Keywords

1 Introduction

Salinity is one of the severe abiotic stresses that limits plant growth and causes a significant loss to crop yield. It is estimated that more than 800 million hectares of land is globally affected by salt stress, in which about 20% of irrigated land is included that accounts for one-third of world’s food production (Rozema and Flowers 2008; Shrivastava and Kumar 2015). Increased soil salinization can rapidly transform a fertile land into barren land due to the deposition of salt in soil and can thus directly impact on the vegetation of that area. Plant growth is retarded by soil salinity, which induces osmotic stress firstly and is followed by ion toxicity. When plants are exposed to high salinity, cell metabolism is immediately affected because of the ion-specific toxicity (Blumwald et al. 2000). Under NaCl stress, plants generally face hyperionic stress due to the accumulation of toxic Na+ and Cl− in cells that impart ionic imbalance. The increase in Na+ concentration within the cell can, in turn, inhibit K+ uptake and can retard the biological activity of many enzymes resulting in less productivity and growth of the plant (James et al. 2011). Salt stress in plants is also characterized by the overproduction of ROS such as superoxide radicals (O2−), hydrogen peroxide (H2O2) and hydroxyl radicals (OH−), which imparts a negative effect on various cellular metabolisms and exerts oxidative damage of different cellular structures like proteins, lipid, DNA, etc. (Bartels and Sunkar 2005; Roychowdhury et al. 2018, 2019).

For the maintenance of normal physiology and metabolism under salt stress, plants strategize to increase their tolerance level through the accumulation of osmolytes, overexpression of antioxidative enzymes and sequestration and removal of toxic ions (Munns and Tester 2008; Roy and Chakraborty 2014). Among these, removal of toxic ion is the most determining factor as excess accumulation of Na+ imposes serious imbalances in plant metabolism (Flowers and Lauchli 1983; Tsunekawa et al. 2009). In salt-tolerant plants, the accumulation of Na+ in the cell is regulated by the rate of net uptake of Na+ by root, the net translocation from root to shoot and lastly the transportation of Na+ from shoot to root through the phloem (Tester and Davenport 2003). The presence of specialized glandular structures in the form of salt glands and bladders in several plants also accounts for removal of excess amount of Na+ from the cells, thereby aiding in maintaining an optimal K+/Na+ ratio (Agarie et al. 2007).

On the basis of salinity tolerance, plants could be broadly classified into glycophytes and halophytes. Unfortunately, all the important cereal crops are glycophytic in nature, and their growth is retarded even at low concentrations of salt (50 mM NaCl). Among the cereals, rye is the most tolerant, followed by barley, maize and rice (Hoang et al. 2016). However, in the order Poales, approximately 8% of the species that include near and distant relatives of cereal crops are halophytic or salt-tolerant in nature and therefore offer a promising system for harbouring salt-tolerant genes for cereal crop improvement (Flowers and Colmer 2008). It has been realized that the ability of these grasses to maintain ion homeostasis in stressed environments is one of the most important mechanisms of salt tolerance. Mostly, in the chloridoid and panicoid grasses, ion homeostasis is mainly achieved by efflux of excess Na+ through bicellular salt glands (Marcum 1999). Several ion transporter genes like SOS1 are involved in the transcription of plasma membrane specific Na+/H+ ion transporters for the secretion of Na+ across the plasma membrane (Qiu et al. 2002). Also, the loading of excess Na+ into the vacuoles of senescing and maturing leaves helps to maintain the turgor pressure of the cell maintaining a high K+ in the cytosol (Storey 1995). Presence of Na+/H+ ion transporters like NHX in the tonoplast plays an important role in the uploading of toxic ions from the cytosol to vacuoles (Jeschke 1984). Many ion transporters from salt-tolerant grasses like Agropyron elongatum, Aeluropus littoralis, Pennisetum glaucum, Puccinellia tenuiflora, Spartina alterniflora, etc. have been deciphered in the last few decades, and their functionality in conferring salinity tolerance has been well explained (Shabala and Mackay 2011).

The understanding of salt-tolerance attributes of ion transporter genes from wild grasses has encouraged the evaluation of these genes in conferring salinity tolerance in cereal crops (Bhatnagar-Mathur et al. 2008). In the past decade, plasma membrane and vacuolar Na+/H+ antiporters have been the centre of attention in transgenic studies for the alleviation of Na+-induced toxicity in plants (Zhang and Blumwald 2001; Vera-Estrella et al. 2005; Liu et al. 2008). This review, therefore, attempts to provide an in-depth status of the ion transporter genes from wild grasses and their potentiality to confer salinity tolerance in crop plants and also to assess the present status of salt-tolerant transgenic cereal crops.

2 Ion Transporters and Their Mechanism of Action in Salinity Tolerance

Several ion transporters, pumps and channel proteins located on the plasma membrane or vacuolar membrane are involved in ionic homeostasis and thus confer salinity tolerance in plants. These ion transporters include high-affinity potassium transporters (HKTs), salt overly sensitive (SOS) and non-selective cation channels (NSCCs) present on the plasma membrane. These are the carrier-type transporter that is involved in Na+ transportation at the cellular level. Other transporters such as Na+/H+ antiporters (NHXs), cation transporter (CAX) and H+ pumps are present on the vacuolar membrane that facilitates compartmentalization of Na+ toxic ions into the vacuoles (Hasanuzzaman et al. 2015; Volkov 2015; Shabala et al. 2015; Anumalla et al. 2016).

A number of plants like rice, wheat and Arabidopsis show the presence of HKTs in their plasma membrane that acts either as a Na+/K+ symporter or Na+ uniporter (Waters et al. 2013). HKT protein family can be divided into two groups: the first group is HKT1 that transports only Na+, and the other group is HKT2 that is involved in the transportation of both Na+ and K+ (Corratge-Faillie et al. 2010). In Arabidopsis, HKT1 was shown to be involved in recirculation of Na+ from shoot to root by loading the excess ion into the phloem and by releasing it into the roots (Berthomieu et al. 2003). It was also reported that HKT1 was involved in Na+ unloading from xylem vessels to xylem parenchyma cells that contribute to salt tolerance in Arabidopsis (Sunarpi et al. 2005).

To date, SOS1, a Na+/H+ antiporter, is the best-known ion transporter, which is involved in Na+ efflux from the cytosol to the apoplast of the cortical cell. It was reported by Shi et al. (2000) that SOS1 found in the root apex of Arabidopsis is the only transporter involved in the Na+ exclusion from the cytosol to the apoplast, functioning as Na+/H+ antiporter. In transgenic plants with SOS1 gene, salt tolerance was found to be enhanced by the overexpression of SOS1 (Yang et al. 2009; Yue et al. 2012; Hasanuzzaman et al. 2015; Anumalla et al. 2016), whereas the loss in function of SOS1 showed hypersensitivity to salt even in the salt-tolerant wild relative of Arabidopsis – i.e. Thellungiella salsuginea (Oh et al. 2009). Similarly, the SOS1 gene isolated from Salicornia brachiata, when overexpressed in tobacco plant, showed better performance under saline environment due to extracellular Na+ effluxing (Yadav et al. 2012). It was reported from Arabidopsis that the activity of SOS1 was regulated by two other proteins, viz. SOS2 and SOS3 (Shi et al. 2000). SOS3 is a calcium-binding protein which in turn induces SOS2, a serine/threonine protein kinase to activate phosphorylation in SOS1, thereby increasing Na+/H+ exchange activity of the SOS1 transporter (Halfter et al. 2000; Liu et al. 2000; Luan et al. 2009).

Non-selective cation channels (NSCCs) play a major role in the regulation of Na+ transportation when the salt concentration is high in the soil. It has been demonstrated in several studies that NSCCs catalyse primary Na+ fluxes under salinity stress (Kronzucker and Britto 2011). According to Tester and Davenport (2003), there are two classes of NSCCs, viz. cyclic nucleotide-gated channels (CNGCs) and glutamate-activated channels (GLRs). Inhibition of cation channels is associated with the amelioration of Na+ toxicity in plants, and external Ca2+ plays an important role in this. Ca2+ and intracellular and extracellular Na+ can all have regulatory roles by blocking the K+-selective outwardly rectifying channel (KOR) and thus preventing excessive Na+ influx and cellular K+ loss (Demidchik and Maathuis 2007). The rice CNGC1 was reported to decline Na+/K+ ratio by decreasing Na+ influx and was observed to be more downregulated in salt-tolerant rice variety than the salt-sensitive variety under salinity stress (Senadheera et al. 2009). NSCC members of glutamate-activated channels from Arabidopsis thaliana, AtGLR3;7, AtGLR1;1 and AtGLR 1;4 when expressed in Xenopus oocytes, were shown to exhibit Na+ permeability, and it was also revealed that these channel proteins could function in the transportation of Na+, K+ and Ca+2 (Roy et al. 2008; Tapken and Hollmann 2008).

Halophytes and salt-tolerant plants can tolerate salinity by lowering net Na+ influx and resist salt stress by the accumulation of excess Na+ inside its vacuole and efflux of excess Na+ outside the cell (Hanana et al. 2007; Munns and Tester 2008). In saline conditions, vacuolar Na+ sequestration reduces the toxicity of cytoplasm and lowers the osmotic potential to maintain turgor pressure. Various transporters such as Na+/H+ antiporters (NHXs), cation antiporters (CAXs) and auto-inhibited Ca+2 ATPase (ACA) are present in the vacuolar membrane for the storage and translocation of Na+ from the cytosol during salt stress. There are also some proton pumps, viz. V-H+-PPase and the vacuolar H+-ATPase in tonoplast that also aids in Na+ compartmentalization. Na+ loading in vacuoles is controlled by NHX proteins which are operated by electrochemical gradient of proton generated across the vacuolar membrane. NHX can function as both Na+/H+ and K+/H+ exchangers under salt conditions (Apse et al. 2003; Leidi et al. 2010). Most of the NHX genes are expressed under salt stress and help in maintaining K+ homeostasis in the cytosol for the growth and development of the plant in saline environments (Pardo et al. 2006; Xu et al. 2013). Maathuis and Amtmann (1999) reported that the Na+ sequestration helped the plants under salt stress in different aspects such as maintenance of ionic homeostasis in the cytosol, enhancing the efflux of Na+ and controlling the osmotic potential of tonoplasm. NHX1, a Na+,K+/H+ exchanger, is the best-known transporter for vacuolar Na+ sequestration, and most of the NHX transporter such as AtNHX1, AtNHX2, AtNHX3, AtNHX4 and OsNHX1 are located on the tonoplast, which was reported from Arabidopsis and rice, and the overexpression of these genes has been shown to improve salinity tolerance (Apse et al. 1999; Chen et al. 2007). The transgenic rice variety carrying the NHX gene exhibited overexpression of OsNHX1 under saline conditions which differentiated it from the wild-type cultivar by its ability to confer higher growth rate and salt tolerance (Fukuda et al. 2004).

Cation transporters (CAXs) are located on the vacuole and belong to a multigene family of ion transporters (Kamiya et al. 2005; Shigaki et al. 2006; Martinoia et al. 2007). During Ca2+ stress, the upregulation of Arabidopsis CAX1 and CAX3 was reported, and most importantly the expression of CAX3 was upregulated and enhanced during Na+ stress as observed in the studies with yeasts (Hirschi 1999; Shigaki and Hirschi 2000). CAX1 and CAX3 showed differential regulation of H+ pumps that can affect H+ coupled ion transport and also helped in the regulation of cellular pH. On the basis of amino acid sequences, CAX family is divided into two groups: Type IA includes CAX1, CAX3 and CAX4, whereas Type IB includes CAX2, CAX5 and CAX6 (Manohar et al. 2011).

Apart from the ion transporters, there are proton pumps that can play an important part in conferring salinity stress tolerance. There are three types of proton pumps such as P-type ATPase, vacuolar H+-pyrophosphatase (V-H+-PPase) and vacuolar H+-ATPase that produces electrochemical gradients throughout the plasma membrane. The P-type ATPase pumps out the cytoplasmic H+ into extracellular space while a pH gradient is maintained between cytoplasm and vacuole by the acidification of lumen through the V-H+-PPase and the vacuolar H+-ATPase (Sze et al. 1999).

3 Cereal Crops and Their Responses to Salinity

Plants attempt a large number of physiological and biochemical strategies to improve their salt tolerance. According to Roy and Chakraborty (2014), salt-tolerance strategies include ion exclusion, osmotic tolerance and tissue tolerance. When exposed to saline soils, plants first experience the osmotic stress which affects plant growth. After that Na+ accumulation causes ion toxicity which upon reaching threshold level negatively impacts ion homeostasis. Plants generally tolerate salinity through osmotic balance, ionic homeostasis or tissue tolerance (Hasanuzzaman et al. 2015; Anumalla et al. 2016). On the basis of salt sensitivity, plants can be divided into two groups: glycophytes, the salt-susceptible plants, and halophytes which tolerate more salinity and can survive in high salt concentration (Tuteja et al. 2011). Most of the cereal crops are glycophytes and are highly affected by soil salinity. Various genotypes of crop plants, however, show varying degrees of growth responses to salinity (Eynard et al. 2005). According to Munns and Tester (2008), plant growth can be affected by salinity in two phases: first is the osmotic phase where the growth of young leaves is inhibited, and second is the ionic phase which speeds up the senescence of leaves. Salinity inhibits shoot elongation by inducing stomatal closure which in turn increases the temperature of leaves (Rajendran et al. 2009; Sirault et al. 2009). The basic mechanism of the ion transporters in cereal crops under salinity stress is illustrated in Fig. 8.1.

Fig. 8.1
figure 1

Illustration of salinity stress responses in a typical cereal plant. The role of few ion transporters which are either upregulated or downregulated under salinity stress is shown. Owing to the susceptibility of cereals to salinity, most of the ion transporters depicted are expected to be downregulated. [A] NSCCs (nonspecific cation channels) are expected to increase the uptake of Na+ ions through roots. [B] Also HKT1 (high-affinity K+ transporters) due to high concentration of Na+ in soil functions in Na+ uptake rather than K+ uptake. [C] In the leaves, SOS pathway involving SOS3, SOS2 and SOS1 (plasma membrane transporters) plays an important role in Na+ ion exclusion. But in cereal crops, due to the absence of salt glands, the exclusion of Na+ is minimal, whereas the SOS1 functions in recirculating the excess Na+ ion back into the conducting tissues. As a whole Na+ ion concentration in plant tissues increases at a high rate inhibiting the normal metabolism. Also the vacuolar proton antiporters (NHX1) which are dependent on ATPases to function are also downregulated under salt stress; therefore the loading of excess Na+ ions into the vacuoles is also not possible

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In rice, salinity shows a negative effect on yield components and slows down the heading (Grattan et al. 2002). Salinity can interfere with the germination of the seed, seedling growth, shoot growth, dry and fresh weight of shoot, size of the leaf, numbers of tillers per plant, number of spikelet and productivity (Lauchli and Grattan 2007; Moradi and Ismail 2007; Roychowdhury et al. 2013; Gupta and Huang 2014). At the flowering stage, the pollen viability can be reduced by salt stress, thus affecting the grain yield (Singh et al. 2004). Chinnusamy et al. (2005) reported that salt tolerance is controlled by multiple genes and known as a complex quantitative trait. Rice is a salt-susceptible cereal plant, mainly at the young stage (Lutts et al. 1995), and also at the mature stage, the productivity is restricted by salinity (Todaka et al. 2012). Maize is also a salt-sensitive crop and exhibits a strong inhibition in shoot growth in the first phase of salt stress (Pitann et al. 2009; Wakeel et al. 2011). According to Omoto et al. (2012), assimilation of carbon in maize is very sensitive during salinity stress. The decrease in photosynthetic pigment and stomatal conductance and impaired activities of photosynthetic enzymes are some key factors which hamper the carbon fixation capacity of maize in the saline environment (Kaya et al. 2010; Qu et al. 2012). In maize, during the reproductive phase, salinity causes a decrease in grain weight and numbers, resulting in an overall reduction in grain yield (Kaya et al. 2013). Under high salt concentration, wheat plant shows a wide range of physiological responses such as decreased water potential in leaf, loss in turgor pressure, stomatal closure, an increase of oxidative stress and production of toxic metabolites (Kingsbury and Epstein 1984; Taiz et al. 2015). Salt stress also declined the content of chlorophyll and carotenoid pigments (Ali et al. 2008) and enhances the formation of ROS (Wahid et al. 2007). It was also reported by Kong et al. (2001) and Sairam et al. (2002) that the proline, soluble sugars, glycine-betaine and abscisic acid content increased during salt stress. Barley is the most tolerant among the cereal crops under salinity stress, but its growth and yield is also affected when it is exposed to a higher level of salt concentrations. Comparing the different genotypes of barley under salt stress indicated that salt tolerance in barley is determined by the abscisic acid pathway (Kamboj et al. 2015). Bakht et al. (2007) observed that the reduction in shoot length of barley and oats was maximum at higher concentration of NaCl (150 mM). Higher level of salt stress increased ion concentration in rooting medium which resulted in the decrease of water potential which in turn was responsible for the shortening of shoot length (Munns et al. 2002).

4 Salinity-Induced Transporters from Wild Relatives of Cereals

The native grasses growing in the coastal area face environmental conditions much different from the plants growing inland. Though there are many plants which can tolerate salt stress up to a threshold level, salt-tolerant grasses specifically are different in their distinct morphology and leaf structure allowing them to exclude excess salt from the interior. Vetiver grass (Vetiveria zizanioides L.) which is a salt-tolerant perennial grass found mostly in tropical and subtropical areas is different from its close relatives in having strong and extensive root system. The root system specifically provides a superior advantage to the plant in adapting and overcoming a wide range of stresses like flood, drought, high temperatures, acidity and salinity (Truong et al. 2002; Zhou and Yu 2009; Ghotbizadeh and Sepaskhah 2015). The wild vetiver grass has a high salinity threshold of about 100 mM NaCl proving its ability to tolerate high salt stress which is achieved by the plant’s ability to maintain an optimum K+/Na+ ratio in its leaves (Liu et al. 2016). It has been observed in Porteresia coarctata - a wild rice variety, which is a highly salt-resistant species growing in the coastal and sub-coastal regions of India, ameliorates Na+ toxicity by the unicellular salt hairs present on the adaxial surface of leaves (Flowers et al. 1990; Ganie et al. 2014). There is a C4 perennial halophytic grass Aeluropus littoralis that grows in salty areas without any toxicity symptoms owing to their ability to adjust the osmolarity of cytoplasm to match the osmolarity of sea water (Touchette 2007; Zouari et al. 2007). Also, most of the other grasses like Sporobolus, Zoysia and Cynodon secrete Na+ through salt gland (Marcum 1999). Greater retention of K+ in shoots is observed in Puccinellia and Thinopyrum by maintaining a negative membrane potential in the root cell (Teakle et al. 2013). A list of salt-tolerant grasses though not exhaustive is provided in Table 8.1, depending upon their ability to grow in high salt concentrations and presence of salt-tolerant attributes specially the salt glands.

Table 8.1 List of wild grasses that could be a source of ion transporter genes for cereal crop improvement
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Among the multimodal strategies of salinity tolerance, management of ionic balance inside the cell seems to be of prime importance. The ion homeostasis is achieved due to the presence of several transporters in the plasma membrane and vacuolar membrane in the shoots and roots of the plants. Transcriptomic analysis in the halophytic turfgrass Sporobolus virginicus has revealed that several ion transporters like high-affinity potassium transporter (HKT), ABC transporters (for Cl− transport) and H+-ATPase were upregulated in both the shoots and roots of the plant under salinity stress (Yamamoto et al. 2015). This indicated efficient machinery of the ion transporters in loading the excess ions into the vacuoles for maintaining an optimum concentration of ions in the cytosol. Class II HKTs which are known to cotransport Na+-K+ in plants have been elucidated from Sporobolus virginicus which are known to maintain ion homeostasis in low K+ saline environments (Tada et al. 2014). The two transporters named as SvHKT2;1 and SvHKT2;2 were shown to be upregulated by high NaCl concentrations and low K+ ion, respectively, thereby regulating ion homeostasis by both outward and inward movements of K+ and Na+ ions. Similarly, transcriptomic analysis of the smooth cord grass – Spartina alterniflora has revealed the importance of several transporter proteins like K+ transporters, H+ transporters and Na+/H+ which played a crucial role in the ion homeostasis and adaptation of this grass in the saline environment (Bedre et al. 2016).

Wang et al. (2005) investigated the K+ selectivity over Na+ ion in Puccinellia tenuiflora in comparison to wheat and found that the accumulation of K+ ion in the former was up to 24% higher than the latter under different NaCl treatments. This revealed the presence of high-affinity K+ transporters in Puccinellia tenuiflora that was induced under NaCl stress, thereby increasing the uptake and accumulation of K+ ion in the shoots. The gene PtAKT1 was found to encode for these channel proteins that maintain a highly selective absorption capacity for K+ ion under high salt stress, which was also confirmed by the presence of a high number of PtAKT1 transcripts in roots of plants subjected to a NaCl concentration greater than 150 mM (Wang et al. 2015). A vacuolar antiporter LfNHX1 from Leptochloa fusca – a halophytic forage grass was shown to be upregulated in response to different levels of salinity which played an important role in the salt tolerance of the grass (Panahi et al. 2013). Real-time PCR analysis of the LfNHX1 gene from the plant treated with 200 and 500 mM NaCl revealed the enhancement in expression of this gene by 4–8 times in comparison to the control plants. In Aeluropus littoralis upregulation of ion transporter genes like SOS1 (encoding plasma membrane antiporters), NHX1 (encoding vacuolar transporters) and VHA (encoding proton pumps) located in the shoots and roots accounted for increased tolerance at high NaCl concentrations (>200 mM) by regulating the cytosolic Na+ ion threshold (Moshaei et al. 2014). Similarly in Aeluropus lagopoides, differential expression of ion transporter genes, viz. SOS1, tonoplast NHX1, HKT and V-ATPases, in shoots and roots of the plants subjected to high NaCl accounted for ion homeostasis which was pivotal in conferring salinity tolerance (Sanadhya et al. 2015a, b). The expression level study of the plasma membrane (CdSOS1) and vacuolar (CdNHX) transporters from Cynodon dactylon and its counterparts in rice revealed the role of these transporters in salt tolerance of the former (Roy and Chakraborty 2018). CdSOS1 expression was found to increase up to 2.5 and 5 fold in plants exposed to 100 and 200 mM NaCl concentrations, respectively. On the other hand, CdNHX expression increased up to 1.5 and 2 fold. In comparison, SOS1 and NHX expressions in rice plants were not upregulated under NaCl treatments. The list of ion transporters that have been characterized by different wild grasses has been summarized in Table 8.2.

Table 8.2 List of ion transporters from wild relatives and their mechanism of action
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5 Transgenic Approaches with Wild Ion Transporter Genes

During salinity stress plant growth is positively induced by the ion transporter proteins, and thus the identification of these proteins and their related gene is essential (Maathuis 2007). The function of salt-tolerant genes and proteins can be monitored by evaluating the salt uptake at the root level, distribution pattern and the cellular influx of Na+ and K+ ions, long-distance transport and compartmentation at cellular as well as tissue level (Flowers and Colmer 2008). There are some useful approaches to improve salt tolerance of plants for introduction and propagation under saline conditions such as genotype screening, the introduction of salt tolerance gene and conventional breeding methods. As conventional breeding takes a long time and can reduce plant vigour, the transgenic approach seems to be the most effective and appropriate technique for the development of salt-tolerant cultivars. Many workers have focused on the ion transporter genes as it regulates the Na+ inclusion and compartmentalization, plays an important role in the maintenance of cellular toxicity and helps in improving salinity tolerance. Therefore, screening the ion transporter genes in the germplasm of the cereal crops and the near and distant relatives of these plants seems to be of great importance in designing salt-tolerant cultivars of cereal crops.

Overexpression of genes encoding vacuolar NHXs in plants treated with salt has shown that these ion transporters could improve the salt tolerance in plants including rice, wheat, tobacco, cotton and tomato up to a certain extent (Apse et al. 1999; Zhang and Blumwald 2001; Zhang et al. 2001; Agarwal et al. 2013). A number of Na+/H+ antiporters have been isolated and characterized by wild salt-tolerant grasses which have been used in transgenic experiments to exhibit their potentiality in increasing the salinity tolerance levels of the transgenic plants. It was observed that the AlTMP2 gene isolated from halophytic C4 grass Aeluropus littoralis, when expressed in transgenic tobacco, improved tolerance against salinity, osmotic, H2O2, heat and freezing stresses during seed germination and seedling stage by improving ion homeostasis and membrane integrity and by regulating some genes related to salt stress (Ben Romdhane et al. 2018). Another gene AlNHX1 from A. littoralis exhibited improved growth in severe saline condition (150 mM NaCl) in transgenic plants which accumulated a higher level of K+ and a lower level of Na+ in the leaves than wild-type plants and thus helped in enhancing salt tolerance (Liu et al. 2014). Qiao et al. (2007) isolated a homolog of Na+/H+ antiporters, AeNHX1 from the root of Agropyron elongatum which promoted salt tolerance in Arabidopsis and Festuca by improving osmotic adjustment and photosynthesis in the transgenic plants under salt stress. The PvNHX1 gene expression in switchgrass (Panicum virgatum L.), when compared to the wild type or transgenic control, showed a better growth-related phenotype and also maintained K+ homeostasis, resulting in more K+ accumulation (Huang et al. 2017). Baisakh et al. (2012) isolated and introduced the SaVHAc1 gene from the halophyte grass Spartina alterniflora in rice where it is observed that the SaVHAc1-expressing plants increased K+/Na+ ratio to protect the cytoplasm from toxic Na+ and maintained higher chlorophyll retention. Zhang et al. (2017) analysed the expression pattern of PtSOS1, PtHKT1;5 and PtNHX1 from Puccinellia tenuiflora under 25 and 150 mM NaCl and observed that the expression of PtSOS1 in roots significantly increased by 6 fold, while PtHKT1;5 increased by 1.2-fold under 25 mM NaCl; in contrast the PtSOS1 increased by 1.4-fold, whereas the PtHKT1;5 increased by 2.2-fold in roots under 150 mM NaCl; on the other hand, compartmentalization of Na+ into vacuole was mediated by PtNHX1. The details of the transgenic experiments are provided in Table 8.3.

Table 8.3 List of transgenic experiments validating the wild genes in improvement of salinity stress tolerance in cereals and other plants
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6 Success Story So Far and the Way Ahead

Many transgenic cereal crops have been produced in the past two decades with enhanced salt tolerance, but the success of these plants in agricultural fields is often less talked about. Though a large amount of government and public funding has been allocated for the development of salt-tolerant designer crops, desired success till today has not been achieved. Among the cereal crops, only transgenic maize carrying the Bt gene for insect resistance has been known to be commercialized in many countries like the USA, Brazil, China, etc. (James 2016). This presents a gloomy picture at present when none of the cereal crops with salt-tolerant genes are in agricultural fields. This is mainly due to the failure in performance of these transgenics in field trials when compared to the laboratory results.

Molecular breeding for imparting salt tolerance in the crop germplasm could also prove to be a significantly viable option. One of the major achievements in imparting salt tolerance has been through the introgression of saltol quantitative trait locus (QTL), which is present in the chromosome 1 of rice and is derived from the near-isogenic lines of the salt-tolerant cultivar of rice ‘Pokkali’ (Linh et al. 2012). The genetic analysis of the saltol region was characterized to be containing shoot K+ content 1 (SKC1), which was eventually cloned to be identified as the gene OsHKT1;5 which encodes for a Na+ transporter regulating K+ homeostasis (Ren et al. 2005). Recently the marker-assisted transfer of this saltol QTL into the aromatic rice Pusa Basmati 1 (PB1) has conferred enhanced tolerance to the cultivar at seedling stage (Singh et al. 2018). In 2013, Grain Research and Development Corporation (GRDC), Government of Australia, initiated a project for the development of salinity-tolerant wheat and barley in the light of huge losses incurred by the Australian farmers due to soil salinity (Roy et al. 2013). This initiative was started to locate the genes responsible for shoot Na+ exclusion and Cl− accumulation for enhancing tissue tolerance to salinity and to provide molecular markers to the breeders for developing salt-tolerant cultivars. Similar target-oriented initiatives could lead to the development of salt-tolerant cereal crops in the near future. One of the major objectives of marker-assisted breeding has been the identification of QTLs related to ion accumulation and exclusion. In this context, at least 14 QTLs from rice and 30 QTLs from barley have been reported to be involved in the regulation of ion movement (Ashraf and Foolad 2012). Some of these QTLs are Nax1 for Na+ exclusion, SKC1/OsKKT8 for K+/Na+ homeostasis, Qkr1 for root K+ uptake and so on.

Therefore, it could be clearly seen that the urge for the search of novel genes and QTLs from near and distant relatives of crop plants has increased quite significantly. The future of cereal crop improvement thus undoubtedly lies in developing a good gene pool of ion transporter genes that can significantly enhance the tolerance ability of the cereal crops when introgressed into the crop genome.

7 Conclusions

Though the current status of salt-tolerant cereal crops presents a shoddy picture, however, the quest for the development of truly salt-tolerant cereal crops is highly desirable keeping in mind the challenges to enhance the crop productivity and to feed the ever-increasing world population. The complexity of salinity responses in the plants has made the picture even worse, and it has also been realized that the transfer of a single gene is not sufficient enough to make the crop plants salt-tolerant in the natural environment. However, there has been a continuous attempt to develop salt-tolerant varieties of cereal crops, and many of the successes in the in vitro conditions have the potentiality to be transformed at the larger scale. The development of modern biological tools has made the scientific community more equipped than ever, and it is high time to use the knowledge gathered during the past few decades to design the cereal crops to sustain their growth and productivity in saline environments. In this context, the ion transporter genes from the wild grasses which are near and distant relatives of cereal crops are likely to play a significant role in the near future for the development of salt-tolerant cereal crops.