Synchronization of oscillatory growth prepares fungal hyphae for fusion

Abstract

Communication is crucial for organismic interactions, from bacteria, to fungi, to humans. Humans may use the visual sense to monitor the environment before starting acoustic interactions. In comparison, fungi lack a visual system, instead, hyphae use a cell-to-cell dialogue based on secreted signaling molecules to orchestrate cell fusion and establish hyphal networks. Hyphae alternate roles as signal-sender and signal-receiver, as can be visualized via the putative signaling protein, Soft, which is recruited in an oscillatory manner to the respective cytoplasmic membrane of interacting hyphae. Here, we show that signal oscillations already occur in single hyphae of Arthrobotrys flagrans in the absence of a potential fusion partner. They occurred in the same phase as growth oscillations. Once two fusion partners came into each others vicinity, their oscillation frequencies slowed down (entrainment phase) and transit into anti-phasic synchronization of the two cells oscillations with frequencies of 130 +/-20 sec. Single-cell oscillations, transient entrainment, and anti-phasic oscillations were reproduced by a mathematical model where nearby hyphae can absorb and secrete a limited molecular signaling component into a shared extra-cellular space. We show that intracellular Ca2+ concentrations oscillate in two approaching hyphae, and depletion of Ca2+ in the surrounding affected vesicle-driven extension of the hyphal tip, abolished single-cell molecular oscillations and the anti-phasic synchronization of two hyphae. Our results suggest that single hyphae engage in a "monologue" that may be used for exploration of the environment and can dynamically shift their extra-cellular signaling systems into a "dialogue" to initiate hyphal fusion. Significance statementCommunication at the cellular level often relies on chemical signal exchange. One prominent example is the fusion of fungal hyphae to form complex hyphal networks. As opposed to mating-type dependent cell fusion, cell-fusion events described here occur in genetically identical cells. Relying only on one chemical signaling channel raises the question of how communication is initiated. We discovered that individual hyphae constantly perform signal oscillations, comparable to a cellular "monologue" until they meet another hypha with which they then coordinate signal oscillations in a cell-to-cell dialogue. We also show that signal oscillations are mechanistically interlinked with calcium-dependent growth oscillations. Although the signaling molecule(s) has not been identified yet, it is highly likely linked to the hyphal growth machinery.