Sterol 3-beta-Glucosyltransferase TRANSPARENT TESTA15 Controls Seed Development and Flavanol Accumulation through its Role in Vacuole Biogenesis and Maintenance in Arabidopsis

Abstract

ABSTRACTThe Arabidopsis sterol 3-beta-glucosyltransferase UGT80B1/TRANSPARENT TESTA15 (TT15) catalyzes sterol glucoside biosynthesis. Its loss of function causes reduced seed size, defective flavanol, polysaccharide and lipid polyester deposition at the seed coat and reduced seed dormancy. How TT15 controls seed development and physiology is unknown. Here we show thattt15mutants exhibit seed lethality with incomplete penetrance and maternal determinism that is correlated with endosperm cellularization defects, together with an increased sensitivity of seed germination to exogenous abscisic acid and paclobutrazol. We also reveal that flavanol deposition in the vacuole duringtt15seed development triggers premature endothelium cell death. An autoimmune-like syndrome characterized by callose and H2O2accumulation was detected in endothelium at the seed abaxial pole. Similar phenotypes were observed withtt9/gfs9,a mutant defective in endomembrane trafficking and homotypic vacuole fusion. Double mutant analysis showed thattt9partially rescuedtt15endothelium phenotypes. Consistent with seed mutant phenotypes,TT15promoter activity was detected in endothelium and endosperm and TT15 protein was located mainly at the vacuolar membrane (tonoplast). Using fluorescence recovery after photobleaching, we demonstrated that tonoplast fluidity was increased intt15roots. Altogether our data suggest that TT15 regulates seed development and flavanol accumulation by modulating vacuole biogenesis and maintenance.