The Arabidopsis KH-domain protein FLOWERING LOCUS Y delays flowering by upregulating FLOWERING LOCUS C family members.

KEY MESSAGE: We identified FLY as a previously uncharacterized RNA-binding-family protein that controls flowering time by positively regulating the expression of FLC clade members. The ability of flowering plants to adjust the timing of the floral transition based on endogenous and environmental signals contributes to their adaptive success. In Arabidopsis thaliana, the MADS-domain protein FLOWERING LOCUS C (FLC) and the FLC clade members FLOWERING LOCUS M/MADS AFFECTING FLOWERING1 (FLM/MAF1), MAF2, MAF3, MAF4, and MAF5 form nuclear complexes that repress flowering under noninductive conditions. However, how FLM/MAF genes are regulated requires further study. Using a genetic strategy, we showed that the previously uncharacterized K-homology (KH) domain protein FLOWERING LOCUS Y (FLY) modulates flowering time. The fly-1 knockout mutant and FLY artificial microRNA knockdown line flowered earlier than the wild type under long- and short-day conditions. The knockout fly-1 allele, a SALK T-DNA insertion mutant, contains an ~ 110-kb genomic deletion induced by T-DNA integration. FLC clade members were downregulated in the fly-1 mutants and FLY artificial microRNA knockdown line, whereas the level of the FLC antisense transcript COOLAIR was similar to that of the wild type. Our results identify FLY as a regulator that affects flowering time through upregulation of FLC clade members.