▪ 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.

Na+ Tolerance and Na+ Transport in Higher Plants

Halophytes, plants that survive to reproduce in environments where the salt concentration is around 200 mM NaCl or more, constitute about 1% of the worlds flora. Some halophytes show optimal growth in saline conditions; others grow optimally in the absence of salt. However, the tolerance of all halophytes to salinity relies on controlled uptake and compartmentalization of Na+, K+ and Cl- and the synthesis of organic compatible solutes, even where salt glands are operative. Although there is evidence that different species may utilize different transporters in their accumulation of Na+, in general little is known of the proteins and regulatory networks involved. Consequently, it is not yet possible to assign molecular mechanisms to apparent differences in rates of Na+ and Cl- uptake, in root-to-shoot transport (xylem loading and retrieval), or in net selectivity for K+ over Na+. At the cellular level, H+-ATPases in the plasma membrane and tonoplast, as well as the tonoplast H+-PPiase, provide the transmembrane proton motive force used by various secondary transporters. The widespread occurrence, taxonomically, of halophytes and the general paucity of information on the molecular regulation of tolerance mechanisms persuade us that research should be concentrated on a number of model species that are representative of the various mechanisms that might be involved in tolerance.

Salinity tolerance in halophytes

Hydroxyl radical scavenging activity of compatible solutes

Lang, Florian, Gillian L. Busch, Markus Ritter, Harald Volkl, Siegfried Waldegger, Erich Gulbins, and Dieter Haussinger. Functional Significance of Cell Volume Regulatory Mechanisms. Physiol. Rev. ...

Functional Significance of Cell Volume Regulatory Mechanisms

Physiological Effects of Gibberellic Acid: I. On Carbohydrate Metabolism and Amylase Activity of Barley Endosperm

Barley (Hordeum vulgare L. var. Prior) leaves converted more 14C-glutamic acid to free proline when water-stressed than when turgid; neither decreased protein synthesis nor isotope trapping by the enlarged free proline pools found in wilted tissue seemed to account for the result. This apparent stimulation of proline biosynthesis in wilted leaves was not observed when radioactive ornithine or P5C (Δ1-pyrroline-5-carboxylate, an intermediate following glutamate in proline synthesis) were used as proline precursors unless proline levels were high as a result of previous water stress. We interpret this to mean that any stimulation of proline synthesis by water stress must act on P5C formation rather than its reduction to proline. Experiments showing greater apparent conversion of 14C-glutamate to proline do not unequivocally prove that proline synthesis is stimulated by water stress, as P5C feeding studies show that proline oxidation is inhibited under comparable conditions. This inhibition could account, at least in part, for increased proline labeling, and must be considered an alternate possibility.

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Effect of Water Stress on Proline Synthesis from Radioactive Precursors

The conversion of proline to glutamic acid and hence to other soluble compounds (proline oxidation) proceeds readily in turgid barley (Hordeum vulgare) leaves and is stimulated by higher concentrations of proline. This suggests that proline oxidation could function as a control mechanism for maintaining low cellular levels of proline in turgid tissue. In water-stressed tissue, however, proline oxidation is reduced to negligible rates. These results are consistent with the idea that proline accumulation results from inactivation by water stress of normal control mechanisms. It seems likely that inhibition of proline oxidation is necessary in maintaining the high levels of proline found in stressed barley leaves.

Inhibition of proline oxidation by water stress.

Two reports have appeared (15, 22) dealing with the effects of gibberellic acid (GA) on intact germinating barley. Both studies were carried out in an attempt to determine whether or not GA influenced the malting behavior of the barley, and extended the earlier observations of Hayashi (6) that GA speeded up germination and the various processes associated with germination, resulting in an enhancement of malt extract yield. GA, in both reports, markedly stimulated a-amylase activity and increased the growth of the leaf. Sandegren and Beling (22) came to the conclusion that the reduction in germination time (or, in other words, the hastening of starch hydrolysis, etc.) was due to GA's effect on embryo growth. The first paper of this series (19) presented evidence of the production of reducing sugars and an increase in amylase activity resulting from the treatment of barley endosperm with GA. Although it has not yet been possible to demonstrate a direct relationship between increased amylase activity and the production of reducing sugars in the intact grain, several results, such as anion and heat effects, and the increased liberation of maltose, lend circumstantial support to the importance of the role of amylase activity in starch hydrolysis in the isolated endosperm. These results also lead to the suggestion that an endogenous gibberellin, such as that present in germinating barley (21), might, in fact, play an important hormonal role during the germination process, and prompted a closer investigation of the changes in starch hydrolyzing activity of solutions obtained from water, and GA-treated barley endosperm. It should be borne in mind that the results discussed here were obtained with barley endosperm alone, i.e., after the embryo had been removed.

Leslie G. Paleg

Papers

Physiological Effects of Gibberellic Acid: I. On Carbohydrate Metabolism and Amylase Activity of Barley Endosperm

Effect of Water Stress on Proline Synthesis from Radioactive Precursors

Inhibition of proline oxidation by water stress.

Physiological Effects of Gibberellic Acid. II. On Starch Hydrolyzing Enzymes of Barley Endosperm

Amino acid and glycine betaine accumulation in cold-stressed wheat seedlings