The physiological and molecular mechanisms of tolerance to osmotic and ionic components of salinity stress are reviewed at the cellular, organ, and whole-plant level. Plant growth responds to salinity in two phases: a rapid, osmotic phase that inhibits growth of young leaves, and a slower, ionic phase that accelerates senescence of mature leaves. Plant adaptations to salinity are of three distinct types: osmotic stress tolerance, Na + or Cl − exclusion, and the tolerance of tissue to accumulated Na + or Cl − . Our understanding of the role of the HKT gene family in Na + exclusion from leaves is increasing, as is the understanding of the molecular bases for many other transport processes at the cellular level. However, we have a limited molecular understanding of the overall control of Na + accumulation and of osmotic stress tolerance at the whole-plant level. Molecular genetics and functional genomics provide a new opportunity to synthesize molecular and physiological knowledge to improve the salinity tolerance of plants relevant to food production and environmental sustainability.

https://rodas5.us.es/file/c2a700e9-8b4a-1970-d343-4a269c302970/1/estres_salino_bibliografia_scorm.zip/files/estres_salino_bibliografia.pdf

Mechanisms of salinity tolerance

We discuss the physical and enzymatic bases of carbone isotope discrimination during photosynthesis, noting how knowledge of discrimination can be used to provide additional insight into photosynthetic metabolism and the environmental influences on that process

Carbon Isotope Discrimination and Photosynthesis

Plant responses to salt and water stress have much in common. Salinity reduces the ability of plants to take up water, and this quickly causes reductions in growth rate, along with a suite of metabolic changes identical to those caused by water stress. The initial reduction in shoot growth is probably due to hormonal signals generated by the roots. There may be salt-specific effects that later have an impact on growth; if excessive amounts of salt enter the plant, salt will eventually rise to toxic levels in the older transpiring leaves, causing premature senescence, and reduce the photosynthetic leaf area of the plant to a level that cannot sustain growth. These effects take time to develop. Salttolerant plants differ from salt-sensitive ones in having a low rate of Na + and Cl ‐ transport to leaves, and the ability to compartmentalize these ions in vacuoles to prevent their build-up in cytoplasm or cell walls and thus avoid salt toxicity. In order to understand the processes that give rise to tolerance of salt, as distinct from tolerance of osmotic stress, and to identify genes that control the transport of salt across membranes, it is important to avoid treatments that induce cell plasmolysis, and to design experiments that distinguish between tolerance of salt and tolerance of water stress.

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.0016-8025.2001.00808.x

Comparative physiology of salt and water stress

Salt and drought stress signal transduction consists of ionic and osmotic homeostasis signaling pathways, detoxification (i.e., damage control and repair) response pathways, and pathways for growth regulation. The ionic aspect of salt stress is signaled via the SOS pathway where a calcium-responsive SOS3-SOS2 protein kinase complex controls the expression and activity of ion transporters such as SOS1. Osmotic stress activates several protein kinases including mitogen-activated kinases, which may mediate osmotic homeostasis and/or detoxification responses. A number of phospholipid systems are activated by osmotic stress, generating a diverse array of messenger molecules, some of which may function upstream of the osmotic stress-activated protein kinases. Abscisic acid biosynthesis is regulated by osmotic stress at multiple steps. Both ABA-dependent and -independent osmotic stress signaling first modify constitutively expressed transcription factors, leading to the expression of early response transcriptional activators, which then activate downstream stress tolerance effector genes.

/pdf/salt-and-drought-stress-signal-transduction-in-plants-4bj87ki2ww.pdf

Salt and drought stress signal transduction in plants

▪ 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.

The role of shoot water status in mediating the decline in leaf elongation rate of nitrogen (N)-deprived barley plants was assessed. Plants were grown at two levels of N supply, with or without the application of pneumatic pressure to the roots. Applying enough pressure (balancing pressure) to keep xylem sap continuously bleeding from the cut surface of a leaf allowed the plants to remain at full turgor throughout the experiments. Plants from which N was withheld required a greater balancing pressure during both day and night. This difference in balancing pressure was greater at high (2.0 kPa) than low (1.2 kPa) atmospheric vapour pressure deficit (VPD). Pressurizing the roots did not prevent the decline in leaf elongation rate induced by withholding N at either high or low VPD. Thus low shoot water status did not limit leaf growth of N-deprived plants.

Does shoot water status limit leaf expansion of nitrogen‐deprived barley?

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1016/0014-5793%2870%2980439-2

Chloroplast and cytoplasmic ribosomes in Euglena gracilis.

Wheat seedlings infected with a pure inoculum of the root-rotting fungus Rhizoctonia solani were grown in pots designed to fit in pressure chambers, to allow the effects of the Rhizoctonia infection on leaf growth to be studied while maintaining the leaves at elevated water status. Wheat was grown to the third leaf stage in soil inoculated with three different levels of Rhizoctonia, and the pots were then pressurised for seven days to maintain the leaf xylem at the point of bleeding (ie. the leaves were at full turgor). The reduction in leaf expansion caused by Rhizoctonia was not overcome by pressurisation, indicating that a reduced supply of water to the leaves was not responsible for reduced leaf growth. The addition of phosphorus to pots marginally deficient in P did not increase the leaf growth of Rhizoctonia-infected plants, despite increased P uptake to the leaves. These results indicate that a reduced supply of water to the leaves and a supply of phosphorus that was bordering on deficient was not the cause of the growth reduction in seedlings with Rhizoctonia infection. The nature of this reduced growth remains uncertain but may involve growth regulators produced by the fungus, or by the plant as a result of the infection process. The mechanism of these growth reductions is of interest as it may provide a key to the development of plant resistance mechanisms.

Does water and phosphorus uptake limit leaf growth of Rhizoctonia-infected wheat seedlings?

Subsoil physicochemical constraints such as primary salinity and high boron (B) can significantly reduce grain yields across wide areas of Australia. Financially viable amelioration options are limited for cropping systems on these soils, which has raised interest in ‘genetic solutions’. Increasing the tolerance of crops to high salinity and boron that typically co-exist within alkaline soils offers the potential for substantial yield benefits. To assess the contribution that genetic variation can make to crop yield, closely related genotypes differing in B and/or Na+ tolerance of bread and durum wheat, barley, and lentil were compared by growing the different lines in intact soil cores of 2 Calcarosol profiles differing in level of subsoil constraints (‘hostile’/’benign’). The hostile profile had salinity increasing to EC1 : 5 ~1.2 dS/m and B ~18 mg/kg to 0.60 m, whereas in the benign soil EC1 : 5 did not exceed ~0.6 dS/m and B ~11 mg/kg. Grain yields were significantly less on the hostile soil than the benign soil for barley (34%), bread wheat (20%), durum wheat (31%), and lentil (38%). Accumulation of B in shoots was significantly lower on the hostile soil across all crop species, indicating high sodium within the soil was associated with inhibited uptake of B in plants. In contrast, accumulation of Na+ was greater for all cereal crops in the hostile soil compared with the benign soil. Lentil plants with reputed sodium tolerance (CIPAL415) produced a significant yield benefit on both the benign and hostile soil over the commercial line, Nugget. The lentil line with combined Na+ and B tolerance (02-355L*03Hs005) also produced an additional yield increase over CIPAL415 on the hostile soil; however, yield was equivalent on the benign soil. For durum wheat, 2 genotypes differing in Na+ tolerance, containing either the Nax1 or Nax2 genes, accumulated less sodium in the straw than the parent cv. Tamaroi within the hostile soil; however, this did not translate to a yield advantage. For barley, there was no difference in either grain yield or B uptake in either the grain or straw between the B-tolerance line 03_007D_087 and its parent cv. Buloke. Similarly, there was no difference in either grain yield or B uptake between the bread wheat Schomburgk and its B-tolerant near-isogenic line BT-Schomburgk. This study suggests that of the cereal lines tested, there was no obvious benefit in lines with potentially improved tolerance for a single, specific subsoil constraint on alkaline soils where multiple potential constraints exist. In contrast, in lentils, incorporating tolerance to Na+ and B did show promise for increased adaptation to soils with subsoil constraints.

Use of genetic tolerance in grain crops to overcome subsoil constraints in alkaline cropping soils

� Scope This review discusses the evidence for Na þ and Cltoxicity and the concept of tissue tolerance in relation to halophytes. � Conclusions The data reviewed here suggest that halophytes tolerate cytoplasmic Na þ and Clconcentrations of 100-200 mM, but whether these ions ever reach toxic concentrations that inhibit metabolism in the cytoplasm or cause death is unknown. Measurements of ion concentrations in the cytosol of various cell types for contrasting species and growth conditions are needed. Future work should also focus on the properties of the tonoplast that enable ion accumulation and prevent ion leakage, such as the special properties of ion transporters and of the lipids that determine membrane permeability.

Rana Munns

Papers

Does shoot water status limit leaf expansion of nitrogen‐deprived barley?

Chloroplast and cytoplasmic ribosomes in Euglena gracilis.

Does water and phosphorus uptake limit leaf growth of Rhizoctonia-infected wheat seedlings?

Use of genetic tolerance in grain crops to overcome subsoil constraints in alkaline cropping soils

REVIEW: PART OF A SPECIAL ISSUE ON HALOPHYTES AND SALINE ADAPTATIONS Sodium chloride toxicity and the cellular basis of salt tolerance in halophytes