Studies on the resistance of wheat and maize to fungal pathogenesis.

Abstract

17-Day-old seedlings of winter-wheat cultivar SST25 were inoculated with an avirulent race of wheat leaf rust, Puccinia recondita f.sp. tritici. After various time intervals the plants were reinoculated with a virulent wheat leaf rust race. No change in latent period or infection type was observed. However, the infection frequency was reduced by approximately 60 per cent. The phytotoxic effects of three mycotoxins of Fusarium spp. (fumonisin B(1)(FB)1) , moniliformin and T-2 toxin), and pathotoxin extracts of Exserohilum turcicum (HT-toxin) and Stenocarpella macrospora (SM-toxin) were studied using callus from the scutella of immature cobs of maize, Zea mays. The callus was grown on modified MS medium containing either 0, 0.1, 1.0, 10, or 100 mg (or ml) toxin per litre. For SM-toxin the concentrations used were 0, 0.01, 0.1, 1.0 or, 10 ml/l. Mass increase of callus on medium containing FB 1, moniliformin, T-2 toxin, and HT-toxin decreased as the concentration of toxin increased, resulting in a significant reduction at the highest toxin level. SM-toxin callsed a slight reduction in mass at 0.01 ml/l, but stimulated growth at 1.0 ml/l. At 10 ml/l a significantly lower callus mass increase was recorded. Transmission electron microscopy studies of FB 1-treated callus showed an increased level of activity in the toxin-treated cells resulting in thicker cell walls, occurrence of starch grains and phenolic substances, when compared to the control. The mitochondria of callus cells were affected by SM-toxin, and starch was found in all toxin treatments. When transferred to toxin-free medium after treatment with FB 1, a complete recovery of the callus occurred at all toxin levels but the highest, although regrowth occurred at this level. Callus treated with SM-toxin retained the same growth rate as during the toxin treatment, and it can be concluded that the toxin has a permanent effect on the growth rate of callus. Maize seedling leaves, injected with a 10g/l FB(1)-solution at the stalk base, showed necrotic areas and chlorotic flecks. The toxin-treated plants were stunted and occasionally produced side shoots. S. macrospora-susceptible and -resistant seedlings, injected in a similar fashion with SM-toxin, gave a different response to the toxin. Susceptible plants were affected by the toxin, while no effects were observed in resistant plants.