Plant Pathogenesis-Related Proteins Induced by Virus Infection

The hypersensitive response triggered by an incompatible plant-pathogen interaction is accompanied by numerous metabolic changes (for reviews see 5, 8, 20, 37, 1 19). One general effect is the induction of enzymes that are involved in the synthesis of ethylene, a plant hormone able to induce many stress responses. The induction of enzymes from the phenylpropanoid path­ way, oxidative enzymes, and hydroxyproline-rich glycoproteins (HRGP; ex­ tensin) is partially related to cell wall modifications that lead to the formation of a physical barrier to further spread of the pathogen from the site of infection. Peroxidases are involved in the polymerization of alcohol de­ rivatives of aromatic compounds such as coumaric, ferulic, and sinapic acid into lignin and suberin and the cross-linking of these polyphenols to extensin molecules from the cell wall matrix. In addition to inducing a defense aimed at localizing the pathogen, various antimicrobial compounds are synthesized. These compounds include phytoalexins synthesized along branches of the phenylpropanoid pathway leading to furano-coumarins or isoflavonoids,