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 33 candidate recessive mutant lines that showed phenotypes similar to those of the constitutive expressor of KNOX genes (Fig.1A, B). Secondly, we examined the expression of OSH1, one of KNOX genes, in leaves of candidate lines by RT-PCR. Finally, we identified 11 lines that ectopically expressed OSH1 in leaves. All of these mutants were segregated as single recessive mutations in their M2 generation, and showed similar phenotype including bladeless leaves, severe dwarfism, dark green leaves, and seedling lethality (Fig.1D-F). These mutants also showed ectopic expression of other members of rice KNOX genes, including OSH6, OSH15 and OSH71, except OSH43, in leaves (Fig.1F). Cloning of causal genes of these mutants is expected to reveal key factors in the repression machinery.

References

Ito Y., M. Eiguchi and N. Kurata, 2001. KNOX homeobox genes are sufficient in maintaining cultured cells in an undifferentiated state in rice. Genesis 30: 231-238.

Jackson D., B. Veit and S. Hake, 1994. Expression of maize KNOTTED1 related homeobox genes in the shoot apical meristem predicts patterns of morphogenesis in the vegetative shoot. Development 120: 405-413.

Mele G., N. Ori, Y. Sato and S. Hake, 2003. The knotted1-like homeobox gene, BREVIPEDICELLUS, regulates cell differentiation by modulating metabolic pathways. Genes and Develop. 17: 2088-2093.

Sakamoto T., N. Kamiya, M. Ueguchi-Tanaka, S. Iwahori and M. Matsuoka, 2001. KNOX homeomain protein directly suppresses the expression of a gibberellin biosynthetic gene in the tobacco shoot apical meristem. Genes and Develop. 15: 581-590.

Sentoku N., Y. Sato and M. Matsuoka, 2000. Overexpression of rice OSH genes induces ectopic shoots on leaf sheaths of transgenic rice plants. Develop. Biol. 220: 358-364.