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

At a certain point in their life cycle, annual plants undergo amajor developmental transition and switch from vegetative to reproductive development. This process is rarely reversible, and ensuring that the timing of this transition is optimal for pollination and seed development is a major factor in reproductive success. Physiological and genetic analysis of flowering has shown that multiple environmental and endogenous inputs influence the timing of the switch. The molecular identity of these different inputs is being dissected using molecular genetic approaches in Arabidopsis. The multiple pathways quantitatively regulate an overlapping set of common targets, the floral pathway integrators, whose activities convert the shoot apical meristem to a reproductive fate. An emerging idea is that changing the predominance of these input pathways could account for much of the plasticity and diversity of flowering time control within and between plant species (Simpson andDean, 2002). During the last few years, the data relevant to a molecular understanding of flowering time control have increased rapidly, making it unwieldy to comprehensively review the field. In addition, there have been numerous and excellent recent reviews on various aspects of flowering time control (Mouradov et al., 2002; Ratcliffe and Riechmann, 2002; Simpson and Dean, 2002; Henderson et al., 2003; Yanovsky and Kay, 2003). To complement these, we have chosen, in this review, to group genes involved in flowering time into pathways that enable the floral transition and those that promote it. We also discuss the role of genes defined through early-flowering mutants. Are they specific floral repressors or more global regulators needed to reset patterns of gene expression?