A Comparison of Purified Host Specific Toxin from Helminthosporium maydis, Race T, and Its Acetate Derivative on Oxidation by Mitochondria from Susceptible and Resistant Plants

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Abstract
NADH or succinate oxidation and malate oxidation were differentially affected in mitochondria from both susceptible and resistant corn by a purified and chemically characterized preparation of host-specific toxin from Bipolaris (Helminthosporium) maydis, race T. NADH and succinate oxidation by susceptible T corn mitochondria were stimulated 50-200% with apparent uncoupling from the cytochrome chain at .simeq. 10-9 M toxin (5-20 ng/ml). Significant inhibition of malate oxidation was observed at slightly higher toxin concentrations, but oxidation was still coupled to ADP utilization. Inhibition of malate oxidation also was observed in N corn (resistant) and soybean mitochondria at .simeq. 1000-fold greater concentrations, but stimulation of NADH and succinate oxidation was not found at any toxin concentration tested. A fully acetylated toxin derivative at .simeq. 1 .mu.g/ml also caused stimulation of NADH or succinate oxidation in T corn mitochondria, but not those of N corn or soybean mitochondria at 100 .mu.g/ml. Malate oxidation was inhibited to the same extent by toxin acetate with mitochondria from T corn, N corn and soybean. The blocking of hydroxyl groups in race T toxin by acetyl functions eliminated selectivity toward malate oxidation only. Inhibition of malate oxidation is either a separate or secondary effect of selective action of toxin on T corn mitochondria, perhaps by interference with transport in or out of the matrix. Sensitivity of T, but not N, corn mitochondria to purified toxin decays within minutes after pellets are suspended in aqueous osmotica, with no obvious change in mitochondrial integrity. The action of race T toxin seems to involve a labile process, such as ion gradient(s), or an unstable structural conformation of T corn mitochondria.