Sporangiospore-yeast transformation of Mucor circinelloides: Ionic circulation, chemical potential, sodium influx rate and morphogenesis

Abstract

Measurement of intracellular ion concentration during sporangiospores-yeast transformation of Mucor circinelloides Tieghem in K⁺- mediated (0.90 to 1.10 g/l) and Na⁺-modulated (0.05 to 0.20 g/l) multiionic broths, pH 4.5, temperature 20°C, showed that (a), transmembrane ion flux was continuous during the growth period (b), Na⁺ efflux varied concurrently with K⁺ influx and vice versa (c), induced morphologies varied: growth spheres, enterothallic-, holothallic-, holoblastic- conidia, septate hyphae with vesicular conidial head groups, and terminal budding yeast cells but protoplasts and granular particles were transient (d), time-course plots of log transformed optical density measurements gave sigmoid growth pattern principally at 1.0 g/l K⁺ and 0.10 g/l Na⁺ incorporation where the induced morphology was mainly terminal budding yeast but primary and secondary growth optima occurred at the other levels and these were thought to result from determinate thallic growth: septate hyphae transformed into enterothallic conidia; cytosolic nucleates/granular particles could be extruded after thallic cell wall lyses or conidial burst, which then collapsed; the granular units thereafter became protoplasts and could assume the yeast morphology (e), ionic profiles showed correlation with growth phases: lag phase was accompanied with Na⁺ efflux with simultaneous influx of K⁺; exponential growth phase- rapid influx of Na⁺ and concurrent efflux of K⁺; stationary phase- rapid efflux of Na⁺ with simultaneous, albeit slow intracellular accumulation of K⁺ . It was strongly suggested that ion fluxes were mediated by the P-type, plasma-membrane localized, cation ATPases. This study shows that morphological expression was correlated with the intracellular ion contents. The chemical potentials (m) were calculated at the pre- logarithmic growth phases and these influenced cytosolic nucleation. As the magnitude of m increased, cytosolic nucleates were less readily formed. Apparently, a balance of Na⁺ and K⁺, deputed as critical concentration (cc), was required for cytosolic nucleation. But transition from protoplasts to the yeast form was thought to be dependent on the magnitude of the Na⁺ influx rate (x). We conclude that highly functional cooperativity existed between m, x, as well as cc, for successful transformation of sporangiospores to terminal budding yeast cells.

Key words: Cytosolic nucleation, ionic flux, critical concentration, chemical potential.