DOWN-REGULATION OF RFL, THE FLO LFY HOMOLOG OF RICE, ACCOMPANIED WITHPANICLE BRANCH INITIATION/

FLORICAULA (FLO) of Antirrhinum and LEAFY (FLY) of Arabidopsis regulat e the formation of floral meristems. To examine whether same mechanism s control floral development in distantly related species such as gras ses, we isolated RFL, FLO-LFY homolog of rice, and examined its expres sion and function. Northern analysis showed that RFL is expressed pred ominantly in very young panicle but not in mature florets, mature leav es, or roots. In situ hybridization revealed that RFL RNA was expresse d in epidermal cells in young leaves at vegetative growth stage. After the transition to reproductive stage, RFL RNA was detected in all lay ers of very young panicle including the apical meristem, but absent in the incipient primary branches. As development of branches proceeds, RFL RNA accumulation localized in the developing branches except for t he apical meristems of the branches and secondary branch primordia. Ex pression pattern of RFL raised a possibility that, unlike FLO and LFY, RFL might be involved in panicle branching. Transgenic Arabidopsis pl ants constitutively expressing RFL from the cauliflower mosaic virus 3 5S promoter were produced to test whether 35S-RFL would cause similar phenotype as observed in 35S-LFY plants. In 35S-RFL plants, transforma tion of inflorescence meristem to floral meristem was rarely observed. Instead, development of cotyledons, rosette leaves, petals, and stame ns was severely affected, demonstrating that RFL function is distinct from that of LFY. Our results suggest that mechanisms controlling flor al development in rice might be diverged from that of Arabidopsis and Antirrhinum.