Two FLX family members are non-redundantly required to establish the vernalization requirement in Arabidopsis.

Studies of natural genetic variation for the vernalization requirement in Arabidopsis have revealed two genes, FRIGIDA and FLOWERING LOCUS C (FLC), that are determinants of the vernalization-requiring, winter-annual habit. In this study, we show that FLOWERING LOCUS C EXPRESSOR-LIKE 4 (FLL4) is essential for upregulation of FLC in winter-annual Arabidopsis accessions and establishment of a vernalization requirement. FLL4 is part of the FLOWERING LOCUS C EXPRESSOR gene family and both are non-redundantly involved in flowering time control. Epistasis analysis among FRIGIDA, FLL4, FLOWERING LOCUS C EXPRESSOR and autonomous-pathway genes reveals that FRIGIDA fve exhibits an extreme delay of flowering compared with fri fve, but mutants in other autonomous-pathway genes do not, indicating that FVE acts most antagonistically to FRIGIDA. FLL4 may represent a new member of a FRI-containing complex that activates FLC.