LEAF POLARITY AND MERISTEM FORMATION IN ARABIDOPSIS

Shoot apical meristems (SAMs) of seed plants are small groups of pluri potent cells responsible for making leaves, stems and flowers. While t he primary SAM forms during embryogenesis, new SAMs, called axillary S AMs, develop later on the body of the plant and give rise to branches. In Arabidopsis plants, axillary SAMs develop in close association wit h the adaxial leaf base at the junction of the leaf and stem (the leaf axil). We describe the phenotype caused by the Arabidopsis phabulosa- 1d (phb-1d) mutation. phb-Id is a dominant mutation that causes altere d leaf polarity such that adaxial characters develop in place of abaxi al leaf characters. The adaxialized leaves fail to develop leaf blades . This supports a recently proposed model in which the juxtaposition o f ad- and abaxial cell fates is required for blade outgrowth. In addit ion to the alteration in leaf polarity, phb-1d mutants develop ectopic SAMs on the undersides of their leaves. Also, the phb-1d mutation wea kly suppresses the shoot meristemless (stm) mutant phenotype, These ob servations indicate an important role for adaxial cell fate in promoti ng the development of axiallary SAMs and suggest a cyclical model for shoot development: SAMs make leaves which in turn are responsible for generating new SAMs.