Overexpression of rice OSH genes induces ectopic shoots on leaf sheaths oftransgenic rice plants

Five rice homeobox (OSH) genes were overexpressed under the control of the cauliflower mosaic virus 35S promoter or the rice actin gene promoter in tr ansgenic rife plants. Almost all of the transgenic plants showed abnormal p henotypes, which could be classified into three types according to their se verity. Plants with the most severe phenotype formed only green organs, wit h many shoot apices on their adaxial sides. Plants with an intermediate phe notype formed bladeless leaves with normally developed leaf sheaths. Plants with a mild phenotype formed normal leaf sheaths and blades, but lacked li gules and showed diffusion of the blade-sheath boundary. The leaf structure of this phenotype was similar to that of dominant maize mutants, such as K n1, Rs1, Lg3, and Lg4. Based on these phenotypes, we suggest that ectopic e xpression of the rice OSH genes interferes with the development of leaf bla des and maintains leaves in less differentiated states. These results are d iscussed in relation to the leaf maturation schedule hypothesis (M. Freelin g et al., 1992, BioEssays 14, 227-236). (C) 2000 Academic Press.