Metabolite-based Defense Mechanisms

The role of 9- and 13-lipoxygenase-derived oxylipins for race-cultivar-specific resistance in potato was analyzed by expressing RNA interference constructs against oxylipin biosynthetic genes in transgenic potato plants carrying the resistance gene R1 against Phytophthora infestans. Down-regulation of 9-lipoxygenase expression resulted in highly reduced levels of 9-hydroxyoctadecatrienoic acid after treatment with the pathogen-associated molecular pattern Pep-13. However, neither 9-lipoxygenase nor 9-divinyl ether synthase RNAi plants exhibited alterations in their resistance to P. infestans. Similarly, successful down-regulation of transcript accumulation of the 13-lipoxygenase pathway genes encoding allene oxide cyclase, 12-oxophytodienoic acid reductase 3 and the jasmonic acid receptor coronatine-insensitive 1 resulted in highly reduced levels of jasmonic acid after Pep-13 treatment. Race-cultivar-specific resistance, however, was not lost in these plants. Our results suggest that neither 9-lipoxygenase-derived oxylipins nor jasmonic acid are required for R-gene-mediated resistance in potato. Importantly, in tobacco, the silencing of 9-lipoxygenase expression was previously demonstrated to suppress race-cultivar-specific resistance. Thus, we conclude a differential requirement of oxylipins for R-gene-mediated resistance in different solanaceous plants.

Inducing systemic resistance responses in crop plants is a promising alternative way of disease management. To understand the underlying signaling events leading to induced resistance, functional analyses of plants defective in defined signaling pathway steps are required. We used potato, one of the economically most-important crop plants worldwide, to examine systemic resistance against the devastating late blight pathogen Phytophthora infestans, induced by treatment with dl-β-aminobutyric acid (BABA). Transgenic plants impaired in either the 9-lipoxygenase pathway, which produces defense-related compounds, or the 13-lipoxygenase pathway, which generates jasmonic acid–derived signals, expressed wild-type levels of BABA-induced resistance. Plants incapable of accumulating salicylic acid (SA), on the other hand, failed to mount this type of induced resistance. Consistently, treatment of these plants with the SA analog 2,6-dichloroisonicotinic acid restored BABA-induced resistance. Together, these results demonstrate the indispensability of a functional SA pathway for systemic resistance in potato induced by BABA.