Abstract
Certain plant varieties typically require prolonged exposure to the cold of winter to become competent to flower rapidly in the spring. This process is known as vernalization. In Arabidopsis thaliana, vernalization renders plants competent to flower by epigenetically silencing the strong floral repressor FLOWERING LOCUS C (FLC). As a result of vernalization, levels of lysine-9 and lysine-27 trimethylation on histone 3, modifications that are characteristic of facultative heterochromatin in plants, increase at FLC chromatin. We have identified a mutant, protein arginine methyltransferase 5 (atprmt5), that fails to flower rapidly after vernalization treatment. AtPRMT5 encodes a type II protein arginine methyltransferase (PRMT) that, in winter-annual strains, is required for epigenetic silencing of FLC and for the vernalization-mediated histone modifications characteristic of the vernalized state. Furthermore, the levels of arginine methylation of FLC chromatin increase after vernalization. Therefore, arginine methylation of FLC chromatin is part of the histone code that is required for mitotic stability of the vernalized state.