Molecular Dissection of Stomatal Infection in the Maize Foliar Pathogen Cercospora zeae-maydis Through Comparative and Functional Genomics Burt H. Bluhm* Department of Plant Pathology, University of Arkansas, Division of Agriculture, Fayetteville, Arkansas 72701, USA * For correspondence, e-mail: [email protected] Abstract Cercospora zeae-maydis causes gray leaf spot, a ubiquitous and often devastating foliar disease of maize. The infection biology underlying gray leaf spot is complex: C. zeae-maydis senses host stomata, which it then grows towards and utilizes as portals of entry into maize leaves, followed by a latent, hemi-biotrophic phase preceding the induction of necrosis. Our focus is to understand the molecular basis of infection in C. zeae-maydis and related Dothidiomycete fungi, with the overarching goal of using this information to improve disease management. To dissect pathogenesis, we have employed a combination of techniques in molecular genetics and functional genomics, which was substantially accelerated by the public release of the C. zeae-maydis genome sequence by DOE-JGI in 2011. Recently, through reverse genetics, we established a genetic linkage between light sensing, the innate circadian clock of C. zeae-maydis, and stomatal perception by the fungus. Additionally, we identified putative epigenetic regulators governing the transition to hemi-biotrophy through a forward genetic screen (tagged mutants with deficiencies in appressorium formation) combined with differential expression profiling via RNA-seq (genes highly expressed in C. zeae-maydis during apressorium formation). The forward genetic component was facilitated by whole-genome resequencing of selected mutants to precisely characterize the site(s) of disruption. Furthermore, we are utilizing comparative genomics to identify putative regulators of pathogenesis in C. zeae-maydis. To this end, we have recently obtained high-quality draft genome sequences for C. zeina (a sibling species of C. zeae-maydis that also causes gray leaf spot of maize), C. sojina (causal agent of frogeye leaf spot of soybean), and C. kikuchii (causal agent of Cercospora leaf blight of soybean). Of particular interest are genes undergoing diversifying selection, displaying patterns of presence/absence that correlate with host range or other relevant parameters, or are unique at the genus or species level. Although many questions remain to be answered, our findings to date have provided the most complete understanding thus far of stomatal infection by a plant pathogenic Ascomycete. Importantly, our findings have led to the formulation of new hypotheses regarding how C. zeae-maydis senses stomata, the genetic and morphogenic basis of the transition to hemi-biotrophy, and mechanisms through which necrosis is induced during the final stages of pathogenesis.