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The effect of a toxin extract of Helminthosporium maydis, race T on K + (86Rb) uptake by excised root segments of normal (N) and Texas cytoplasmic male-sterile (T) versions of corn inbred W64A was investigated. The uptake of K+ was inhibited in both N and T roots by the toxin. This was true for both basal (freshly excised) and augmented (pretreated with aeration) K+ uptake. Augmented uptake was more toxin-sensitive than basal uptake (irrespective of cytoplasm type), and the augmented uptake in T roots was seven to eight times more toxin-sensitive than in N roots.Specific zones of roots differed in their basal and augmented K+ uptake rates as well as their toxin sensitivities. The root apex of T was more sensitive to toxin than the apex of N roots when basal K+ uptake was measured. In mature zones of the root, T was more sensitive than N when augmented rates were measured. During the development of the augmented K+ uptake capacity in either N or T roots, the sensitivity to the toxin did not change; uptake in N roots was inhibited by 10 to 25% and uptake in T roots was inhibited by 70 to 80%. The difference in toxin sensitivity of K+ uptake between N and T roots may be due to N possessing a protective mechanism which is deficient in T.The extreme susceptibility of Texas cytoplasmic male-sterile (T) maize to Southern Corn Leaf Blight is cytoplasmically conditioned and inherited in association with the non-Mendelian determinant of male sterility (12,18). T versions of a given maize inbred are more susceptible to Helminthosporium maydis, race T than normal cytoplasmic (N) versions (7, 10). The fungal pathogen synthesizes a toxin complex which accounts for this difference in levels of susceptibility of the host cytoplasmic types (7,8 Table I and Figure 1, the absorption period was 60 min during which time some augmentation occurred. The segments were exposed to label and toxin simultaneously. Labeled segments were washed for 20 min in 40 ml of cold I mM CaSO4, weighed, and assayed for " Rb content by liquid scintillation spectrometry. Transfer of segments between solutions was by filtration on a Buchner funnel. Influx of K+ and 86Rb were taken to be equivalent using tissue grown in the absence of exogenous K+ (17).Basal rates of K+ uptake were measured in freshly cut root segments, whereas augmented rates were measured after aeration of segments in distilled H20. This augmented uptake potential has been referred to as in "induced" ion uptake (19), or as "aging" (1, 16) and may be an important feature of the transport process.Helminthosporium maydis Nisikado and Miyake, race T, was grown for 8 to 10 days in shake culture (100 rpm, 24 C) on Fries medium (26), without micronutrients. The culture fluid was filtered through cheesecloth, concentrated to one-tenth volume by flash evaporation at 45 C, and precipitated (12 hr, 4 C) with two volumes of cold methyl alcohol. The precipitate was removed by centrifugation and the methyl alcohol was removed from the aqueous-methyl alcohol phase by flash evaporation. The rema...

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