Modification of 13-hydroperoxide lyase expression in olive affects plant growth and results in altered volatile profile.

The C6 aldehydes, alcohols, and the corresponding esters are the most important compounds of virgin olive oil aroma. These C6 volatile compounds are synthesized via the 13-hydroperoxide lyase (13-HPL) branch of the lipoxygenase pathway. In this investigation, a functional analysis of the olive (Olea europaea L.) 13-HPL gene by its overexpression and silencing in olive transgenic lines was carried out. With this aim, sense and RNAi constructs of the olive 13-HPL gene were generated and used for the transformation of embryogenic olive cultures. Leaves from overexpressing lines showed a slight increase in 13-HPL gene expression, whereas RNAi lines exhibited a strong decrease in their transcript levels. Quantification of 13-HPL activity in two overexpressing and two RNAi lines showed a positive correlation with levels of transcripts. Interestingly, RNAi lines showed a high decrease in the content of C6 volatiles linked to a strong increase of C5 volatile compounds, altering the volatile profile in the leaves. In addition, the silencing of the 13-HPL gene severely affected plant growth and development. This investigation demonstrates the role of the 13-HPL gene in the biogenesis of olive volatile compounds and constitutes a functional genomics study in olive related to virgin olive oil quality.

Study of the olive ß-glucosidase gene family putatively involved in the synthesis of phenolic compounds of virgin olive oil.

BACKGROUND: Hydrolysis of the fruit phenolic glucosides occurring during the oil extraction process is the main biochemical reaction affecting the biosynthesis and accumulation of secoiridoid compounds in virgin olive oil. An integrated approach at the molecular, biochemical, and metabolic level was used to study the olive ß-glucosidase gene family in seven olive cultivars selected by their different phenolic profiles. RESULTS: Eight ß-glucosidase genes have been identified by in silico analysis of an olive transcriptome. Their expression levels were analyzed by reverse transcription quantitative polymerase chain reaction in olive fruits at different ripening stages: I, green fruits, 16-19 weeks after flowering (WAF); II, yellow-green fruits, 22-25 WAF; III, turning fruits, 28-31 WAF; and IV, fully ripe fruits, 35-40 WAF. Gene expression was compared with the level of ß-glucosidase activity in the fruit and with the phenolic composition of fruits and oils from different olive cultivars. Phylogenetic analysis of the encoded proteins and differences found among the ß-glucosidase genes based on Gene Ontology enrichment analysis data suggests maximum involvement of two genes, OeBGLU1A and OeBGLU1B, in the phenolic composition of virgin olive oil. Positive correlation coefficients were found within each olive cultivar between OeBGLU1A and OeBGLU1B gene expression data and the phenolic content of the oil. CONCLUSION: The results obtained suggest that the expression pattern of specific ß-glucosidase genes may be an accurate predictor for the phenolic content of virgin olive oil that could be used in olive breeding programs. © 2021 Society of Chemical Industry.

An Aromatic Aldehyde Synthase Controls the Synthesis of Hydroxytyrosol Derivatives Present in Virgin Olive Oil.

The phenolic composition of virgin olive oil (VOO) is strongly determined by the content and distribution of secoiridoid phenolic glucosides present in the olive fruit. Among them, oleuropein is the most abundant in olive mesocarp and is characterized by containing an hydroxytyrosol residue in its chemical structure. Hydroxytyrosol-containing molecules are those that exhibit the most important biological activities in virgin olive oil. In this regard, we identified an aromatic aldehyde synthase gene (OeAAS) from an olive transcriptome, which was synthesized, expressed in Eschrichia coli, and purified its encoded protein. The recombinant OeAAS is a bifunctional enzyme catalyzing decarboxylation and amine-oxidation reactions in a single step. OeAAS displays strict substrate specificity for l-DOPA to form 2,4-dihydroxyphenylacetaldehyde, the immediate precursor of hydroxytyrosol. In addition to the biochemical characterization of the enzyme, the expression analysis carried out in different olive cultivars and ripening stages indicate that OeAAS gene is temporally regulated in a cultivar-dependent manner. High correlation coefficients were found between OeAAS expression levels and the phenolic content of olive fruits and oils, which supports a key role for OeAAS in the accumulation of hydroxytyrosol-derived secoiridoid compounds in olive fruit and virgin olive oil.

Volatile Compound Profiling by HS-SPME/GC-MS-FID of a Core Olive Cultivar Collection as a Tool for Aroma Improvement of Virgin Olive Oil.

Virgin olive oil (VOO) is the only food product requiring official sensory analysis to be classified in commercial categories, in which the evaluation of the aroma plays a very important role. The selection of parents, with the aim of obtaining new cultivars with improved oil aroma, is of paramount importance in olive breeding programs. We have assessed the volatile fraction by headspace-solid-phase microextraction/gas chromatography-mass spectrometry-flame ionization detection (HS-SPME/GC-MS-FID) and the deduced aroma properties of VOO from a core set of olive cultivars (Core-36) which possesses most of the genetic diversity found in the World Olive Germplasm Collection (IFAPA Alameda del Obispo) located in Cordoba, Spain. The VOO volatile fractions of Core-36 cultivars display a high level of variability. It is mostly made of compounds produced from polyunsaturated fatty acids through the lipoxygenase pathway, which confirms to be a general characteristic of the olive species (Olea europaea L.). The main group of volatile compounds in the oils was six straight-chain carbon compounds derived from linolenic acid, some of them being the main contributors to the aroma of the olive oils according to their odor activity values (OAV). The high level of variability found for the volatile fraction of the oils from Core-36 and, therefore, for the aroma odor notes, suggest that this core set may be a very useful tool for the choice of optimal parents in olive breeding programs in order to raise new cultivars with improved VOO aroma.