The Biosynthesis Pattern and Transcriptome Analysis of Sapindus saponaria Oil

Abstract

The Sapindus saponaria (soapberry) kernel is rich in oil that has antibacterial, anti-inflammatory, and antioxidant properties, promotes cell proliferation, cell migration, and stimulates skin wound-healing effects. S. saponaria oil has excellent lubricating properties and is a high-quality raw material for biodiesel and premium lubricants, showing great potential in industrial and medical applications. Metabolite and transcriptome analysis revealed patterns of oil accumulation and composition and differentially expressed genes (DEGs) during seed development. Morphological observations of soapberry fruits at different developmental stages were conducted, and the oil content and fatty acid composition of the kernels were determined. Transcriptome sequencing was performed on kernels at 70, 100, and 130 days after flowering (DAF). The oil content of soapberry kernels was lowest at 60 DAF (5%) and peaked at 130 DAF (31%). Following soapberry fruit-ripening, the primary fatty acids in the kernels were C18:1 (oleic acid) and C18:3 (linolenic acid), accounting for an average proportion of 62% and 18%, respectively. The average contents of unsaturated fatty acids and saturated fatty acids in the kernel were 86% and 14%, respectively. Through the dynamic changes in fatty acid composition and DEGs analysis of soapberry kernels, FATA, KCR1, ECR, FAD2 and FAD3 were identified as candidate genes contributing to a high proportion of C18:1 and C18:3, while DGAT3 emerged as a key candidate gene for TAG biosynthesis. The combined analysis of transcriptome and metabolism unveiled the molecular mechanism of oil accumulation, leading to the creation of a metabolic pathway pattern diagram for oil biosynthesis in S. saponaria kernels. The study of soapberry fruit development, kernel oil accumulation, and the molecular mechanism of oil biosynthesis holds great significance in increasing oil yield and improving oil quality.

Keywords: Sapindus saponaria; biodiesel; fatty acid; key enzyme genes; lipid metabolism; oil biosynthesis; soapberry saponin; transcriptome analysis; triacylglycerol; woody oil plant.

Figure 1

Phenotypic characteristics, oil content, and fatty acid dynamics of S. saponaria fruit at different developmental stages: (A) phenotypic characteristics of S. saponaria fruits; (B) changes in fresh weight of 100 fruits of S. saponaria; (C) changes in oil content in S. saponaria kernels; (D) the gas chromatography peak plot of S. saponaria kernel oil in 130 DAF; and (E) the fatty acid content of S. saponaria kernels. Note: ** means oil content with significant difference, *** means oil content with very significant difference, ns means oil content with insignificant differences. C18:1: oleic acid, C18:3: linolenic acid, C18:2: linoleic acid, C20:0: arachidic acid, C16:0: palmitic acid, C18:0: stearic acid.

Figure 2

Analysis of DEGs in the transcriptome of S. saponaria kernels: (A) trend of DEGs in S. saponaria kernels transcriptome; (B) statistics of up-regulation and down-regulation of DEGs in the transcriptome of S. saponaria kernels; (C) Venn diagram of DEGs in the transcriptome of S. saponaria kernels; and (D) volcano plots showing up-regulation and down-regulation of DEGs in S. saponaria kernels. Note: grey represents the actual FPKM of DEGs, while black represents the average FPKM value in (A); up means significantly up-regulated genes, down means significantly down-regulated genes, and ns (dotted line) means genes with insignificant differences in (D).

Figure 3

GO and KEGG annotation of DEGs in the transcriptome of S. saponaria kernels: (A) annotation on GO functional classification of DEGs in S. saponaria kernels; and (B) enrichment of KEGG metabolic pathway of DEGs in S. saponaria kernels.

Figure 4

Schematic diagram of the biosynthetic metabolic pathway of FA and TAG in S. saponaria kernels. Note: pyruvate dehydrogenase complex: PDC; acetyl-CoA carboxylase: CAC3; ACP-S-malonyltransferase: FabD; 3-oxoacyl-ACP synthase: KAS; 3-oxoacyl-ACP reductase: FabG; 3-hydroxyacyl-ACP dehydratase: FabZ; enoyl-ACP reductase I: FabI, MOD1; acyl-ACP desaturase: SAD; fatty acyl-ACP thioesterase A: FATA; fatty acyl-ACP thioesterase B: FATB; long-chain acyl-CoA synthetase: LACS; very-long-chain 3-oxoacyl-CoA reductase: ECR; very-long-chain enoyl-CoA reductase: KCR1; ω-6 FA desaturase: FAD2; ω-3 FA desaturase: FAD3; acetaldehyde dehydrogenase: ALDH; alcohol dehydrogenase: AKR; glycerol kinase (ATP: glycerol-3-phosphotransferase): GLPK; glycerol-3-phosphate acyltransferase: GPAT; lysocardiolipin and lysophospholipid acyltransferase: LPAT; Diacylglycerol diphosphate phosphatase: LPP3; diacylglycerol O-acyltransferase 3: DGAT3; diacylglycerol O-acyltransferase 2: DGAT2; phospholipid: diacylglycerol acyltransferase: PDAT; sn-Glycerol 3-phosphate: G-3-P; lysophosphatidic acids: LPA; Phosphatidic acid: PA; Diacylglycerol: DAG; Triacylglycerol: TAG; phosphatidylcholine: PC; glycerophosphodiester phosphodiesterase: GDPD1; sn-Glycero-3-phosphoethanolamine: Glycerophosphoethanolamine; sn-Glycero-3-phosphocholine: Glycerophosphocholine; phosphatidylcholine diacylglycerol cholinephosphotransferase: ROD1, PDCT; lysophosphatidylcholine acyltransferase: LPCAT; free fatty acid: FFA; lysophosphatidylcholine: LPC. The vertically stacked blocks represent different genes encoding enzymes with this catalytic function, and the horizonal direction represents different stages.

Figure 5

qPCR validation of six candidate genes associated with oil biosynthesis during S. saponaria kernel development.