Evidence for homology of flowering-time genes VFR2 from Brassica rapa and FLC from Arabidopsis thaliana

Variation in flowering time is important for the adaptation of plant species to different natural and agricultural environments. We previously identified VFR2 as one of two major quantitative trait loci (QTLs) controlling vernalization-responsive flowering time in a segregating population derived from a cross of annual and biennial Brassica rapa. The region containing VFR2 is homologous to a region in Brassica napus that controls the same trait, and also to a region on chromosome 5 of Arabidopsis thaliana that contains several flowering-time loci. In order to determine precisely the allelic effects and map position of VFR2, we backcrossed the late allele into an early flowering line and obtained monogenic segregation for flowering time in a BC3S1 population. The two homozygous genotypic classes differed by 43 and 95 days to flowering in the field and growth chamber, respectively; and the effect of the late allele was almost completely additive. DNA probes that were previously shown to detect RFLP loci in the VFR2 region, or in the homologous regions of B. napus or A. thaliana (including two DNA clones of flowering-time genes) were used to construct a high-resolution map around VFR2. An RFLP detected by an A. thaliana cDNA clone of flowering focus C (FLC) co-segregated exactly with VFR2 in 414 gametes analyzed (<0.24 cM). FLC is a repressor of flowering and is required for the winter-annual habit of late-flowering ecotypes of A. thaliana. The regulation of FLC RNA in B. rapa was consistent with that seen in A. thaliana; RNA levels were up-regulated in the biennial parent and down-regulated by cold treatment. Thus, VFR2 appears to be homologous to FLC and may control flowering time though a similar mechanism as in A. thaliana.