Multiple pathways in the decision to flower: Enabling, promoting, and resetting

In this review, the network of pathways that controls the timing of the transition to flowering has been divided into those that enable and those that promote the floral transition (Figure 4). The interaction of these different pathways changes in response to different environmental and endogenous cues to generate the plasticity and diversity of the flowering response. The predominance of the different floral pathways also must change over the life cycle of the plant. During early vegetative development, floral repressors in the enabling pathways overcome any promotive cues, ensuring that a sufficiently long vegetative phase occurs for the necessary energy reserves to be accumulated. During the later stages of vegetative development, the activity of the floral repressors declines and there is a progressive activation of floral promoters until a quantitative threshold is reached and the transition of the meristem from a vegetative to a reproductive state occurs. The pattern of gene expression in the flowers then must be reset in the gametes and developing embryos so that the next generation can determine its own "right time to flower." Therefore, the plant life cycle can be viewed in three sequential floral phases resulting from the changing predominance of three activities: resetting, repression, and promotion (Figure 4). Judging from how fast our knowledge has progressed in the last few years, progress in determining the molecular basis of how these different phases are regulated and maintained should be rapid.