FLC-Independent Vernalization Responses

Varieties of many plant species show a requirement for prolonged exposure to low temperatures in order to accelerate flowering, a process termed vernalization. In Arabidopsis, the early-flowering phenotype of vernalized plants results from the combined action of three MADS-domain proteins - FLC, AGL19 and AGL24, each assigned to an independent vernalization pathway. Both AGL19 and AGL24 function to promote flowering, and are activated during vernalization, while FLC acts to delay flowering and therefore is repressed by a vernalizing-treatment. One conspicuous attribute of vernalization is a delayed effect that is coupled to a cellular memory-mechanism. For both FLC and AGL19 pathways this cellular memory has been found to be based on epigenetic modifications. One model is that two distinct histone-modifying Polycomb repressive complexes – the VRN2- and EMF2-complexes - introduce repressive histone H3 lysine 27 trimethylation marks at specific locations in the FLC and AGL19 chromatin, respectively, leading to mitotically stable transcriptional repression. Vernalization acts differentially on each complex, and the coordinated action of both is necessary for a complete vernalization response. As homologs of the Arabidopsis vernalization genes are being identified in other species, it may soon be revealed whether the same mechanisms are shared by distinct plant groups. However, it is believed that vernalization responses evolved independently in different plant groups, and in grasses, epistatic interactions between two loci, VRN1 and VRN2 that are unrelated to the Arabidopsis VRN1 and VRN2 genes, mainly determine the vernalization requirement. Whether epigenetic mechanisms are also involved in the vernalization response outside Arabidopsis remains to be determined. Importantly, VRN1 in grasses encodes a MADS-domain protein. Thus, MADSdomain proteins play central roles in various vernalization pathways.