The king of the Mediterranean region

Autumn in the South of Italy means a lot of nice things: a still relatively warm weather, the bright warm colours of leaves of deciduous plants slowly turning into a colourful carpet, the smell of roasted chestnuts in the streets, etc. This time of the year is also linked to a very important economic activity: the collection of olives and their processing to give olive oil.

The main components of olive oil are triacylglycerols. A triacylglycerol (or triglyceride) is a molecule made up by a unit of glycerol esterified with three units of fatty acids. The most abundant fatty acid in olive oil is oleic acid. Besides this main components, olive oil also contains free fatty acids, tocopherols (i.e. Vitamin E), low amounts of phenolic compounds and other chemicals present in traces or low amount (1). Its composition depends on many factors, including the way oil is produced, but even more important the geographical origin and the cultivar of olives from which it is obtained (1).

Olive oil is an important component of Mediterranean cuisine and not only. Health promoting effects have been associated to its use (2). These properties are a result of its chemical components, which in turn, depend on the ability of olive trees to produce such compounds.

Olea europaea plants, indeed, produce several different chemicals. This evergreen tree species is native to the Mediterranean basin and it is thought to have been domesticated in the third millennium BC at the latest (3). Olive trees are nowadays cultivated in several regions of the world, as far as they have the proper climatic conditions. About 90% of olives are used for olive oil production, while 10% are used as table olives. Not only the cultivation of olive trees, but also the use of olives to obtain the oil is a tradition that dates back to very long ago. Just recently a bottle containing olive oil that had been found at the Vesuvius archaeological sites in Pompeii, was analysed and identified as the oldest residue of olive oil in the world found in bulk amount (4).

Ripe olive fruits are rich of course in triacylglycerols and sugars, but they also contain some amounts of phenolic compounds. Among these, many flavonoids, tyrosol, hydroxytytosol and secoiridoid derivatives, like oleuropein (5, 6). Many of these metabolites are also found in the leaves and in other plant organs (5, 7).
These compounds are very important since they have very interesting biological activities. Flavonoids are known antioxidants, and, among the specialized metabolites, maybe the most widespread chemicals throughout the plant kingdom. However, there will surely be the chance in the future to further talk about these compounds.

Among the chemicals typically found in olives and in olive leaves, oleuropein might be linked to the ethnobotanical use of Olea europaea as an antidiabetic. The compound was shown to lower blood glucose levels in animal experiments (8). Furthermore, this compound is also known for its blood pressure-lowering effects, justifying the traditional use of olive leaf in the treatment of mild hypertension (9). Oleuropein is, however, rapidly degraded by colonic microflora producing one of its most important metabolites, hydroxytyrosol (10), which is also by itself present already in the olives, in the olive leaf and in olive oil. This molecule is of high interest to the pharmaceutical industry first of all because of its anti-inflammatory and antimicrobial properties (11). The compound also seems to have a role against cardiovascular diseases, cancer and it seems to be involved in neuroprotection (11).

Besides the possible positive effects on human health, it would be interesting to understand why the plant produces these compounds. For what concerns oleuropein, together with a structurally related compound, ligstroside, it might protect olive leaf and fruit against the attacks of insects and microbes, with oleuropein and ligstroside acting as precursors of the defensive compounds (12). It is therefore likely that these secoiridoids are hydrolysed or anyway metabolized to give different, more reactive, chemicals upon attack. However, as it often happens, the story might be more complicated than this. It has been reported that oleuropein actually attracts the females of the insect Dacus oleae (i.e. the olive fruit fly), while several of its derivatives exert a strong chemotactile repulsion on the same species (which means that when these derivatives are present, the insect perceives them and gets repelled). Well, it seems that after oviposition by a female of Dacus oleae, this insect regurgitates small droplets of olive juice around the oviposition hole. This juice contains those compounds derived from oleuropein that act as repellent, therefore preventing other females from laying eggs on the same fruit (13), with advantages for the progeny of the fly that arrived first.

Like oleuropein, also the other specialized metabolites produced by the king of the Mediterranean trees probably have a role, ranging from the physiological to the ecological level, although often still unknown.

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