Mechanisms underlying vernalization-mediated VERNALIZATION INSENSITIVE 3 (VIN3) induction in Arabidopsis thaliana
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Vernalization is defined as the response to prolonged cold exposure required for acquiring the molecular competence necessary to undergo floral transition. FLOWERING LOCUS C (FLC), a potent floral repressor in Arabidopsis, is highly expressed before vernalizing cold treatment but is repressed during prolonged vernalization. VERNALIZATION INSENSITIVE 3 (VIN3) is a Plant HomeoDomein (PHD)- containing protein that is required for establishing vernalization-mediated repression of FLC. The induction of VIN3 is one of the earliest molecular events in vernalization response and its expression is intimately linked to prolonged cold exposure. However, mechanisms underlying VIN3 induction remain poorly understood. The constitutive repression of VIN3 in the absence of cold is due to multiple repressive components, including a transposable element-derived sequence, LIKE-HETEROCHROMA TIN PROTEIN 1 (LHP1), and POLYCOMB REPRESSION COMPLEX 2 (PRC2). Furthermore, the full extent of VIN3 induction by vernalization requires activating complex components, including EARLY FLOWERING 7 (ELF7) and EARLY FLOWERING IN SHORT DAYS (EFS). Dynamic changes in the histone modifications present at VIN3 chromatin during vernalization were also observed, indicating that chromatin changes play a critical role in regulating VIN3 induction. However, VIN3 induction by vernalization still occurs in the absence of activation complexes and de- repression of VIN3 in the absence of the repressive complexes is not sufficient for achieving complete induction. Thus, unknown cold-influenced regulators responsible for achieving maximum VIN3 induction during vernalization must exist. Therefore, forward genetic screening was undertaken to elucidate upstream regulators of VIN3. Molecular characterization of T-DNA mutant populations elucidated two interesting mutants: a mutant that ectopically expressed VIN3 before cold (ectopic VIN3 induction, evi1) and mutants that failed to induce VIN3 during vernalization (defects in VIN3 induction, dvi1). FLC is over-expressed in dvi1 despite its failure to induce VIN3 expression during vernalization, suggesting that this mutant may regulate both VIN3 and FLC. In evi1, FLC is hyper-repressed after 40 days of vernalization, leading to an acceleration of flowering time. These results indicate that regulators of VIN3 in the vernalization pathway exist and that these regulators may use different mechanisms in order to influence VIN3 expression.