5-bisphosphate carboxylase/oxygenase, water potential, soluble sugars and proline content, and POX and SOD activities were increased in soybean leaves under drought stress significantly when BRs were applied than soybean leaf treated with drought stress only. However, malondialdehyde (MDA) content and electrical conductivity of soybean leaves were decreased in BRs treated leaves but biomass accumulation and seed yield were increased in both control and drought-stressed soybean plants (Zhang et al., 2008b). On the other hand, activities of CAT, POX, SOD, and proline content were increased and reduction in yield loss in mustard plants were recorded when treated with HBL and/or under drought stress (Fariduddin et al., 2009b). Erectness of leaf in rice is very important as more the erectness of leaf more photosynthesis and less transpiration of water will happen resulting in more yield is ensured with erected leaf specially under drought stress conditions where BRs play a vital role in ensuring higher erectness of leaf (Lang et al., 2004). In a study it was found that BRs have positive effect on variation of cell-wall building-block and modification of the membrane system therefore providing the first line of defense against any environmental stresses (Clouse, 1996) and also regulate transcription factors YODA (YDA) and MAPK (mitogen-activated protein kinase) to reduce stomatal conductance which in turn increases salt and drought tolerance capacity of plants (Kim et al., 2012). Crosstalk of BRs and CK to induce drought stress tolerance in plants has been proven by numerous authors (Peleg et al., 2011). For example, increased expression of a key enzyme for CK synthesis is isopentenyl transferase which increases drought tolerance by its gene under drought induced or senescence induced-promoter concurrently increase in protein encoding genes involved in BRs synthesis and their signaling signifying a cross talk between BRs and CK resulting enhancing drought stress tolerance (Sharma et al., 2017). Some proteins playing a vital role in transduction of signal under cold stress are lipocalins and ABA stress ripening-like protein, REMORIN is a vital membrane skeleton protein involved with biotic and abiotic-stress responses, and dehydrin-proteins are the key proteins associated with drought stress response all of which are involved in BRs leading stress tolerance (Li et al., 2012b). Squalene synthase is an enzyme which is RNA interference–mediated disruption in rice catalyzes staring reaction in the isoprenoid metabolic pathway for BLs and sterol biosynthesis leading to decreased stomatal conductance thus enhancing drought tolerance (Manavalan et al., 2012). WRKY is a critical compound in the complex stress-responsive signaling pathways expression, which is induced by BR (brassinosteroids) exogenous treatment in rice (Xiao et al., 2009; Chen et al., 2012) therefore OsWRKY08 confers drought, salt, and cold tolerance by regulating auxin signaling and stress responsive genes (RD and COR) (Song et al., 2010; Chen et al., 2012). NaCl (100 mM root application) toxicity downgrades photosynthetic pigments contents, photosynthetic electron transport and PS II, yields and oxidation of the plastoquinone (PQ) in potato plants but application of 24-EB (24 epibrassinolide) (10−10 M) under this stress challenge increases PS II photochemistry efficiency, PQ pool oxidation and decreases leaf osmotic potential (Kolomeichuk et al., 2020). Linum usitatissimum L. plants when exposed to salinity stress (150 mM) by applying methylation-sensitive amplified polymorphisms technique modification on the extent and pattern of DNA cytosine methylation by reduced methylation of CCGG sequences while seed priming with 24-EBL (10−8 M) plays a vital role in mitigating this stress by regulating epigenetic modification and initiation of methylation (Amraee et al., 2019). For increasing tolerance of Glycine max L. against salinity (100 mM), 24-epibrassinolide (EBL) exogenous application (10−7M) plays a unique role by increasing mineral nutrient uptake, reducing Na+ accumulation, modification of osmolytes, major antioxidant enzymes activities enhancement, increasing nonenzymatic antioxidants levels. and increasing growth and photosynthesis (Alam et al., 2019). Seed treatment and foliar application of 24-EBL (1.5 and 2.0 μM) has potentiality to regulate major physio-biochemical traits and oxidative defense system of maize (var. PR32T83 and PR34N24) under different salinity levels (1.1 and 8.0 dS/m) for mitigating salt toxicity but without EBL treatment fresh and dry mass of plant, PS-II content, chlorophyll contents, leaf water potential, and leaf K and Ca levels were decreased while membrane permeability, lipid peroxidation, activities of H2O2, SOD, POD, and CAT, leaf Na and Cl, proline, GB and leaf sap osmotic pressure were found to be increased (Kaya et al., 2018). Organic pollutants (OPs) exposer (2,4,6-trichlorophenol, chlorpyrifos and oxytetracycline) decreased root elongation by separating the redox homeostasis and secondary metabolism mechanisms in Cucumis sativus L. grown under vermiculite in hydroponic culture resulting in improved activities of MDA, H2O2, and NO but exogenous 24-EBL noticeably reduced the accumulations of H2O2, NO, and MDA through modulating the genes expression associated with detoxification systems and antioxidant (Ahammed et al., 2017). In imidacloprid-challenged (300 mg/kg soil) environment, B. juncea L. exhibited a reduction in protein and 21 amino acid contents but application of 24-EBL (100 nM) as a seed presoaking agent restoration of total protein and amino acid contents was recorded thus, enhancement of pesticide tolerance by applying EBL (Sharma et al., 2017). Toxic effects of Cr (10 mg/kg soil) in tomato plants were remediated by applying 24-EBL (10−7 M) for 8 h as seed priming by regulating physiological, metabolic, and defense mechanisms of crops (Jan et al., 2020). Exposer of two varieties of Vigna radiata (PDM-139 and T-44) under Ni toxicity (0, 50, 100, or 150 mg/kg) and protecting capacity of two doses of HBL (10−8 or 10−6 M) was tested and conferred that HBL is effective in restraining Ni stress up to 100 mg/kg concentration of Ni by increasing growth, nodulation, photosynthesis, and yield attributes of plants (Yusuf et al., 2014). Cucumis sativus L. plants seedlings were uncovered to two chilling pressure (10/8 and 5/3 °C for 18 h) to contemplate the ameliorative job of HBL (10−8 or 10−6 M) applied at 30-day stage through examining the photosynthesis, proteins, growth and biochemical parameters. Chilling pressure decreases growth, chlorophyll, net photosynthesis, productivity of PS II, and modifications in nitrate reductase (NR) and carbonic anhydrase (CA) exercises. Conversely, the exercises antioxidant enzymes CAT, POD, and SOD alongside the proline content expanded in chilling stress. HBL supply applied a defensive job through the kept up higher upsides of antioxidant enzymes and proline content under chilling stress (Fariduddin et al., 2011). Solanum lycopersicum exposed to Cd stress (3 and 9 mg/kg sand) resulted in significant decrease in N metabolism associated enzymes in plant growth chamber but treating with HBL (10−8 M) enhanced growth, photosynthesis, protein, carbohydrate, inorganic N content, and N assimilating enzymes (Singh et al., 2017). Brassica juncea plants under the integrated effect of salt (180 mM)-temperature (4 °C or high 44 °C) induced oxidative stress decreased crop height but improved H2O2 content and the activities of SOD, CAT, APX, GR, DHAR, and MDHAR. Seed priming with HBL (0, 10−6, 10−9, and 10−12 M) increased growth parameters and decreased H2O2 but the plants response was dose dependent (Kaur et al., 2018). BRs have also been shown to increase the behavior of