‘Diacylglycerol acyltransferase (DGAT)’ acts as a key rate-limiting enzyme that catalyzes the final step of the de novo biosynthesis of triacylglycerol (TAG). The study was to characterize the function of the DGAT3 gene in Paeonia rockii, which is known for its accumulation of high levels of unsaturated fatty acids (UFAs). We identified a DGAT3 gene which encodes a soluble protein that is located within the chloroplasts of P. rockii. Functional complementarity experiments in yeast demonstrated that PrDGAT3 restored TAG synthesis. Linoleic acid (LA, C18:2) and α-linolenic acid (ALA, C18:3) are essential unsaturated fatty acids that cannot be synthesized by the human body. Through the yeast lipotoxicity test, we found that the yeast cell density was largely increased by adding exogenous LA and, especially, ALA to the yeast medium. Further ectopic transient overexpression in Nicotiana benthamiana leaf tissue and stable overexpression in Arabidopsis thaliana indicated that PrDGAT3 significantly enhanced the accumulation of the TAG and UFAs. In contrast, we observed a significant decrease in the total fatty acid content and in several major fatty acids in PrDGAT3-silenced tree peony leaves. Overall, PrDGAT3 is important in catalyzing TAG synthesis, with a substrate preference for UFAs, especially LA and ALA. These results suggest that PrDGAT3 may have practical applications in improving plant lipid nutrition and increasing oil production in plants.

Tree peony seeds, traditionally used for edible oil production, are rich in α-linolenic acid (ALA). However, little attention is paid to their development as a healthcare food due to their bitter and astringent taste. The aim of this study was to optimize the debittering process of peony seeds on the basis of maintaining nutritional value and to identify the compounds that cause the taste of bitterness. We first optimized the debittering process by orthogonal experiments which reduced the polyphenol content by 90.25%, and we measured the main nutritional value of fatty acid composition, indicating that the high content of ALA is not affected by debittering. Then, we identified and determined the types and content of polyphenols, the metabolites causing bitter taste, in the samples based on LC-ESI-QQQ-MS. Principal component analysis (PCA) and orthogonal partial least square discriminant analysis (OPLS-DA) were used to compare and analyze the seeds at different stages of debittering. Thirty-eight key metabolites were identified, of which paeoniflorin, taxifolin, alibiflorin, protocatechuic acid, benzoyl paeoniflorin, quercetin-3-galactoside, and oxpaeoniflorin were significantly compared, and most of them were positively correlated with bitter taste. These results are conducive to the exploration and study of the bitter taste and nutritional value of tree peony seeds in the future.