Regulation of flowering in temperate cereals

Long exposure to cold (vernalization) accelerates flowering in winter cereals, a process regulated by the VRN1 (≈AP1), VRN2, and VRN3 (≈FT) vernalization genes. Flowering during the fall is prevented by the VRN2 downregulation of VRN3 and low VRN1 transcription. Vernalization induces VRN1, which is followed by the downregulation of VRN2, thereby releasing VRN3. In the longer days of spring, photoperiod genes PPD1 and CO upregulate VRN3, which induces VRN1 above the threshold levels required for flowering initiation. VRN3 transcription is modulated through interactions involving CCT-domain proteins and HAP2/HAP3/HAP5 complexes coded by multiple genes. The vast number of HAP–CCT combinations can provide the flexibility required for integrating seasonal cues and different stress signals in the regulation of the transition to flowering.