The Role of Phytoalexins in the Resistance of Soybean Leaves to Bacteria: Effect of Glyphosate on Glyceollin Accumulation

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Abstract
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 aminooxyphenylpropionic acid all reported to inhibit phenylpropanoid biosynthesis in other plants, were not effective inhibitors of glyceollin production in soybean leaves. Glyophosate inhibits 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 2-fold higher than in untreated leaves, but compatible-cell populations were 8-fold 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. Glyceollin accumulation is probably a component, but not the only mechanism, of resistance to bacteria in hypersensitive soybean leaves.