Dissecting the roles of osmolyte accumulation during stress

Abstract

Many plants accumulate organic osmolytes in response to the imposition of environmental stresses that cause cellular dehydration. Although an adaptive role for these compounds in mediating osmotic adjustment and protecting subcellular structure has become a central dogma in stress physiology, the evidence in favour of this hypothesis is largely correlative. Transgenic plants engineered to accumulate proline, mannitol, fructans, trehalose, glycine betaine or ononitol exhibit marginal improvements in salt and/or drought tolerance. While these studies do not dismiss causative relationships between osmolyte levels and stress tolerance, the absolute osmolyte concentrations in these plants are unlikely to mediate osmotic adjustment. Metabolic benefits of osmolyte accumulation may augment the classically accepted roles of these compounds. In re-assessing the functional significance of compatible solute accumulation, it is suggested that proline and glycine betaine synthesis may buffer cellular redox potential. Disturbances in hexose sensing in transgenic plants engineered to produce trehalose, fructans or mannitol may be an important contributory factor to the stress-tolerant phenotypes observed. Associated effects on photoassimilate allocation between root and shoot tissues may also be involved. Whether or not osmolyte transport between subcellular compartments or different organs represents a bottleneck that limits stress tolerance at the whole-plant level is presently unclear. None the less, if osmolyte metabolism impinges on hexose or redox signalling, then it may be important in long-range signal transmission throughout the plant.Key-words: betaine; cold stress; drought; fructans; mannitol; osmolytes; proline; salinity; sugar signalling; trehalose. Abbreviations INTRODUCTIONEnvironmental stress is the major factor limiting plant productivity . Abiotic stresses which cause depletion of cellular water (drought, high soil salinity and temperature extremes) are responsible for the greatest agricultural losses. Upon exposure to these prevalent stresses, many plants accumulate organic osmolytes, most commonly polyhydroxylic compounds (saccharides and polyhydric alcohols) and zwitterionic alkylamines (amino acids and quaternary ammonium compounds).Several recent reviews discuss osmolyte accumulation in plants (Ingram & Bartels 1996;Serrano 1996). It is generally accepted that the increase in cellular osmolarity which results from the accumulation of non-toxic (thus 'compatible') osmotically active solutes is accompanied by the influx of water into, or at least a reduced efflux from, cells, thus providing the turgor necessary for cell expansion. None the less, a conclusive demonstration that osmotic adjustment contributes to fitness in stressful environments has yet to be achieved (Munns 1993). Since all subcellular structures must exist in an aqueous environment, tolerance to dehydration also depends on the ability of cells to maintain membrane integrity and prevent protein denaturation. Hypotheses that attribut...

Proline, Sugars, and Antioxidant Enzymes Respond to Drought Stress in the Leaves of Strawberry Plants

“…Maintaining water potential for plants under water deficit is important for growth. This can be realized by accumulating of compatible cytoplasmic solutes such as soluble sugars, proline, organic acids, and glycine betaine (Hare et al, 1998). The most important plant response to drought and other abiotic stress is one of the excessive productions of different types of metabolites and solute (Ashraf and Harris, 2008).…”

“…Proline and total sugars accumulate to high levels in drought-stressed plants of many speciess (Hessini et al, 2008). These solutes are hypothesized to benefit drought-stressed plant cells by acting as the cell osmotic regulation substances and contribute to water retention (Hare et al, 1998) and achieving the protection and stabilization of macromolecules from drought-stress induced damage (Bohnert and Jensen, 1996). Proline is compatible solutes in the protection for the folded protein structure against denaturation, interacts with phospholipids to stabilize cell membranes, function as a hydroxyl radical scavenger, and play a role as a source of energy and nitrogen (Samaras, 1995).…”

Over‐expression of the water and salt stress‐regulated Asr1 gene confers an increased salt tolerance

“…Up-regulated salt stress operates in almost all aspects of plant function and metabolism including signal transduction, ion homeostasis, carbohydrate and nitrogen metabolism, photosynthesis, growth and development. In addition to abiotic stress-induced signal transduction (Rock 2000;Shinozaki & Yamaguchi-Shinozaki 2000;Xiong & Zhu 2001), special interest has been focused on enzymes catalysing the synthesis of compatible solutes, that is, small molecules which accumulate in response to stress (Hare, Cress & Van Staden 1998;Bohnert & Shen 1999), protective proteins (Hasegawa et al . 2000), enzymes involves in the scavenging of reactive oxygen species (ROS) which accumulate after exposure to stress (Bohnert & Sheveleva 1998), and ion and water transport systems (Blumwald 2000;Johansson et al .…”

Shoot Epicatechin and Epigallocatechin Contents Respond to Water Stress in Tea [Camellia sinensis(L.) O. Kuntze]

“…Plants are known to accumulate organic osmolytes such as proline, glycine betaine, non-reducing sugars, and polyols 1,2) in response to stress factors. Though these organic compounds are species-specific their role is not clearly defined, but it is generally accepted that they contribute to ameliorating stress in plants.…”

“…Though these organic compounds are species-specific their role is not clearly defined, but it is generally accepted that they contribute to ameliorating stress in plants. [2][3][4] Most stress-related organic compounds are secondary plant metabolites, and tea (Camellia sinensis) contains large amounts of polyphenols, mainly catechins, that belong to the flavan-3-ol class. Flavonoids play a key role in quality determination in black tea, 5) and in fruits, 6) but their role as indicators of desiccation tolerance in tea has not been explored.…”