Effect of extracellular calcium chloride on sporangiospore-yeast transformation of Rhizopus stolonifer

Abstract

Previous studies showed that zinc and myoinositol, which are intracellular, serve as components of signalling elements in many eukaryotes, and participate in stimulating the induction and proliferation of yeast cells from sporangiospores of fungi. It was thought that a transmembrane-pH-gradient, similar to what is obtained in bacterial mitochondrion or proton-substrate symport in yeasts, permitted influx/efflux of materials into the cell that triggered the induction and subsequent proliferation of yeast cells. To examine this model further, this study evaluated the ability of sporangiospores of Rhizopus stolonifer to undergo morphogenetic transformation in the presence of different levels of extracellular calcium (0.0, 0.20, 0.25, 0.50, 1.0, 1.5 and 1.8 mM). It was found that calcium supported yeast induction and proliferation to varying extent. Ca²⁺ at 0.50 and1.8 mM, which was outstanding, was compared with broth treatment at 0.25 mM ZnSO₄: pH 4.5 which gave optimum biomass in a previous study. Medium pH was further varied: pH 4.2, 4.5 and 5.0. The results showed that there was interaction between the level of divalent cations and pH. At all levels tested, Ca²⁺ was better stimulatory than Zn²⁺. Direct induced yeast cell count, which eliminated possible impact of protoplasts and calcium phosphate precipitate on optical density measurements, made these results more valid. Treatment at 0.50 mM Ca²⁺: pH 5.0 gave the most profound result. This was therefore chosen for further biochemical work.

Key words: Rhizopus stolonifer, synthetic broth, dimorphism, extracellular Ca²⁺, sporangiospores, induced yeast cells.