Selection of T‐Cytoplasm Maize Callus Cultures Resistant to Helminthosporium maydis Race T Pathotoxin¹

Texas male‐sterile cytoplasm ( cms ‐T) maize ( Zea mays L.) is susceptible to Helminthosporium maydis (Nisikado and Miyake) rate T and its pathotoxin; whereas nonsterile (N) cytoplasm maize is resistant. Callus cultures of cms ‐T and N versions of A619 maize were established to study the effect of the toxin on maize callus and to select for toxin‐resistant cms ‐T callus. Toxin in the culture medium inhibited cms ‐T growth, while N grew normally. The percent inhibition of cms ‐T callus growth depended on the toxin concentration. Complete inhibition was obtainable. Selection of toxin‐resistant A619( cms ‐T) callus began with cultures grown on sublethal toxin concentrations. The fastest growing callus or callus sectors were subeultured for additional selection. After four subculture cycles (160 days on toxin), 14 callus clones were selected for growth analysis in the absence and presence of toxin concentrations lethal to unselected A619( cms ‐T) callus. Growth rates at these toxin concentrations were 53 to 111% of the growth without toxin. This high degree of toxin resistance remained stable for 127 days growth in the absence of toxin. The in vitro effects of toxin on NADH oxidation, oxidative phosphorylation, malate‐driven 2,6‐dichlorophenol indophenol reduction, and swelling of mitochondria isolated from unselected A619(N) and A619( cms ‐T) callus and from resistant A619( cms ‐T) callus were determined. These four activity parameters of mitochondria from susceptible A619( cms ‐T) callus were significantly affected by toxin added to the reaction medium. Mitochondria isolated from A619(N) and resistant A619( cms ‐T) callus were not affected by toxin addition. This association of mitochondrial insensitivity to the toxin with resistance at the callus level may help clarify the role of mitochondria in resistance to this disease.