FILAMENTOUS FLOWER, a meristem and organ identity gene of Arabidopsis, encodes a protein with a zinc finger and HMG-related domains

Distinctive from that of the animal system, the basic plan of the plant bod y is the continuous formation of a structural unit, composed of a stem with a meristem at the top and lateral organs continuously forming at the meris tem. Therefore, mechanisms controlling the formation, maintenance, and deve lopment of a meristem will be a key to understanding the body plan of highe r plants. Genetic analyses of filamentous flower (fil) mutants have indicat ed that FIL is required for the maintenance and growth of inflorescence and floral meristems, and of floral organs of Arabidopsis thaliana. Pa encodes a protein carrying a zinc finger and a HMG box-like domain, which is known to work as a transcription regulator. As expected, the FIL protein was sho wn to have a nuclear location. In situ hybridization clearly demonstrated t hat mt is expressed only at the abaxial side of primordia of leaves and flo ral organs. Transgenic plants, ectopically expressing FIL, formed filament- like leaves with randomly arranged cells at the leaf margin. Our results in dicate that cells at the abaxial side of the lateral organs are responsible for the normal development of the organs as well as for maintaining the ac tivity of meristems.