A RICE HOMEOBOX GENE, OSH1, IS EXPRESSED BEFORE ORGAN DIFFERENTIATIONIN A SPECIFIC REGION DURING EARLY EMBRYOGENESIS

Homeobox genes encode a large family of homeodomain proteins that play a key role in the pattern formation of animal embryos, By analogy, ho meobox genes in plants are thought to mediate important processes in t heir embryogenesis, but there is very little evidence to support this notion. Here we described the temporal and spatial expression patterns of a rice homeobox gene, OSH1, during rice embryogenesis. lit situ hy bridization analysis revealed that in the wild-type embryo, OSH1 was f irst expressed at the globular stage, much earlier than organogenesis started, in a ventral region where shoot apical meristem and epiblast would later develop, This localized expression of OSH1 indicates that the cellular differentiation has already occurred at this stage, At la ter stages after organogenesis had initiated, OSH1 expression was obse rved in shoot apical meristem [except in the L1 (tunica) layer], epibl ast, radicle, and their intervening tissues in descending strength of expression level with embryonic maturation. We also performed in situ hybridization analysis with a rice organless embryo mutant, orl1, that develops no embryonic organs. In the orl1 embryo, the expression patt ern of OSH1 was the same as that in the wild-type embryo in spite of t he lack of embryonic organs. This shows that OSH1 is not directly asso ciated with organ differentiation, but may be related to a regulatory process before or independent of the organ determination, The results described here strongly suggest that, like animal homeobox genes, OSH1 plays an important role in regionalization of cell identity during ea rly embryogenesis.