FLYING SAUCER 1 is a transmembrane RING protein involved in cell wall biosynthesis in the Arabidopsis thaliana seed coat

The plant cell wall is a complex and dynamic network of polysaccharides and structural proteins, which lies outside the plasma membrane and provides strength and protection. The mechanical properties of the cell wall depend largely on the structure and organization of its components. Pectins form the gel matrix in which all other wall components are embedded and changes in their degree of methylesterification (DM) impact wall strength and adhesion. Low DM pectin molecules can be linked together via calcium bridges to form strong gels, but very few mutants affecting pectin methylesterification have been identified. During my MSc research, I characterized flying saucer 1 (fly1), a novel Arabidopsis thaliana seed coat mutant, which displays primary cell wall detachment, reduced mucilage extrusion, and increased mucilage adherence. These defects appear to result from a lower DM in mucilage, and can be intensified by addition of Ca²⁺ ions or completely rescued by treatment of seeds with cation chelators. The FLY1 gene encodes a protein with multiple transmembrane spans that is targeted to the secretory pathway and contains a RING-H2 domain, which generally facilitates protein-protein interactions. FLY1-YFP fusion proteins localize to small intracellular compartments in seed coat epidermal cells at the stage of mucilage biosynthesis. TUL1, a previously described FLY1 yeast ortholog, is a Golgi-localized E3 ligase involved in the trafficking of membrane proteins. I propose that FLY1 promotes pectin methylesterification in seed coat epidermal cells, potentially through interactions with pectin methyltransferase enzymes in the Golgi apparatus. Co-expression analysis suggests that FLY1 and FLY2, its only paralog, may play partially redundant roles in xylem development. These genes may be regulated by KNAT7, a transcription factor that controls secondary wall biosynthesis in both xylem and seed coat cells. The binding partners of the FLY1 protein and its precise molecular function remain to be determined.Science, Faculty ofBotany, Department ofGraduat