Key genes for olive breeding: role of olive polyphenol oxidase genes

The phenolic components of virgin olive oil (VOO) are key to the organoleptic and functional quality of this emblematic food of the Mediterranean diet. Olive breeding for improving VOO phenolic fraction, including both phenolic compounds with secoiridoid structures and tocopherols, depends largely on the identification and characterization of the key genes determining the biosynthesis of these compounds in the olive fruit as well as those that transform these phenolic components during the oil extraction process. However, there is very little knowledge about the key genes/enzymes controlling these metabolic processes. In this work, the role of olive polyphenol oxidase (PPO) in the metabolism of olive phenolic compounds has been studied. Three PPO genes have been identified from a transcriptome obtained from six olive cultivars with highly contrasting phenolic contents. These genes were expressed in Escherichia coli and the functional identity of the recombinant proteins verified using olive phenolic substrates. The OePPO1 and OePPO2 proteins exhibit diphenolase activity, while OePPO3 is a tyrosinase that not only possesses diphenolase but also monophenolase activity, able to catalyse the hydroxylation of tyrosol to form hydroxytyrosol. The role of these PPO genes in the metabolism of phenolic compounds in the olive is analysed and the potential use of this information for olive breeding is discussed.