Isolation and Biological Activities of Four Selective Toxins from Helminthosporium carbonum

A new and simpler purification procedure was developed for host selective toxins from Helminthosporium carbonum race 1. Four analogs or forms of toxin with the same selectivity as the fungus were isolated from culture fluids; two forms (HC toxins III and IV) have not been reported by other workers. Crystals of the major form of toxin (HC toxin I) were recovered in high yields (>80 milligrams per liter of culture fluid) without the use of high performance or preparative thin layer liquid chromatography. ED(50) values, based on inhibition of root growth of susceptible seedlings, for HC toxins I, II, III, and IV were 0.2, 0.4, 2.0, and 20 micrograms per milliliter, respectively. The specific activity of crystalline HC toxin I matched the most active preparation reported previously; the preparation of HC toxin II was more active than that reported previously. Resistant seedlings tolerated 100-fold higher concentrations of each form of toxin than did susceptible seedlings. Hydrolysis of the epoxide group of HC toxin I to a diol destroyed toxicity to susceptible and resistant seedlings. The data suggest that the same mechanisms are affected in resistant and susceptible plants.