Reduction of Adenosine Triphosphate Levels in Susceptible Maize Mesophyll Protoplasts by Helminthosporium maydis Race T Toxin

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Abstract
H. maydis race T (HMT) toxin caused a reduction in the steady-state ATP levels when leaf mesophyll protoplasts isolated from maize [Zea mays] containing Texas male-sterile (T) but not male-fertile (N) cytoplasm were incubated in the dark. At a toxin concentration 10 times the mean effective dose for inhibition of root growth, the ATP levels began to fall in 30-90 s, fell by 50% in about 4 min, and reached 23% of the original levels in 100 min. This is faster than any previously observed response of whole cells or tissues to HMT toxin. In protoplasts incubated in the light, ATP levels were 25% higher than in the dark and were either unaffected or only slightly diminished by toxin. 3-(3,4-Dichlorophenyl)-1,1-dimethylurea (DCMU), an inhibitor of photosynthetic electron transport, overcame the effect of light on both toxin-treated and control protoplasts. Oligomycin, an inhibitor of mitochondrial ATP synthesis, mimicked the effects of toxin in the dark, in the light and in the light plus DCMU, but it was not specific for T cytoplasm. During the first 24 h of culture, ATP levels in control protoplasts increased in both the light and dark. In the dark, ATP was not detectable after 24 h incubation in the presence of toxin, whereas in the light a substantial amount of ATP remained. The hypothesis that mitochondria in vivo are inhibited by HMT toxin was supported. Other responses of cells and tissues to toxin can be explained in terms of reduced ATP levels.

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