PmaI-dependent pH gradients initiate inter- and intraspecies germling fusion in filamentous fungi

Somatic cell fusion among spore germlings is a complex process in filamentous fungi that enables the formation of a syncytial unit and facilitates the exchange of genetic material apart from sexual reproduction. In this study, we investigated the molecular mechanisms underlying germling fusion in the maize pathogen Colletotrichum graminicola . Based on secretomes harvested during the germling fusion process of fusion-competent oval and fusion-deficient falcate conidia, we identified pH as a communication signal. We showed that the plasma membrane localized ATPase PmaI is a probable source for pH gradient formation. Our results also suggest that the a-pheromone receptor CgSte3 mediates sensing of acidic pH gradients, activating downstream the Cell Wall Integrity Mitogen Activated Protein Kinase pathway. We propose a new model for germling interactions, in which pH serves as an interaction initiation signal, resulting in the activation of the cell-dialogue. Our findings have implications for our understanding of fungal communication and pathogenicity, and highlight the importance of pH as a signal molecule in germling fusion.