Functional analysis of FRIGIDA using naturally occurring variation in Arabidopsis thaliana

The FRIGIDA locus (FRI, AT4G00650) has been extensively studied in Arabidopsis thaliana because of its role creating flowering time diversity. The FRI protein regulates flowering induction by binding partner proteins on its N-terminus and C-terminus domains and creating a supercomplex that promotes transcription of the floral repressor FLOWERING LOCUS C (FLC). Despite the knowledge accumulated on FRIGIDA (FRI), the function of the highly conserved central domain of the protein is still unknown. Functional characterization of naturally occurring DNA polymorphisms can provide useful information about the role of a protein and the localization of its operative domains. For FRI, loss-of-function mutations are positively selected and widespread in nature, making them a powerful tool to study the function of the different domains of the protein. Here we explore natural sequence variation in the FRI locus in more than 1000 Arabidopsis accessions. We identify 127 mutations that alter the FRI protein, including 60 that had never been described before. We defined 103 different alleles of FRI and study their association with variation in flowering time. We confirmed these associations by cloning 22 different alleles and expressing them in a common null genetic background. Our analysis pinpoints two single amino acid changes in the central domain that render the protein non-functional. We show that these two mutations determine the stability and cellular localization of the FRI protein. In summary, our work makes use of natural variants at the FRI locus to help understanding the function of the central domain of the FRI protein.