OsEXPA4andOsRWC3are involved in asymmetric growth during gravitropic bending of rice leaf sheath bases

Expression of an expansin gene ( OsEXPA4 ) and a plasma membrane aquaporin ( OsRWC3 ) were differentially induced in the abaxial (lower) side of rice leaf sheath bases during gravitropism. Expression of both of these genes was induced by GA 3 . β‐glucuronidase (GUS) activity in transgenic rice expressing a GUS reporter gene under the control of the gibberellin (GA)‐responsive OsRWC3 promoter was investigated during gravitropic curvature and found to be enhanced in the abaxial side of graviresponding leaf sheath bases. Ancymidol, a GA biosynthesis inhibitor, reduced the expression gradient of OsEXPA4 and OsRWC3 . The osmotic potential was transiently lower on the abaxial side of gravistimulated leaf sheath bases, and the gravitropic curvature was inhibited by HgCl 2 and phloretin, two known inhibitors of aquaporins. These data suggest that asymmetric GA redistribution following gravistimulation results in the localized expression of OsEXPA4 and OsRWC3 , that in turn might bring about differential cell wall loosening, cell expansion and water influx via aquaporins, thus leading to gravitropic bending.