SPECIFIC BINDING OF A VERTICILLIUM-DAHLIAE PHYTOTOXIN TO PROTOPLASTS OF COTTON, GOSSYPIUM-HIRSUTUM

The occurrence of specific, high-affinity binding sites for a protein- lipopolysaccharide (PLP) phytotoxin purified from culture filtrates of a virulent Verticillium dahliae isolate has been demonstrated in cott on protoplasts. Binding of the I-125-radiolabelled PLP-complex to prot oplasts from cotyledon tissue was saturable and with an affinity (K-d = 17.3 nM) comparable with the concentration required for biological a ctivity. A single class of binding site, accessible at the surface of the intact protoplasts, was found and the maximal number of binding si tes were estimated as 2.41x10(-16) moles per protoplast. The binding a ffinity to protoplasts proved near identical to that found with purifi ed plasma membrane fractions from roots. When cultivars exhibiting res istance or susceptibility towards the pathogen were compared, no signi ficant differences were found in the affinity of binding, but five tim es as many binding sites per protoplast and sixteen times as many bind ing sites per mg membrane protein were found in the resistant cultivar .