Upon floral induction, the primary shoot meristem of an Arabidopsis plant begins to produce flower meristems rather than leaf primordia on its flanks. Assignment of floral fate to lateral meristems is primarily due to the cooperative activity of the flower meristem identity genes LEAFY (LFY), APETALA1 (AP1), and CAULIFLOWER. We present evidence here that AP1 expression in lateral meristems is activated by at least two independent pathways, one of which is regulated by LFY. In lfy mutants, the onset of AP1 expression is delayed, indicating that LFY is formally a positive regulator of AP1. We have found that AP1, in turn, can positively regulate LFY, because LFY is expressed prematurely in the converted floral meristems of plants constitutively expressing AP1. Shoot meristems maintain an identity distinct from that of flower meristems, in part through the action of genes such as TERMINAL FLOWER1 (TFL1), which bars AP1 and LFY expression from the influorescence shoot meristem. We show here that this negative regulation can be mutual because TFL1 expression is downregulated in plants constitutively expressing AP1. Therefore, the normally sharp phase transition between the production of leaves with associated shoots and formation of the flowers, which occurs upon floral induction, is promoted by positive feedback interactions between LFY and AP1, together with negative interactions of these two genes with TFL1.

SummaryMADS-domain-containing transcription factors comprise a large family of regulators that have diverse roles in plant development, including the regulation of flowering time. AGAMOUS-LIKE 20/SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 (SOC1) and FRUITFUL act to promote flowering, whereas FLOWERING LOCUS C (FLC), FLOWERING LOCUS M/MADS AFFECTING FLOWERING1, and SHORT VEGETATIVE PHASE are inhibitors of flowering. Here we report that AGAMOUS-LIKE 24 (AGL24) also plays a role in the regulation of flowering time. agl24 mutants are late flowering and overexpression of AGL24 causes early flowering in wild-type and late-flowering-mutant backgrounds. The effect of AGL24 overexpression is most pronounced in autonomous-pathway-mutant and FRIGIDA-containing backgrounds. The behavior of AGL24 is most similar to that of SOC1. Like SOC1, AGL24 mRNA levels are upregulated by vernalization. Unlike SOC1, however, AGL24 mRNA levels are not affected by FLC, and therefore AGL24 may represent an FLC-independent target of the vernalization pathway. There is also evidence for cross-talk between AGL24 and SOC1. When overexpressed, SOC1 and AGL24 are able to upregulate each other's expression. Thus, AGL24 represents another component in a network of MADS-domain-containing transcription factors that regulate flowering time.

SummaryIn Arabidopsis, the closely related APETALA1 (AP1) and CAULIFLOWER (CAL) MADS-box genes share overlapping roles in promoting¯ower meristem identity. Later in¯ower development, the AP1 gene is required for normal development of sepals and petals. Studies of MADS-domain proteins in diverse species have shown that they often function as heterodimers or in larger ternary complexes, suggesting that additional proteins may interact with AP1 and CAL during¯ower development. To identify proteins that may interact with AP1 and CAL, we used the yeast two-hybrid assay. Among the ®ve MADS-box genes identi®ed in this screen, the SEPALLATA3 (SEP3) gene was chosen for further study. Mutations in the SEP3 gene, as well as SEP3 antisense plants that have a reduction in SEP3 RNA, display phenotypes that closely resemble intermediate alleles of AP1. Furthermore, the early¯owering phenotype of plants constitutively expressing AP1 is signi®cantly enhanced by constitutive SEP3 expression. Taken together, these studies suggest that SEP3 interacts with AP1 to promote normal¯ower development.