Suppression of induced resistance in cucumber through disruption of the flavonoid pathway

ABSTRACT In this study, cucumber plants (Cucumis sativus) expressing induced resistance against powdery mildew (caused by Podosphaera xanthii) were infiltrated with inhibitors of cinnamate 4-hydroxylase, 4-coumarate:CoA ligase (4CL), and chalcone synthase (CHS) to evaluate the role of flavonoid phytoalexin production in induced disease resistance. Light and transmission electron microscopy demonstrated ultrastructural changes in inhibited plants, and biochemical analyses determined levels of CHS and beta-glucosidase enzyme activity and 4CL protein accumulation. Our results showed that elicited plants displayed a high level of induced resistance. In contrast, down regulation of CHS, a key enzyme of the flavonoid pathway, resulted in nearly complete suppression of induced resistance, and microscopy confirmed the development of healthy fungal haustoria within these plants. Inhibition of 4CL ligase, an enzyme largely responsible for channeling phenylpropanoid metabolites into the lignin pathway, had little effect on induced disease resistance. Biochemical analyses revealed similar levels of 4CL protein accumulation for all treatments, suggesting no alterations of nontargeted functions within inhibited plants. Collectively, the results of this study support the idea that induced resistance in cucumber is largely correlated with rapid de novo biosynthesis of flavonoid phytoalexin compounds.