BIOLOGICAL-ACTIVITY OF THE ISOMERIC FORMS OF HELMINTHOSPORIUM-SACCHARI TOXIN AND OF HOMOLOGS PRODUCED IN CULTURE

The effect of H. 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 3 isomeric bis-5-O-(.beta.-galactofuranosyl)-.beta.-galactofuranosides (A, B and C) differing in the position of 1 double bond in the sesquiterpene aglycone. Maximum inhibition of dark CO2 fixation in susceptible sugar cane (CP52-68) occurred within 30-40 min, and amounts necessary to reach 50% inhibition values typically were approximately 1.7 .mu.M for natural toxin mixture (.simeq. 2:3:5 mixture of isomers A:B:C) and 4, 6 and 0.7 .mu.M 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 2 .alpha.-glucopyranose units as well as 4 .beta.-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 min 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 .mu.M, 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.