ناقل سدیم در گیاهان، ژن های مختلف و توابع فیزیولوژیکی Sodium Transporters in Plants. Diverse Genes and Physiological Functions

One of the important questions to be addressed with respect to salinity stress in plants is the identification of channels and transporters responsible for toxic Na1 influx into root cells. Classical 22Na1 influx studies showed multiple kinetic components of Na1 influx into barley roots (Rains and Epstein, 1965, 1967). Furthermore, single locus mutations that greatly diminish Na1 influx into plants have not been found, suggesting that several parallel redundant pathways exist (Schroeder et al., 1994). Na1 influx currents have been characterized in electrophysiological studies in root cortex cells of wheat (Tyerman et al., 1997), maize (Roberts and Tester, 1997), and barley suspensioncultured cells (Amtmann et al., 1997). These studies suggest that Na1 influx is mediated by voltage-independent, nonselective cation channels (named VIC or NSC; Tyerman and Skerrett, 1999). Calcium inhibition of Na1 influx in wheat was observed (Tyerman et al., 1997; Buschmann et al., 2000; Davenport and Tester, 2000). The NSC current in wheat cortex cells is weakly voltage dependent and nonselective among monovalent cations (Davenport and Tester, 2000). Furthermore, in wheat root cortex cells K1 deprivation was shown to enhance Na1 influx currents, providing evidence that K1 starvation-induced transporters contribute to Na1 influx (Buschmann et al., 2000). However, the molecular identity of VIC/NSC remains unknown and more than one transporter gene may contribute to this activity. A cDNA was isolated from wheat that mediates low-affinity K1 and cation transport in yeast and was named LCT1 (Schachtman et al., 1997). Analysis of the secondary structure of LCT1 predicts the presence of 8 to 10 hydrophobic domains with a hydrophilic N terminus. The hydrophobic region is distinctive and novel compared to other transporter genes. LCT1 mRNA is detected at low levels in wheat roots and leaves. LCT1 functions as a nonselective cation permeable transporter in yeast mediating not only K1 influx but also Rb1, Na1, Cd21, and Ca21 transport (Schachtman et al., 1997; Clemens et al., 1998). LCT1 expression rendered yeast more salt sensitive (Amtmann et al., 2001). However, further analyses will be required to determine to which plant membrane and cell types LCT1 is targeted and physiological roles of LCT1 remain to be identified. VIC/NSC currents in Arabidopsis are downregulated by the addition of cAMP and cGMP (Maathuis and Sanders, 2001; Fig. 1). Furthermore, ۲۲Na1 tracer influx experiments show reduction in Na1 influx in the presence of cyclic nucleotides and salt tolerance of Arabidopsis plants was improved (Maathuis and Sanders, 2001). These results support the hypothesis that cyclic nucleotide inhibited channels may contribute to VIC/NSC currents (Fig. 1). The Arabidopsis genome includes 20 cyclic nucleotide gated channel-like genes (Ma¨ser et al., 2001), and their roles in root Na1 influx remain to be determined.