Molecular genetic studies of the memory of winter.

Many plant species have evolved the ability to flower in the proper season by sensing environmental cues. The prolonged cold of winter is one such cue that certain plants use to acquire competence to flower the following spring. For example, biennials and winter annuals become established in one growing season and often flower quickly in the early spring of the following year to complete their life cycles. The process by which exposure to prolonged cold establishes competence to flower is known as vernalization. Many studies, starting with the classic work of Lang and Melchers, have shown that the vernalized state can be stable; i.e. after exposure to cold has ended, competence to flower, in certain species, can persist for many months and throughout many cell divisions in the shoot apical meristem. Thus, plants can exhibit a 'memory of winter' and vernalization can result in an epigenetic switch in the classic sense of the term: a change that is stable in the absence of the inducing signal. The nature of this epigenetic switch in Arabidopsis thaliana is discussed here.