Mechanism Of SKC1, A Rice Quantitative Trait Locus For Salt Tolerance, And Analysis Of Expression Pattern Of OsHKT Genes

Salt stress is believed to cause the major problems in agriculture by reducing crop growth and productivity. Rice plants are relatively susceptible to soil salinity, while NaCl is a major salt that causes this problem. Salt tolerance, like other important agronomic traits in crop plants, is complex trait controlled by quantitative trait loci (QTL). To understand the molecular basis of K+/Na+ homeostasis which is involved in salt tolerance, we cloned a major QTL, SKC1, which maintained shoot K+ content in salt-tolerant variety Nona Bokra under salt stress. The SKC1 gene encoded a sodium selective transporter belonging to HKT family (OsHKT8). We transformed Nona Bokra SKC1 (NSKC1) into another salt-susceptible japonica variety Zhonghua11. The shoot K+ concentration of transformants was substantially higher than that of vector control in the presence of 140 mM NaCl. The result of genetic complementation test confirmed the mapping and cloning of SKC1. To investigate the SKC1 spatial expression pattern, we analyzed transgenic rice plants containing theβ-glucuronidase (GUS) reporter gene under the control of the SKC1 promoter. SKC1 was preferentially expressed in the parenchyma cells surrounding the xylem vessels in the tissue of shoot and root, suggesting a function for SKC1 in the control of long distance transport that mediates K+/Na+ selectivity. There were four amino-acid substitutions between NSKC1 and KSKC1 (Koshihikari, a salt-susceptible japonica variety). Patch clamp analysis indicated that both NSKC1 and KSKC1 were sodium-selective transporters, but NSKC1 was more active than KSKC1. We constructed NSKC1-GFP and KSKC1-GFP fusion protein to analyze the efficiency of SKC1 targeted to the plasma membrane of Xenopus oocytes. The equal strength of green fluorescence on plasma membrane indicates that the different sodium-selective activity between NSKC1 and KSKC1 may results from four amino acid substitutions rather than transporter quantity. Under salt stress, seedlings carrying NSKC1 (natural allele) exhibited more tolerant to salinity than those carrying KSKC1, resulting from greater Na+ extraction from xylem sap by NSKC1. Our work showed compelling evidences that SKC1 was a key determinant to shoot K+ homeostasis in rice under salt stress, providing a potential tool for improving salt tolerance in crops.The genome of rice japonica variety Nipponbare contains seven HKT genes, which encode functional transporters of HKT family. In addition to SKC1/OsHKT8, we analyzed the expression patterns of six OsHKT genes form Nona Bokra by the promoter-β-glucuronidase (GUS) fusion construct. These genes in rice were expressed in various organs, including roots, stems, leaves and spikelets, but their expression patterns were different from each other. These results may provide new insights for further functional analysis.