C6‐volatiles derived from the lipoxygenase pathway induce a subset of defense‐related genes

Abstract

Summary: Six‐Carbon (C 6‐) volatiles, including the aldehydes trans‐2‐hexenal, hexanal and cis‐3‐hexenal, as well as their corresponding alcohols, are produced from damaged or wounded plant tissue as a product of the enzymatic activity of hydroperoxide lyase (HPL), a component of the lipoxygenase (LOX) pathway. Aerial treatment of Arabidopsis seedlings with 10 μM concentrations of trans‐2‐hexenal induces several genes known to be involved in the plant’s defense response, including phenylpropanoid‐related genes as well as genes of the LOX pathway. Genes encoding the pathogenesis‐related proteins PR‐1 or PR‐2, however, were not induced. Trans‐2‐hexenal induction thus closely mimics the group of genes induced by methyl jasmonate (MeJA), also a LOX‐derived volatile. However, unlike MeJA, trans‐2‐hexenal did not induce hydroxymethylglutaryl‐coenzyme A reductase (HMGR) or thionin2–1. The inductive effect seemed to be limited to C 6‐related volatiles, as C 8‐, C 9‐ and other related volatiles did not induce LOX mRNA levels. As has been demonstrated for MeJA, trans‐2‐hexenal quantitatively reduced wild‐type seed germination. Trans‐2‐hexenal also reduced the germination frequency of the MeJA resistant Arabidopsis mutant, jar1–1, supporting the notion that trans‐2‐hexenal and MeJA are recognized via different mechanisms. In addition, trans‐2‐hexenal had a moderate inhibitory effect on root length relative to similar concentrations of MeJA and was approximately 10‐fold less effective than MeJA at inducing anthocyanin accumulation in Arabidopsis seedlings. These results suggest that C 6‐volatiles of the LOX pathway act as a wound signal in plants, but result in a moderate plant response relative to MeJA at both the physiological and molecular level.