Assay for Helminthosporium maydis Toxin-binding Activity in Plants

A relatively rapid and sensitive assay is described for assessing the binding of Helminthosporium maydis Race T(14) C-toxins I and II to plant components. The technique is a modification of the one of Haddad and Birge (J. Biol. Chem. 250: 299-303, 1975), and utilizes dextran-coated charcoal as an adsorbent for the unreacted toxin and employs a Millipore filter to isolate the protein-toxin complex.Extracts from susceptible corn line W64A Tms possess a protein primarily localized in the cytosol which is relatively heat-insensitive, ficin- and papain-sensitive, and binds toxins I and II at half-saturation in the order of 0.1 mm. The toxin-binding activities of the extracts of various corn lines and other species are not correlated to resistance or susceptibility to H. maydis Race T, nor to sensitivity to the toxins. These findings are discussed relative to the function of the binding protein and cellular sensitivity to the toxins.