Genetic analysis of the floral initiation process (FLIP) in Arabidopsis

Abstract

Within the Arabidopsis inflorescence, two distinct developmental phases exist. The early inflorescence phase is characterized by nodes bearing coflorescences and leaves, and the late inflorescence phase by nodes bearing flowers. Four genes, TERMINAL FLOWER 1, LEAFY, APETALA1 and APETALA2 are necessary to initiate the switch from formation of early to formation of late inflorescence nodes at the appropriate time. We have investigated the relative roles of these genes in development by isolating and characterizing new alleles of TERMINAL FLOWER 1, LEAFY and APETALA1, and by constructing double mutants to test gene interactions. We suggest that the TERMINAL FLOWER 1 gene product is part of a mechanism that controls the timing of phase-switching in Arabidopsis. We propose that this mechanism involves factor(s) whose activity changes in response to shoot development and environmental variation. TERMINAL FLOWER 1 influences phase transitions in Arabidopsis, and appears to regulate the timing of expression of LEAFY, APETALA1 and APETALA2. LEAFY, APETALA1 and APETALA2 have partially redundant functions in initiating the floral program. In the absence of any one of the three genes, there is a gradual transition from coflorescence to flower-like lateral shoots. This suggests that (1) LEAFY, APETALA1 and APETALA2 are required in combination to ensure that the floral program is initiated rapidly and completely and (2) in the absence of one of the three genes, the others are activated slowly in response to the mechanism controlling timing of phase switching. Besides their role in establishing the floral program, phenotypes of flower-like lateral shoots in mutant inflorescences suggest that all three, LEAFY, APETALA1 and APETALA2, influence expression of whorl identity genes. Loss of LEAFY results in decreased Class B gene expression, as well as altered expression patterns of Class A and Class C genes. In the absence of either APETALA2 or APETALA1, reproductive organs develop in the perianth whorls, suggesting that both genes should be considered Class A organ identity genes, restricting Class C gene expression to inner whorls.