Expression of alpha-expansin genes in young seedlings of rice (Oryza sativa L.).

Rice is the only cereal in which germination and coleoptile elongation occur in hypoxia or anoxia. Little is known of the molecular basis directly underlying coleoptile cell extension. In this paper, we describe the expression of alpha-expansin genes in embryos during seed development and young seedlings grown under various oxygen concentrations. The genes Os-EXP2 and Os-EXP1 were predominantly expressed in the developing seeds, mainly in newly developed leaves, coleoptiles, and seminal roots. These expansins expressed in the developing seeds may give cells the potential to expand after seed imbibition begins. In coleoptiles, Os-EXP4 and Os-EXP2 mRNAs were greatly induced by submergence, while they were weakly detected in aerobic or anoxic conditions. Under submerged soil conditions, the signals hybridized with probes Os-EXP4 and Os-EXP2 in coleoptiles were strongest when coleoptiles elongated in the water layer. These data show that expansin gene expression is highly correlated with coleoptile elongation in response to oxygen concentrations. The Os-EXP4 gene was also expressed in leaves, mesocotyls, and coleorhizas of young seedlings. The growth of these tissues was also correlated with the presence of expansins. Therefore, the evidence derived from this study clearly demonstrates that expansins are indispensable for the growing tissues of rice seedlings.