Photo and hormonal control of meristem identity in the Arabidopsis flower mutants apetala2 and apetala1

We have analyzed the contributions of phytochrome and gibberellin signal transduction to the control of flower meristem identity in the Arabidopsis mutants apetala1 (apl) and apetala2 (ap2). ap1 flowers are partially defective for the establishment of flower meristem identity and are characterized by the production of ectopic secondary or axillary flowers and by branching. Axillary flower production is also induced in ap2-1 flowers by short-day photoperiod and is suppressed by hy1, a mutation blocking phytochrome activity. The production of axillary flowers by ap2-1 is also suppressed by exogenous gibberellins and by spindly (spy), a mutation that activates basal gibberellin signal transduction in a hormone-independent manner. Ectopic axillary flower production and floral branching by ap1 flowers are also suppressed by spy. We conclude that gibberellins promote flower meristem identity and that the inflorescence-like traits of ap2-1 and apl-l flowers are due in part to SPY gene activity.