Introduction
A doubling of global food production is likely necessary to meet the needs of a growing human population, which is expected to increase up to 9.3 billion by 2050 . However, abiotic stresses such as drought, salinity, and high temperatures are negatively impacting and exerting a drag on increasing the yield of major crops. Among them, salinity is the most important factor. Over 800 million hectares (6%) of the land throughout the world and 45 million hectares (20%) of irrigated land are affected by salinity resulting in a reduction of crop yield (). Most food crop species such as rice (Oryza sativa L.), wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), and sorghum (Sorghum bicolor L.), etc. are glycophytes, therefore are not capable of growing in a saline environment. Of these crops, rice is highly sensitive to salinity, with a threshold of 3 dSm-1 (deciSiemens per meter) electrical conductivity of saturated extract (ECe), which corresponds to approximately 30 mM NaCl for most cultivated varieties compared to 6-8 dSm-1 (60-80 mM NaCl) for wheat (Chinnusamy, Jagendorf, & Zhu, 2005; Munns, 2005). This sensitivity is variable at different growth stages in rice (Munns & Tester, 2008). In particular, rice is a more sensitive to salinity at the seedling stage, but it becomes moderately salinity insensitive at the tillering stage (Walia et al., 2005). Thus, even slightly higher salinity concentrations in soil than optimal levels can lead to retarded growth in rice plants at the seedling stage.
Roots are the first tissue exposed to salinity stress in soil and several genes have been identified along with their functional mechanisms to avoid the toxic effect of high salinity in roots. Both Salt Overlay Sensitive genes in rice (OsSOS1/OsNHX7 ) andArabidopsis (AtSOS1/AtNHX7 ) encode a plasma membrane Na+/H+ antiporter, which have important roles in Na+ extrusion in the roots under salinity conditions (Chinnusamy et al., 2005; Ding & Zhu, 1997).AtSOS1 is mainly expressed in the root epidermis and xylem parenchyma and extrude the excess of Na+ ions from root epidermal cells (Martinez-Atienza et al., 2007). Thus, theatsos1 mutants are salinity hypersensitive due to reduced rate of Na+ extrusion resulting in increased Na+ concentration in aerial parts of the atsos1mutant (Shi, Quintero, Pardo, & Zhu, 2002). The salinity hypersensitive phenotype of the atsos1 mutant was complemented by overexpression of the OsSOS1 in Arabidopsis indicating that both OsSOS1 and AtSOS1 participate in the root-shoot translocation of Na+ under salinity stress conditions (Shi, Ishitani, Kim, & Zhu, 2000). After Na+ uptake by the roots, a fraction of Na+ is sequestrated into vacuoles through vacuolar Na+/H+ antiporters of theAtNHX1 and OsNHX1 , which are expressed in root, shoot, leaf, and flower tissues in Arabidopsis (Martinez-Atienza et al., 2007) and the stelar tissue, lateral roots, and vascular bundles in the shoot of rice seedlings, respectively (Apse, Aharon, Snedden, & Blumwald, 1999; Fukuda, Nakamura, Hara, Toki, & Tanaka, 2011). Thus, these two Na+/H+ antiporters of SOS1 and NHX1 are involved in critical roles of Na+exclusion and sequestration to reduce salinity toxicity in plant, respectively.
Several ion channels and carrier-type transporters also have been identified whose functional roles involve Na+ uptake in plants. Among these, the HKT family is quite diverse in functions and well-characterized in several crop species. This HKT family is further divided into two distinct classes (HKT1 and HKT2) based on their transport characteristics (Almeida, Oliveira, & Saibo, 2017; Fukuda et al., 2004; Platten et al., 2006). Most members of class I transporters, HKT1s, including AtHTK1;1 in Arabidopsis (Maser et al., 2002; Sunarpi et al., 2005), OsHKT1;1 , OsHKT1;3 ,OsHKT1;4 and OsHKT1;5 in rice (Berthomieu et al., 2003; Cotsaftis, Plett, Shirley, Tester, & Hrmova, 2012; Jabnoune et al., 2009), and TaHKT1;4 and TaHKT1;5 in wheat (Byrt et al., 2007; Ren et al., 2005) have been implicated in controlling Na+ accumulation in shoot as Na+selective exclusion transporters for enhancing salinity stress tolerance. The class II transporters of HKT are only found in monocot species (Huang et al., 2006). All members of identified HKT2 including OsHKT2;1 and OsHKT2;2 in rice (Platten et al., 2006; Yao et al., 2010), TaHKT2;1 in wheat (Horie et al., 2007), and HvHKT2;1 in barley (Schachtman & Schroeder, 1994) are clearly shown to be involved in mediating Na+ influx in root tissues under K+ starvation conditions. Although the mechanisms regulating the transport activity of the most HKT genes are unknown, one magnesium transporter has been reported, OsMGT1 (OsMRS2-1) protein is involved in enhancing OsHKT1;5 activity in rice (Mian et al., 2011).
Magnesium ion (Mg2+) is one of the most abundant free divalent cations and essential macronutrient for plants. Mg2+ is essential for photosynthesis as a central metal for chlorophylls and acts as a cofactor for structural conformation for many enzymes in catalytic processes (Chen et al., 2017). Thus, Mg2+ deficiency in plants generally results in a reduction of root and shoot growth and necrosis in leaves due to the decline of chlorophyll and carbon fixation (Hermans et al., 2010; Shaul, 2002). Several ionomic analysis showed that Mg2+ concentrations decreased significantly with increasing Na+ levels in many plant species including rice (Hakim et al., 2014; Hermans & Verbruggen, 2005; Munns & Tester, 2008; Talei, Kadir, Yusop, Valdiani, & Abdullah, 2012; Yildirim, Karlidag, & Turan, 2009). This reduced Mg2+ uptake might be due to the suppressive effect of Na+ or transport activities of Na+ and Mg2+transport could compete with each other under salinity stress condition, but actual mechanisms remain unclear. Alternately, there is Na+/Mg2+ antiporter which plays a major role in Mg2+ extrusion in humans (Akter & Oue, 2018), however, this antiporters have not yet been discovered in plants. The bacterial CorA protein and its yeast homologs of CorA-type transporters of Alr1 and Mrs2 proteins are well characterized as Mg2+ transporters in all living organisms (Sontia & Touyz, 2007). In plants, there are 11 AtMRS2/MGT and 9 OsMRS2/MGT homologs of bacterial CorA-type transporter in Arabidopsis and rice, respectively (Knoop, Groth-Malonek, Gebert, Eifler, & Weyand, 2005; L. Li, Tutone, Drummond, Gardner, & Luan, 2001; Saito et al., 2013). The CorA-type MRS2/MGT proteins have a unique topology with two C-terminal transmembrane (TM) domains and the conserved Gly-Met-Asn (GMN) tripeptide motif is located at the end of the first TM domain that is thought to be essential for Mg2+ transport activity. A distant CorA homolog in Salmonella typhimurium (ZntB), which alters the GMN-motif to GIN-motif, has been reported as putative zinc transporter involved in Zn2+ and Cd2+ transport activity (Knoop et al., 2005; Schock et al., 2000). However, homologs of CorA-like ZntB transporters in plant species have not reported for their exact roles.