Tolerance/adaptation mechanisms in plants to salt stress

In general, the salt stress response of plants consists of ion homeostasis, osmotic adjustment and detoxification (Fig. 1).

3. TISSUE TOLERANCE OF SODIUM IONS

• At the cellular level, high amounts of Na+ and Cl− arriving in leaves can be tolerated by anatomical adaptations and intracellular partitioning.
• Dicotyledonous halophytes exemplify two types of anatomical adaptations:
• Salt-induced increase in cell size due to increases in vacuole volume (succulence)
• The excretion of Na+ and Cl− by salt glands (modified trichomes) or bladders (modified epidermal cells).
• The effect of salinity on intercellular partitioning of ions has been particularly studied in barley, a cereal known for its ability to tolerate high leaf tissue concentrations of Na+ and Cl−.
• In salt-treated barley, there is a greater accumulation of Cl− in epidermal compared with mesophyll cells.
• The converse is true for K+, that is, there is a greater accumulation of K+ in mesophyll compared with epidermal cell

Predominant salt-tolerance mechanisms operating in plant

Intracellular Compartmentation of Na

• Na+ must be partitioned within cells so that concentrations in the cytoplasm are kept low, possibly as low as 10–30 mM.
• No direct measurements of cytosolic concentrations in leaves have been reported, but in roots, direct measurements of cytosolic Na+ in salt-treated plants via the use of ion-sensitive microelectrodes indicate cytosolic Na+ concentrations range from 10 to 30 mM
• However, the concentration at which Na+ becomes toxic is not well defined.
• In vitro studies showed Na+ starts to inhibit most enzymes at concentrations approaching 100mM, although some enzymes are sensitive to lower concentrations .
• The concentration at which Cl− becomes toxic is even less well defined, but is probably similar to that for Na+ .
• Even K+ starts to inhibit enzymes at concentrations above 100 mM
• Ideally, Na+ and Cl− should be largely sequestered in the vacuole of the cell. That this sequestering occurs is indicated by the high concentrations of Na+ found in leaves that are still functioning normally.
• Concentrations well over 200 mM on a tissue basis are common, yet these same concentrations will completely repress enzyme activity in vitro and are beyond all known direct measurements of cytosolic Na+ in both eukaryotic and prokaryotic cells, other than the extremely halophilic prokaryotes
• Importantly, enzymes in halophytes are not more tolerant of salt in vitro than the corresponding enzymes in nonhalophytes, suggesting compartmentation of Na+ is an essential mechanism in all plants, rather than a result of the evolution of tolerance of enzymatic functions in plants from saline environments.
• Increased efficiency of intracellular compartmentation may explain differences in salinity tolerance between closely related species. This hypothesis is supported by findings of a much greater salt stress– induced Na+/H+ antiporter activity in the salt-tolerant species Plantago maritima than in the salt-sensitive species Plantago media

Mechanisms of Salinity Tolerance Other than Na+ Exclusion

1. Osmotic tolerance.

• A close association likely exists between osmotic tolerance and tissue tolerance of Na+, because genotypes that tolerate high internal Na+ concentrations in leaves by compartmentalizing it in the vacuole should also be more tolerant of the osmotic stress owing to their elevated osmotic adjustment.

2. Potassium retention in the cytosol

• The concentration of K+ in the cytoplasm relative to that of Na+ may be a contributing factor to salinity tolerance
• plant salinity tolerance may be achieved not only by cytosolic Na+ exclusion but also by efficient cytosolic K+ retention. A strong positive correlation between shoot K+ concentration and genotype’s salinity tolerance was reported for a wide range of plant species

3. Cl− tolerance.

• The question is often asked: “Why focus only on Na+, why not also consider Cl−?” This question relates particularly to species that accumulate high concentrations of Cl− and not Na+ in leaves, such as soybean, woody perennials such as avocado, and those species that are routinely grown on Cl−-excluding rootstocks such as grapevines and citrus. For these species, Cl− toxicity is more important than Na+ toxicity.
• However, this statement does not imply that Cl− is more metabolically toxic than Na+, rather these species are better at excluding Na+ from the leaf blades than Cl−. For example, Na+ does not increase in the leaf blade of grapevines until after several years of exposure to saline soil, then the exclusion within the root, stem, and petiole breaks down, and Na+ starts to accumulate in the leaf blade, whereas leaf blade Cl− concentrations increase progressively.
• Thus, Na+ may be a more toxic solute, but because the plant is managing the Na+ transport better than Cl− transport, Cl− becomes the potentially more toxic component.

4. Oxidative stress tolerance

• The main line of defence that protect cells against oxidative injury are various antioxidant components – a number of enzymes and low molecular weight compounds capable of quenching ROS without themselves undergoing conversion to a destructive radical.
• Both enzymatic and non-enzymatic components contribute to detoxification of ROS species. This includes;

(i) superoxide dismutase (SOD; found in all cellular compartments);

(ii) water–water cycle (in chloroplasts);

(iii) the ascorbate–glutathione cycle (in chloroplasts, mitochondria, cytosol and apoplastic space);

(iv) Glutathione peroxidase (GPX);

(v) Catalase (CAT; both in peroxisomes)

(vi) Ascorbate peroxidase (APX)

• The most important Non-enzymatic antioxidants: Acrobate and Gluathione

5. Anatomical Adaptation

Leaf succulence

• Leaf succulency is a term used to describe thickening of leaf tissues and the resultant increase in the volume of leaf sap.
• The physiological rationale beyond this phenomenon is a significant increase in the cell (and, hence, vacuole) size leading to the possibility of more efficient intracellular Na+ sequestration in this organelle.
• In glycophytes, leaf succulence is typically achieved by increasing the size of mesophyll cells.

Salt glands and bladders

Gene expression in response to salinity stress

• Plant molecular biology and stress physiology is a fast-expanding research area, providing new insights into the plant responses to salinity and in identifying genetic determinants that affect salt tolerance.
• Genes that are upregulated by salt stress belong to several groups, based on their possible functionality, e.g.
• enzymes (involved in the biosynthesis of osmolytes, hormones, detoxification and general metabolism),
• transporters (ion transporters ABC transporters, and aquaporins),
• regulatory molecules such as transcription factors, protein kinase and phosphatases.
• The most common and widely reported genes that are stress-regulated are perhaps the Late Embryogenesis Abundant proteins or LEA-like genes.
• LEA proteins have been found to protect other proteins from aggregation, from desiccation or osmotic stresses associated with high salinity

A generalized schematic representation of salinity stress tolerance mechanism in a plant.

Transcriptomic approaches

• Expression of numerous plant genes is regulated by salt stress and investigators are currently interested in studying these differentially regulated genes, their mode of regulation and functions of their products in plant stress tolerance. A general assumption has been that up-regulated genes may be important for stress related studies.
• Expression profiling has become an important tool to study how an organism responds to environmental changes.
• Certain techniques have been employed to identify genes whose expression is differentially regulated in response to various environmental stresses in higher plants.

Such methods include

• Differential Display Polymerase Chain Reaction (DDPCR),
• Suppression Subtractive Hybridization (SSH),
• Serial Analysis of Gene Expression (SAGE),
• DNA Chip, Microarray,
• cDNA Amplified Fragment Length Polymorphism (AFLP),
• qRT–PCR, etc.

Proteomic approaches

• Although transcriptome analysis using microarray and SAGE technologies is a potential tool for identification of stress responsive genes, the reliance on these techniques as a sole tool for profiling gene expression has a number of limitations.
• Identification of the predicted gene products at the protein level bridges the gap between genome sequencing data and protein function.
• Proteome analysis has become an indispensable source of information for protein expression

Figure. Functions of salinity stress inducible proteins in stress tolerance and response.

How to analyze stress related proteome in plants?

Figure 6. Overview of the common steps involved in proteomic

analysis (which typically include separation by 1D- or 2Delectrophoresis, followed by protein identification using spectra generated by MALDI-TOF/MS or ESI-MS/MS.

Amelioration of Salinity Stress

• Seed priming
• Calcium and divalent cations
• Compatible solutes
• Polyamines
• Hormones