Induction of cinnamate 4-hydroxylase and phenylpropanoids in virus-infected cucumber and melon plants

In the present work, we have looked for the nature of the phenylpropanoids biosynthesized during the plant-pathogen reaction of two systems, Cucumis sativus and Cucumis melo infected with either prunus necrotic ringspot virus (PNRSV) or melon necrotic spot virus (MNSV), respectively. An accumulation of p-coumaric, caffeic and/or ferulic acids was observed in infected plant extracts hydrolysed with β-glucosidase or esterase. Analysis of undigested samples by HPLC/ESI revealed that these compounds are mainly forming esters with glucose: 1-O-coumaroyl-β-glucose, 1-O-caffeoyl-β-glucose, and 1-O-feruloyl-β-glucose. Cinnamic acid 4-hydroxylase (C4H, EC 1.14.13.11), the second enzyme of the plant phenylpropanoid pathway, plays a pivotal role in the synthesis of these hydroxycinnamic acids. Thus, we have isolated and characterised a cDNA clone encoding this enzyme from PNRSV-infected cucumber, and a partial cDNA from MNSV-infected melon leaves. The deduced amino acid sequence revealed a notable degree of identity with homologous C4H enzymes from other plant species. In agreement with the induction of the phenylpropanoids presently described, it is reported that in cucumber and melon leaves, both viral infections studied induced C4H mRNA expression. A similar induction was observed for the first phenylpropanoid biosynthetic enzyme, phyenylalanine ammonia-lyase (PAL, EC 4.3.1.5), and for chitinase and peroxidase defence-related genes.