The Hydroperoxide Lyase Branch of the Oxylipin Pathway and Green Leaf Volatiles in Plant/Insect Interaction

The lipoxygenase (LOX) catalysed peroxidation of polyunsaturated fatty acids (PUFA) is a key starting point in defence mechanisms common to plant, animal and at least some microorganisms. In human, the peroxidation of arachidonic acid finally leads to the biosynthesis of important defence effectors such as leukotrienes and lipoxins. In plant, the most common substrates of lipoxygenases are represented by linoleic (C18:2) and linolenic (C18:3) acids. Differently from auto-oxidation reactions which produce a huge number of hydroperoxide derivatives, the oxidation reaction catalysed by LOXs is positional and stereo-specific. Indeed, only 9- or 13-hydroperoxides (with a prevalence of the S steroisomers) are produced from these substrates. PUFA hydroperoxides are per se signal molecules or can be used as substrates for a number of enzymatic reactions carried out by other enzymes of the LOX pathway. At the end, an array of volatile and non volatile compounds, collectively known as phyto-oxylipins are produced upon specific stress signals, by the contribution of a multitude of enzymes localised in different subcellular compartments. Thanks to the excellent work of several groups of scientists around the world, our knowledge on the contribution of the oxylipin pathway on plant defence mechanisms dramatically increased in recent years. In the present contribution, we'll focus our attention on the hydroperoxide lyase branch of the plant oxylipins pathway, responsible for the synthesis of volatile aldehydes, alcohols and other related compounds which are important constituents of fruit aromas and the green leaf volatiles.