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Silicon-Mediated Accumulation of Flavonoid Phytoalexins in Cucumber

First and fourth authors: Centre de Recherche en Horticulture, Département de Phytologie, Université Laval, Québec, Canada G1K 7P4; second author: Canadian Forest Service, Great Lakes Forestry Centre, Sault Ste. Marie, Canada P6A 5M7; and third author: Cereal Research Centre, Agriculture and Agri-Food Canada, Winnipeg, Canada R3T 2M9

The controversial role of silicon in plant disease resistance, described mostly as a passive mechanical protection, has been addressed. Conclusive evidence is presented that silicon is involved in the increased resistance of cucumber to powdery mildew by enhancing the antifungal activity of infected leaves. This antifungal activity was attributable to the presence of low-molecular-weight metabolites. One of these metabolites, described here as a phytoalexin, was identified as a flavonol aglycone rhamnetin (3,5,3′,4′-tetrahydroxy-7-O-methoxyflavone). This is the first report of a phytoalexin for this chemical group in the plant kingdom and of a flavonol phytoalexin in cucumber, a chemical defense long believed to be nonexistent in the family Cucurbitaceae. The antifungal activity of leaf extracts was better expressed after acid hydrolysis, extending to another plant species the concept that some phytoalexins are synthesized as glycosylated phytoalexins or their precursors.

Additional keywords: induced resistance , phenolics .