The cytoplasmic free Ca(2+) could play an important role for salt tolerance in rice root (Oryza sativa L.). Here, we compared the expression profiles of two putative developmentally regulated plasma membrane polypeptides (DREPP1 and DREPP2) in rice roots of salt-tolerant cv. Pokkali and salt-sensitive cv. IR29. The messenger RNA (mRNA) for OsDREPP1 could be detected in all parts of root and did not change upon salt stress, whereas the mRNA for OsDREPP2 was detected only in root tips. The transcript level of OsDREPP2 first disappeared upon salt stress, then recovered in Pokkali, but not recovered in IR29. The gene-encoding OsDREPP2 was cloned from cv. Pokkali and expressed in Escherichia coli, and its biochemical properties were studied. It was found that OsDREPP2 is a Ca(2+)-binding protein and binds also to calmodulin (CaM) as well as microtubules. The mutation of Trp4 and Phe16 in OsDREPP2 to Ala decreased the binding of DREPP2 to Ca(2+)/CaM complex, indicating the N-terminal basic domain is involved for the binding. The binding of OsDREPP2 to microtubules was inhibited by Ca(2+)/CaM complex, while the binding of double-mutant OsDREPP2 protein to microtubules was not inhibited by Ca(2+)/CaM complex. We propose that CaM inhibits the binding of DREPP2 to cortical microtubules, causes the inhibition of microtubule depolymerization, and enhances the cell elongation.
Keywords: Ca2+-binding protein; Calmodulin; Cell elongation; DREPP; Microtubule; Oryza sativa; Salt stress.