Alternaria brassicicola – Brassicaceae pathosystem: insights into the infection process and resistance mechanisms under optimized artificial bio-assay

Abstract

Heavy losses incited yearly by Alternaria brassicicola on the vegetable Brassicaceae have prompted our search for sources of genetic resistance against the pathogen and the resultant disease, dark leaf spot. We optimized several parameters to test the performance of the plants under artificial inoculations with this pathogen, including leaf age and position, inoculum concentration, and incubation temperature. Using these optimized conditions, we screened a collection of 38 Brassicaceae cultigens with two methods (detached leaf and seedlings). Our results show that either method can be used for the A. brassicicola resistance breeding, and that the plant genotype was crucial in determining its response to the pathogen. The bio-assays for A. brassicicola resistance were run under more stringent lab conditions than the field tests (natural epidemics), resulting in identification of two interspecific hybrids that might be used in breeding programs. Based on the results of the biochemical analyses, reactive oxygen species and red-ox enzymes interplay has been suggested to determine the outcome of the plant-A. brassicicola interplay. Confocal microscopy analyses of the leaf samples provided data on the pathogen mode of infection: Direct epidermal infection or stomatal attack were related to plant resistance level against A. brassicicola among the cultigens tested. Further, the microscopic analyses suggested rapid actin network activation of the host cells around the papillas deposited under the pathogen appressorium.