Biological Activity of the Isomeric Forms of Helminthosporium sacchari Toxin and of Homologs Produced in Culture

The effect of Helminthosporium sacchari (HS) toxin isomers and related, pathogen-produced compounds on dark CO₂ fixation in HS-susceptible sugar cane leaf slices was investigated. HS toxin consists of a mixture of three isomeric bis-5-O-(β-galactofuranosyl)-β-galactofuranosides (A, B, and C) differing in the position of one double bond in the sesquiterpene aglycone. Maximum inhibition of dark CO₂ fixation in susceptible sugar cane (CP52-68) occurred within 30 to 40 minutes, and amounts necessary to reach 50% inhibition values typically were approximately 1.7 micromolar for natural toxin mixture ( 2:3:5 mixture of isomers A:B:C) and 4, 6, and 0.7 micromolar for isomers A, B, and C, respectively. Other fractions from cultures of the pathogen consist of comparable mixtures of sesquiterpene isomers but have only 1, 2, or 3 galactofuranose units (HS₁, HS₂, HS₃) or two α-glucopyranose units as well as four β-galactofuranose units (HS₆). The lower toxin homologs were not toxic to clone CP52-68, but protected sugar cane from the effects of toxin. Minimum ratios of protectant: toxin giving 95% protection were approximately 50:1, 6:1, and 12:1 for HS₁, HS₂, and HS₃, respectively. HS₂ and HS₃ protected when added up to 12 minutes after toxin as well as when added with or before toxin. Some common plant galactopyranosides were not toxic and did not protect at 500:1 molar excess. The sample of HS₆ was toxic at 500 micromolar, and did not protect against HS toxin. With the availability of purified, homogeneous preparations of HS toxin, homologs, and chemically modified or synthetic analogs, the dark CO₂ fixation assay should prove to be a useful tool for understanding the mode of action of HS toxin.