The VERNALIZATION INDEPENDENCE 4 gene encodes a novel regulator of FLOWERING LOCUS C

The late-flowering, vernalization-responsive habit of many Arabidopsis ecotypes is mediated predominantly through repression of the floral programme by the FLOWERING LOCUS C (FLC ) gene. To better understand this repressive mechanism, we have taken a genetic approach to identify novel genes that positively regulate FLC expression. We identified recessive mutations in a gene designated VERNALIZATION INDEPENDENCE 4 (VIP4 ), that confer early flowering and loss of FLC expression in the absence of cold. We cloned the VIP4 gene and found that it encodes a highly hydrophilic protein with similarity to proteins from yeasts, Drosophila , and Caenorhabditis elegans . Consistent with a proposed role as a direct activator of FLC , VIP4 is expressed throughout the plant in a pattern similar to that of FLC . However, unlike FLC , VIP4 RNA expression is not down-regulated in vernalized plants, suggesting that VIP4 is probably not sufficient to activate FLC , and that VIP4 is probably not directly involved in a vernalization mechanism. Epistasis analysis suggests that VIP4 could act in a separate pathway from previously identified FLC regulators, including FRIGIDA and the autonomous flowering promotion pathway gene LUMINIDEPENDENS . Mutants lacking detectable VIP4 expression flower earlier than FLC null mutants, suggesting that VIP4 regulates flowering-time genes in addition to FLC . Floral morphology is also disrupted in vip4 mutants; thus, VIP4 has multiple roles in development.