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+.