Channel formation in phospholipid bilayer membranes by the toxin ofHeminthosporium maydis,race T

Summary Southern Corn Leaf Blight is caused by a toxin produced by a virulent form ofHelminthosporium maydis (Race T). The toxin has been shown to uncouple oxidative phosphorylation and dissipate Ca²⁺ gradients in mitochondria isolated from susceptible, but not resistant, corn. The possibility that the toxin acted by increasing the permeability of membranes to ions was tested using a planar bilayer membrane system. Addition of the toxin to the bilayer system, under voltage-clamp conditions, resulted in stepwise increases in current across the phospholipid bilayer, a response characteristic for channel formers. Single-channel conductance in 1m KCl is 27 pS which corresponds to 1.7×10⁷ ions sec^–1 channel^–1 at 100 mV applied potential. The toxin channels are: (i) fairly uniform in conductance, (ii) ideally selective for K⁺ over Cl^–, and (iii) most conductive to H⁺. The channel showed the following selectivity for alkali metal cations: Rb⁺>K⁺>Cs⁺>Na⁺>Li⁺ (169731) based on the most frequently observed conductance in 1m chloride salts. The toxin showed no voltage dependence over the range of –100 to +100 mV. Channel formation was also a property of a synthetic analog (Cmpd IV) of the toxin. The ability of the native toxin to form channels may be a mode of toxin action on mitochondrial membranes from susceptible corn.     

Reaction of common bean callus to culture filtrate of Colletotrichum lindemuthianum: Differences in the composition and toxic activity of fungal culture filtrates

Culture filtrate of Colletotrichum lindemuthianum caused dark brown lesions on the lower surface veins of bean leaves. This phenomenon was used as a bioassay to study the production of toxic fungal metabolites. Calli from anthracnose-susceptible bean cultivars 'Collacia', 'Andecha' and 'Seronda' were sensitive to a 12.5% solution of race 38 filtrate or to a 25% solution of race 7 filtrate. In contrast, calli from anthracnose-resistant bean genotypes A 247, TU, PI 207262, 'Collacia' × 'Tu', 'Collacia' × AB 136 and 'Collacia' × PI 207262 did not develop browning. Culture filtrates were passed through an ionic-exchange resin and a gel filtration resin. Toxic activity of fractions from two races of the fungal pathogen was different, although in both races slight necrosis was produced by the same nine fractions. Pathogenicity could be related with common substances and toxicity could be identified with differential compounds. This revised version was published online in June 2006 with corrections to the Cover Date.