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

Northern corn leaf blight is caused by Exserohilum turcicum , mainly controlled by the use of resistant cultivars. Maize lines carrying individual resistance genes B37 Ht1 , B37 Ht2 , B37 Ht3 and B37 Htn1 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 B37 Ht1 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 B37 Ht2 , B37 Ht3 , and B37 Htn1 (54 - 81%) at 5 dpi. Curiously, the reduction in k for B37 and B37 Ht1 (up to 95%) was higher at 14 dpi then at 5 dpi (up to 81%). At 6 dpi, low levels of H2 O2 were detected in B37 Ht1 , in contrast to B37 Htn1 , where a high H2 O2 level and peroxidase activity were observed. Sporulation rate on B37 Ht1 , B37 Ht3 and B37 Htn1 decreased by 92% compared to the susceptible control, while strong sporulation occurred in lesions on line B37 Ht2 . 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.