Ca2+ Signalling Differentially Regulates Germ-Tube Formation and Cell Fusion in Fusarium oxysporum.

Smija M Kurian,Alexander Lichius,Nick D Read

doi:10.3390/jof8010090

Abstract

Fusarium oxysporum is an important plant pathogen and an emerging opportunistic human pathogen. Germination of conidial spores and their fusion via conidial anastomosis tubes (CATs) are significant events during colony establishment in culture and on host plants and, hence, very likely on human epithelia. CAT fusion exhibited by conidial germlings of Fusarium species has been postulated to facilitate mitotic recombination, leading to heterokaryon formation and strains with varied genotypes and potentially increased virulence. Ca2+ signalling is key to many of the important physiological processes in filamentous fungi. Here, we tested pharmacological agents with defined modes of action in modulation of the mammalian Ca2+ signalling machinery for their effect on germination and CAT-mediated cell fusion in F. oxysporum. We found various drug-specific and dose-dependent effects. Inhibition of calcineurin by FK506 or cyclosporin A, as well as chelation of extracellular Ca2+ by BAPTA, exclusively inhibit CAT induction but not germ-tube formation. On the other hand, inhibition of Ca2+ channels by verapamil, calmodulin inhibition by calmidazolium, and inhibition of mitochondrial calcium uniporters by RU360 inhibited both CAT induction and germ-tube formation. Thapsigargin, an inhibitor of mammalian sarco/endoplasmic reticulum Ca2+ ATPase (SERCA), partially inhibited CAT induction but had no effect on germ-tube formation. These results provide initial evidence for morphologically defining roles of Ca2+-signalling components in the early developmental stages of F. oxysporum colony establishment—most notably, the indication that calcium ions act as self-signalling molecules in this process. Our findings contribute an important first step towards the identification of Ca2+ inhibitors with fungas-specific effects that could be exploited for the treatment of infected plants and humans.

Highlights

Ca2+ signalling is known to play a major role in regulating important morphogenetic and physiological processes in filamentous fungi
We have reported the absence of conidial anastomosis tubes (CATs) fusion as an inhibition of the first stage in CAT-mediated cell fusion— which is CAT induction/CAT formation—because we did not find any condition involving the tested pharmacological agents in which CATs were formed but were unable to fuse
As CCH1 is a component of highaffinity Ca2+ uptake system (HACS), which functions under low extracellular Ca2+ concentrations, our results indicate the requirement of a higher extracellular Ca2+ concentration for CAT induction

Summary

Introduction

Ca2+ signalling is known to play a major role in regulating important morphogenetic and physiological processes in filamentous fungi. With cell-symmetry breaking through the emergence of a germ tube and/or CAT as the initiating process, is the first step in the life cycle of most filamentous fungi, including the economically important fungal pathogen Fusarium oxysporum. Microconidia of F. oxysporum undergo cell-cell fusion via conidial anastomosis tubes (CATs), which has been postulated to promote efficient colonization and horizontal gene transfer (HGT), leading to heterokaryon formation [12,13]. The process of HGT is important, as it can lead to strains with acquired or altered virulence and resistance [14]. This is important in the context of acquiring resistance to antifungal drugs.

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