Control of flowering time: interacting pathways as a basis for diversity.

Flowering is controlled by environmental conditions and developmental regulation. The complexity of this regulation is created by an intricate network of signaling pathways. Arabidopsis is an excellent model system in which to approach this complexity, because it responds to many of the environmental conditions that control flowering in other species, and genetic tools are well developed. Studies in Arabidopsis have led to the identification of components within individual signaling pathways that affect flowering, and to their positioning within molecular hierarchies. Furthermore, distinct signaling pathways are known to converge on the activation of the same flowering-time genes. This convergence of pathways on a common set of genes may enable the integration of different responses, so that the plant can produce a coordinated flowering response under conditions in which multiple environmental parameters are changing simultaneously. Also, genetic analysis of Arabidopsis varieties showing natural variation in flowering time has demonstrated how the activity of these pathways can be altered in nature and how balancing the effects of different environmental stimuli on flowering time is important in plants adapting to growth in different geographical locations. At present, the full complexity of the flowering network can only be approached in Arabidopsis where the necessary tools are available, and extensive efforts are being made to describe related pathways. For example, photoreception, circadian clock regulation, growth regulator synthesis and response, chromatin structure, and response to low temperatures play important roles in flowering-time control and are studied extensively in Arabidopsis. Nevertheless, there is a need to understand how the full diversity in flowering responses is generated. For example, Arabidopsis responds to photoperiod, but all ecotypes are long-day plants that flower earlier under long than short days, whereas many other species show the reverse response. Also, all Arabidopsis thaliana ecotypes are annual plants, and understanding the perennial habit will require a different model species. To understand the diversity in flowering responses, there is a need to look to other species, and here we compare detailed information from Arabidopsis with the emerging picture of the genetic control of flowering in short-day plants. The control of flowering has been reviewed frequently over the last few years (Koornneef et al., 1998b; Simpson et al., 1999; Reeves and Coupland, 2000; Samach and Coupland, 2000; Araki, 2001). However, progress has been rapid, and in the following review we emphasize results that have emerged recently. In particular, we summarize the current understanding of the signaling pathways involved in flowering control in Arabidopsis, and stress how alterations in the balance of activity between pathways can give rise to dramatically different flowering behaviors, for example between ecotypes. We also address recent progress in describing how these pathways function in species that show different responses to environmental conditions than does Arabidopsis.