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.

Substantial advances have been made in breeding wheat for dry environments that will also improve performance in saline environments. These genetic gains have been made by conventional breeding. Further gains in productivity will come from the addition of traits that increase the efficiency of water use in dry soils, and control the uptake of salts from saline soils. Conventional breeding methods will continue to be important to provide farmers with higher yielding varieties in dry or saline soils which are resistant to current diseases and which have the grain quality demanded by competitive markets. Trait-based breeding approaches, which often utilize molecular markers to improve selection efficiency, are starting to deliver new and significant gains. To target the most important traits, it is important to know how they will influence yield. Is it through more water use, more efficient use of water or a higher harvest index? For example, the trait of ‘early vigour’ may be an advantage in some years but in others may lead to the exhaustion of soil water and a low yield. The challenge for breeders will be to efficiently integrate trait-based and molecular methods to increase yield in dry and saline environments

Recent Advances in Breeding Wheat for Drought and Salt Stresses

https://api.research-repository.uwa.edu.au/ws/files/33274066/Setter_et_al_2016_salt_tolerance_date_of.pdf

Salt tolerance, date of flowering and rain affect the productivity of wheat and barley on rainfed saline land

Utilizing genetic resources to enhance productivity of salt-prone land.

. Slightly vacuolated cells, i.e. microalgae and meristematic cells of vascular plants, maintain low Cl− and Na+ concentrations even when exposed to a highly saline environment. The factors regulating the internal ion concentration are the relative rate of volume expansion, the membrane permeability to ions, the electrical potential, and the active ion fluxes.

For ion species which are not actively transported, a formula is developed which relates the internal concentration to the rate of expansion of cell volume, the permeability of membranes to that ion, and the electrical potential. For example, when the external concentration of Cl− is high, and Cl− influx is probably mainly passive, the formula predicts that rapid growth keeps the internal Cl− concentration lower than that in a non-growing cell with the same electrical potential; this effect is substantial if the plasmalemma has a low permeability to Cl−.

For ion species which are actively transported, the rate of pumping must be considered. For instance Na+ concentrations are kept low mainly by an efficient Na+ extrusion pump which works against the electric field across the membrane. The requirement for Na+ extrusion is related to the external Na+ concentration, the rate of expansion of cell volume, the membrane permeability, and the electrical potential. It is possible that microalgae have a more positive electrical potential than many other plant cells; if so, requirements for high rates of active Na+ extrusion will be lower. The required rates of Na+ extrusion are lower during rapid growth, provided that the permeability of the plasmalemma to Na+ is low.

The energy required for the regulation of Cl− and Na+ concentrations is low, especially in rapidly expanding cells where Na+ extrusion requires only 1–2% of the energy normally produced in respiration. The exclusion of these ions, however, must be accompanied by the synthesis of enough organic compounds to provide adequate osmotic solutes for the increases in volume accompanying growth. This process reduces the substrates available for respiration and synthesis of cell constituents, but the reduction is not prohibitively large—even for cells growing in 750 mol m−3 NaCl, the carbohydrate accumulated as osmotic solute is only 10% of that consumed in respiration.

Interactions between growth, Cl− and Na+ uptake, and water relations of plants in saline environments. I. Slightly vacuolated cells

An important trait associated with the salt tolerance of wheat is the exclusion of sodium ions (Na+) from the shoot. We have previously shown that the sodium transporters TmHKT1;5-A and TaHKT1;5-D, from Triticum monoccocum (Tm) and Triticum aestivum (Ta), are encoded by genes underlying the major shoot Na+-exclusion loci Nax1 and Kna1, respectively. Here, using heterologous expression, we show that the affinity (K
 m) for the Na+ transport of TmHKT1;5-A, at 2.66 mM, is higher than that of TaHKT1;5-D at 7.50 mM. Through 3D structural modelling, we identify residues D471/a gap and D474/G473 that contribute to this property. We identify four additional mutations in amino acid residues that inhibit the transport activity of TmHKT1;5-A, which are predicted to be the result of an occlusion of the pore. We propose that the underlying transport properties of TmHKT1;5-A and TaHKT1;5-D contribute to their unique ability to improve Na+ exclusion in wheat that leads to an improved salinity tolerance in the field.

/pdf/structural-variations-in-wheat-hkt1-5-underpin-differences-3xgsghl79p.pdf

Rana Munns

Papers

Recent Advances in Breeding Wheat for Drought and Salt Stresses

Salt tolerance, date of flowering and rain affect the productivity of wheat and barley on rainfed saline land

Utilizing genetic resources to enhance productivity of salt-prone land.

Interactions between growth, Cl− and Na+ uptake, and water relations of plants in saline environments. I. Slightly vacuolated cells

Structural variations in wheat HKT1;5 underpin differences in Na + transport capacity.