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Ambient temperature during olive fruit growth modulates oil content and fatty acid composition. However, the mechanisms behind their regulation are not completely understood. In this work, the transcriptional regulation of oil synthesis and fatty acid desaturation were evaluated by manipulating air temperature in the field. Three-year-old trees of cultivars ‘Arbequina’ and ‘Coratina’ were placed in open-top chambers (OTC) where control or elevated air temperatures were applied early in the oil accumulation phase, and fruits were sampled after 10 or 36 days of exposure. Upon elevated temperature, the oil content in ‘Coratina’ mesocarp tissue decreased less than in ‘Arbequina’ compared to their respective controls. However, the oleic acid content of ‘Coratina’ was more sensitive to temperature. The cultivar-dependent responses were mainly explained by differential gene expression. In ‘Arbequina’, the downregulation of genes involved in triacylglycerol biosynthesis (OeDGAT1-2, OeDGAT2, OePDAT1-1, and OePDAT1-2), upon elevated temperature explains the decrease in oil content, whereas the downregulation of the fatty acid desaturases genes (OeSAD1, OeSAD3, OeFAD2-1, OeFAD2-2, and OeFAD2-5) decreased the oleic/linoleic ratio. In contrast, in ‘Coratina’, OePDAT1-2 downregulation explains the fluctuation in oil content, but the increased expression of three members of the OeFAD2 gene family was responsible for the oleic/linoleic ratio. This knowledge contributes to developing an integrated strategy for olive production under high temperatures, and the analysis of the expression of key genes could be used as a selection criterion in genetic improvement programs for the adaptation of olive cultivars to global warming.

The relative contribution of carbon sources generated from leaves and fruits photosynthesis for triacylglycerol biosynthesis in the olive mesocarp and their interaction with water stress was investigated. With this aim, altered carbon source treatments were combined with different irrigation conditions. A higher decrease in mesocarp oil content was observed in fruits under girdled and defoliated shoot treatment compared to darkened fruit conditions, indicating that both leaf and fruit photosynthesis participate in carbon supply for oil biosynthesis being leaves the main source. The carbon supply and water status affected oil synthesis in the mesocarp, regulating the expression of DGAT and PDAT genes and implicating DGAT1-1, DGAT2, PDAT1-1, and PDAT1-2 as the principal genes responsible for triacylglycerol biosynthesis. A major role was indicated for DGAT2 and PDAT1-2 in well-watered conditions. Moreover, polyunsaturated fatty acid content together with FAD2-1, FAD2-2 and FAD7-1 expression levels were augmented in response to modified carbon supply in the olive mesocarp. Furthermore, water stress caused an increase in DGAT1-1, DGAT1-2, PDAT1-1, and FAD2-5 gene transcript levels. Overall, these data indicate that oil content and fatty acid composition in olive fruit mesocarp are regulated by carbon supply and water status, affecting the transcription of key genes in both metabolic pathways.

The release of new olive cultivars with an increased squalene content in their virgin olive oil is considered an important target in olive breeding programs. In this work, the variability of the squalene content in a core collection of 36 olive cultivars was first studied, revealing two olive cultivars, 'Dokkar' and 'Klon-14', with extremely low and high squalene contents in their oils, respectively. Next, four cDNA sequences encoding squalene synthases (SQS) were cloned from olive. Sequence analysis and functional expression in bacteria confirmed that they encode squalene synthases. Transcriptional analysis in distinct olive tissues and cultivars indicated that expression levels of these four SQS genes are spatially and temporally regulated in a cultivar-dependent manner and pointed to OeSQS2 as the gene mainly involved in squalene biosynthesis in olive mesocarp and, therefore, in the olive oil. In addition, the biosynthesis of squalene appears to be transcriptionally regulated in water-stressed olive mesocarp.

Three different cDNA sequences, designated OepPDAT1-1, OepPDAT1-2, and OepPDAT2, encoding three phospholipid:diacylglycerol acyltransferases (PDAT) have been isolated from olive (Olea europaea cv. Picual). Sequence analysis showed the distinctive features typical of the PDAT family and together with phylogenetic analysis indicated that they encode PDAT. Gene expression analysis in different olive tissues showed that transcript levels of these three PDAT genes are spatially and temporally regulated and suggested that, in addition to acyl-CoA:diacylglycerol acyltransferase, OePDAT1-1 may contribute to the biosynthesis of triacylglycerols in the seed, whereas OePDAT1-2 could be involved in the triacylglycerols content in the mesocarp and, therefore, in the olive oil. The relative contribution of PDAT and acyl-CoA:diacylglycerol acyltransferase enzymes to the triacylglycerols content in olive appears to be tissue-dependent. Furthermore, water regime, temperature, light, and wounding regulate PDAT genes at transcriptional level in the olive fruit mesocarp, indicating that PDAT could be involved in the response to abiotic stresses. Altogether, this study represents an advance in our knowledge on the regulation of oil accumulation in oil fruit.

The effect of three irrigation strategies on the olive oil quality in a hedgerow ‘Arbequina’ olive orchard with 1667 trees ha−1 was assessed. Olive trees were fully irrigated, with the crop water demand replaced through daily irrigation (FI), or irrigated according to two regulated deficit irrigation (RDI) treatments, in which the total irrigation supplies amounted to 60% and 30% of the water supplied to the FI treatment. Oil content, fatty acid composition and desaturase genes expression in fruits from the three treatments were analysed. Although the total oil content and the amounts of the diverse lipid classes were not affected by the different irrigation treatments, changes in the fatty acid composition during the mesocarp development and ripening in the three irrigation treatments were detected. The 30RDI strategy caused a decrease in the linoleic acid content in olive mesocarp and, therefore, in the olive oil. The lack of correlation with the expression of known oleate desaturase genes (OeFAD2-1, OeFAD2-2 and OeFAD6) points out to an un-identified oleate desaturase gene that could be repressed by water stress. Linolenic acid content and expression levels of plastidial linoleate desaturase genes (OeFAD7-1 and OeFAD7-2) increased under 30RDI conditions at the beginning of fruit development. However, this effect was not noticeable in the olive oil because it was not observed during fruit ripening. All these data enlighten the molecular mechanisms related to the fatty acid desaturation in the oil of olive trees under water stress and point out to the 30RDI as suitable strategy when olive quality is the main production target.

Fatty acids are the main components of the olive oil and their composition has a critical influence on the oil quality. However, oil quality evaluation has not been frequently included in the selection of new bred cultivars. This can be due to the difficulties in analyzing oil quality in large set of genotypes and also to the long juvenile period of olive seedlings. Therefore, the identification of molecular markers associated to olive oil quality traits could facilitate their selection in breeding programs of this species. In the present work, the identification of the first QTLs for fatty acids on olive oil is reported. They have been located in a linkage map of a ‘Picual’ × ‘Arbequina’ progeny of the olive breeding program of Córdoba. Correlations among fatty acids are in agreement with previous reports of breeding progenies. QTLs found for oleic and linoleic acids explained 41.1 and 69.7% of the total variability, respectively, and were co-localized in the same linkage groups. In the same region, QTLs for monounsaturated, polyunsaturated and oleic/linoleic ratio were also identified. In other linkage groups, three QTLs for linolenic and one for palmitoleic acid were also located explaining 15.0–28.0% of the total variability. These results could be useful to increase the efficiency of breeding programs aimed at selecting new cultivars with high oleic acid content, and, therefore, with enhanced nutritional properties and oxidative stability of the olive oil.

The specific contribution of different stearoyl-ACP desaturase (SAD) genes to the oleic acid content in olive (Olea europaea) fruit has been studied. Toward that end, we isolated three distinct cDNA clones encoding three SAD isoforms from olive (cv. Picual), as revealed by sequence analysis. The expression levels of olive SAD genes were determined in different tissues from Picual and Arbequina cultivars, including developing mesocarp and seed, together with the unsaturated fatty acid content. Lipid and gene expression analyses indicate that OeSAD2 seems to be the main gene contributing to the oleic acid content of the olive fruit and, therefore, of the virgin olive oil. This conclusion was confirmed when the study was extended to Hojiblanca, Picudo, and Manzanilla cultivars. Furthermore, our data indicate that the olive microsomal oleate desaturase gene OeFAD2-2, but not OeSAD2, is responsible for the linoleic acid content in the virgin olive oil.