Rice Sulfoquinovosyltransferase SQD2 Is Invovled In Osmotic Stress And Sugar Metabolism Through Flavonoid Glycosylation

Sulfoquinovosyltransferase 2(SQD2)catalyzes the final-step reaction in the sulfoquinovosyldiacylglycerol(SQDG)biosynthetic pathway in Arabidopsis.SQDG is a sulfur-containing anionic glycerolipid of photosynthetic membranes and is involved in phosphate starvation due to its substitution for phosphatidylglycerol(PG)to maintain negative charge of thylakoid membrane surface.Loss of SQD2 results in growth arrest under phosphate starvation in Arabidopsis.Rice contains three SQD2 s that are more complicated than Arabidopsis SQD2.The roles of rice SQD2 s remain unknown.In this study,rice SQD2 s were characterized.The results showed that different SQD2 s exhibited unique functions.Except for the role of the SQDG synthesis in adapting to phosphate starvation,rice SQD2 s also possess an activity toward flavonoid glycosylation and are involved in osmotic stress and sugar metabolism.The main results are as follows:The subcellular localization of three rice SQD2 s are distinguishable.SQD2.2 and SQD2.3 are localized to cytoplasm.SQD2.1 and UDP-sulfoquinovose synthase(SQD1),the enzyme catalyzes the second step reaction of SQDG synthesis pathway,are localized to chloroplasts.Loss of either SQD1 or SQD2.1 resulted in defective SQDG in plants.The rice sqd2.1 mutants exhibited growth arrest and reduced phosphate content relative to wild type(WT)plants under phosphate starvation.The results suggest that rice SQD2.1 is solely responsible for SQDG synthesis in response to phosphate starvation.The study also showed that rice SQD2.1 and Arabidopsis SQD2 have glycosylation activity toward flavonoids,and this activity,not its SQDG synthesis,improves plant growth under salt and drought stresses.The gene knockout,overexpression and genetic complementation for SQD2.1,as well as SQD2.1enzymatic analysis,revealed that rice SQD2.1 possesses dual activities for SQDG synthesis and flavonoid glycosylation.Knockout and overexpression(OE)of SQD2.1decreased and increased,respectively,the level of apigenin 7-O-glucoside(A7G),and altered rice response to drought and high salinity.Exogenous supplementation of A7G to plants restored the stress response of sqd2.1 in rice and Atsqd2 in Arabidopsis to that of WT.SQD2-deficient rice and Arabidopsis plants displayed more oxidative damage whereas increased SQD2 expression or supplementation of its product A7G attenuated oxidative damage compared to WT plants.Collectively,this study identifies a novel activity and role of SQD2 in plant response to drought and high salinity and shows that the glycosidic flavonoid A7G produced by SQD2 plays a positive role in reactive oxygen species scavenging to protect cells against oxidative damages under hyperosmotic stress.Seed setting is an important trait that contributes to seed yield and relies greatly on starch accumulation.In this study,rice SQD2.2 is involved in seed setting and flavonoid accumulation.Rice SQD2.2 is localized to the cytoplasm,and the SQD2.2transcript was highest in leaves.Rice SQD2.2-OE plants exhibited a decreased seed setting rate and diminished tiller number simultaneously with an increased glycosidic flavonoid level compared with WT plants.SQD2.2 catalyzes the glycosylation of apigenin to produce A7 G using UDPG as a sugar donor,but it failed to compensate for SQDG synthesis in the Arabidopsis Atsqd2 mutant.Furthermore,A7 G inhibited starch synthase(SS)activity in a concentration-dependent manner,and SQD2.2-OE plants exhibited reduced SS activity accompanied by a significant reduction in starch levels and an elevation in soluble sugar levels relative to WT plants.Both adenosine diphosphate glucose(ADPG)and UDPG levels in SQD2.2-OE plants were notably lower than those in WT plants.Taken together,rice SQD2.2 exhibits a novel role in flavonoid synthesis and plays an important role in mediating sugar allocation between primary and secondary metabolism in rice.