Temporal relationship between the transcription of two Arabidopsis MADS box genes and the floral organ identity genes.

MADS box genes play important roles in specifying floral meristem and floral organ identity. We characterized the temporal and spatial expression patterns of two members of this gene family, AGL4 and AGL5 (for AGAMOUS [AG]-like). AGL4 RNA initially accumulates after the onset of expression of the floral meristem identity genes but before the onset of expression of the floral organ identity genes. AGL4 is, therefore, a putative target of the floral meristem identity genes and/or a potential regulator of the floral organ identity genes. AGL5 is initially expressed early in carpel development, shortly after the onset of AG expression. The loss of AGL5 expression in flowers of ag mutants, the activation of AGL5 by ectopic expression of AG, and the specific binding of AG to an element in the AGL5 promoter identify AGL5 as a putative direct target of AG. Our study provides possible links between the establishment of floral meristem and floral organ identity as well as subsequent steps in flower development.