▪ Abstract Plant responses to salinity stress are reviewed with emphasis on molecular mechanisms of signal transduction and on the physiological consequences of altered gene expression that affect biochemical reactions downstream of stress sensing. We make extensive use of comparisons with model organisms, halophytic plants, and yeast, which provide a paradigm for many responses to salinity exhibited by stress-sensitive plants. Among biochemical responses, we emphasize osmolyte biosynthesis and function, water flux control, and membrane transport of ions for maintenance and re-establishment of homeostasis. The advances in understanding the effectiveness of stress responses, and distinctions between pathology and adaptive advantage, are increasingly based on transgenic plant and mutant analyses, in particular the analysis of Arabidopsis mutants defective in elements of stress signal transduction pathways. We summarize evidence for plant stress signaling systems, some of which have components analogous to t...

Plant cellular and molecular responses to high salinity.

Salt tolerance and salinity effects on plants: a review.

Cold, salinity and drought stresses: an overview.

The human ATP-binding cassette (ABC) transporter superfamily

Environmental stresses come in many forms, yet the most prevalent stresses have in common their effect on plant water status. The availability of water for its biological roles as solvent and transport medium, as electron donor in the Hill reaction, and as evaporative coolant is often impaired by environmental conditions. Although plant species vary in their sensitivity and response to the decrease in water potential caused by drought, low temperature, or high salinity, it may be assumed that all plants have encoded capability for stress perception, signaling, and response. First, most cultivated species have wild relatives that exhibit excellent tolerance to abiotic stresses. Second, biochemical studies have revealed similarities in processes induced by stress that lead to accumulated metabolites in vascular and nonvascular plants, algae, fungi, and bacteria (Csonka, 1989; Galinski, 1993; Potts, 1994). These metabolites include nitrogen-containing compounds (proline, other amino acids, quaternary amino compounds, and polyamines) and hydroxyl compounds (sucrose, polyols, and oligosaccharides) (McCue and Hanson, 1990). Accumulation of any single metabolite is not restricted to taxonomic groupings, indicating that these are evolutionarily old traits. Third, molecular studies have revealed that a wide variety of species express a common set of genes and similar proteins (for example, Rab-related proteins and dehydrins) when stressed (Skriver and Mundy, 1990; Vilardell et al., 1994). Although functions for many of these genes have not yet been unequivocally assigned, it is likely, based on their characteristics, that these proteins play active roles in the response to stress. Learning about the biochemical and molecular mechanisms by which plants tolerate environmentat stresses is necessary for genetic engineering approaches to improving crop performance under stress. By investigating plants under stress, we can learn about the plasticity of metabolic pathways and the limits to their functioning. Also, questions of an ecological and evolutionary nature need investigation. Are the genes that confer salt tolerance on halophytes and/or drought tolerance on xerophytes evolutionarily ancient genes that have been selected against in saltand drought-sensitive plants (glycophytes) for the sake of productivity? Or have some species obtained nove1 genes in their evolutionary history that have enabled them to occupy stressful environments? How will the

Adaptations to Environmental Stresses.

Trehalose-6-phosphate synthase, catalyzing the reaction between UDP-glucose and glucose 6-phosphate and forming trehalose 6-phosphate, was isolated and partially purified (30-fold) from the phototrophic, haloalkaliphilic bacteriumEctothiorhodospira halochloris. The activity is stabilized by 20mM MgCl2, 50mM NaCe and 2M glycine betaine. The molecular weight was 63000. The enriched enzyme had a MgCl2 optimum at 3–6mM, a pH optimum at 7.5 (in Tris-HCl buffer) and a temperature optimum at 50°C. The Km-values were 1.5×10−3M for UDP-glucose and 2×10−3M for glucose 6-phosphate. The enzyme showed a salinity dependence with optimal concentrations between 100 and 300mM salt. Higher concentrations of salt resulted in a decrease in activity. In the presence of inhibitory salt concentrations the compatible solute glycine betaine had a protective effect with a maximum between 0.5 and 2.0M.

Biosynthesis and function of trehalose in Ectothiorhodospira halochloris

Trehalase, which hydrolyzes the disaccharide trehalose to α-d-glucose was isolated and partially purified (124-fold) from the phototrophic halo-alkaliphilic bacterium Ectothiorhodospira halochloris. The molecular mass was determined to be 480,000 and the isoelectric point pH 5.6. Temperature optimum was found to be 40°C and the pH-optimum 7.8–8.1. In spite of its high Km-value of 0.5 M, trehalase of E. halochloris was shown to be specific for trehalose. Trehalase is activated by phosphate which is, however, not involved in the reaction mechanism. The enzyme is activated by the compatible solute betaine and inhibited by salts. In the presence of betaine the Km-value is lowered from 0.5 M to 0.16 M; moreover, betaine partially protects enzymatic activity from salt inhibition. The findings indicate that betaine might regulate the trehalose level in the cells by affecting trehalase activity.

Degradation of the compatible solute trehalose in Ectothiorhodospira halochloris: isolation and characterization of trehalase

The aim of this work was to study the physiological response of Lactobacillus plantarum subjected to osmotic stress in the presence of three structurally related compatible solutes. Either betaine, choline or acetylcholine was accumulated by osmotically stressed cells when provided in the chemically defined medium. Choline and acetylcholine were accumulated to maximum concentrations of 139 and 222 μmol g (dry weight) of cells−1 respectively and were not converted to betaine. Addition of 0.5 mM choline or 0.5 mM acetylcholine to the medium increased the growth rates of cells in media with various amounts of added sodium chloride. Both choline and acetylcholine are positively charged compounds; therefore, it was presumed that charged intracellular solutes could counterbalance the excess of positive charge. Intracellular inorganic ion levels (K+, SO2−4, PO3−4 and Cl−) of cells cultured under conditions of osmotic stress remained similar in the presence of either betaine, choline or acetylcholine. However, cells cultured in the presence of choline or acetylcholine accumulated an additional quantity of approximately 125 or 200 μmol.glutamate (dry weight) cells−1 respectively, as compared to cells grown in the presence of betaine. Hence glutamate appears to be the counterion for choline and acetylcholine. This is the first study demonstrating accumulation of choline and acetylcholine in lactic acid bacteria subjected to osmotic stress.

Choline and acetylcholine: novel cationic osmolytes in Lactobacillus plantarum

On a first view there is apparently little correlation between taxonomy and halophily The distribution of different compatible solutes in the eubacterial phyla Proteobacteria and Firmacutes follows a pattern, however, that at least follows certain regularities: (1) All eubacteria that gain energy from photosynthesis or respiration and are capable of haloadaptation are able to accumulate and/or synthesize compatible solutes (2) When a eubacterium can grow in a non-complex medium, it usually can synthesize ectoine Growth on yeast extract does not necessarily exclude ectoine synthesis (3) Extreme halophily in eubacteria is always accompanied by glycine betaine synthesis The complete synthesis of glycine betaine from CO2 or simple carbon compounds has only been proven for cyanobacteria, Ectothiorhodospiraceae and Actinopolyspora halophila (4) A not yet identified compound “Y” occurs preferably in Firmacutes; Bacillus species accumulate and synthesize proline and glutamate as compatible solutes

Halophily, Taxonomy, Phylogeny and Nomenclature

In the course of a screening programme for novel natural substances acting as compatible solutes, an unusual amino acid derivative was detected in several halophilic Gram-positive eubacteria. The compound was isolated and subsequently identified as N
δ-acetylornithine using spectroscopic (NMR, MS) and chromatographic methods. So far, N
δ-acetylornithine has only been described as an excretion product of some bacteria under certain growth conditions. This report describes, for the first time, its intracellular accumulation and use as a compatible solute in halophilic/halotolerant eubacteria.

Erwin A. Galinski

Papers

Biosynthesis and function of trehalose in Ectothiorhodospira halochloris

Degradation of the compatible solute trehalose in Ectothiorhodospira halochloris: isolation and characterization of trehalase

Choline and acetylcholine: novel cationic osmolytes in Lactobacillus plantarum

Halophily, Taxonomy, Phylogeny and Nomenclature

Identification of N -acetylornithine as a novel osmolyte in some Gram-positive halophilic eubacteria