The importance of lipoxygenase control in the production of green leaf volatiles by lipase-dependent and independent pathways

Abstract

Green leaf volatiles (GLVs) are six-carbon volatile compounds. They are formed from fatty acids by a dioxygenation reaction catalyzed by lipoxygenase and a subsequent cleavage reaction catalyzed by hydroperoxide lyase. GLVs are involved in direct and indirect plant defense against herbivores and pathogens. In intact plant tissues, GLVs are usually present at low concentrations, but upon wounding, GLVs are synthesized rapidly: within seconds to minutes. It has been hypothesized that this 'GLV burst' is supported by activation of pre-existing enzymes on endogenous substrates; however, the detailed mechanism of the GLV burst has not been elucidated. Recently, we found that a certain portion of GLVs is formed without liberation of free fatty acids from lipids. Accordingly, we hypothesized that lipoxygenase plays an essential role in the GLV burst. In particular, direct oxygenation by lipoxygenase on membrane lipids seems to be responsible. Lipoxygenase is also a target for controlling GLV levels in food derived from plants.

Arabidopsis lipoxygenase 2 is essential for formation of green leaf volatiles and five‐carbon volatiles

“…LOX2 is most accountable for the biosynthesis of JA after mechanical wounding of Arabidopsis leaves . HPL is under regulation of JA signaling . Therefore, it was anticipated that low amounts of GLVs in lox2‐1 mutant were caused by low HPL activity because of relatively low JA concentrations in this mutant.…”

“…Lipoxygenases are the enzymes that catalyze oxygenation of polyunsaturated fatty acids to form corresponding fatty acid HPOs. Acyl groups in glycerolipids are also oxygenated to form corresponding lipid HPOs by some LOXs . The HPOs are in most cases further metabolized to yield a wide range of bioactive compounds called oxylipins.…”

Green leaf volatile production by plants: a meta‐analysis

“…) (Matsui, ). While it has been assumed that FAs are hydrolysed from membrane lipids (Matsui et al ., ) before further enzymatic reaction, little is known about the identity of specific lipases that play a role in the liberation of FAs for the GLV pathway (Mwenda & Matsui, ). In Arabidopsis , the supply of free FAs for the biosynthesis of the plant hormone jasmonic acid (JA), which is formed via another branch of the oxylipin pathway, is regulated by a number of different lipases, all belonging to the class of phospholipases‐A1 (Bonaventure, ).…”

“…This figure focuses on the formation of the most abundant unsaturated C6‐volatiles. For more detailed information on the biosynthesis of GLVs, see recent reviews (Mwenda & Matsui, ; ul Hassan et al ., ). Molecular oxygen is introduced into α‐linolenic acid and the resulting hydroperoxide (13(S)‐hydroperoxy‐9Z,11E,15Z‐octadecatrienoic acid; 13‐HPOT) is cleaved into a C12‐(9 Z ‐traumatin) and a C6‐( Z ‐3‐hexenal) compound.…”

“…Although, there is some disagreement within the scientific community on this terminology, we will use, for the sake of brevity and uniformity, the term (3Z):(2E)-enal isomerase and the abbreviation HI (hexenal isomerase) for this double bond migration. While extensive research has been done on plant LOXs and HPLs (Andreou & Feussner, 2009;Hughes et al, 2009;Mwenda & Matsui, 2014;ul Hassan et al, 2015), much less is known about the enzymes that are involved in the reduction of C6aldehydes to their corresponding alcohols. For many years it was assumed that this conversion was catalysed by an alcohol dehydrogenase (ADH; Hatanaka, 1993); however, an Arabidopsis mutant lacking ADH activity could not fully abolish the concentrations of Z-3-HOL, indicating that either there is gene redundancy or there are other enzymes involved as well.…”