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 cotton protoplasts. Binding of file ¹²⁵I-radiolabelled PLP-complex to protoplasts from cotyledon tissue was saturable and with an affinity (K(d) = 17.3 nM) comparable with the concentration required for biological activity. A single class of binding site, accessible at the surface of the intact protoplasts, was found and the maximal number of binding sites were estimated as 2.41x10^{- 16} moles per protoplast. The binding affinity to protoplasts proved near identical to that found with purified plasma membrane fractions from roots. When cultivars exhibiting resistance or susceptibility towards the pathogen were compared, no significant differences were found in the affinity of binding, but five times as many binding sites per protoplast and sixteen times as many binding sites per mg membrane protein were found in the resistant cultivar.