ACTIVATION OF FLORAL MERISTEM IDENTITY GENES IN ARABIDOPSIS

THE Arabidopsis floral meristem identity genes APETALA1 (AP1) and LEAF Y (LFY) confer floral identity on developing floral primordia(1-4), wh ereas TERMINAL FLOWER (TFL) is required to repress their expression wi thin shoot and inflorescence meristems(1,5). LFY and AP1 are expressed in floral primordia in response to environmental conditions, such as day length, which regulate the onset of flowering, and presumably also in response to the action of genes that influence flowering time. How ever, the relationship between these flowering-time genes and the flor al meristem-identity genes has been difficult to assess because flower ing time is determined by several interacting genetic pathways(6,7). H ere we describe a method to regulate expression of the flowering-time gene CONSTANS (CO) and demonstrate that CO expression is sufficient to trigger flowering, irrespective of day length. In response to CO expr ession, transcription of LFY and TFL is initiated rapidly, whereas tra nscription of AP1 occurs much later. We propose that CO acts within a genetic pathway that is sufficient to activate LFY and TFL transcripti on, but that rapid activation of AP1 requires an additional pathway.