Antagonistic interactions between Arabidopsis K-homology domain genes uncover PEPPER as a positive regulator of the central floral repressor FLOWERING LOCUS C.

Plant floral transition is a major developmental switch regulated by an integrated network of pathways. Arabidopsis FLOWERING LOCUS K (FLK), a protein with three KH RNA-binding domains, operates in the autonomous flowering-promotive pathway by decreasing the transcript levels of the key flowering repressor FLOWERING LOCUS C (FLC). Here we report that PEPPER (PEP), an FLK paralog previously shown to affect vegetative and pistil development, antagonizes FLK by positively regulating FLC. Lack of PEP function rescues the flk late-flowering phenotype with a concomitant decrease in FLC RNA levels. Loss of HUA2, another FLC activator encoding an RNA-binding protein, further rescues flk, being flk hua2 pep triple mutants virtually wild-type regarding flowering time. Consistently, PEP overexpression determines high levels of FLC transcripts and flowering delay. Genetic and molecular analyses indicate that FLK and PEP act independently of FCA, another important FLC repressor in the autonomous pathway. In addition, we present data suggesting that PEP may affect FLC expression at both transcriptional and post-transcriptional levels. Overall, our results uncover PEP as a new factor for FLC upregulation, underscoring the importance of RNA-binding activities during developmental timing of flowering.