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Constitutive Accumulation of a Resveratrol-Glucoside in Transgenic Alfalfa Increases Resistance to Phoma medicaginis

May 2000 , Volume 13 , Number  5
Pages  551 - 562

John D. Hipskind and Nancy L. Paiva

The Samuel Roberts Noble Foundation, P.O. Box 2180, 2510 Sam Noble Parkway, Ardmore, OK 73402, U.S.A.

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Accepted 14 January 2000.

Alfalfa (Medicago sativa) was transformed with a peanut (Arachis hypogaea) cDNA encoding resveratrol synthase (RS) transcriptionally regulated by an enhanced Cauliflower mosaic virus (CaMV) 35S promoter. Transgenic plants accumulated a new compound, not present in wild-type or vector-transformed alfalfa, that was identified as trans-resveratrol-3-O-β-D-glucopyranoside (RGluc) by high-pressure liquid chromatography (HPLC), UV, 1H- and 13C-nuclear magnetic resonance (NMR) analyses. RGluc concentration was highest in the youngest leaves (>15 μg per g fresh weight) and oldest stem internode segments (>10 μg per g fresh weight) while roots contained only trace amounts (<0.2 μg per g fresh weight). RS transcript levels were highest in leaves and stems, with comparatively little transcript accumulation in the roots, while an inverse pattern was observed for chalcone synthase (CHS) transcript levels. CHS directly competes with RS for the metabolic precursors p-coumaroyl CoA and malo-nyl CoA, and may also contribute to the developmental variations in RGluc levels by limiting the availability of substrates. Agar-plate bioassays indicated that both RGluc and resveratrol greatly inhibit hyphal growth of the alfalfa fungal pathogen Phoma medicaginis. Subsequently, RGluc-containing leaves were wound inoculated and showed a significant reduction (relative to control leaves) in the size of necrotic lesions, intensity of adjacent chlorosis, and number of fungal reproductive structures (pycnidia). Decreasing sporulation of this pathogen may greatly reduce disease spread and severity throughout the field.

Additional keywords: disease resistance, phytoalexin, secondary metabolism, stilbene.

© 2000 The American Phytopathological Society