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Incompatibility between proliferation and plant invasion is mediated by a regulator of appressorium formation in the corn smut fungus Ustilago maydis

Significance Many phytopathogenic fungi need specialized structures, called appressoria, to penetrate the plant before infection starts. For these structures, the fungus must carefully control when and where its cells divide, and it has been reported in different fungal phytopathogens that appressorium formation is subordinated to the cell cycle control. However, the molecular details about how this control operates were unknown. We described in the corn smut fungus U. maydis that Biz1, a regulator for the formation of appressorium, is phosphorylated by the kinase responsible for cell cycle progression. Biz1 is part of a negative feedback loop responsible for the final decision to invade or not the plant tissue. Targeting this process will lead to the development of antipenetrant fungicides. Plant pathogenic fungi often developed specialized infection structures to breach the outer surface of a host plant. These structures, called appressoria, lead the invasion of the plant by the fungal hyphae. Studies in different phytopathogenic fungi showed that appressorium formation seems to be subordinated to the cell cycle. This subordination ensures the loading in the invading hypha of the correct genetic information to proceed with plant infection. However, how the cell cycle transmits its condition to the genetic program controlling appressorium formation and promoting the plant’s invasion is unknown. Our results have uncovered how this process occurs for the appressorium of Ustilago maydis, the agent responsible for corn smut disease. Here, we described that the complex Clb2-cyclin-dependent kinase (Cdk)1, one of the master regulators of G2/M cell cycle progression in U. maydis, interacts and controls the subcellular localization of Biz1, a transcriptional factor required for the activation of the appressorium formation. Besides, Biz1 can arrest the cell cycle by down-regulation of the gene encoding a second b-cyclin Clb1 also required for the G2/M transition. These results revealed a negative feedback loop between appressorium formation and cell cycle progression in U. maydis, which serves as a “toggle switch” to control the fungal decision between infecting the plant or proliferating out of the plant.