Inhibition of Corn Callus Growth by Helminthosporium carbonum Race 1 Toxin
Wolf, S. J. | Earle, E. D.
Plant tissue culture has potential application in the development of disease-resistant crops and also in the study of host-pathogen interactions in vitro. Pathogen-produced toxins (phytotoxins) can be used as pathogen surrogates in such work. Helminthosporium carbonum Race 1, the causal agent for Helminthosporium leaf spot of corn (Zea mays L.), produces a phytotoxin (HC toxin) known to be the pathogenicity factor for that disease. Tissue cultures were established from corn genotypes susceptible or resistant to H. carbonum Race 1. A toxin preparation with an effective dose (ED₅₀,) of 2 μg mL⁻¹ for seedling roots was incorporated into callus growth and regeneration media. Growth of callus derived from genotypes susceptible to the pathogen was inhibited at 5 μg mL⁻¹ toxin, whereas callus from a resistant genotype was inhibited only at 50 μg mL⁻¹ toxin. Regeneration of plants from callus of susceptible genotypes was also inhibited at 5 μg mL⁻¹ toxin, but regeneration of a resistant genotype was not inhibited at 20 μg mL⁻¹ toxin. A total of 5676 calli from the susceptible lines were exposed to toxin concentrations of 2, 5, and 10 μg mL⁻¹. Both 3 and 15-mo old calli were used. Some cultures were treated with the mutagens sodium azide and ethyl methane sulfonate before exposure to toxin. No resistant callus or plants were recovered from any of the treatments. Failure to recover resistant callus might be due to an inability to identify resistant cells within a relatively slow-growing callus cell population.
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