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.