Interactions among APETALA1, LEAFY, and TERMINAL FLOWER1 specify meristem fate

Upon floral induction, the primary shoot meristem of an Arabidopsis plant b egins to produce flower meristems rather than leaf primordia on its Ranks. Assignment of floral fate to lateral meristems is primarily due to the coop erative activity of the flower meristem identity genes LEAN (LFY), APETALA1 (AP1), and CAULIFLOWER, We present evidence here that AP1 expression in la teral meristems is activated by at least two independent pathways, one of w hich is regulated by LFY, In lfy mutants, the onset of AP1 expression is de layed, indicating that LN is formally a positive regulator of AP1. We have found that AP1, in turn, can positively regulate LN because LN is expressed prematurely in the converted floral meristems of plants constitutively exp ressing AP1, Shoot meristems maintain an identity distinct from that of flo wer meristems, in part through the action of genes such as TERMINAL FLOWER1 (TFL1), which bars AP1 and LN expression from the inflorescence shoot meri stem, We show here that this negative regulation can be mutual because TFL1 expression is downregulated in plants constitutively expressing AP1, There fore, the normally sharp phase transition between the production of leaves with associated shoots and formation of the flowers, which occurs upon flor al induction, is promoted by positive feedback interactions between LN and AP1, together with negative interactions of these two genes with TFL1.