12 Rice Genetics Newsletter Vol. 24 2. Identification of mutants with impaired regulation of KNOX genes expression

Knotted1-like homeobox (KNOX) genes are expressed in the shoot apical meristem (SAM) to maintain cells in a meristematic state, and are repressed in leaf founder cells on the flanks of the SAM (Jackson et al. 1994). Previous studies revealed that KNOX gene repression in leaf primordia is required to control hormonal levels and cell differentiation in leaf development (Sakamoto et al. 2001, Mele et al. 2003). Since expression patterns of KNOX genes in the shoot apex are correlated with phyllotaxy and plastochron, elucidation of a regulatory mechanism of KNOX gene expression would bring progress in understanding the mechanisms of early leaf development, phyllotaxy and plastochron. Although some genes have been reported to be necessary to repress KNOX genes in leaf primordia, a regulatory system of KNOX gene expression is still unclear. In rice, it was reported that constitutive expression of KNOX genes caused typical defects on leaf morphogenesis like bladeless leaves and invasion of sheath region into blade region (Sentoku et al. 2000, Ito et al. 2001, Fig.1A, B). These characteristics could be useful criteria for screening of mutants misexpressing KNOX genes in leaves. To understand a regulatory system of KNOX gene expression, we screened a rice Tos17 mutant population for mutants that misexpress KNOX genes in leaves. In the primary screening, we selected 3