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Reversible Effects of Toxin from Helminthosporium maydis Race T on Oxidative Phosphorylation by Mitochondria from
M A Bednarski1, S Izawa, R P Scheffer
1Department of Botany and Plant Pathology, Michigan State University, East Lansing, Michigan 48824.
Abstract:
Host-selective toxin from Helminthosporium maydis race T inhibited oxidative phosphorylation (AT(32)P formation) and stimulated ATPase activity by mitochondria from male-sterile (T) but not from normal (N) cytoplasm maize (Zea mays L.). Toxin increased the rate of NADH oxidation, but succinate oxidation was slightly, and malate-pyruvate oxidation was strongly inhibited as the associated ATP formation was abolished. There was a 1-minute lag before toxin gave maximal stimulation of NADH oxidation; the responses to 2,4-dinitrophenol and valinomycin were immediate. There was also a delay in the effect of toxin on ATP formation. T mitochondria were more sensitive than were N mitochondria to uncoupling by nigericin plus K(+); there was no evidence, however, that the action of toxin is related to that of nigericin or other ionophores. With NADH as the substrate, the degree of uncoupling increased with increases in toxin concentration up to a saturating level; kinetics of the response suggested reversibility. T mitochondria exposed to toxin for 5 minutes regained normal rates of respiration and of ATP formation when they were washed with toxin-free medium, showing that the uncoupling effect is reversible. Evidently HM-T toxin does not bind firmly to its site(s) of action, in contrast to reports for another hostselective toxin.
Insights
The Helminthosporium maydis race T toxin disrupts energy production in T cytoplasm maize mitochondria. This toxin reversibly inhibits ATP formation and alters respiration, showing it doesn't bind tightly to its target.
Area of Science:
- Biochemistry
- Plant Pathology
- Molecular Biology
Background:
- Host-selective toxins are crucial virulence factors in plant-pathogen interactions.
- Mitochondrial function is essential for plant energy metabolism and stress response.
- Cytoplasmic male sterility in maize (Zea mays L.) is associated with specific mitochondrial genomes.
Purpose of the Study:
- To investigate the effects of Helminthosporium maydis race T toxin on mitochondrial oxidative phosphorylation and ATPase activity in maize.
- To determine the mechanism of action of the toxin, particularly its interaction with T and N cytoplasm mitochondria.
- To assess the reversibility and binding characteristics of the toxin's effects.
Main Methods:
- Isolation and characterization of mitochondria from T and N cytoplasm maize.
- Measurement of oxygen consumption (respiration) with various substrates (NADH, succinate, malate-pyruvate).
- Assay of ATP formation and ATPase activity in response to toxin exposure.
- Kinetic analysis and reversibility studies using toxin-free washing.
Main Results:
- The toxin specifically inhibited oxidative phosphorylation (ATP formation) and stimulated ATPase activity in T cytoplasm mitochondria, but not in N cytoplasm mitochondria.
- Toxin increased NADH oxidation rate but inhibited succinate and malate-pyruvate oxidation, abolishing associated ATP formation.
- The toxin's uncoupling effect was concentration-dependent, reversible upon washing, and exhibited a lag phase, suggesting it does not bind tightly to its site of action.
Conclusions:
- Helminthosporium maydis race T toxin targets mitochondrial energy transduction in T cytoplasm maize.
- The toxin's reversible action and lack of firm binding differentiate it from other host-selective toxins.
- Understanding these molecular interactions is key to developing disease resistance strategies in maize.
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