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The Role of Phytoalexins in the Resistance of Soybean Leaves to Bacteria: Effect of Glyphosate on Glyceollin Accumulation. M. J. Holliday, Department of Plant Pathology, University of California, Riverside 92521, Current address of senior author: E. I. du Pont de Nemours & Co., Biochemistry Department, Experimental Station, Wilmington, DE 19898; N. T. Keen, Department of Plant Pathology, University of California, Riverside 92521. Phytopathology 72:1470-1474. Accepted for publication 14 April 1982. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-1470.

The herbicide glyphosate (N-phosphonomethyl glycine) inhibited glyceollin accumulation in hypersensitive soybean leaves inoculated with an incompatible race of Pseudomonas syringae pv. glycinea. Aminooxyacetic acid, aminooxyphenylpropionic acid, and benzyloxycarbonyl amino-oxyphenylpropionic acid all reported to inhibit phenylpropanoid biosynthesis in other plants, were not effective inhibitors of glyceollin production in soybean leaves. Glyophosate has been shown to inhibit the conversion of shikimate to chorismate in other plants and this mechanism of action may explain the inhibition of glyceollin accumulation in soybean. Significantly, glyphosate did not block hypersensitive host cell necrosis in leaves inoculated with incompatible bacteria. Incompatible bacterial-cell populations in resistant leaves treated with glyphosate were twofold higher than in untreated leaves, but compatible-cell populations were eightfold higher than incompatible-cell populations in glyphosate-treated leaves. Thus, inhibition of glyceollin accumulation by glyphosate only partially prevented the expression of resistance to bacteria. Pretreatment of plants with phenylalanine and tyrosine, however, restored glyceollin accumulation in glyphosate-treated plants and resulted in complete resistance expression to incompatible bacteria. The results suggest that glyceollin accumulation is a component, but not the only mechanism, of resistance to bacteria in hypersensitive soybean leaves.