▪ 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.

Photoreduction of dioxygen in photosystem I (PSI) of chloroplasts generates superoxide radicals as the primary product. In intact chloroplasts, the superoxide and the hydrogen peroxide produced via the disproportionation of superoxide are so rapidly scavenged at the site of their generation that the active oxygens do not inactivate the PSI complex, the stromal enzymes, or the scavenging system itself. The overall reaction for scavenging of active oxygens is the photoreduction of dioxygen to water via superoxide and hydrogen peroxide in PSI by the electrons derived from water in PSII, and the water-water cycle is proposed for these sequences. An overview is given of the molecular mechanism of the water-water cycle and microcompartmentalization of the enzymes participating in it. Whenever the water-water cycle operates properly for scavenging of active oxygens in chloroplasts, it also effectively dissipates excess excitation energy under environmental stress. The dual functions of the water-water cycle for protection from photoinihibition are discussed.

THE WATER-WATER CYCLE IN CHLOROPLASTS: Scavenging of Active Oxygens and Dissipation of Excess Photons

Roles of glycine betaine and proline in improving plant abiotic stress resistance

Salt tolerance and salinity effects on plants: a review.

In the last decade, our understanding of the processes underlying plant response to drought, at the molecular and whole-plant levels, has rapidly progressed. Here, we review that progress. We draw attention to the perception and signalling processes (chemical and hydraulic) of water deficits. Knowledge of these processes is essential for a holistic understanding of plant resistance to stress, which is needed to improve crop management and breeding techniques. Hundreds of genes that are induced under drought have been identified. A range of tools, from gene expression patterns to the use of transgenic plants, is being used to study the specific function of these genes and their role in plant acclimation or adaptation to water deficit. However, because plant responses to stress are complex, the functions of many of the genes are still unknown. Many of the traits that explain plant adaptation to drought - such as phenology, root size and depth, hydraulic conductivity and the storage of reserves - are those associated with plant development and structure, and are constitutive rather than stress induced. But a large part of plant resistance to drought is the ability to get rid of excess radiation, a concomitant stress under natural conditions. The nature of the mechanisms responsible for leaf photoprotection, especially those related to thermal dissipation, and oxidative stress are being actively researched. The new tools that operate at molecular, plant and ecosystem levels are revolutionising our understanding of plant response to drought, and our ability to monitor it. Techniques such as genome-wide tools, proteomics, stable isotopes and thermal or fluorescence imaging may allow the genotype-phenotype gap to be bridged, which is essential for faster progress in stress biology research.

Understanding plant responses to drought — from genes to the whole plant

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.

Strategies for engineering water-stress tolerance in plants

The accumulation of sugar alcohols and other low molecular weight metabolites such as proline and glycine-betaine is a widespread response that may protect against environmental stress that occurs in a diverse range of organisms. Transgenic tobacco plants that synthesize and accumulate the sugar alcohol mannitol were engineered by introduction of a bacterial gene that encodes mannitol 1 -phosphate dehydrogenase. Growth of plants from control and mannitol-containing lines in the absence and presence of added sodium chloride was analyzed. Plants containing mannitol had an increased ability to tolerate high salinity.

https://science.sciencemag.org/content/sci/259/5094/508.full.pdf

Stress protection of transgenic tobacco by production of the osmolyte mannitol.

Hydroxyl radicals may be responsible for oxidative damage during drought or chilling stress. We have shown that the presence of mannitol in chloroplasts can protect plants against oxidative damage by hydroxyl radicals (B. Shen, R.G. Jensen, H.J. Bohnert [1997] Plant Physiol 113: 1177-1183). Here we identify one of the target enzymes that may be protected by mannitol. Isolated thylakoids in the presence of physiological concentrations of Fe2+ generated hydroxyl radicals that were detected by the conversion of phenylalanine into tyrosine. The activity of phosphoribulokinase (PRK), a thiol-regulated enzyme of the Calvin cycle, was reduced by 65% in illuminated thylakoids producing hydroxyl radicals. Mannitol (125 mM) and sodium formate (15 mM), both hydroxyl radical scavengers, and catalase (3000 units mL-1) prevented loss of PRK activity. In contrast, superoxide dismutase (300 units mL-1) and glycine betaine (125 mM) were not effective in protecting PRK against oxidative inactivation. Ribulose-1,5-bisphosphate carboxylase/oxygenase activity was not affected by hydroxyl radicals. We suggest that the stress-protective role of mannitol may be to shield susceptible thiol-regulated enzymes like PRK plus thioredoxin, ferredoxin, and glutathione from inactivation by hydroxyl radicals in plants.

Mannitol Protects against Oxidation by Hydroxyl Radicals.

To investigate the potential role of a polyol, mannitol, in oxidative stress protection, a bacterial mannitol-1-phosphate dehydrogenase gene was targeted to chloroplasts by the addition of an amino-terminal transit peptide. Transgenic tobacco (Nicotiana tabacum) lines accumulate mannitol at concentrations ranging from 2.5 to 7 [mu]mol/g fresh weight. Line BS1-31 accumulated approximately 100 mM mannitol in chloroplasts and was identical to the wild type in phenotype and photosynthetic performance. The presence of mannitol in chloroplasts resulted in an increased resistance to methyl viologen (MV)-induced oxidative stress, documented by the increased retention of chlorophyll in transgenic leaf tissue following MV treatment. In the presence of MV, isolated mesophyll cells of BS1-31 exhibited higher CO2 fixation than the wild type. When the hydroxyl radical probe dimethyl sulfoxide was introduced into cells, the initial formation rate of methane sulfinic acid was significantly lower in cells containing mannitol in the chloroplast compartment than in wild-type cells, indicating an increased hydroxyl radical-scavenging capacity in BS1-31 tobacco. We suggest that the chloroplast location of mannitol can supplement endogenous radical-scavenging mechanisms and reduce oxidative damage of cells by hydroxyl radicals.

http://www.plantphysiol.org/content/plantphysiol/113/4/1177.full.pdf

Richard G. Jensen

Papers

Adaptations to Environmental Stresses.

Strategies for engineering water-stress tolerance in plants

Stress protection of transgenic tobacco by production of the osmolyte mannitol.

Mannitol Protects against Oxidation by Hydroxyl Radicals.

Increased Resistance to Oxidative Stress in Transgenic Plants by Targeting Mannitol Biosynthesis to Chloroplasts