Toxic and protective effects of analogues of Helminthosporium sacchari toxin on sugarcane tissues.
Host-selective toxin and 3 groups of smaller, structurally related compounds were isolated from culture fluids of H. [Drechslera] sacchari. The lower mol. wt. compounds (toxoids) differed from HS toxin in the number of galactose units/molecule. They protected sugarcane tissues from toxin, as shown by an assay based on toxin-induced loss of electrolytes. Toxoid III (3 units of galactose/molecule) was most effective; it gave 90% protection at a 24:1 ratio, on a molar basis. Toxoid II (2 units of galactose) was intermediate, and toxoid I (1 unit) the least effective. Protection was apparent when tissues were exposed to toxoid III for 1 h, rinsed to remove free toxoid, then exposed to toxin. These results suggest that toxoids act as competitive inhibitors of HS toxin. This is consistent with the hypothesis that toxin binds to a receptor molecule in susceptible cells. Toxoids I and II induced no loss of electrolytes from any of the tested sugarcane clones. Toxoid III caused losses and runner lesions on clones NG 77-234 and NG 77-82, but not on clones Co 453 and NG 77-103; all 4 clones are equally susceptible to D. sacchari.