Inheritance Analysis Of Salt Tolerance Of Rice And Cloning Of Salt Tolerance-Related Genes

A F7 Recombinant inbred line (RIL) from the cross of JCQ (japonica) and IR26 (indica) was used to analyze the inheritance of rice salt tolerance by genetic model of major-gene and polygene. Under 0.8% NaCl stress, the two traits of rating of salt tolerance (STR) and Na+: K+ in roots are controlled by two major genes and with effect of polygenes. The two traits of dry weight of shoots (DWS) and dry weight of roots(DWR) are controlled by polygenes completely.Na+/H+ antiporter catalyzes the countertransport of Na+ and H+ across membranes. We isolated a rice cDNA sequence which contains 3596bp with an open reading frame of 3426bp by RT-PCR, and denominated as OsNHAl. OsNHAl encodes a polypeptide of 1142 amino acid with 11 transmembrane domains in the N-terminal part and a hydrophilic tail in C-terminal part. The transmembrane region of OsNHAl has high sequence similarities to plasma membrane Na+/H+ antiporter from Arabidopsis thaliana. Phylogenetic analysis showed that OsNHAl clusters with plasma membrane Na+/H+ antiporters and is more distantly related to a cluster of vacuolar Na+/H+ antiporters. OsNHAl gene expression was up-regulated by salt stress, but not by drought stress. According to the marker-based rice chromosome 12 PAC / BAC physical map supplied by Japan Rice Genome Project (RGP: http://rgp.dna.affrc.go.jp ), OsNHAl is further mapped between RFLP marker E321S and E541S on the terminus of chromosome 12. Semi-quantitive RT-PCR results revealed that OsNHAl expression was up-regulated by NaCl stress in roots stems and leaves, but the level was higher in roots than in stems and in leaves. These results suggest that the product of OsNHAl functions as a plasma membrane Na+/H+ antiporter to mediate Na+ efflux and plays an important role in salt tolerance of rice.A tonoplast Na+/H+ antiporter gene 0sNHX2 was isolated through the method mentioned above. 0sNHX2 contains 1726bp and encodes a protein of 544 amino acid residues with a calculated molecuteir weight of 60KD and isoelectric point of 7.5. Hydrophobicity plot analysis showed the OsNHX2 has 11 putative transmembrane domains by TMpred program.Database search revealed the deduced amino acid sequence (OsNHX2) has 78% similarity to tonoplast Na+/H+ antiporter (OsNHXl) from rice and 77% similarity to Na+/H+ antiporter (AtNHXl) from Arabidopsis thaliana. Phylogenetic analysis showed that OsNHX2 clusters with vacuolar Na+/H+ antiporters and is more distantly related to a cluster of plasma membrane Na+/H+ antiporters. Semi-quantitive RT-PCR results revealed that 0sNHX2 expression was up-regulated by NaCl stress in roots stems and leaves, but the level was higher in leaves than in stems and in roots. These results suggest that the product of the 0sNHX2 functions as a vacuolar Na+/H+ antiporter to compartmentalize Na+ into the vacuole.