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Maize Tumors Caused by Ustilago maydis Require Organ-Specific Genes in Host and Pathogen

Tailor-Made Tumor The biotrophic smut pathogen Ustilago maydis specifically infects the important crop plant, maize. The pathogen elicits large tumors on all aerial maize organs by redirecting primordia into a tumor pathway, and maize developmental mutants can disrupt tumor progression. Skibbe et al. (p. 89) examined gene expression in parallel in both the host plant and the smut pathogen and found that organ-specific gene expression patterns were required in both for tumor formation. Thus, fungal pathogens may exert distinct effects on different organs and tissues in plants, perhaps explaining the diverse pathologies that can be produced in diseased plants. Transcriptionally different expression occurs between infected maize tissues and the corn smut infecting these tissues. Infection of maize by corn smut (Ustilago maydis) provides an agronomically important model of biotrophic host-pathogen interactions. After penetration of the maize epidermis, fungal colonization of host tissue induces tumor formation on all aerial maize organs. We hypothesized that transformation of different primordia into plant tumors would require organ-specific gene expression by both host and pathogen and documented these differences by transcriptome profiling. Phenotypic screening of U. maydis mutants deleted for genes encoding secreted proteins and maize mutants with organ-specific defects confirmed organ-restricted tumorigenesis. This is the foundation for exploring how individual pathogen effectors, deployed in an organ-specific pattern, interact with host factors to reprogram normal ontogeny into a tumor pathway.