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

Marco Taurino

doi:10.4172/2329-9029.1000102

Abstract

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.

Highlights

Plant HPLs are Involved in Stress Induced Biosynthesis of Volatile AldehydesHydroperoxide lyases (HPL) catalyse the cleavage of hydroperoxides into short chain aldehydes and π-oxo fatty acids (Figure 1)
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
Unlike Hydroperoxide lyase (HPL), which cleaves hydroperoxides, allene oxide synthase (AOS) transforms them into allene oxides, which are converted into jasmonic acid; whereas divinyl ether synthase (DES) converts them into divinyl ethers, showing antifungal properties (Figure 1)

Summary

Plant HPLs are Involved in Stress Induced Biosynthesis of Volatile Aldehydes

Hydroperoxide lyases (HPL) catalyse the cleavage of hydroperoxides into short chain aldehydes and π-oxo fatty acids (Figure 1). Some HPLs show a strict specificity for 13-hydroperoxides with consequent production of (Z)-3-hexenal and 12-oxo-(Z)-9-dodecenoic acid starting from 13-hydroperoxy-derivatives of linolenic acids [1,2]. All these volatile compounds are important constituents of the green leaf volatiles (GLV) and are rapidly released by plant in response to mechanical damage or herbivores attack [1,2]. Together with 13-HPL, 9/13-HPLs have been reported They were initially thought to be restricted to the Cucurbitaceae family, even though their occurrence in other plant species such as Medicago spp., rice, almond and grape have been later reported [3,4,5,6,7]. The phylogenetic analysis carried out on a number of plant HPLs (Figure 2) clearly confirms the presence of these two distinct groups inside plant HPLs

Endocellular Localisation of HPLs

Linolenic acid

Physiological Roles of Oxylipins Produced via the HPL Branch

ZmHPL OsHPL

Concluding Remarks