Little is known about the signals that govern the network of meristem and organ identity genes that control f lower development. In Arabidopsis, we can induce a heterochronic switch from f lower to shoot development, a process known as f loral meristem reversion, by manipulating photoperiod in the f loral homeotic mutant agamous and in plants heterozygous for the meristem identity gene leafy. The transformation from f lower to shoot meristem is suppressed by hy1, a mutation blocking phytochrome activity, by spindly, a mutation that activates basal gibberellin signal transduction in a hormone independent manner, or by the exogenous application of gibberellins. We propose that LFY and AG play an important role in the maintenance of f lower meristem identity and that f loral meristem reversion in heterozygous lfy and in ag f lowers is regulated by a phytochrome and gibberellin signal transduction cascade.Plant growth and morphogenesis is controlled by meristems, organized tissues containing pluripotent stem cells whose identities and activities are regulated by intrinsic and environmental signals. In Arabidopsis, the shoot apical meristem undergoes two phases, vegetative and inflorescence; both phases are characterized by reiterative and indeterminate patterns of growth and organogenesis (1). The vegetative meristem produces a compact rosette consisting of a short stem and a variable number of leaves. By contrast, the inflorescence meristem produces an elongated stem punctuated by narrow cauline leaves, lateral secondary shoots, and flowers that are derived from the flanks of the inflorescence meristem. Although closely related spatially and by cell lineage to the inflorescence meristem, the floral meristem proceeds along a determinate developmental pathway producing four compact whorls of organs (four sepals, four petals, six stamens, and two carpels). At the completion of organogenesis, the floral meristem is thought to be depleted or its activity is suppressed (2, 3). The transition from vegetative to inflorescence shoot meristem is controlled by environmental signals including photoperiod and temperature (4-6), by intrinsic growth regulators such as the gibberellins (6-8), and by a system of flowering time genes (9). The inflorescence meristem in turn produces an indeterminate number of floral meristems. Genetic and molecular studies have shown that the establishment of floral meristem identity is governed by a network of genes including APETALA1 (AP1), APETALA2 (AP2), CAULI-FLOWER (CAL), CLAVATA1 (CLV1), CLAVATA3 (CLV3), and LEAFY (LFY) (3,(10)(11)(12)(13)(14)(15)(16)(17)(18), and several models have been proposed to explain how these genes function together (17,19,20). By comparison, much less is known about the signals and genes required for the maintenance of flower meristem identity.Recently several genes have been implicated in the maintenance of flower meristem identity including LFY and AGA-MOUS (AG) (3,15,17,(21)(22)(23). The LFY gene encodes a novel polypeptide that is reported to have DNA bin...

We have analyzed the contributions of phytochrome and gibberellin signal transduction to the control of flower meristem identity in the Arabidopsis mutants apetala1 (ap1) 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 flower by ap2-1 is also suppressed by exogenous gibberellins and by spindly (spy), a mutation that activates basal gibberellin signal transduction in 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 ap1-1 flowers are due in part to SPY gene activity.

We have analyzed the contributions of phytochrome and gibberellin signal transduction to the control of flower meristem identity in the Arabidopsis mutants apetala7 (ap7) and apetala2 (ap2). ap7 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 ap2-7 flowers by short-day photoperiod and is suppressed by hy7, a mutation blocking phytochrome activity. The production of axillary flowers by ap2-7 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 ap7 flowers are also suppressed by spyLWe conclude that gibberellins promote flower meristem identity and that the inflorescence-like traits of ap2-7 and ap7-7 flowqrs are due in part to SPY gene activity.\