THE ROUGH SHEATH2 GENE NEGATIVELY REGULATES HOMEOBOX GENE-EXPRESSION DURING MAIZE LEAF DEVELOPMENT

Leaves of higher plants are produced in a sequential manner through th e differentiation of cells that are derived from the shoot apical meri stem, Current evidence suggests that this transition from meristematic to leaf cell fate requires the down-regulation of knotted1-like homeo box (knox) gene expression. If knox gene expression is not repressed, overall leaf shape and cellular differentiation within the leaf are pe rturbed. In order to identify genes that are required for the aquisiti on of leaf cell fates, we have genetically screened for recessive muta tions that confer phenotypes similar to dominant mutations (e.g. Knott ed1 and Rough sheath1) that result in the ectopic expression of class I knox genes. Independently derived mutations at the rough sheath2 (rs 2) locus condition a range of pleiotropic leaf, node and internode phe notypes that are sensitive to genetic background and environment. Phen otypes include dwarfism, leaf twisting, disorganized differentiation o f the blade-sheath boundary, aberrant vascular patterning and the gene ration of semi-bladeless leaves. knox genes are initially repressed in rs2 mutants as leaf founder cells are recruited in the meristem, Howe ver, this repression is often incomplete and is not maintained as the leaf progresses through developement, Expression studies indicate that three knox genes are ectopically or over-expressed in developing prim ordia and in mature leaves. We therefore propose that the rs2 gene pro duct acts to repress knox gene expression (either directly or indirect ly) and that rs2 gene action is essential for the elaboration of norma l leaf morphology.