Involvement of Ca2+ in the regulation of apical growth and branching in the pathogenic fungus Aspergillus fumigatus
Calcium has been shown to play important roles in the control of tip growth in a wide range of cell types. This thesis sets out to test the effects of exogenous Ca2+ on various morphological events during the life cycle of the human pathogenic fungus Aspergillus fumigatus. The Ca2+ chelating agent EGTA induced marked morphological effects on colonies of A.fumigatus. That is radial growth rate decreased and branching frequency increased with increasing concentration of EGTA. Ion substitution experiments indicated that these effects were due to the chelation of Ca2+ and not Mg2+, Zn 2+, Cu2+ or Fe2+. However, Mn2+ was able to substitute for Ca2+. Calcium ions were required for germ tube emergence but not for the preceding period of spherical growth in spores of A.fumigatus. That is the rate of spherical growth was independent of exogenous [Ca2+ ] but germ tube emergence was retarded and spores became more swollen as exogenous Ca2+ decreased. The effect of applied electrical fields on spores of A.fumigatus was investigated. Germ tubes were anodotropic and polarisation increased with increasing field strength. The galvanotropic response was dependent on Ca2+ and polarisation decreased with exogenous [Ca2+ ]. The effect of exogenous [Ca2+] on growth kinetics of colonies of A.fumigatus growing on solid medium was investigated. When colonies were incubated on medium buffered at pCa 4-8 (10-4-10 -8 M) hyphal growth unit length and mean hyphal extension rate decreased linearly with the log of Ca2+ concentration. In contrast the specific growth rate remained constant over the range pCa 4-7 and was only reduced when colonies were incubated in medium buffered at pCa 8. This suggests that exogenous Ca2+ acts on processes that govern apical growth and branching but do not affect growth per se. The effect of a broad range of Ca2+-channel blockers and calmodulin antagonists on the growth and morphology of colonies of A.fumigatus were also investigated.