Expressions of OsHKT1, OsHKT2, and OsVHA are differentially regulated under NaCl stress in salt-sensitive and salt-tolerant rice (Oryza sativa L.) cultivars

Under NaCl-dominated salt stress, the key to plant survival is maintaining a low cytosolic Na+ level or Na+/K+ ratio. The OsHKT1, OsHKT2, and OsVHA transporter genes might play important roles in maintaining cytosolic Na+ homeostasis in rice (Oryza sativa L. indica cvs Pokkali and BRRI Dhan29). Upon NaCl stress, the OsHKT1 transcript was significantly down-regulated in salt-tolerant cv. Pokkali, but not in salt-sensitive cv. BRRI Dhan29. NaCl stress induced the expression of OsHKT2 and OsVHA in both Pokkali and BRRI Dhan29. In cv. Pokkali, OsHKT2 and OsVHA transcripts were induced immediately after NaCl stress. However, in cv. BRRI Dhan29, the induction of OsHKT2 was quite low and of OsVHA was low and delayed, compared with that in cv. Pokkali. OsHKT2 and OsVHA induction mostly occurred in the phloem, in the transition from phloem to mesophyll cells, and in the mesophyll cells of the leaves. The vacuolar area in cv. Pokkali did not change under either short- (5-10 min) or long-term (24 h) salt stress, although it significantly increased 24 h after the stress in cv. BRRI Dhan29. When expressional constructs of VHA-c and VHA-a with YFP and CFP were introduced into isolated protoplasts of cvs Pokkali and BRRI Dhan29, the fluorescence resonance energy transfer (FRET) efficiency between VHA-c and VHA-a upon salt stress decreased slightly in cv. Pokkali, but increased significantly in cv. BRRI Dhan29. The results suggest that the salt-tolerant cv. Pokkali regulates the expression of OsHKT1, OsHKT2, and OsVHA differently from how the salt-sensitive cv. BRRI Dhan29 does. Together, these proteins might confer salt tolerance in Pokkali by maintaining a low cytosolic Na+ level and a correct ratio of cytosolic Na+/K+.