Epigenetic regulation in the control of flowering.

for the reproductive success of plants. Plants need to gauge when both environmental and endogenous cues are optimal before undergoing the switch from vegetative to reproductive development. To achieve this, a complex regulatory network has evolved consisting of multiple pathways that quantitatively regulate a set of genes—the floral pathway integrators (Simpson and Dean 2002). The activity of these genes causes the transition of the shoot apical meristem to reproductive development and the production of flowers rather than leaves. The major environmental cues that regulate flowering are day length (photoperiod), light quality, and temperature. Temperature plays different roles in the flowering process. First, in all plants, ambient temperature during the growing season affects growth rate and time to flower (Blázquez et al. 2003). Second, many plant species, particularly those growing in high latitudes, require a prolonged period (weeks to several months) of cold temperature (2–10oC) before they will flower. This process, called vernalization, is an adaptation to ensure that plants overwinter before flowering, thus aligning flowering and seed production with the favorable environmental conditions of spring. A requirement for vernalization has also been extensively selected for in many crop plants, including wheat, Brassica species (Fig. 1A), barley (Fig. 1B), and sugar beet (Fig. 1C). Indeed, breeders have been so successful at introducing a strong vernalization requirement into crops that many accessions of wheat and oilseed rape will not flower without vernalization and so, if sown in spring, would stay vegetative through the whole summer, only flowering the following year after winter. The small garden weed Arabidopsis thaliana is an ideal model to study the molecular basis of this process as many Arabidopsis thaliana ecotypes also have a vernalization requirement and these have a very different growth form without vernalization to the classic rapid-cycling ecotypes used in the lab (Fig. 2C). The physiological properties of vernalization in Arabidopsis match those of all other plants: It is a quantitative response, with increasing weeks of cold progressively accelerating flowering (Figs. 1E, 2A); it is reversible if vernalized seeds are subsequently subjected to a brief period of heat stress; and it is not graft-transmissible (unlike the photoperiodic flowering signal). Importantly, the vernalization response is mitotically stable; the prolonged cold stimulus happens at one stage of development with flowering often occurring many months later. Also, cuttings from vernalized plants “remember” that they have experienced winter and flower at the appropriately early time. However, the process is reset at some point during meiosis or seed development so that seedlings need to be vernalized each generation to align flowering with spring. Epigenetic Regulation in the Control of Flowering