Diversity and biosynthesis of compatible solutes in hyper/thermophiles.

The accumulation of compatible solutes, either by uptake from the medium or by de novo synthesis, is a general response of microorganisms to osmotic stress. The diversity of compatible solutes is large but falls into a few major chemical categories, such as carbohydrates or their derivatives and amino acids or their derivatives. This review deals with compatible solutes found in thermophilic or hyperthermophilic bacteria and archaea that have not been commonly identified in microorganisms growing at low and moderate temperatures. The response to NaCl stress of Thermus thermophilus is an example of how a thermophilic bacterium responds to osmotic stress by compatible solute accumulation. Emphasis is made on the pathways leading to the synthesis of mannosylglycerate and glucosylglycerate that have been recently elucidated in several hyper/thermophilic microorganisms. The role of compatible solutes in the thermoprotection of these fascinating microorganisms is also discussed.     

Exogenously supplied compatible solutes rapidly ameliorate NaCl-induced potassium efflux from barley roots

It has been suggested that the role of compatible solutes in plant stress responses is not limited to conventional osmotic adjustment, but also includes some other regulatory or osmoprotective functions. In this study, we hypothesized that one such function is in maintaining cytosolic K+ homeostasis by preventing NaCl-induced K+ leakage from the cell, a feature that may confer salt tolerance in many species, particularly in barley. This hypothesis was investigated using the non-invasive microelectrode ion flux (MIFE) measuring technique. We show that low (0.5-5 mM) concentrations of exogenously supplied proline or betaine significantly reduced NaCl-induced K+ efflux from barley roots in a dose-response manner. This effect was instantaneous, implying that large intracellular concentrations of compatible solutes are not required for an amelioratory role. Exogenously supplied betaine also significantly enhanced NaCl-induced H+ efflux, but only in pre-incubated roots, implying some alternative mechanism of regulation. Sap K+ and Na+ analysis and membrane potential measurements are also consistent with the model that one function of compatible solutes is in maintaining cytosolic K+ homeostasis by preventing NaCl-induced K+ leakage from the cell, possibly through the enhanced activity of H+-ATPase, controlling voltage-dependent outward-rectifying K+ channels and creating the electrochemical gradient necessary for secondary ion transport processes. These data provide the first direct evidence for regulation of ion fluxes across the plasma membrane by physiologically relevant low concentrations of compatible solutes.     

Microbial behaviour in salt-stressed ecosystems

Abstract: Salt stress is primarily osmotic stress, and halophilic/halotolerant microorganisms have evolved two basic mechanisms of osmoadaplation: the KCI-type and the compatible-solute type, the latter representing a very flexible mode of adaptation making use of distinct stabilizing properties of compatible solutes. A comprehensive survey, using HPLC and NMR methods, has revealed the full diversity of euhacterial compatible solutes found in nature. With the exception of proline (a proteinogenic amino acid) they are characterized as amino acid derivatives of the following types: betaines, ectoines, N-acetylated diamino acids and N-derivatized carboxamides of glutamine. From our present knowledge of hiosynthetic pathways it appears that, apart from glycine betaine, all nitrogen-containing compatible solutes originate from two major pathways (the aspartate branch and the glutamate branch). Uptake of compatible solutes from the growth medium (environment) seems to have preference over de novo synthesis. Therefore in the natural ecosystem the solutes of primary producers (mainly glycine betaine), which are readily excreted upon dilution stress, certainly play an important role as a 'preferred' solute source for heterolrophic organisms, and as a 'vital' source for organisms unable to synthesize their own compatible solutes.     

Osmoadaptation mechanisms in prokaryotes: distribution of compatible solutes

Microorganisms respond to osmotic stress mostly by accumulating compatible solutes, either by uptake from the medium or by de novo synthesis. These osmotically active molecules preserve the positive turgor pressure required for cell division. The diversity of compatible solutes is large but falls into a few major chemical categories; they are usually small organic molecules such as amino acids or their derivatives, and carbohydrates or their derivatives. Some are widely distributed in nature while others seem to be exclusively present in specific groups of organisms. This review discusses the diversity and distribution of known classes of compatible solutes found in prokaryotes as well as the increasing knowledge of the genes and pathways involved in their synthesis. The alternative roles of some archetypal compatible solutes not subject to osmoregulatory constraints are also discussed.     

Activity regulation of the betaine transporter BetP of Corynebacterium glutamicum in response to osmotic compensation

As a response to hyperosmotic stress bacterial cells accumulate compatible solutes by synthesis or by uptake. Beside the instant activation of uptake systems after an osmotic upshift, transport systems show also a second, equally important type of regulation. In order to adapt the pool size of compatible solutes in the cytoplasm to the actual extent of osmotic stress, cells down-regulate solute uptake when the initial osmotic stress is compensated. Here we describe the role of the betaine transporter BetP, the major uptake carrier for compatible solutes in Corynebacterium glutamicum, in this adaptation process. For this purpose, betP was expressed in cells (C. glutamicum and Escherichia coli), which lack all known uptake systems for compatible solutes. Betaine uptake mediated by BetP as well as by a truncated form of BetP, which is deregulated in its response to hyperosmotic stress, was dissected into the individual substrate fluxes of unidirectional uptake, unidirectional efflux and net uptake. We determined a strong decrease of unidirectional betaine uptake by BetP in the adaptation phase. The observed decrease in net uptake was thus mainly due to a decrease of Vmax of BetP and not a consequence of the presence of separate efflux system(s). These results indicate that adaptation of BetP to osmotic compensation is different from activation by osmotic stress and also different from previously described adaptation mechanisms in other organisms. Cytoplasmic K+, which was shown to be responsible for activation of BetP upon osmotic stress, as well as a number of other factors was ruled out as triggers for the adaptation process. Our results thus indicate the presence of a second type of signal input in the adaptive regulation of osmoregulated carrier proteins.     

Identification of trehalose as a compatible solute in different species of acidophilic bacteria

The major industrial heap bioleaching processes are located in desert regions (mainly Chile and Australia) where fresh water is scarce and the use of resources with low water activity becomes an attractive alternative. However, in spite of the importance of the microbial populations involved in these processes, little is known about their response or adaptation to osmotic stress. In order to investigate the response to osmotic stress in these microorganisms, six species of acidophilic bacteria were grown at elevated osmotic strength in liquid media, and the compatible solutes synthesised were identified using ion chromatography and MALDI-TOF mass spectrometry. Trehalose was identified as one of, or the sole, compatible solute in all species and strains, apart from Acidithiobacillus thiooxidans where glucose and proline levels increased at elevated osmotic potentials. Several other potential compatible solutes were tentatively identified by MALDITOF analysis. The same compatible solutes were produced by these bacteria regardless of the salt used to produce the osmotic stress. The results correlate with data from sequenced genomes which confirm that many chemolithotrophic and heterotrophic acidophiles possess genes for trehalose synthesis. This is the first report to identify and quantify compatible solutes in acidophilic bacteria that have important roles in biomining technologies.     

叶绿体对盐胁迫的某些生理适应机制

本文介绍了近些年来有关叶绿体对盐胁迫的一些生理适应机制的研究,包括：（１）叶绿体的离子调节;（２）叶绿体的渗透调节;（３）叶绿体内相容溶质对光合酶的保护作用。最后讨论了这方面研究所存在的、今后急需要研究解决的几个问题。     

Physiology of halophytes

Summary The cellular basis of salt tolerance in halophytes depends upon the compartmentation of ions necessary for osmoregulation in vacuoles and upon osmotic adjustment of the cytoplasm by compatible solutes. The central role played by Na⁺ and Cl^– in osmotic adjustment suggests that the transport of these ions and its regulation must be of primary importance in the physiology of the plant as a whole. There have been few investigations into the regulation of leaf ion concentrations, but such data as are in the literature suggest that limiting xylem Na⁺ (and Cl^–) concentrations, together with continued leaf expansion, are particularly important. The role of phloem in retranslocation is uncertain due to lack of data. Decreases in transpiration rate per unit area of leaf help to lower the ion input into leaves. Any linked reductions in photosynthesis appear to be due to decreases in stomatal frequency.     

Solutes Involved in Osmotic Adjustment to Increasing Salinity in Suspension Cells of Alternanthera philoxeroides Griseb

Cell recovery from osmotic stress was studied in suspension cell cultures from Alternanthera philoxeroides [Mart.] Griseb. Changes in different classes of cellular solutes were measured after cells were transferred from 0 to 200 mM NaCl (high salt) to obtain an integrated picture of the solute pools involved in osmotic adjustment. By 2 h, cellular [Na⁺] and [Cl⁻] had increased several-fold, potentially accounting for the osmotic adjustment that produced a rapid recovery of cell turgor. There was a four-fold increase in the concentration of quaternary ammonium compounds (QAC) by 12 h and a slower increase for several days afterward. Betaine aldehyde dehydrogenase (BADH) is required for synthesis of glycine betaine, a QAC produced by a range of organisms in response to osmotic stress. Western-blot analysis for BADH suggested that glycine betaine was a significant component of the QAC solutes. The amount of BADH was generally similar at different sampling times for control and high salt cells, unlike previous reports of stimulation by osmotic stress in intact plants of some species. Between 3 and 7 days after cell transfer to high salt, other organic solutes increased in concentration and [Na⁺] and [Cl⁻] decreased. In A. philoxeroides, high [Na⁺] and [Cl⁻] produce rapid osmotic adjustment but organic solutes apparently replace these potentially harmful inorganic ions after the recovery of turgor.     

Activity regulation of the betaine transporter BetP of Corynebacterium glutamicum in response to osmotic compensation

As a response to hyperosmotic stress bacterial cells accumulate compatible solutes by synthesis or by uptake. Beside the instant activation of uptake systems after an osmotic upshift, transport systems show also a second, equally important type of regulation. In order to adapt the pool size of compatible solutes in the cytoplasm to the actual extent of osmotic stress, cells down-regulate solute uptake when the initial osmotic stress is compensated. Here we describe the role of the betaine transporter BetP, the major uptake carrier for compatible solutes in Corynebacterium glutamicum, in this adaptation process. For this purpose, betP was expressed in cells (C. glutamicum and Escherichia coli), which lack all known uptake systems for compatible solutes. Betaine uptake mediated by BetP as well as by a truncated form of BetP, which is deregulated in its response to hyperosmotic stress, was dissected into the individual substrate fluxes of unidirectional uptake, unidirectional efflux and net uptake. We determined a strong decrease of unidirectional betaine uptake by BetP in the adaptation phase. The observed decrease in net uptake was thus mainly due to a decrease of V_max of BetP and not a consequence of the presence of separate efflux system(s). These results indicate that adaptation of BetP to osmotic compensation is different from activation by osmotic stress and also different from previously described adaptation mechanisms in other organisms. Cytoplasmic K⁺, which was shown to be responsible for activation of BetP upon osmotic stress, as well as a number of other factors was ruled out as triggers for the adaptation process. Our results thus indicate the presence of a second type of signal input in the adaptive regulation of osmoregulated carrier proteins.     

Accumulation of the compatible solutes, glycine-betaine and ectoine, in osmotic stress adaptation and heat shock cross-protection in the biocontrol agent Pantoea agglomerans CPA-2

AIMS: The effect of modifying the water activity (a(w)) of Pantoea agglomerans growth medium with the ionic solute NaCl on water stress resistance, heat-shock survival and intracellular accumulation of the compatible solutes glycine-betaine and ectoine were determined. METHODS AND RESULTS: The bacterium was cultured in an unmodified liquid medium or that modified with NaCl to 0.98 and 0.97 a(w), and viability of cells evaluated on a 0.96 a(w)-modified solid media to check water stress tolerance. Cells grown under ionic stress had better water stress tolerance than control cells. These cells also had cross-protection to heat stress (30 min, 45 degrees C). The modified cells accumulated substantial amounts of the compatible solutes glycine-betaine and ectoine in contrast to the control cells, which contained little or none of these two compounds. CONCLUSIONS: Improvement in osmotic and thermal tolerance of cells of the biocontrol agent P. agglomerans by modifying growth media with the ionic solute NaCl was achieved. The compatible solutes glycine-betaine and ectoine play a critical role in environmental stress tolerance improvement. SIGNIFICANCE AND IMPACT OF THE STUDY: This approach provides a method for improving the physiological quality of inocula and could have implications for formulation and shelf-life of biocontrol agents.     

Compatible solute biosynthesis in cyanobacteria

Compatible solutes are a functional group of small, highly soluble organic molecules that demonstrate compatibility in high amounts with cellular metabolism. The accumulation of compatible solutes is often observed during the acclimation of organisms to adverse environmental conditions, particularly to salt and drought stress. Among cyanobacteria, sucrose, trehalose, glucosylglycerol and glycine betaine are used as major compatible solutes. Interestingly, a close correlation has been discovered between the final salt tolerance limit and the primary compatible solute in these organisms. In addition to the dominant compatible solutes, many strains accumulate mixtures of these compounds, including minor compounds such as glucosylglycerate or proline as secondary or tertiary solutes. In particular, the accumulation of sucrose and trehalose results in an increase in tolerance to general stresses such as desiccation and high temperatures. During recent years, the biochemical and molecular basis of compatible solute accumulation has been characterized using cyanobacterial model strains that comprise different salt tolerance groups. Based on these data, the distribution of genes involved in compatible solute synthesis among sequenced cyanobacterial genomes is reviewed, and thereby, the major compatible solutes and potential salt tolerance of these strains can be predicted. Knowledge regarding cyanobacterial salt tolerance is not only useful to characterize strain-specific adaptations to ecological niches, but it can also be used to generate cells with increased tolerance to adverse environmental conditions for biotechnological purposes.     

Solute compatibility with enzyme function and structure: rationales for the selection of osmotic agents and end-products of anaerobic metabolism in marine invertebrates.

The major nitrogenous osmolytes present in the cells of marine invertebrates, notably the free amino acids glycine, alanine and proline, and trimethylamine oxide and betaine, are highly compatible with proper enzyme function and structure. These nitrogenous osmolytes display either non-perturbing or, in some cases, favorable effects on enzyme-substrate and enzyme-cofactor complex formation, catalytic velocity and protein structural stability. In contrast, inorganic salts (KCl and NaCl) and certain of the free amino acids which play only a minor osmotic role, e.g., arginine and lysine, have strongly perturbing effects on one or more of these enzymic parameters. The compatible nitrogenous solutes therefore are suitable for use at high (several tenths molar) concentrations and at widely varying concentrations in osmo-conforming species. Certain nitrogenous solutes, especially trimethylamine oxide, betaine and glutamate, offset some of the perturbing effects of inorganic ions on enzyme function. The selective accumulation of osmolytes thus involves not only the concentration of non-perturbing solutes, but also a balanced accumulation of solutes with opposing effects on enzymes. The selection of end-products of anaerobic metabolism also appears to be based, in part, on considerations of solute compatibility with enzyme function. Octopine is a non-perturbing solute, whereas arginine, which is condensed with pyruvate to form octopine, is very strongly perturbing. Succinate has marked stabilizing effects on protein structure. We conclude that the composition of the intracellular fluids of marine invertebrates reflects selection for osmolytes and end-products whose net effects create a cellular microenvironment which is conducive to optimal enzyme function and structure. The accumulation of compatible solutes may preclude the necessity for widespread changes in protein structure in adapting to concentrated or highly variable osmotic environments.     

Heterosides – compatible solutes occurring in prokaryotic and eukaryotic phototrophs

The acclimation to osmotic and/or salt stress conditions induces an integrated response at different cellular levels. One acclimation strategy relies on the massive accumulation of low molecular mass compounds, so‐called compatible solutes, to balance osmotic gradients and to directly protect critical macromolecules. Heterosides are compounds composed of a sugar and a polyol moiety that represent one chemical class of compatible solutes with interesting features. Well‐investigated examples are glucosylglycerol, which is found in many cyanobacteria, and galactosylglycerols (floridoside and isofloridoside), which are accumulated by eukaryotic algae under salt stress conditions. Here, we review knowledge on physiology, biochemistry and genetics of heteroside accumulation in pro‐ and eukaryotic photoautotrophic organisms.     

Compatible solute addition to biological systems treating waste/wastewater to counteract osmotic and other environmental stresses: a review

This study reviews the addition of compatible solutes to biological systems as a strategy to counteract osmolarity and other environmental stresses. At high osmolarity many microorganisms accumulate organic solutes called “compatible solutes” in order to balance osmotic pressure between the cytoplasm and the environment. These organic compounds are called compatible solutes because they can function inside the cell without the need for special adaptation of the intracellular enzymes, and also serve as protein stabilizers in the presence of high ionic strength. Moreover, the compatible solutes strategy is regularly being employed by the cell, not only under osmotic stress at high salinity, but also under other extreme environmental conditions such as low temperature, freezing, heat, starvation, dryness, recalcitrant compounds and solvent stresses. The accumulation of these solutes from the environment has energetically a lower cost than de novo synthesis. Based on this cell mechanism several studies in the field of environmental biotechnology (most of them on biological wastewater treatment) employed this strategy by exogenously adding compatible solutes to the wastewater or medium in order to alleviate environmental stress. This current paper critically reviews and evaluates these studies, and examines the future potential of this approach. In addition to this, a strategy for the successful implementation of compatible solutes in biological systems is proposed.     

乳酸菌渗透胁迫相关相容性溶质及其转运机制研究进展

渗透胁迫严重影响乳酸菌的生理功能, 制约着相关产品开发。近年来相容性溶质作为响应乳酸菌渗透胁迫的重要功能物质受到广泛关注。本文综述了国内外与乳酸菌响应渗透胁迫相关相容性物质的种类和特性, 在此基础上进一步介绍了其转运机制, 并对后续研究进行展望。     

Stress response by solute accumulation in archaea

The accumulation of organic solutes is a prerequisite for osmotic adjustment of all organisms. Archaea synthesize unusual solutes such as beta-amino acids, Nepsilon-acetyl-beta-lysine, mannosylglycerate and di-myo-inositol phosphate but, as in other cells, uptake of solutes such as glycine betaine is preferred over de novo synthesis. Study of the molecular basis of osmoadaptation and its regulation in archaea is still in its infancy, but genomics and functional genome analyses combined with classical biochemistry shed light on the processes that confer osmoadaptation in archaea. Most interestingly, some solutes are not only produced in response to salt but also to temperature stress.     

An osmoregulated dipeptide in stressed Rhizobium meliloti.

One common mechanism of cellular adaptation to osmotic stress is the accumulation of organic solutes in the cytosol. We have used natural-abundance 13C nuclear magnetic resonance to identify all organic solutes that accumulate to significant levels in Rhizobium meliloti. Our studies led to the discovery of a new dipeptide, N-acetylglutaminylglutamine amide (NAGGN), which is accumulated during osmotic stress. Only rarely have peptides been shown to function in bacteria, and furthermore, this is the first example of a peptide playing a role in osmoregulation. Evidence for the biological role of NAGGN in osmotic-stress protection is presented.