The ability to adapt to altered availability of free water is a fundamental property of living cells. The principles underlying osmoadaptation are well conserved. The yeast Saccharomyces cerevisiae is an excellent model system with which to study the molecular biology and physiology of osmoadaptation. Upon a shift to high osmolarity, yeast cells rapidly stimulate a mitogen-activated protein (MAP) kinase cascade, the high-osmolarity glycerol (HOG) pathway, which orchestrates part of the transcriptional response. The dynamic operation of the HOG pathway has been well studied, and similar osmosensing pathways exist in other eukaryotes. Protein kinase A, which seems to mediate a response to diverse stress conditions, is also involved in the transcriptional response program. Expression changes after a shift to high osmolarity aim at adjusting metabolism and the production of cellular protectants. Accumulation of the osmolyte glycerol, which is also controlled by altering transmembrane glycerol transport, is of central importance. Upon a shift from high to low osmolarity, yeast cells stimulate a different MAP kinase cascade, the cell integrity pathway. The transcriptional program upon hypo-osmotic shock seems to aim at adjusting cell surface properties. Rapid export of glycerol is an important event in adaptation to low osmolarity. Osmoadaptation, adjustment of cell surface properties, and the control of cell morphogenesis, growth, and proliferation are highly coordinated processes. The Skn7p response regulator may be involved in coordinating these events. An integrated understanding of osmoadaptation requires not only knowledge of the function of many uncharacterized genes but also further insight into the time line of events, their interdependence, their dynamics, and their spatial organization as well as the importance of subtle effects.

Osmotic Stress Signaling and Osmoadaptation in Yeasts

A fellow of my acquaintance, on seeing a colleague drink undiluted water (55.5 molal), has been known to comment in disapproval that water at such a concentration should not be used for that purpose and that its main function is for putting around the outside of boats. He conceded that dilution with a little salt is acceptable for boats but for no other purpose. The proponent of this philosophy is not a biologist and it is unlikely that many biologists would accept his generalization without some qualification. Nevertheless, it is a point of view. Another point of view with which all biologists might not agree, at least initially, is one which I wish to advance in this review. It is that, notwithstanding the indispensability of water in living systems and the unique properties of solvent water, quantitative variations in the amount of water available are of less direct microbiological significance than is generally conceded.

https://ro.uow.edu.au/cgi/viewcontent.cgi?article=1991&context=smhpapers

Microbial water stress.

Publisher Summary This chapter emphasizes the measurement of plant water potential following the realization of the importance of atmospheric demand in determining plant water deficits. The total water potential concept determines the movement of water through the soil-plant-atmosphere continuum in response to gradients in water potential and of the soil, plant, and atmospheric factors that influence the development of water deficits in plants. A water deficit occurs whenever water loss exceeds absorption. The use of total water potential as the best single indicator of plant water status has its limitations while attempting to understand the effect of water deficits on the various physiological processes involved in plant growth. The chapter discusses the effects of water deficits on crop growth, crop development, and crop yield. The differences in response of plants grown under controlled conditions and in the field are discussed. Some difficulties in extrapolating from controlled environments to the field are presented in the chapter.

Crop Water Deficits

MOST microorganisms show optimal growth at low osmotic pressures, but even so are often able to grow, though more slowly, in the presence of concentrations of environmental solutes sufficiently high to reduce the water activity to values as low as 0.86 (Table 1). The ability of osmophilic yeasts to grow at low water activities depends on the accumulation of high concentrations of polyhydric alcohols inside the cell1, and similarly, halophilic bacteria accumulate potassium ions2. Amino acids, especially proline, have been shown to stimulate growth3 and respiration4 of some bacteria at low water activities, and to accumulate in response to increased environmental sodium chloride5 and, more particularly, to osmotic dehydration (my unpublished results). I now report that growth of non-halophilic bacteria at low water activities seems to depend on the ability of the cell to balance the environmental osmotic pressure by intracellular accumulation of amino acids, and on the types of amino acid which accumulate.

Role of amino acids in osmoregulation of non-halophilic bacteria

Publisher Summary The response of a fungus to osmotic stress involves the integrated function of many components of cell metabolism. The dehydration stress is countered by an important mechanism that entails accumulation of polyols, primarily glycerol, to achieve an internal environment that is conducive for enzyme function and growth under water stress. The changes in the composition of the cytoplasm are controlled by systems for biosynthesis of polyols and for transport of inorganic ions. The intracellular retention of glycerol is controlled at the level of glycerol efflux and by systems for glycerol uptake. The osmoregulatory processes require energy to drive transport and carbon supply for polyol formation. The capacity for substrate uptake under osmotic stress and the efficiency operating the osmoregulatory processes are important in setting the limits for growth at low water potentials. The fungal response to changes in the external water potential must involve sensing as well as transduction of the received signal. Combined genetic and physiological analysis is required for a deeper understanding of fungus-water relations. Analysis at this level has revealed sequential induction of osmotically controlled genes in enteric bacteria and given exciting insights in signal transduction and regulation of the process.

Physiology of osmotolerance in fungi

Several factors contributed to differences in intracellular composition between sugar-tolerant (osmophilic) and nontolerant species of yeast. One such factor was the difference in accumulation of those nonelectrolytes whose uptake was not dominated by vigorous metabolism. In such cases (lactose and glycerol), the sugar-tolerant species had a much lower capacity for the solute than did the nontolerant species. Sucrose uptake was consistently different between all sugar-tolerant strains on the one hand and all nontolerant strains on the other. The difference was attributable in part to metabolism of sucrose by the nontolerant yeasts. The major difference between the two types of yeast, however, was the presence of one or more polyhydric alcohols at high concentrations within each of the sugar-tolerant strains but none of the nontolerant strains. In most cases the major polyol was arabitol. The solute concentration (and, hence, water availability) of the growth medium affected both the amount of arabitol produced by Saccharomyces rouxii and the proportion retained by the yeast after brief washing with water at 0 C. When the yeast was suspended in a buffer at 30 C, the polyol leaked out at a slow, constant, reproducible rate. The polyene antibiotic amphotericin B caused rapid release of polyol by the yeast, the rate being proportional to amphotericin concentration. Contact of the yeast with glucose (1 mM) caused an extremely rapid ejection of polyol which lasted less than 40 s. Some implications of these results are discussed, as is the role of the polyol as a compatible solute in determining the water relations of the yeast.

A. D. Brown

Papers

Microbial Water Relations: Features of the Intracellular Composition of Sugar-Tolerant Yeasts