Fig. 2. Schematic representations of Arabidopsis plants summarizing the genetic control of vernalization requirement and response. The flowering phenotype of Arabidopsis is represented as either a rapid cycler (e.g. top right), which produces a flowering inflorescence, or as a winter annual accession (e.g. top left), which continues to produce rosette leaves. Rapid-cycling accessions do not require a vernalization treatment to flower early and are commonly used as laboratory backgrounds. By contrast, the majority of Arabidopsis accessions are winter annuals, which flower late unless they have been exposed to a prior vernalization treatment. Typically, 6 weeks of growth at 4°C produces a saturated vernalization response in Arabidopsis. Growth habit is indicated either with (+VRN) or without (–VRN) a vernalization treatment. When both FRI and FLC are active, the plant is vernalization responsive, as is found in many winter annual accessions. Mutations in either fri or flc can lead to rapid cycling. A vernalization-responsive FRI FLC accession is rendered insensitive to vernalization by a vrn mutation. Finally, a rapid-cycling fri FLC genotype becomes a winter annual background in the presence of an autonomous pathway mutation such as fca.