Molecular responses of maize to its foliar pathogen, Cercospora zeina

Abstract

The defence response of maize (Zea mays) to its foliar pathogen, Cercospora zeina, is not well characterized at the molecular and genetic level. C. zeina causes grey leaf spot (GLS), and high infection levels result in reduced crop yield. A molecular genetic study of the interaction between maize and C. zeina required a method to quantify the fungus within infected leaves. A quantitative PCR assay was developed for this purpose. It was based on amplification of a fungal cytochrome P450 reductase (cpr1) gene fragment, which was normalized using an amplified maize glutathione S-transferase III (gst) gene fragment. The assay was specific to C. zeina and a related species, Cercospora zeae-maydis, not yet found in Africa. In addition, a melt curve analysis enabled discrimination between these species. There was no amplification from a range of other maize foliar pathogens. The fungal quantification assay was successfully tested on glasshouse grown maize inoculated with C. zeina, and infected field grown maize. This assay can be implemented to quantify C. zeina at early stages of infection due to its sensitivity, and in the field due to its specificity. The fungal quantification assay was used to map GLS resistance QTL from a field trial of a sub-tropical maize RIL population in KwaZulu-Natal. We hypothesized that QTL involved in limiting C. zeina growth maize in leaves could be detected using in planta fungal quantity and lesion area data from digital image analysis, and that they would correspond with GLS resistance QTL previously mapped in the same population. Three QTL were mapped for in planta fungal quantity and one for lesion area. The strongest effect QTL was located on chromosome six. It was detected using both the fungal quantification assay and the lesion area data, and overlapped with a GLS resistance QTL from the same population in the same environment. Thus QTL can be successfully mapped from in planta fungal quantification and lesion area data and these QTL correspond to GLS resistance QTL. The molecular response of maize to C. zeina was studied using expression profiling of a pooled bulk of resistant RILs versus a pooled bulk of susceptible RILs from the same population used for GLS QTL mapping. We aimed to find genes with differential expression between the resistant and susceptible bulks during GLS field infection. Additionally, we hypothesized that genes contributing to the QTL effect could be identified within QTL. Candidate genes for the resistance response included an RPP13-like gene and an mlo gene. The genome positions of the differentially expressed genes were compared to the genome positions of the QTL. Candidate genes coinciding with QTL included a malectin containing RLK, an EDR1-like gene, a GTPase gene, a cytochrome b561 gene, and a chorismate synthase gene. Comparison of gene ontologies from all the genes differentially expressed between the resistant and susceptible bulks indicated that cell death was a likely strategy for resistance. The resistant response is probably an early response, with a later and continuing response to biotic stress in the susceptible maize plants.