The genes that determine the transition from shoot apical meristems to inflorescence meristems and from inflorescence meristems to floral meristems have been studied in model organisms such as Arabidopsis.

MERISTEM IDENTITY GENES Expression of two meristem identity genes initiates a cascade of further gene expression that leads to flower formation. These genes encode the transcription factors LEAFY and APETALA1, which together are necessary and sufficient for determining the transition to flowering. Evidence for the roles of these factors comes from both genetic and plant transformation experiments. For example, a mutant allele of the APETALA1 gene leads to continued vegetative growth, even if conditions are suitable for flowering. However, if the wild-type APETALA1 gene is coupled with a constitutive (always on) promoter and used to transform Arabidopsis plants, the plants will flower prematurely, regardless of environmental conditions. This is powerful evidence that APETALA1 plays a role in switching meristem cells from a vegetative to a reproductive fate (see Figure 37.7B).

FLORAL ORGAN IDENTITY GENES The products of the meristem identity genes trigger the expression of floral organ identity genes. They encode transcription factors that determine whether cells in the floral meristem will be sepals, petals, stamens, or carpels. For example, AGAMOUS is a *class C gene that causes cells determined to be part of the flower to form stamens and carpels. Floral organ identity genes often become activated in response to external cues (day length, temperature) or internal cues (hormones). We begin with day length, or photoperiod.

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