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.