The role of vegetative cell fusions in the lifestyle of the wheat fungal pathogenZymoseptoria tritici

The ability of fungal cells to undergo cell fusion allows them to maximize their overall fitness. In this study, we characterized the role of the so gene orthologous in Zymoseptoria tritici and the biological contribution of vegetative cell fusions in the lifestyle of this latent necrotrophic fungus. Firstly, we show that Z. tritici undergoes self-fusion between distinct cellular structures and its mechanism is dependent on the initial cell density. Next, the deletion of ZtSo resulted in the loss of cell-to-cell communication affecting both hyphal and germlings fusion. We show that Z. tritici mutants for MAP kinase-encoding ZtSlt2 (orthologous MAK-1) and ZtFus3 (orthologous MAK-2) genes also fail to undergo self-stimulation and self-fusion, demonstrating the functional conservation of this signaling mechanism across species. Additionally, the ΔZtSo mutant was severely impaired in melanization, which leads us to identify a trade-off between fungal growth and melanization. Though it has been proposed that So is a scaffold protein for MAP kinase genes from the CWI pathway, its deletion did not affect the cell wall integrity of the fungus. Finally, we demonstrated that anastomose is dispensable for pathogenicity, but essential for the fruiting body development and its absence abolish the asexual reproduction of Z. tritici . Taken together, our data show that ZtSo is required for fungal development, while vegetative cell fusions are essential for fungal fitness.