Document Type : Research Article
Authors
1 Department of Plant Production and genetic (Biotechnology), Faculty of Agriculture, Jahrom University, Jahrom, Iran
2 Department of Plant Production and Genetic (Biotechnology), Faculty of Agriculture, Jahrom University, Jahrom, Iran
3 Department of Biology, Faculty of Basic Sciences, Semnan University, Semnan, Semnan, Iran.
Abstract
Introduction
Olive (Olea europaea L.) is a fruit-bearing tree of global significance due to the high quantity and quality of oil produced from its fruits. Fatty acid desaturase (FAD) genes are responsible for catalyzing the conversion of linoleic acid to α-linolenic acid. In olives, the FAD7 and FAD3 genes encode omega-3 fatty acid desaturases that catalyze the conversion of linoleic acid (omega-6) to α-linolenic acid (omega-3), occurring respectively in plastids and the endoplasmic reticulum. In olive fruit, oil accumulates in two distinct tissues: the mesocarp, which contains the highest concentration of oil, and the seed, which is enclosed by a hard endocarp. Due to the significance of the mesocarp for oil extraction in olives, most previous research has focused on oil biosynthesis within the fruit's mesocarp tissue, while the biosynthetic process in the seed has been comparatively less studied.
Materials and Methods
This study investigated the expression patterns of OeFAD7 and OeFAD3 genes during different fruit ripening stages in two tissues, mesocarp and seed, in four olive cultivars: two native (‘Mari’ and ‘Shengeh’) and two imported (‘Arbequina’ and ‘Koroneiki’). Total RNA was extracted from both seed and mesocarp tissues of the cultivars at five developmental time points: 64, 90, 120, 150, and 180 days after full bloom (DAFB). Subsequently, cDNA was synthesized and gene expression was analyzed using specific primers for OeFAD7-1 and OeFAD3A. The OeQUB2 gene was employed as a reference for evaluating relative expression levels. Each experimental sample was evaluated in triplicate. Data analysis was performed using SAS software, and mean values were compared using Tukey's test.
Results and Discussion
Transcripts of both OeFAD3 and OeFAD7 genes were consistently detected in the two examined tissues across all sampling times and in all olive cultivars, indicating stable, though quantitatively variable, patterns of gene expression. Results revealed significant differences in gene expression across tissues, ripening stages, and among cultivars. While OeFAD7 exhibited higher expression in mesocarp tissue compared to seeds, OeFAD3 was more strongly expressed in seeds than in mesocarp. On the other hand, the findings of this study revealed that the expression level of the OeFAD3 gene in the seed of the fruit was significantly higher than in the mesocarp. Moreover, in most sampling stages, the expression of this gene in the seed exceeded that of the OeFAD7 gene in the same tissue. Previous studies on the expression of this gene in various olive tissues also indicated that the OeFAD3A isoform exhibited the highest expression in the seed and young olive leaves, while its transcripts were extremely low and nearly undetectable in the fruit mesocarp. Expression of OeFAD7 in the mesocarp showed a distinct upregulation at a specific growth stage, occurring either at 90 or 120 DAFB depending on the cultivar. The ‘Koroneiki’ cultivar demonstrated the highest transcript abundance for both genes and displayed a relatively distinct expression profile during fruit ripening compared to the other cultivars. Furthermore, the comparison of the expression levels of oleate desaturase genes in this study aligns with previous research findings regarding the fatty acid composition of the studied cultivars. In general, the native cultivars showed lower expression levels of the target genes compared to the imported ones, although the ‘Mari’ cultivar exhibited higher gene expression than ‘Shengeh’ in most developmental stages. Based on these findings, it can be proposed that OeFAD7 plays a more prominent role in the conversion of linoleic acid to α-linolenic acid in the mesocarp, while OeFAD3 is more pivotal in α-linolenic acid biosynthesis within the seed.
Conclusion
The data generated in this study provide valuable insights about tissue specific expression of oleate desaturases in olive trees. In conclusion, the distinct spatiotemporal expression profiles of FAD7 and FAD3 during olive fruit development underscore their pivotal roles in α-linolenic acid biosynthesis and identify them as valuable targets for breeding and biotechnological strategies aimed at enhancing olive oil composition and quality. Based on these findings, it can be concluded that the OeFAD7 gene plays a critical role in the fatty acid desaturation process within the mesocarp tissue of olive fruit, whereas the OeFAD3 gene has greater significance in this process within the seed. Overall, the data from this study highlight the substantial impact of these genes on linoleic acid degradation in olive oil, thereby enhancing its quality index in terms of the ratio of oleic acid to linoleic and linolenic acids.
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