Characterization of the KNOX class homeobox genes Oskn2 and Oskn3 identified in a collection of cDNA libraries covering the early stages of rice embryogenesis

For identification of genes involved in embryogenesis in the model cereal r ice, we have constructed a collection of cDNA libraries of well-defined sta ges of embryo development before, during and after organ differentiation. H ere, we focus on the possible role of KNOX (maize Knotted1-like) class home obox genes in regulation of rice embryogenesis. Three types of KNOX clones were identified in libraries of early zygotic embryos. Two of these, Oskn2 and Oskn3, encode newly described KNOX genes, whereas the third (Oskn1) cor responds to the previously described OSH1 gene. In situ hybridizations show ed that during the early stages of embryo development, all three KNOX genes are expressed in the region where the shoot apical meristem (SAM) is organ izing, suggesting that these genes are involved in regulating SAM formation . Whereas OSH1 was previously proposed to function also in SAM maintenance, Oskn3 may be involved in patterning organ positions, as its expression was found to mark the boundaries of different embryonic organs following SAM f ormation. The expression pattern of Oskn2 suggested an additional role in s cutellum and epiblast development. Transgenic expression of Oskn2 and Oskn3 in tobacco further supported their involvement in cell fate determination, like previously reported for Knotted1 and OSH1 ectopic expression. Whereas Oskn3 transformants showed the most pronounced phenotypic effects during v egetative development, Oskn2 transformants showed relatively mild alteratio ns in the vegetative phase but a more severly affected flower morphology. T he observation that the KNOX genes produce similar though distinct phenotyp ic reponses in tobacco, indicates that their gene products act on overlappi ng but different sets of target genes, or that cell-type specific factors d etermine their precise action.