Specific binding of a fungal glucan phytoalexin elicitor to membrane fractions from soybean Glycine max

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Abstract
Treatment of soybean tissues with elicitors results in the production of phytoalexins, one of a number of inducible plant defense reactions against microbial infections. The present study uses a .beta.-1,3-[3H]glucan elicitor fraction from Phytophthora megasperma f. sp. glycinea, a fungal pathogen of soybean, to identify putative elicitor targets in saturable high-affinity elicitor binding site(s) in membrane fractions of soybean roots. Highest binding activity is associated with a plasma membrane-enriched fraction. The apparent Kd value for .beta.-glucan elicitor binding is .apprxeq. 0.2 .times. 10-6 M and the maximum number of binding sites is 0.5 pmol per mg of protein. Competition studies with the [3H]glucan elicitor and a number of polysaccharides demonstrate that only polysaccharides of a branched .beta.-glucan type effectively displace the radiolabeled ligand from membrane binding. Differential displacing activity of the glucans on P. megasperma elicitor binding corresponds closely to their respective ability to elicit phytoalexin production in a cotyledon bioassay.