Effects of Helminthosporium maydis race T toxin on protoplasts of resistant and susceptible corn (Zea mays)

Leaf protoplasts from resistant (N, C, and S cytoplasm) and susceptible (T cytoplasm) corn inbred W182B (Zea mays L.) exhibited a differential response after exposure to Helminthosporium maydis race T toxin. The volume of untreated protoplasts increased twofold during the first 24 h after isolation and by 48–72 h, most protoplasts exhibited an extensive network of cytoplasmic strands and the chloroplasts were distributed throughout the cytoplasm by cyclosis. However, susceptible protoplasts exposed to ≥2 μg toxin/ml failed to increase in volume and cytoplasmic streaming was rarely observed. By 72 h there was extensive degradation and collapse of susceptible protoplasts exposed to toxin. Resistant protoplasts exposed to up to 100 μg toxin/ml exhibited a twofold volume increase and were indistinguishable from untreated protoplasts. Susceptible protoplasts exposed to 0.1 μg toxin/ml also appeared unaffected, but at 1.0 μg toxin/ml an intermediate response was observed. The differential response to H. maydis race T toxin of protoplasts from resistant and susceptible corn correlates well with those effects observed in intact plant tissues and may serve to explain further the mode of action of the toxin on susceptible corn cells.