Cation Transporters in Plants

Cation Transporters in Plants presents expert information on the major cation transporters, along with developments of various new strategies to cope with the adverse effects of abiotic and biotic stresses. The book will serve as a very important repository for the scientist, researcher, academician and industrialist to enhance their knowledge about cation transport in plants. Further, applications listed in the book will facilitate future developments in crop designing strategies. This comprehensive resource provides an alternative strategy for abiotic and biotic stress management in agricultural and horticultural crops.

In addition, it will further improve basic knowledge om the origin and mechanism of cation homeostasis and their role in developmental transition and stress regulation.

• Contains in-depth knowledge about various cation transporters in plants
• Provides information about important macro and micronutrient cation transporters and their applications in the agricultural and biotechnology sectors
• Facilitates agricultural scientists and industries in future crop designing strategies
• Provides an alternative strategy for abiotic and biotic stress management in agricultural and horticultural crops

• Language: English
• Publisher: Academic Press
• Release date: Nov 19, 2021
• ISBN: 9780323885737

Book preview

Chapter 1

Cation transporters in plants: an overview

Alok Sharma¹, Himanshu Sharma¹, ² and Santosh Kumar Upadhyay¹,    ¹Department of Botany, Panjab University, Chandigarh, India,    ²I.K. Gujral Punjab Technical University, Chandigarh, India

Abstract

Cation transporters are involved in the movement of various ions in the plants. These are known to play pivotal roles in many physiological processes like cell signaling, cellular integrity, osmoregulation, nutrition uptake, stress tolerance, ion homeostasis, and photosynthesis. The plants require a well-developed system to maintain ion balance, as their scarcity and surplus accumulation can cause severe problems in normal plant growth and development. Hence, in this chapter, we have discussed various transporters and their types involved in the uptake, transport and efflux of a number of cations like potassium (K), sodium (Na), calcium, (Ca), magnesium (Mg), cobalt (Co), copper (Cu), nickel (Ni), iron (Fe), zinc (Zn), boron (B), aluminum (Al), molybdenum (Mo), cadmium (Cd), arsenic (As), and antimony (Sb). These transporters are not always specific to a particular cation rather sometimes show affinity to other similar cations as well. For instance, some K+ transporters also facilitate the cotransport of Na+ ions. Till now, various cation transporters like K+, Ca²+, Na+, and Fe²+ are well explored, while others have a huge scope of further understanding. Therefore this chapter will provide aid in the understanding of several important ion transporters and their roles in plant development and defense mechanisms.

Keywords

Cation; cell signaling; homeostasis; stress; transporters

1.1 Introduction

Plants require various ions for their physiological and biochemical processes. A number of cations like calcium (Ca), copper (Cu), cobalt (Co), iron (Fe), magnesium (Mg), manganese (Mn), potassium (K), sodium (Na), nickel (Ni), and zinc (Zn) are found to be crucial for the plant development. Ca, Mg, and K are required in macroquantity, while Cu, Fe, Mn, Ni, and Zn are vital in trace amounts (Pilon, Cohu, Ravet, Abdel-Ghany, & Gaymard, 2009). Cobalt and Calcium are recognized as beneficial elements (Pilon-Smits, Quinn, Tapken, Malagoli, & Schiavon, 2009). These cations are well known for their involvement in various cellular components and pathways. Ca and Mg are found to be part of various enzymes and responsible for their catalytic activities (Maathuis, 2009). Furthermore, K is known to be involved in osmoregulation, catalytic activity, and electrochemical balance mechanism of plants (Wang & Wu, 2013). Unlike macronutrients, micronutrients are not directly involved in catalytic activities and osmoregulation, but they act as cofactors for enzymes, protein structure stabilizers, and enzyme activators (Hänsch & Mendel, 2009; Pilon et al., 2009; Pinto & Ferreira, 2015). Moreover, transporters of several trace elements like boron, aluminum, molybdenum, chromium, arsenic, and antimony have also been discussed in the chapter. Intriguingly, some of them are involved in biochemical processes and some can cause toxicity in the plants (Poschenrieder, Busoms, & Barceló, 2019).

Cation transport mechanisms are crucial for the uptake and transport of various micro and macronutrients in plants. Toxicity as well as the deficiency of cation challenges the normal growth and development of plants. Specialized cation transport mechanisms have been developed in plants, which can maintain a balance between the deficiency and toxicity of these ions. Several transporters involved in ion homeostasis have been discussed in the chapter.

1.2 Monovalent cation transporters

1.2.1 Potassium transporters

K+ is one of the most important cations required for the normal functioning of plants. A higher concentration of K+ in various vegetative and reproductive tissues showed their participation in the cellular, molecular, and physiological processes of plants (Amtmann, Troufflard, & Armengaud, 2008). Plants acquire K+ from the soil by only means of the root system. The majority of K+ attainment is carried through the cells of the epidermis and cortical tissues (Ashley, Grant, & Grabov, 2006). The transport of K+ is carried out by both passive and active pathways. Channels are responsible for passive transport, while different transporters are involved in the active transport of K+ in plants.

Mainly, the transportation of K+ in plants is facilitated by the K+ transporters. K+ homeostasis is maintained by four major transporter families, including HAK/KT/KUP, high-affinity K+ transporters (HKT)/TRK, K+ efflux antiporter (KEA), and cation/hydrogen exchanger (CHX) 7 (Gupta et al., 2008; Mäser et al., 2001). Complexities in the nomenclature of these families are attributed to the use of different names in the studies of fungal HAK, Arabidopsis KT, and bacterial KUP (Bañuelos, Klein, Alexander-Bowman, & Rodríguez-Navarro, 1995; Li, Xu, Alli, & Yu, 2018; Li et al., 2018; Quintero & Blatt, 1997; Schleyer & Bakker, 1993). We have briefly discussed here a few potassium transporters, further details are given in Chapters 2–4, and 7.

1.2.1.1 HAK/KT/KUP

The HAK/KT/KUP group is the largest group of K+ transporters. This is further divided into four clusters: I, II, III, and IV (Gupta et al., 2008). The HAK/KUP/KT is found to be involved in plant growth and development and various stress resistance mechanisms, including salt stress and osmotic regulation (Li et al., 2018). Till now, the HAK/KUP/KT family has been studied in numerous plant species, for instance, Arabidopsis, rice, maize, peach, pear, and wheat (Ahn, Shin, & Schachtman, 2004; Amrutha, Sekhar, Varshney, & Kishor, 2007; Cheng et al., 2018; Gupta et al., 2008; Li et al., 2018; Rubio, Guillermo, & Rodríguez-Navarro, 2000; Song, Ma, & Yu, 2015; Zhang et al., 2012). In the case of plants, on the basis of their homology with bacterial KUP and fungal HAK, the HAK/KUP/KT family was first identified in barley and Arabidopsis (KUP1/KT1, KUP2/KT2; K+ transporter) (Quintero & Blatt, 1997; Santa-María, Rubio, Dubcovsky, & Rodríguez-Navarro, 1997). In 2000, Rubio et al. reported the higher expression of HAK genes in shoot and root of Arabidopsis. In their study, they have identified a total of 13 HAK potassium transporters genes in Arabidopsis thaliana. Moreover, they reported that only some plant HAK genes like AtHAK5 and HvHAK2 of A. thaliana and Hordeum vulgare, respectively, were functional in trk1trk2 mutant of yeast (Rubio et al., 2000). Furthermore, different studies reported a total of 16, 21, 21, 24, 45, 44, 22, 27, and 56 KT/HAK/KUP genes in Prunus persica, Pyrus betulifolia, Gossypium arboreum, Gossypium raimondii, Gossypium hirsutum, and Gossypium barbadense, Ipomea, Oryza sativa, and Triticum aestivum, respectively (Jin et al., 2020; Li et al., 2018; Song et al., 2015; Xu et al., 2020). Upregulation of these genes in fruit, flower, cold stress, drought stress, salt stress, low K+ condition, and fiber development has been reported in various studies, which suggests their involvement in plant development and K+ transport (Jin et al., 2020; Li et al., 2018; Song et al., 2015; Xu et al., 2020).

1.2.1.2 High-affinity K+ transporters

The Trk/HKT superfamily involves the TrkH and KtrB (bacteria), HKT (plant), and fungal Trk transporters (Durell, Hao, Nakamura, Bakker, & Guy, 1999). Trk/Ktr/HKT transporters are the main factors for osmotic control, pH homeostasis, and drought and salt tolerance in bacteria, archaea, fungi, and plants (Bafeel, 2013; Cheng et al., 2018; Zhang et al., 2008). These transporters have a wide range of selectivity, that is, they might be the symporter of Na+/K+, Na+ uniporter, and divalent cation transporter.

These transporters are tetrameric in nature, and two transmembranes (TM) domains and a loop form a single motif (Schachtman & Schroeder, 1994). The available reports support the classification of HKTs into three subfamilies (i.e., I, II, and III) through phylogenetic analysis (Horie et al., 2011; Tada et al., 2019). A highly conserved serine (Ser) residue in the first MPAM motif is present in subfamily I of HKT transporters, while subfamily II members mainly have glycine (Gly) residue (Su, Luo, Lin, Ma, & Kabir, 2015). Members of subfamily III are similar to subfamily II with typical features of GlyGlyGlyGly type. The substitution of glycine and serine amino acids in these groups might be responsible for their different cation selectivity (Horie et al., 2007). On the basis of these features, the plant HKT transporters can be divided into two groups, that is, SerGlyGlyGly and GlyGlyGlyGly type. The SerGlyGlyGly is found to be exclusive for Na+ uniport, whereas GlyGlyGlyGly showed permeability for K+ and Mg²+/Ca²+ as well, OsHKT2;1, came out to be an exception in this context as it shows three modes of ion selectivity based on the concentration of external Na+ and K+ (Horie et al., 2001, 2011; Yao et al., 2010). The diverse selectivity of GlyGlyGlyGly type is might be due to their comparatively more flexible structure because of the presence of glycine in place of serine residues (Su et al., 2015).

TaHKT2;1 of wheat is reported to be permeable for both Na+ and K+ uptake (Gassmann, Rubio, & Schroeder, 1996). Similarly, PutHKT2;1 of a salt-tolerant plant Puccinellia tenuiflora showed transportability for both Na+ and K+ in transformed yeast and Arabidopsis (Ardie, Xie, Takahashi, Liu, & Takano, 2009). Similarly, the overexpression of HvHKT2;1 from barley showed the cotransport ability of these genes for both Na+ and K+ and conferred the salt tolerance in Xenopus oocytes (Mian et al., 2011). In various other studies, the role of type II HKT transporters in the cotransport of Na+ and K+ cations and stress resistance has been discussed (Rubio, Gassmann, & Schroeder, 1995; Tada & Ohnuma, 2020; Tada et al., 2019). For more information, please refer to the Chapter 3.

1.2.1.3 Cation-proton antiporters

Cation-proton antiporters (CPAs) superfamily involves the two subgroups, that is, CPA1 and CPA2 (Saier, 2000). The CPA family is known to be involved in the transport of monovalent cations in both prokaryotic and eukaryotic systems. Their localization is found to be in the plasma membrane (PM), vacuolar and organelle membranes. They are well known for their involvement in the homeostasis of pH and various cations in a cell (Brett, Donowitz, & Rao, 2005; Chanroj et al., 2012). The CPA1 family comprises Na+/H+ exchanger (NHX), whereas CHX and KEA fall under the CPA2 group (Chanroj et al., 2012; Ye, Yang, Xia, & Yin, 2013).

1.2.1.3.1 K+ efflux antiporter

KEA encoding genes in higher plants were first classified in the A. thaliana (Mäser et al., 2001). Based on the homology with bacterial potassium antiporters, these are clustered into K1 and K2 subgroups. The proteins clustered with AtKEA1–3 and AtKEA4–6 have been placed in clade 1 and clade 2, respectively. Clade 1 proteins showed homology with bacterial proteins, EcKefB and EcKefC, and possess a complete KTN domain. In the case of clade II proteins, they are similar to metazoa protein TMCO3 and lack the KTN domain (Chanroj et al., 2012). In 2012, the functional characterization of AtKEA2 was performed for the first time (Aranda-Sicilia et al., 2012). In their experiment, they have reported a higher expression of AtKEA2 in leaves and found it to be localized in the chloroplast. Moreover, they found the involvement of AtKEA2 in K+/H+ exchange activity, which ultimately leads to pH and cation homeostasis (Aranda-Sicilia et al., 2012). Furthermore, the role of KEA1, KEA2, and KEA3 in chloroplast integration and its normal functioning was also reported (Kunz et al., 2014). The association of KEA transporters in some important cellular processes like photosynthesis and nodulation was also reported by different research groups (Armbruster et al., 2014; Rehman et al., 2017). Several genome-wide studies explored the total number of KEA genes in various plants, ranging from algae to angiosperms (Sharma et al., 2020; Wang et al., 2020; Ye et al., 2013; Zhou et al., 2016). Moreover, a detailed description has been provided in the Chapter 7.

1.2.1.3.2 Cation/H+ exchanger

CHX is another important monovalent cation transporter family of CPA2 group. These proteins are involved in the transportation of important cations like Na+, K+, and H+ (Sze et al., 2004). CHX proteins consist of 10–12 membrane-spanning domains with a carboxy tail (Zhao, Li, Motes, Park, & Hirschi, 2015). Various studies performed the functional characterization of CHX genes of different plants. In 2001, Maser et al. identified a total of 28 CHX (AtCHX1–28) genes in A. thaliana (Mäser et al., 2001). Further, the functional characterization of these AtCHX genes was carried out by various research groups in their studies. In a number of studies, the participation of AtCHX genes in various physiological and reproductive processes was reported (Lu et al., 2011; Padmanaban et al., 2007; Zhou et al., 2016). Moreover, AtCHX13, AtCHX14, AtCHX17, AtCHX20, and AtCHX23 were found to be responsible for the transport and distribution of K+ (Cellier et al., 2004; Hall, Evans, Newbury, & Pritchard, 2006; Ye et al., 2013; Zhao et al., 2008). Besides these studies, various groups also conducted the genome-wide identification of the CHX gene family in a number of plant species, which showed the variable number of CHX genes in the different group of plants (Ma, Wang, Zhong, Cramer, & Cheng, 2015; Sharma et al., 2020; Wang et al., 2020; Ye et al., 2013; Zhou et al., 2016).

1.2.2 Sodium transporters

Na+ is one of the major causes of soil salinity, which affects the growth and development of plants. Although salinity stress is a result of accumulation of various ions; however, NaCl is the leading factor for the saline nature of the soil (Zhang, Flowers, & Wang, 2010). It has been observed that a higher amount of Na+ leads to osmotic imbalance and ion toxicity, which in turn leads to plant growth irregularities (Blumwald, 2000). There are various ways of Na+ transportation in plants, for instance, non-selective cation channels (NSCCs), low-affinity cation transporters (LCTs), KUP/HAK/KT, AKT, and NHX.

1.2.2.1 Na+/H+ exchanger

NHX family transporters are the member of the CPA1 class of monovalent cation-proton transporters and are possibly present in all the organisms (Brett et al., 2005; Sharma et al., 2020). These genes are thought to be evolved from the prokaryotic Na-proton antiporter (NhaP) genes (Brett et al., 2005; Mäser et al., 2001). For the first time in plants, a gene named as NHX1 was reported in the A. thaliana (Gaxiola et al., 1999). Later on, a total of eight NHX genes were identified in A. thaliana. Based on their cellular location, these eight genes are classified into three different classes. The vacuolar (AtNHX1–4) and endosomal (AtNHX5 and AtNHX6) localized genes come under class one and two, respectively, while the third class found to be localized in the PM and includes the SOS1/AtNHX7 and AtNHX8 genes (Bassil, Coku, & Blumwald, 2012). The working mechanism of these differently localized NHX proteins varies and the PM-localized NHX proteins export the Na+ outside of the cell and import an H+. However, the AtNHX5 and AtNHX6 and SOS1/AtNHX7 and AtNHX8 work by transporting the Na+/K+ in the vacuole and endosomes, respectively, and in turn export the H+ ion in the cytosol (Bassil et al., 2012). The presence of NHX genes in lower to higher plants supported their conserved and decisive roles in all the plant groups (Chanroj et al., 2012). The NHX genes were studied for their various roles in plant development and stress management. The involvement of NHX transporters in the various physiological processes and stress resistance is reported in various studies (Barragán et al., 2012; Hanana et al., 2007; Zhang et al., 2017). Recent advancements in bioinformatics lead to the various genome-wide studies of NHX genes in a number of plants, which showed their occurrence in a diverse group of plants (Ma et al., 2015; Sharma et al., 2020; Wang et al., 2020; Ye et al., 2013; Zhou et al., 2016). Furthermore, the descriptive information of the NHX family has been given in Chapter 6 (Sharma, Sharma, & Upadhyay, 2021).

1.2.2.2 Low-affinity cation transporters and salt overly sensitive

LCTs are also known to be involved in Na+ influx. These were first to be isolated from wheat and named as LCT1. These proteins show some homology with NSCCs (Amtmann et al., 2001). The expression of wheat LCT1 in yeast depicted their role in Na+ transport, which caused the reduction of K+: Na+ ratio in the cell (Amtmann et al., 2001). However, the specific role and mechanism of TaLCT1 is still uncertain as Plett and Moller in 2010 argued that it is not directly involved in Na+ transport (Plett & Møller, 2010). Moreover, the cations like Ca+ and Cd²+ was also found to be transported by TaLCT1 (Antosiewicz & Hennig, 2004). Further, there is a need for the detailed study of the mechanism and ion specificity of LCTs in the future to attain a complete understanding of these transporters.

Another less explored group of Na+ transporter is SOS. In 1996 Wu et al. identified the SOS1 phenotype for the first time (Wu, Ding, & Zhu, 1996). In addition to SOS1, the SOS complex involves the SOS2 and SOS3, which are calcium-binding protein and serine/threonine kinases, respectively (Qiu et al., 2004; Quintero, Ohta, Shi, Zhu, & Pardo, 2002). Initially, the SOS1 was thought to be involved in K+ transport; however, later studies advised the SOS1 as PM Na+/H+ antiporter (Shi, Ishitani, Kim, & Zhu, 2000). The specific role of AtSOS1 is still ambiguous; however, it is predicted to be involved in the export of Na+ from root to soil and loading of Na+ into xylem tissue. These two different ways of work are dependent on the concentration of salt, that is, under high salt conditions, these act by efflux of Na+ in soil, while medium salt condition favors the xylem distribution of Na+ (Shi, Ishitani, Kim, & Zhu, 2000). However, some studies suggested the contrary results of SOS1 under different salt stress conditions (OlÍas et al., 2009). In a recent study, Mahi et al. published the role of SOS1 in salt stress resistance of O. sativa via Na+ homeostasis and sensing (Mahi et al., 2019).

1.2.2.3 HAK/KT/KUP and HKT

The structural and physicochemical similarity of K+ and Na+ makes them suitable for being a competitor for transportation. Hence, the transport of Na+ is also carried out by the various K+ transporters. Several studies suggested the involvement of the HAK/KT/KUP group in both K+ and Na+ transportation (Ardie, Liu, & Takano, 2010; Horie et al., 2011; Zhang, Flowers, & Wang, 2013), while some studies fail to provide a clear idea about the involvement of these genes in Na+ influx (Nieves-Cordones, Alemán, Martínez, & Rubio, 2010). The expression of AtHKT1;1 of A. thaliana and its orthologous genes in rice (OsHKT1;1) showed the Na+ transport activity (Haro, Bañuelos, Senn, Barrero-Gil, & Rodríguez-Navarro, 2005; Jabnoune et al., 2009). However, group 2 members of rice (OsHKT2;1, OsHKT2;2, and OsHKT2;4) and barley (HvHKT2;1) showed cotransportation of Na+ and K+ (Mian et al., 2011; Sassi et al., 2012; Yao et al., 2010). In addition to HAK/KT/KUP, under HKT, the subclass SerGlyGlyGly are Na+ specific transporters in both monocots and dicot plants (Deinlein et al., 2014; Garciadeblás, Senn, Bañuelos, & Rodríguez-Navarro, 2003). Moreover, OsHKT2;1, HvHKT2;1, and TaHKT2;1, of rice, barley, and wheat, respectively, are found to be involved in the uptake of Na+ (Haro et al., 2005; Horie et al., 2007; Laurie et al., 2002).

1.3 Divalent cation transporters

1.3.1 Calcium transporters

Calcium is a key macronutrient in the cell and plays a crucial role by means of a nutrient and a secondary messenger (Sanders, Pelloux, Brownlee, & Harper, 2002). As an essential nutrient, calcium is important for the structural integrity and metabolic processes of the cell. However, in signaling, calcium acts as a sensor for external stimuli and binds to various calcium-binding proteins, which ultimately leads to gene expression via activation of the various kinases (Zhang, Du, & Poovaiah, 2014). The increased concentration of Ca²+ under the influence of various plant hormones, nutrients, and diverse stresses was reported, which also proposed their role in signaling mechanisms (Kader & Lindberg, 2010; Tuteja & Sopory, 2008). Generally, Ca²+ is present in the higher concentration in soil and some plant organelle like vacuole and endoplasmic reticulum (0.1–10 mM) as compared to the cytoplasm (50–150 nM) (Stael et al., 2012). However, plants have developed various transportation ways to maintain Ca²+ homeostasis in the plant cell (Taneja and Upadhyay, 2021a; Upadhyay, 2021). In the plants, Ca²+ is transported by three types of transporters: Ca²+ permeable channels, Ca²+-ATPases, and Ca²+/cation antiporters (CaCAs) (Axelsen & Palmgren, 2001; Kaur, Madhu, Taneja, & Upadhyay, 2021; Taneja & Upadhyay, 2021b). The Ca²+ permeable channels allow a passive influx of Ca²+ through the vacuolar membrane and other endomembranes, while the active transportation is performed by the other two classes. The detailed account of calcium transporters and their applications can be found in an earlier book (Upadhyay, 2021; Kaur et al., 2020; Kaur, Madhu, & Upadhyay, 2021).

1.3.1.1 Ca²+-ATPases

The Ca²+-ATPases (PMCAs) perform the efflux of Ca²+ from the cytoplasm and show some characteristic features like phospho-aspartate enzyme formation, presence of conserved motifs and vanadate inhibition (Huda, Banu, Tuteja, & Tuteja, 2013; Taneja & Upadhyay, 2018). PMCAs are categorized into two types: type IIA (P2A) and type IIB (P2B). The presence or absence of the N-terminal autoinhibitory domain is the basis of their classification. Various genome-wide and characterization studies indicated the involvement of PMCAs in stomatal regulation, hormonal signaling, tissue development, reproduction, and abiotic and biotic stresses (Axelsen & Palmgren, 2001; Goel, Taj, Pandey, Gupta, & Kumar, 2011; Huda et al., 2013; Taneja & Upadhyay, 2018).

1.3.1.2 Ca²+/cation antiporters

The second class of active transporter of calcium is CaCAs, which facilitates the transport of Ca²+ outside the cell in turn of an influx of H+, Na+, or K+ cations. The CaCA superfamily is found to be present in bacteria, plants, and animals (Emery, Whelan, Hirschi, & Pittman, 2012; Pittman & Hirschi, 2016a; Taneja, Tyagi, Sharma, & Upadhyay, 2016). Based on their evolutionary relatedness and functions, it is categorized into five different families, including Na+/Ca²+ exchanger (NCX), YRBG, cation/Ca²+ exchanger (CCX), Na+/Ca²+, K+ exchanger (NCKX), and H+/cation exchanger (CAX) (Emery et al., 2012; Pittman & Hirschi, 2016a). Moreover, in the case of land plants, the two additional types, that is, EF-hand domain-containing CAX (EF-CAX) proteins and Mg²+/H+ exchanger (MHX) proteins, are also present (Emery et al., 2012; Taneja & Upadhyay, 2018).

YRBG family is limited to the prokaryotic group only, while the NCX and NCKX are absent in higher plants and found to be present in algae and animals. However, CCX, CAX, EF-CAX group is present in the higher plants also and are responsible for Ca²+ transportation, while the MHX group is involved in the Mg²+ homeostasis (Emery et al., 2012; Khananshvili, 2013; Lytton, 2007; Pittman & Hirschi, 2016a; Shigaki, Rees, Nakhleh, & Hirschi, 2006).

In 2004 Platy et al. performed the functional characterization of a mammalian CCX protein, which showed that these proteins mediate the Na+(Li+)/Ca²+ exchange activity (Palty et al., 2004). Although an Arabidopsis CCX3 protein exhibited the H+/K+ activity and was unable to transport Ca²+, which suggested the diversity among the functions of CCX (Morris et al., 2008). Additionally, various genome-wide studies identified the different number of CCX genes in plants, including Glycine max, T. aestivum, and Solanum lycopersicum (Amagaya, Shibuya, Nishiyama, Kato, & Kanayama, 2019; Emery et al., 2012; Taneja & Upadhyay, 2018).

CAX proteins are ubiquitous in nature and comprised the largest group of the CaCA superfamily (Emery et al., 2012; Pittman & Hirschi, 2016b; Taneja et al., 2016). Phylogenetic relatedness classified the CAX group into three types: type1, type2, and type3. In plants, usually, type 1 CAX proteins are present, which can be categorized into 1A and 1B groups (Emery et al., 2012; Shigaki et al., 2006). Functionally, the 1A type proteins, like AtCAX1 and AtCAX3, are specific to Ca²+, while type 1B proteins, for instance, AtCAX2 and AtCAX5, showed the affinity for Ca²+, Cd²+, and Mn²+ (Conn et al., 2011a, 2011b; Edmond et al., 2009; Hirschi, Korenkov, Wilganowski, & Wagner, 2000; Shigaki, Pittman, & Hirschi, 2003). On the contrary, the expression studies in the heterologous system found the transportation of multiple ions by type 1A proteins. Due to their wide range of selectivity, recently these are referred to as CAX (Mei et al., 2009).

NCL proteins are newly identified members of CaCA superfamily and are involved in Na+/Ca²+ exchange function (Wang & Wu, 2013). This NCL group is distinct to the CAX group because of the occurrence of a long cytoplasmic loop with EF-hand domains (Emery, Whelan, Hirschi, & Pittman, 2012; Pittman & Hirschi, 2016a). The role of AtNCL proteins under abiotic stress and biotic stress was also studied by some groups (Li et al., 2016; Wang et al., 2012).

1.3.2 Magnesium transporters

Mg²+ is the most bountiful free divalent cation in cells and associated with enzymatic activities, membrane integrity, photosynthesis, etc. (Le-Gong et al., 2008; Shaul, 2002). Either the deficiency or toxicity of Mg²+ can negatively affect plant growth, so Mg²+ homeostasis is an important phenomenon for plants. The Mg²+ can be transported into the root passively through Mg-permeable cation channels (Karley & White, 2009). However, the major contribution in Mg²+ transportation is achieved by the Mg²+ active transporters (Gebert et al., 2009). In plants, the main family involved in Mg²+ transportation is Mg²+ transporters (MGTs), which is a homolog of bacterial CorA proteins (Li, Tutone, Drummond, Gardner, & Luan, 2001). Till now, in A. thaliana, various MGT transporters have been identified (Conn et al., 2011a; Gebert et al., 2009; Le-Gong, Yong-Hua, Dong-Ping, Julie, & Sheng, 2008; Mao et al., 2014; Oda et al., 2016; Xu et al., 2015a). These transporters genes are found to be expressed in different tissues and facilitate the Mg²+ translocation in plants. For instance, MGT5 showed the maximum expression in pollen cells and are involved in the reproduction (Le-Gong et al., 2008), while MGT6 and MGT7 are highly expressed in roots and showed their role in Mg²+ homeostasis (Gebert et al., 2009; Mao et al., 2014; Oda et al., 2016). The detailed information of Mg²+ transporters has been provided in the Chapter 8.

1.3.3 Manganese transporters

Manganese (Mn) is one of the essential micronutrients, known to be involved in various plant growth and development processes like photosynthesis, lipid biosynthesis, and oxidative stress, where it acts as a cofactor (Socha & Guerinot, 2014). Mn deficiency in plant leads to decreased growth, yield and make them susceptible for pathogen attacks. Mn exists in many oxidation states but plants absorb Mn²+ form via root cells (Pittman, 2005). In the plants, various transporter gene families are involved in Mn transport. But there are only a few Mn specific transporters are available, as Mn can be transported by the transporters of other divalent cation such as Fe. The NRAMP (natural resistance-associated macrophage protein) transporter family was studied in Arabidopsis (Thomine, Wang, Ward, Crawford, & Schroeder, 2000). A total of six NRAMP genes have been identified, out of which three genes were functionally characterized (AtNRAMP1, AtNRAMP3, and AtNRAMP4 genes) (Thomine et al., 2000). AtNRAMP1 was reported to be localized in the PM and believed to transport Mn in root under Mn deficiency condition, while AtNRAMP3 and AtNRAMP4 export Mn from vacuole of mesophyll cells of the leaf (Lanquar et al., 2010). In rice, seven NRAMP genes were found and four were functionally characterized. OsNRAMP3 transporter is localized on the node of rice and transports Mn from the xylem to young tissues and panicles under the deficient condition (Yamaji, Sasaki, Xia, Yokosho, & Ma, 2013), whereas OsNRAMP5 gene expresses in roots under Fe and Mn deficient condition (Sasaki, Yamaji, Yokosho, & Ma, 2012).

In the case of the YSL (yellow stripe-like) transporter family, AtYSL4 and AtYSL6 were found to be localized in the vacuoles of Arabidopsis and known to play roles in Mn transport (Conte et al., 2013). In rice, two OsYSL2 and OsYSL6 proteins are believed to have roles in Mn homeostasis (Koike et al., 2004). Zinc-regulated transporter and iron-regulated transporter-like proteins (ZIP) family is also involved in Mn transport. In Arabidopsis, an iron-regulated transporter (IRT) AtIRT1 (Arabidpsis thaliana IRT1), which has a high affinity to Fe, is found to have a low affinity for Mn transport as well (Eide, Broderius, Fett, & Guerinot, 1996; Vert et al., 2002). Another ZIP family transporter member in Arabidopsis, AtZIP1, AtZIP2, AtZIP5, AtZIP6, AtZIP7, and AtZIP9 has been known to perform a function in Mn transport into cytoplasm across root and shoot of the plant (Milner, Seamon, Craft, & Kochian, 2013). In the case of rice, OsZIP5 transporter gene is found to be slightly induced in the roots upon Mn deficiency (Lee, Kim, Lee, Guerinot, & An, 2010). Besides Arabidopsis, ZIPs members have been studied in plants like H. vulgare (HvIRT1), Medicago truncatula (MtZIP4 and MtZIP7), Pisum sativa (PsIRT1), and S. lycopersicum (LeIRT1 and LeIRT2) and known to have roles in Mn transport (Socha & Guerinot, 2014).

The CAX (cation exchanger) family is known to play role in calcium transport (Emery et al., 2012). Different studies revealed the role of these transporters in Mn transport. For instance, as in Arabidopsis, AtCAX2, AtCAX4, and AtCAX5 were found to be involved in Mn transport from the cytoplasm to vacuole (Edmond et al., 2009; Schaaf et al., 2002; Shigaki, Pittman, & Hirschi, 2003). The CCX family is also known to have a slight role in the transport of Mn as it majorly transports calcium (Emery et al., 2012; Shigaki et al., 2006). AtCCX3 of Arabidopsis transport Mn and its expression was produced in root and flower (Morris et al., 2008). CDF/MTP (cation diffusion facilitator/metal tolerance protein) transporter family members export metal ions from the cytoplasm. The first Mn-CDF transporter, ShMTP8, has been identified in Stylosanthes hamata (Delhaize, Kataoka, Hebb, White, & Ryan, 2003). Total two, two, four, and five Mn-CDF transporters have been found in poplar (PtMTP11.1 and PtMTP11.2), beet root (BmMTP10 and BmMTP11), Arabidopsis (AtMTP8, AtMTP9, AtMTP10, and AtMTP11), and rice (OsMTP8, OsMTP8.1, OsMTP9, OsMTP11, and OsMTP11.1), respectively, which have roles in Mn transport (Chen et al., 2013; Delhaize et al., 2007; Peiter et al., 2007). The P-type ATPases family is also involved in the transport of Mn ions. Two proteins AtECA1 (ER-type calcium ATPases) and AtECA3 are reported to be localized in the endoplasmic reticulum and Golgi complex, respectively. Both proteins pump Mn from the cytosol (Liang, Cunningham, Harper, & Sze, 1997; Mills et al., 2008). VIT (vacuolar iron transporter)/CCC1 (Calcium sensitive complementary 1) also transports Mn as along with Fe into vacuoles. The best-studied example is AtVIT1, which is involved in Mn uptake into vacuoles (Kim et al., 2006). For more comprehensive information, please refer to the Chapter 13.

1.3.4 Iron transporters

Iron (Fe) is an essential micronutrient, which is required for various vital cellular processes like photosynthesis, respiration, nitrogen fixation, hormone, and DNA synthesis in plants (Briat & Lobréaux, 1997). Many cellular processes depend upon Fe as it has the ability to transfer electrons by shuttling from its reduced state (Fe²+) and its oxidized state (Fe³+). However, molecular Fe is used directly as a cofactor by some enzymes and maximum proteins use Fe-containing cofactors. These can be categorized into two main forms, Fe heme and Fe sulfur clusters. Fe heme is found in cytochromes, while Fe sulfur is used by various biosynthetic enzymes (Thomine & Lanquar, 2011). Though Fe is one of the utmost abundant metals in the earth’s crust but its availability to plant is very low. The availability of Fe depends on two factors, soil redox potential and pH. At high pH, Fe is present mainly in insoluble ferric oxide form, while at low pH it gets freed from oxide and become available in the soil which further uptake by roots (GuerinotYi, 1994). Due to low availability of Fe in soil, plants have evolved two strategies to overcome this barrier. Strategy I is generally used by nongraminaceous monocot and dicot plants, while strategy II includes all graminaceous monocots, especially cereal crops. Strategy I involve three steps of Fe uptake: first, release protons for the acidification of rhizophore (Santi & Schmidt, 2009), secondly reduced Fe³+ to Fe²+ form with the use of ferric-chelate reductase 2 (FRO2) (Robinson, Procter, Connolly, & Guerinot, 1999), and lastly Fe²+ transportation across the PM of root using IRT1 (Eide et al., 1996; Vert et al., 2002). IRT1 is high-affinity transporter for Fe²+, identified in A. thaliana (Eide et al., 1996). Strategy II is a chelation-based strategy in which plants first secrete Fe³+ specific phytosiderophores (PS) to form Fe³+–PS chelate complex. Further, this complex is reimported into root cells via yellow stripe 1 (YS 1) transporter (Curie et al., 2001). This transporter was first identified in the maize mutant (ys1) (Curie et al., 2001). In rice, 18 YS 1-like genes (OsYSLs) encoding transporters have been identified (Koike et al., 2004). However, rice can also transport Fe through OsIRT1 and OsIRT2 transporters (Ishimaru et al., 2006). In barley root, HvYS1, Fe³+–PS uptake transporter has been studied (Murata et al., 2006). Plant vacuole has VITs, which are used to store metal, as Fe can cause cellular toxicity. In A. thaliana, under high iron concentration, AtVIT1, AtVTL, AtMEB1 and AtMEB2, AtVTL2 and AtVTL5 (Gollhofer, Timofeev, Lan, Schmidt, & Buckhout, 2014; Kim et al., 2006; Yamada, Nagano, Nishina, Hara-Nishimura, & Nishimura, 2013) are known to involved in iron transport. In the case of rice, OsVIT1 and OsVIT2, vacuolar iron transporters, have been identified which were located on flag leaf blades and sheath of a plants (Zhang et al., 2012). BnMEB2, vacuolar iron transporter protein of Brassica napus, was identified and known to have roles in the enhancement of tolerance of iron toxicity in transgenic Arabidopsis (Zhu et al., 2016). In T. aestivum, TaVIT2 transporter protein was recognized which have roles in iron transport (Connorton et al., 2017). Many other transporters like ferroportin FPN2 (IREG2), AtNRAMP3, and AtNRAM4 have been identified for their roles in iron transport (Lanquar et al., 2005; Morrissey et al., 2009; Schaaf et al., 2006). Maximum leaf iron is located in the chloroplast. The synthesis of heme and Fe–S cluster assembly occurs in the chloroplast (Shingles, North, & McCarty, 2002). AtFRO7, ferric-chelate reductase, is used to transport Fe²+ into the chloroplast in developing seeds (Jeong et al., 2008). Additionally, PIC1 (Permease In Chloroplast) transporter is present in the inner membrane of the chloroplast. AtPIC1 transporter has been identified in Arabidopsis to transport iron inside the chloroplast (Duy et al., 2007). In Arabidopsis, AtATM3 is known to be involved in the export of Fe–S cluster from mitochondria (Kispal, Csere, Prohl, & Lill, 1997, 1999). Furthermore, the AtIRT2 gene sequesters the Fe accumulation under Fe deficient condition (Vert, Briat, & Curie, 2001). Additionally, FDR3 (Ferric reductase defective 3) protein has been isolated which transports citrate in the xylem. The iron moves through the xylem as a ferric citrate complex (Durrett, Gassmann, & Rogers, 2007). FPN1 is another transporter, located on the PM, used to load iron in the xylem for root-to-shoot transportation (Morrissey et al., 2009). AtOPT3, an OligoPeptide Transporter, was first discovered in Arabidopsis involved in Fe transportation (Wintz et al., 2003). In the case of Golgi complex, BCD1, the multidrug and toxic compound extrusion (MATE) family transporter, found to be involved in the homeostasis of Fe in Arabidopsis (Seo et al., 2012). The detailed knowledge of iron uptake and homeostasis has been provided in Chapter 9, Mechanism of Iron Uptake and Homeostasis in Plants (Shumayla & Upadhyay, 2021).

1.3.5 Zinc transporters

Zinc (Zn) is another essential micronutrient required for normal growth and development in plants. It is involved in the regulation of enzymes as it is a crucial component of biological metabolic enzymes (Maret, 2004). Plants have developed a multiform transport system to transport Zn ions. It is taken as a free ion form or complexed with PS (Sinclair & Krämer, 2012). ZIP transporter family (Zn-regulated, iron-regulated transporters-like proteins) regulate the transport of Zn across the plant cytoplasm. AtIRT1 is the first identified Zn transporter that is located in root, member of ZIP family, mediate the transport of Zn as well as Fe in A. thaliana (Barberon et al., 2011; Shanmugam et al., 2011). Similarly, AtZIP1 and AtZIP2, AtIRT1, AtIRT2, and AtIRT3 also have the ability to transport Zn into roots (Lin et al., 2009; Milner et al., 2013; Vert et al., 2009). In rice, the members of ZIP family (OsZIP1, OsZIP3, OsZIP4, and OsZIP5) were found to have roles in transport of Zn (Kavitha, Kuruvilla, & Mathew, 2015; Lee et al., 2010; Sasaki, Yamaji, Mitani-Ueno, Kashino, & Ma, 2015). Another transporter of Zn belongs to NRAMP family, AtNRAMP1 has been identified in Arabidopsis, which was able to mediate the influx of Zn into root cells (Cailliatte, Schikora, Briat, Mari, & Curie, 2010). Zn is also transported in bound form with PS (Zn–PS complex) by YS and YSL family transporters (Araki, Murata, & Murata, 2011; Murata et al., 2008). In root vacuoles, another vacuole membrane transporter of Zn, AtMTP1 transporter, has been found, which perform an important role in detoxification of excess of Zn in Arabidopsis (Kawachi et al., 2009). Additionally, tonoplast localized AtMTP3, AtPCR2, AtFDR3 are also found to have functions in Zn partitioning in Arabidopsis (Arrivault, Senger, & Krämer, 2006). After loading into the xylem, Zn can be transported to the above tissues by members of YS, YSL, IRT, ZIP, ZIF, and HMA family transporters (Chu et al., 2010; Haydon et al., 2012; Morel et al., 2009; Sinclair & Krämer, 2012). In rice, OsVIT1 and OsVIT2 contribute to transport Zn into vacuoles (Zhang et al., 2012).

1.3.6 Nickel transporters

Nickel (Ni) is an essential trace element, present in the Ni²+ oxidation state in the environment, which is the most available form for plants. These are an essential component of the urease enzyme and hydrolyze the ammonia and bicarbonate (Polacco, Mazzafera, & Tezotto, 2013). It can be transported symplastically, through AtIRT1, into the root cells (Nishida, Aisu, & Mizuno, 2012). YS1 family is found to be involved in the translocation of Ni in the form of Ni–PS complex through ZmYS1 of maize (Murata et al., 2008). Moreover, in hyperaccumulator plant, Thlaspi caerulescens, YSL family transporters also transport Ni, through TcYSL5 and TcYSL7 proteins, which are located on vascular bundles of shoots and central cylinder of the roots (Gendre et al., 2007). It can also form complexes with other molecules like citrate and histidine during translocation from root to shoots (Centofanti et al., 2013; Richau et al., 2009).

1.3.7 Copper transporters

Copper (Cu) is one of the essential micronutrients known to be entailed in various processes of plant growth and development (Bertrand & Poirier, 2005). The excess or deficiency of Cu may affect the plant metabolism. Thus Cu transporters are known to maintain Cu homeostasis. Mostly, Cu²+ form is available in soil but in less availability of Cu²+, it gets reduced to Cu+ (cuprous ion). The reduction process is executed by ferric reductase oxidases, studied in Arabidopsis (AtFRO2, AtFRO3, AtFRO4, and AtFRO5) (Bernal et al., 2012; Mukherjee, Campbell, Ash, & Connolly, 2006). The plants mostly absorb Cu+ form through Cu-specific transporters, COPT family (copper transporter). Six members have been identified in Arabidopsis, which are known to be involved in the movement of Cu (Andrés-Colás, Perea-García, Puig, & Peñarrubia, 2010; Yamasaki, Hayashi, Fukazawa, Kobayashi, & Shikanai, 2009). Members of the ZIP family in Arabidopsis and YS and YSL family in maize and barley are also reported to be involved in Cu transport (Araki et al., 2011; Murata et al., 2008). HMA5 (heavy metal-associated domain) present on tonoplast also participate in the influx of Cu during its high supply (Andres-Colas et al., 2006). The comprehensive knowledge of copper transporters and their mechanisms has been provided in the Chapter 12 of COPT/Ctr family transporters.

1.3.8 Cobalt transporters

Cobalt (Co) is a beneficial element, known to perform important functions in living organisms. In plants, especially in legume nodules, Co is found to be involved in symbiotic nitrogen fixation (Marschner, 2012). Cobalt exists in divalent form and is mostly available to the plants in acidic soil. As pH increases, the availability of Co gets decreased (Lange et al., 2014). The mechanism Co²+ transport is not clearly understood, but few transporters genes are involved, which translocate Co²+ in the plant. AtIRT1 (iron-regulated transporter 1) of Arabidopsis mediates the transport of both Co²+ and Fe²+ in root cells (Vert et al., 2002). Other family members like ZmYS1 and AtNRAM1 participate in the uptake of Co as a Co–PS complex in the roots of rice and Arabidopsis, respectively. (Cailliatte et al., 2010; Murata et al., 2008). Another tonoplast located ferroportin transporter, FPN2, transport the Co inside the vacuoles (Morrissey et al., 2009). Additionally, the PM-localized FPN1 transporter on stele also transports cobalt. When Co is present in a higher amount, the MTP1 transporter facilitates the translocation of Co into vacuoles (Wang et al., 2018).

1.4 Trivalent cation transporters

1.4.1 Boron transporters

Boron (B) is a micronutrient, which is required for the plant development and found in both the lower and higher group of plants (Takano, Miwa, & Fujiwara, 2008). In soil, B exists in various forms like water-soluble form, nonspecific and specific observed forms, Fe–Al and Mn oxide-bound forms, and B residues (Rehman et al., 2018; Xu, Wang, & Meng, 2001). Main function of B is in the strengthening of the cell wall by cross-linking of pectin through bonding to rhamnogalacturonan-II (Kobayashi, Matoh, & Azuma, 1996; O’Neill, Eberhard, Albersheim, & Darvill, 2001). In addition to this, B is reported to be involved in many physiological and cellular processes like photosynthesis, food partitioning, protein synthesis, etc. (Goldbach, 1997; Goldbach et al., 2001; Rehman et al., 2018). The B concentration should be in a balance state as both deficiency and excess are harmful to plant. To maintain its homeostatic condition, plants have three ways to transport B, that is, passive transport, facilitated diffusion, and active transport (Takano et al., 2002). The passive diffusion is the simplest one and follow the concentration gradient principle. This type of transport is found to occur in lower plants and higher plants (under adequate B). The passive diffusion carries the B, from the soil to root and in the long-distance transport through xylem and phloem.

The facilitated diffusion is carried out by the boric acid channels under the inadequate B concentration. Boric acid channels show homology to aquaporins, which is a member of the major intrinsic protein (MIP) superfamily, which further divides into subfamilies (Park & Saier, 1996). In NIP subclass, specifically, the subtype NIP II is responsible for the B transport from soil to different parts of the plant via apoplastic pathways. However, there is still doubt about their specificity to B only. Moreover, several studies reported the role of different type of boric acid channels in several plants (Hanaoka, Uraguchi, Takano, Tanaka, & Fujiwara, 2014; Sabir, Gomes, Loureiro-Dias, Soveral, & Prista, 2020; Takano et al., 2006).

Another major class of B transporter is active transporters, and they are responsible for the efflux of B, from endodermal cells to xylem vessels. For the first time, an Arabidopsis active transporter (BOR1) was studied and found to be responsible for the B transport in the xylem (Takano et al., 2002). Till now, in A. thaliana, a total of 7 BOR genes have been identified, while a variable number of BOR genes were reported in a number of monocot and dicot plants (Leaungthitikanchana et al., 2013; Luo, Liang, Wu, & Mei, 2019; Martínez-Cuenca, Primo-Capella, & Forner-Giner, 2019). Furthermore, Chapter 16, Role of BOR family proteins in boron (B) transport in plants, will provide detailed knowledge of boron family transporters in plants (Sharma, Sharma, Sidhu, & Upadhyay, 2021).

1.4.2 Aluminum transporters

Aluminum is the third most abundant element and present in the form of Al³+. The Al³+ form is toxic to plant as it affects the root growth and absorption capability of plants (Kochian, Piñeros, & Hoekenga, 2005; Poschenrieder et al., 2019; Tesfaye, Temple, Allan, Vance, & Samac, 2001). According to the hypothesis proposed by Exley and Mold, the Al can pass the PM in four ways: diffusion in neutral form, in the form of organic Al complex, active transportation, and through channels (Exley & Mold, 2015). However, a plant can transport the Al in the form of OA–Al compound as it is the detoxified form (Horst, Wang, & Eticha, 2010; Kochian, Hoekenga, & Piñeros, 2004; Ryan et al., 2011). The OA (organic acids) which plants commonly used are malate, oxalate, and citrate (Delhaize, Ryan, & Randall, 1993; Jian et al., 2005; Pellet, Grunes, & Kochian, 1995). The ALMT (Al-activated Malate Transporter) is responsible for the malate efflux from the root, which is used to form the OA–Al complex (Wang et al., 2018). Moreover, MATE proteins are also found to help in the release of organic ions (Furukawa et al., 2007; Liu, Magalhaes, Shaff, & Kochian, 2009; Magalhaes et al., 2007; Wang et al., 2007). For the transport of OA–Al complex, the role of transporter proteins like, NRAT;1, NIP1;2 is reported in rice and Arabidopsis, respectively (Jixing, Naoki, & Feng, 2014; Wang et al., 2017).

1.4.3 Molybdenum transporters

In soil, molybdenum is present in the two soluble forms, that is, MoO4²− or HMoO4−. In A. thaliana, molybdenum/sulfate transporter MOT1/SULT5.2 and MOT2/SULTR51 are known to be involved in the molybdenum transport (Bittner, 2014; Poschenrieder et al., 2019).

1.4.4 Chromium transporters

Chromium (Cr) transport mechanism is still not well understood, but few studies proposed PS-mediated transport of Cr(III) (Poschenrieder et al., 2019; Zhang et al., 2007).

1.4.5 Arsenic and antimony transporters

Arsenic in the form of As(III) is taken by the aquaglyceroporins, a member of the aquaporin family (Bhattacharjee, Mukhopadhyay, Thiyagarajan, & Rosen, 2008). Additionally, the member of NIP (Nodulin 26-like Intrinsic Protein), LSI1 (Low Silicon 1), TIP4, ACR3, and OCT4 were reported to be involved in uptake, transport, and efflux of As(III) (Cai et al., 2019; Xu et al., 2015b). However, Sb transport is less explored and supposed to move in a similar fashion to arsenic because of its homology (Bienert et al., 2008).

1.5 Conclusions

Cation homeostasis is an important requirement for the growth and development of plants. There are a number of roles that are played by the cations of different elements. Plants have evolved specialized mechanisms to achieve cation homeostasis and to regulate their transport. A number of cation transporters have been identified and studied by various research groups. The advancement of bioinformatics tools made it easier to look into the genome of different plants for these transporters. Molecular techniques have explained the hidden facts about the different cation specific and nonspecific transporters. The transporters discussed in this chapter enhance our knowledge about cation transport. Cations like potassium (K), sodium (Na), calcium, (Ca), magnesium (Mg), cobalt (Co), copper (Cu), nickel (Ni), iron (Fe), zinc (Zn), boron (B), aluminum (Al), molybdenum (Mo), cadmium (Cd), arsenic (As) and antimony (Sb) are required to be regulated and transported. The studies that unveiled the roles of these transporters have opened avenues for many new researches. The predicted role of cation transporters in the management of various stress conditions will be helpful in various plant improvement programs.

Acknowledgments

The authors are thankful to Panjab University, Chandigarh, India for providing the research facility and infrastructure. AS and HS are thankful to CSIR for senior research fellowships. HS is also thankful to IKGPTU, Jalandhar for Ph.D. registration.

References

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