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The transition from vegetative to reproductive development in plants is controlled by environmental and endogenous cues to optimize reproductive fitness. These multiple cues need to be integrated temporally and spatially for the proper developmental regulation of the initiation of flower formation during the meristem identity (MI) transition. In Arabidopsis, the transcription factors LEAFY (LFY), APETALA1 (AP1) and FRUITFULL (FUL) play a key role in this process. How the relevant environmental and endogenous cues together trigger LFY, AP1 and FUL upregulation is still not fully understood. Inductive photoperiod activates expression of all three MI regulators, while gibberellin hormone signaling upregulates LFY expression. Recently, we and others identified a new pathway that activates LFY, AP1, and FUL expression during the MI transition (Wang et al., 2009; Yamaguchi et al., 2009).
Overexpression of the micro RNA regulated SQUAMOSA PROMOTER BINDING PROTEIN-LIKE 3 (SPL3) transcription factor causes precocious developmental phase transitions. Genetic analysis revealed that LFY, AP1 and FUL are required for the precocious MI transition of SPL3 overexpressors. In addition, ful mutants suppress the early flowering phenotype of SPL3 overexpressors. SPL3 directly binds to the regulatory regions of LFY, AP1 and FUL and up-regulates their expression together with closely related SPL transcription factors. We show that the SPL pathway activates initiation of flower formation in parallel to the known photoperiod pathway. Using the FUL promoter as a model, we are further investigating the individual contributions of the relevant inductive pathways towards upregulation of an MI regulator and initiation of flower formation.

References
Wang et al., Cell 2009 138 738-749
Yamaguchi et al., Dev Cell. 2009 17 268-278