Initiation of shoot apical meristem in rice: characterization of four SHOOTLESS genes

The regulatory mechanism of shoot apical meristem (SAM) initiation is an im portant subject in developmental plant biology. We characterized nine reces sive mutations derived from four independent loci (SHL1-SHL4) causing the d eletion of the SAM. Radicles were produced in these mutant embryos. Concomi tant with the loss of SAM, two embryo-specific organs, coleoptile and epibl ast, were lost, but the scutellum was formed normally. Therefore. different iation of radicle and scutellum is regulated independently of SAM, but that of coleoptile and epiblast may depend on SAM. Regeneration experiments usi ng adventitious shoots from the scutellum-derived calli showed that no adve ntitious shoots were regenerated in any shl mutant. However, small adventit ious leaves were observed in both mutant and wild-type calli, but they soon became necrotic and showed no extensive growth. Thus, leaf primordia can i nitiate in the absence of SAM, but their extensive growth requires the SAM. An in situ hybridization experiment using a rice homeobox gene, OSH1, as a probe revealed that shl1 and shl2 modified the expression domain of OSH1, but normal expression of OSH1 was observed in shl3 and shl4 embryos. Accord ingly, SHL1 and SHL2 function upstream of OSH1, and SHL3 and SHL4 downstrea m or independently of OSH1, These shl mutants are useful for elucidating th e genetic program driving SAM initiation and for unraveling the interrelati onships among various organs in grass embryos.