Abstract
Seed setting is an important trait that contributes to seed yield and relies greatly on starch accumulation. In this study, a sulfoquinovosyl transferase-like protein, designated as SQD2.2 involved in seed setting and flavonoid accumulation, was identified and characterized in rice. Rice SQD2.2 is localized to the cytoplasm, and the SQD2.2 transcript was highest in leaves. Rice SQD2.2-overexpressing (OE) plants exhibited a decreased seed setting rate and diminished tiller number simultaneously with an increased glycosidic flavonoid level compared with wild-type (WT) plants. SQD2.2 catalyzes the glycosylation of apigenin to produce apigenin 7-O-glucoside using uridine diphosphate-glucose (UDPG) as a sugar donor, but it failed to compensate for sulfoquinovosyldiacylglycerol (SQDG) synthesis in the Arabidopsis sqd2 mutant. Furthermore, apigenin 7-O-glucoside 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.
Highlights
Cereal grain, mostly in the form of starch, is a major product that supplies energy for human activity, feedstock and industrial materials
Phylogenetic analysis showed that SQD2.2 is classified into the group of SQD2 homologs in Arabidopsis and Synechococcus elongatus PCC7924 but is relatively distant from the UDP-sugar: glycosyltransferase (UGT) involved in flavonoid glycosylation (Fig. S2)
In Arabidopsis and spinach, SQDG is exclusively synthesized in chloroplasts, as demonstrated by the chloroplast localization of all enzymes involved in SQDG synthesis[29, 32,33,34]
Summary
Mostly in the form of starch, is a major product that supplies energy for human activity, feedstock and industrial materials. Among the three SQDG-deficient mutants of Arabidopsis, the severe growth defect and lack of the novel anionic glycolipid glucuronosyldiacylglycerol (GlcADG) were found only in the sqd[2] mutants under phosphate-limited conditions[34], suggesting that the function of Arabidopsis SQD2 (AtSQD2) is required for SQDG synthesis and for GlcADG synthesis in Arabidopsis These results indicate that the SQD2 enzyme may have diverse substrates in plants. SQD2.2-overexpressing (OE) plants had a significant reduction in ADPG and UDPG compared with wild-type (WT) plants, suggesting that enhanced production of flavonoid by SQD2.2 results in sugar substrate competition and a reduced requirement for ADPG for starch synthesis. These results suggest that rice SQD2 exhibits multifaceted enzymatic properties with different substrate selections and regulates the partitioning of carbon between primary and secondary metabolism in rice
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