Abstract
BackgroundSea buckthorn is a woody oil crop in which palmitoleic acid (C16:1n7, an omega-7 fatty acid (FA)) contributes approximately 40% of the total FA content in berry pulp (non-seed tissue). However, the molecular mechanisms contributing to the high accumulation of C16:1n7 in developing sea buckthorn berry pulp (SBP) remain poorly understood.ResultsWe identified 1737 unigenes associated with lipid metabolism through RNA-sequencing analysis of the four developmental stages of berry pulp in two sea buckthorn lines, ‘Za56’ and ‘TF2–36’; 139 differentially expressed genes were detected between the different berry pulp developmental stages in the two lines. Analyses of the FA composition showed that the C16:1n7 contents were significantly higher in line ‘Za56’ than in line ‘TF2–36’ in the mid-late developmental stages of SBP. Additionally, qRT-PCR analyses of 15 genes involved in FA and triacylglycerol (TAG) biosynthesis in both lines revealed that delta9-ACP-desaturase (ACP-Δ9D) competed with 3-ketoacyl-ACP-synthase II (KASII) for the substrate C16:0-ACP and that ACP-Δ9D and delta9-CoA-desaturase (CoA-Δ9D) gene expression positively correlated with C16:1n7 content; KASII and fatty acid elongation 1 (FAE1) gene expression positively correlated with C18:0 content in developing SBP. Specifically, the abundance of ACP-Δ9D and CoA-Δ9D transcripts in line ‘Za56’, which had a higher C16:1n7 content than line ‘TF2–36’, suggests that these two genes play an important role in C16:1n7 biosynthesis. Furthermore, the high expressions of the glycerol-3-phosphate dehydrogenase (GPD1) gene and the WRINKLED1 (WRI1) transcription factor contributed to increased biosynthesis of TAG precursor and FAs, respectively, in the early developmental stages of SBP, and the high expression of the diacylglycerol O-acyltransferase 1 (DGAT1) gene increased TAG assembly in the later developmental stages of SBP. Overall, we concluded that increased ACP-Δ9D and CoA-Δ9D levels coupled with decreased KASII and FAE1 activity is a critical event for high C16:1n7 accumulation and that the coordinated high expression of WRI1, GPD1, and DGAT1 genes resulted in high oil accumulation in SBP.ConclusionOur results provide a scientific basis for understanding the mechanism of high C16:1n7 accumulation in berry pulp (non-seed tissue) and are valuable to the genetic breeding programme for achieving a high quality and yield of SBP oil.
Highlights
Sea buckthorn is a woody oil crop in which palmitoleic acid (C16:1n7, an omega-7 fatty acid (FA)) contributes approximately 40% of the total FA content in berry pulp
To explore whether transcriptional regulation is involved in controlling C16:1n7 biosynthesis and TAG accumulation in sea buckthorn berry pulp (SBP), we analysed the dynamic patterns in oil content and FA composition, especially for C16:1n7, in FA composition and oil content during berry pulp development in sea buckthorn Because berry pulp and seed oils have unique bioactive compounds (FAs, phytosterols, carotenoids, and tocopherols), sea buckthorn berries have been used for hundreds of years in Russia and China for medicinal and nutritional purposes
In our previous study, we found that the high accumulation of C18 unsaturated fatty acids (UFAs) (88% of the total FAs) in sea buckthorn seed oil was due to low expression of the Delta9-CoA desaturase (CoA-Δ9D) gene coordinated with high expression of the KAS 3-ketoacyl-ACP synthase II (II), acyl carrier protein (ACP)-Δ9D, Fatty acid desaturase 2 (FAD2), and Fatty acid desaturase 3 (FAD3) genes [28]
Summary
Sea buckthorn is a woody oil crop in which palmitoleic acid (C16:1n7, an omega-7 fatty acid (FA)) contributes approximately 40% of the total FA content in berry pulp (non-seed tissue). Bioactive oils (2–38%) with high levels of the uncommon fatty acid (FA) palmitoleic acid (approximately 40%) [2,3,4], carotenoids, flavonol glycosides, and tocopherol [5,6,7], can be extracted from the orange or red berry pulp. C16:1n7 is rarely synthesized in common oil crops, and few plants are known to accumulate C16:1n7 Those that do include cat’s claw (Doxantha unguis-cati), which contains 64% C16:1n7 [12], followed by Tasmanian waratah (Telopea truncata) (45%) and macadamia nuts (Macadamia integrifolia) (30%) [13, 14]. The seeds of these plants are the source of C16:1n7, and no reports have addressed C16:1n7 biosynthesis and accumulation mechanisms in SBP and non-seed tissues
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