Meristem identity genes must be active for the primordia formed at the flanks of the apical meristem to become floral meristems. (Recall that an apical meristem that is forming floral meristems on its flanks is known as an inflorescence meristem.) For example, mutants of Antirrhinum (snapdragon) that have a defect in the meristem identity gene FLORICAULA develop an inflorescence that does not produce flowers. Instead of causing floral meristems to form in the axils of the bracts, the mutant floricaula gene results in the development of additional inflorescence meristems at the bract axils. The wild-type floricaula (FLO) gene controls the determination step in which floral meristem identity is established.
In Arabidopsis, AGAMOUS-LIKE 201 (AGL20), APETALA1 (AP1), and LEAFY (LFY) are all critical genes in the genetic pathway that must be activated to establish floral meristem identity. LFY is the Arabidopsis version of the snapdragon FLO gene. AGL20 plays a central role in floral evocation by integrating signals from several different pathways involving both environmental and internal cues (Borner et al. 2000). AGL20 thus appears to serve as a master switch initiating floral development.
Once activated, AGL20 triggers the expression of LFY, and LFY turns on the expression of AP1 (Simon et al. 1996). In Arabidopsis, LFY and AP1 are involved in a positive feedback loop; that is, AP1 expression also stimulates the expression of LFY.
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