Structural and biochemical studies on FRIGIDA flowering time regulator

Abstract

The flowering of plant is amazing and beautiful phenomenon. Interactive molecular pathways determine when to flower based on the environmental and endogenous signals. Flowering type of Arabidopsis can be separated by two groups. Summer annuals flower early while winter annuals show late-flowering phenotype after vernalization. The major determinant of flowering type is FLOWERING LOCUS C (FLC) which represses flowering by inhibiting floral pathway integrators. FRIGIDA (FRI), increases transcription level of FLC and causes late-flowering phenotype in Arabidopsis. However, the molecular mechanism of how FRI up-regulates FLC chromatin is not well understood. To investigate the function of FRI in flowering time regulation, I solved the crystal structure of FRI. The structure shows a novel fold and contains 14 $\alpha -helices$ forming extended helical bundle structure. Based on structural analysis and sequence conservation of FRI, we found a conserved region on the surface of the structure coincides with positive charged region. To identify the conserved positive surface is important for the function of FRI, we generated mutants by mutating the conserved positive amino acids to Ala and introduced the mutant genes into first generation (T1) transgenic plants for flowering time analysis. We showed that the conserved region we observed on the surface plays a crucial role in flowering time regulation. In addition, I suggested structure-based explanation of naturally occurring nonfunctional FRI. Furthermore, EARLY FLOWERING IN SHORT DAYS (EFS) which is also related to regulation of FLC, is a histone methyltransferase which has SET domain and methylates histone H3 Lys4 and Lys36. We showed that FRI increases the activity of EFS by in vitro histone methyltransferase assay. In addition, we observed that EFS methylates histone H3 Lys36 not Lys4 in our in vitro methylation assay condition. This structural analysis of FRI provides insights to study flowering time regulation by FRI and EFS.