Osmotic adjustment in the filamentous fungus Aspergillus nidulans

Abstract

Aspergillus nidulans was shown to be xerotolerant, with optimal radial growth on basal medium amended with 0.5 M NaCl (osmotic potential [*,] of medium, -3 MPa), 50% optimal growth on medium amended with 1.6 M NaCl (*s of medium, -8.7 MPa), and little growth on medium amended with 3.4 M NaCl (*, of medium, -21 MPa). The intracellular content of soluble carbohydrates and of selected cations was measured after growth on basal medium, on this medium osmotically amended with NaCl, KCI, glucose, or glycerol, and also after hyperosmotic and hypoosmotic transfer. The results implicate glycerol and erythritol as the major osmoregulatory solutes. They both accumulated during growth on osmotically amended media, as well as after hyperosmotic transfer, except on glycerol-amended media, in which erythritol did not accumulate. Furthermore, they both decreased in amount after hypoosmotic transfer. With the exception of glycerol, the extracellular osmotic solute did not accumulate intracellularly when mycelium was grown in osmotically amended media, but it accumulated after hyperosmotic transfer. It was concluded that the extracellular solute usually plays only a transient role in osmotic adaptation. The intracellular content of soluble carbohydrates and cations measured could reasonably account for the intracellular osmotic potential of mycelium growing on osmotically amended media.The ability of microorganisms to grow under conditions of low osmotic (solute) potential has received considerable attention (5, 6, 10). For microorganisms with a cell wall, it is accepted that for active growth the internal water potential must be slightly lower than that external to the cell, so that water will tend to flow into the cell and generate the turgor potential needed for growth. For such microorganisms to grow under conditions of low osmotic potential, this low internal potential is generated largely by the intracellular accumulation of one or more low-molecular-weight solutes or osmoregulators. Although any solute will contribute to the water potential, only a restricted group, the so-called compatible solutes, can accumulate to high concentrations without interfering with enzyme activity and intracellular metabolism.We became interested in the mechanisms whereby ascomycetous filamentous fungi adapt to osmotic stress after we found, with three species, that most mutants resistant to the dicarboximide group of agricultural fungicides are abnormally sensitive to low osmotic potential (3). Because the mode of action of the dicarboximides, which include iprodione, procymidone, and vinclozolin, is not yet known (4), it was anticipated that an understanding of how these fungi adapt to osmotic stress would aid studies of their mode of action. Little data are available on osmotic adaptation in the ascomycetous fungi, although recent studies implicate glycerol as the major osmoregulator in this group (9,12,18,19). In this study, we report on osmotic adaptation in a wild-type strain of the fungus Aspergillus nidulans. MATERIALS AND METHODSOrgan...

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