EXPANSINS AND INTERNODAL GROWTH OF DEEP-WATER RICE

The distribution and activity of the cell wall-loosening protein expan sin is correlated with internodal growth in deepwater rice (Oryza sati va L.). Acid-induced extension of native cell walls and reconstituted extension of boiled cell walls were confined to the growing region of the internode, i.e. to the intercalary meristem (IM) and the elongatio n zone. Immunolocalization by tissue printing and immunoblot analysis, using antibody against cucumber expansin 29 as a probe, confirmed tha t rice expansin occurred primarily in the IM and elongation zone. Rice expansin was localized mainly around the vascular bundles at the base of the IM and along the inner epidermal cell layer surrounding the in ternodal cavity. Submergence greatly promoted the growth of rice inter nodes, and cell walls of submerged internodes extended much more in re sponse to acidification than did the cell walls of air-grown internode s. Susceptibility of cell walls to added expansin was also increased i n submerged internodes, and analysis by immunoblotting showed that cel l walls of submerged internodes contained more expansin than did cell walls of air-grown internodes. Based on these data, we propose that ex pansin is involved in mediating rapid internodal elongation in submerg ed deepwater rice internodes.