Photosynthetic costs and impact on epidemiological parameters associated with Ht resistance genes in maize lines infected with Exserohilum turcicum

Abstract

Northern corn leaf blight is caused by Exserohilum turcicum, mainly controlled by the use of resistant cultivars. Maize lines carrying individual resistance genes B37Ht1, B37Ht2, B37Ht3 and B37Htn1 express different defense symptoms having impact on the photosynthetic activity, the accumulation of reactive oxygen species and on epidemiological parameters. Plants were inoculated with a race 0 isolate of E. turcicum conferring a compatible interaction with B37 and incompatible interactions with plants carrying resistance genes. Five days post inoculation (dpi), the resistant lines displayed reduction in leaf CO2 assimilation (A) of 30 to 80% compared to healthy plants. At 14 dpi, inoculated plants of B37Ht1 showed a significant decrease in A, similar to B37 (up to 94%). The instantaneous carboxylation efficiency (k) was significantly reduced on inoculated plants of the lines B37Ht2, B37Ht3, and B37Htn1 (54 - 81%) at 5 dpi. Curiously, the reduction in k for B37 and B37Ht1 (up to 95%) was higher at 14 dpi then at 5 dpi (up to 81%). At 6 dpi, low levels of H2O2 were detected in B37Ht1, in contrast to B37Htn1, where a high H2O2 level and peroxidase activity were observed. Sporulation rate on B37Ht1, B37Ht3 and B37Htn1 decreased by 92% compared to the susceptible control, while strong sporulation occurred in lesions on line B37Ht2. The resistance in maize to E. turcicum conferred by Ht resistance genes is associated with photosynthetic costs and may have quite contrasting effects on host physiology and major epidemiological parameters, like sporulation, which contributes inoculum for secondary infections.