Flowering on time: genes that regulate the floral transition. Workshop on the molecular basis of flowering time control.

Successful sexual reproduction in plants depends on the recognition of favourable environmental conditions and the integration of that information with endogenous developmental cues. Flowering in higher plants involves the transition of a vegetative meristem, producing leaves and stems, into a floral meristem, producing flowers. Most of our understanding of the regulation of the floral transition has been developed in the past 10 years using molecular genetic approaches in the model plant Arabidopsis thaliana, a rosette plant in which the initiation of flowering is followed by the elongation of the main stem. The two main environmental factors that promote flowering in Arabidopsis are long days (Figure 1) and exposure to low temperatures (vernalization). Genetic and physiological analysis of flowering time in Arabidopsis has led to the identification of a large number of flowering-time genes (>80) that regulate flowering time in response to environmental and endogenous cues (reviewed by Simpson et al., 1999). Regulation occurs through a complex network of genetic pathways, with two main pathways mediating environmental responses (the long-day and vernalization pathways) and two pathways that function independently of environmental cues: the autonomous pathway, which promotes flowering under all conditions, and the gibberellin (GA) pathway, which is needed for flowering under non-inductive short-day conditions. These pathways converge in the induction of floral meristem identity genes and the floral transition (Figure 2). Many flowering-time genes have been cloned recently, and the primary aim of the EMBO workshop in Norwich, and this report, was to highlight the most recent advances in our understanding of the molecular mechanisms regulating the floral transition.