Establishment of a vernalization requirement in <i>Brachypodium distachyon</i> requires <i>REPRESSOR OF VERNALIZATION1</i>

Abstract

A requirement for vernalization, the process by which prolonged cold exposure provides competence to flower, is an important adaptation to temperate climates that ensures flowering does not occur before the onset of winter. In temperate grasses, vernalization results in the up-regulation of <i>VERNALIZATION1</i> (<i>VRN1</i>) to establish competence to flower; however, little is known about the mechanism underlying repression of <i>VRN1</i> in the fall season, which is necessary to establish a vernalization requirement. Here, we report that a plant-specific gene containing a bromo-adjacent homology and transcriptional elongation factor S-II domain, which we named <i>REPRESSOR OF VERNALIZATION1</i> (<i>RVR1</i>), represses <i>VRN1</i> before vernalization in <i>Brachypodium distachyon</i> That <i>RVR1</i> is upstream of <i>VRN1</i> is supported by the observations that <i>VRN1</i> is precociously elevated in an <i>rvr1</i> mutant, resulting in rapid flowering without cold exposure, and the rapid-flowering <i>rvr1</i> phenotype is dependent on <i>VRN1</i> The precocious <i>VRN1</i> expression in <i>rvr1</i> is associated with reduced levels of the repressive chromatin modification H3K27me3 at <i>VRN1</i>, which is similar to the reduced <i>VRN1</i> H3K27me3 in vernalized plants. Furthermore, the transcriptome of vernalized wild-type plants overlaps with that of nonvernalized <i>rvr1</i> plants, indicating loss of <i>rvr1</i> is similar to the vernalized state at a molecular level. However, loss of <i>rvr1</i> results in more differentially expressed genes than does vernalization, indicating that <i>RVR1</i> may be involved in processes other than vernalization despite a lack of any obvious pleiotropy in the <i>rvr1</i> mutant. This study provides an example of a role for this class of plant-specific genes.