Dr. Simoglou

thank you so much

and Opinions more than other scientists Please

The main mechanism for a plant to address for abiotic stress are: Stress tolerance (development physiological responses to counteract the stress such as increasing internal solute concentration, closing stomata, etc); Stress avoidance (developing physical barriers such as engrossing cell walls, and even altering their entire architecture (as Cactacaea ); and Stress scape that consists in accelerate the life cycle, meaning in early flowering times to not deal directly with the stress and ensure also that the yielded offspring will not face the abiotic stress to survive.

Dr. Barraza,

thank you so much for your review

Stress tolerance and Avoidance are two common mechanisms...

Depends what exactly you are asking; for example, I am working on transgenerational mechanisms of stress tolerance, and to me changes in DNA methylation and histone modifications at specific genomic regions, together with activation of specific non-coding RNAs are the main mechanisms.

Dear, Kovalchuk

Your work in the molecular level, which is part of the mechanism is important but I am asked about the mechanism in general,I will write a short review about it and I'm interested in your opinion it will

Thank you

2.1.1 Mechanisms of salt tolerance

Mechanisms of salt tolerance, not yet completely clear, can be explained to some extent by stress adaptation effectors that mediate ion homeostasis, osmolyte biosynthesis, toxic radical scavenging, water transport and long distance response coordination (Hasegawa et al., 2000 ). also, Salinity tolerance may be defined as the ability of a plant to grow and complete its life cycle under stressful salt conditions like NaCl or with association of other salts (Yadav et al ,2011 ).

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−. ( Munns and Tester, 2008).

2.1.1.1. Osmatic Stress tolearance

The decreased rate of leaf growth after an increase in soil salinity is primarily due to the osmotic effect of the salt around the roots. A sudden increase in soil salinity causes leaf cells to lose water, but this loss of cell volume and turgor is transient Within hours, cells regain their original volume and turgor owing to osmotic adjustment, but despite this, cell elongation rates are reduced (Cramer , 2002, Fricke and Peters, 2002 ; Passioura and Munns, 2000).

Salts may build up in the apoplast and dehydrate the cell, they may build up in the cytoplasm and inhibit enzymes involved in carbohydrate metabolism, or they may build up in the chloroplast and exert a direct toxic effect on photosynthetic processes (Munns and Tester, 2008).

The most dramatic and readily measurable whole plant response to salinity is a decrease in stomatal aperture. Stomatal responses are undoubtedly induced by the osmotic effect of the salt outside the roots. Salinity affects stomatal conductance immediately, firstly and transiently owing to perturbed water relations and shortly afterward owing to the local synthesis of ABA (Fricke et al. 2004).

The reduced rate of photosynthesis increases the formation of reactive oxygen species (ROS), and increases the activity of enzymes that detoxify these species (Ahmad et al. 2008; Foyer and Noctor, 2005).

When plants acclimate to a changed environment, they undergo adjustments in leaf morphology, chloroplast pigment composition, and in the activity of biochemical processes that prevent oxidative damage to photosystems. The two processes that avoid photoinhibition owing to excess light are heat dissipation by the xanthophyll pigments and electron transfer to oxygen acceptors other than water. The latter response necessitates the upregulation of key enzymes for regulating ROS levels such as superoxide dismutase, ascorbate peroxidase, catalase, and the various peroxidases (Apel and Hirt, 2004; Gill and Tuteja, 2010 ).

Long-distance signaling of salinity stress to the shoot from the roots, mediated at least in part by ABA, cells in the roots initially must sense both the ionic and osmotic components of the addition of Na+ and then respond rapidly to changes in its external concentration. The responses root cells need to make are necessary not only to maintain their own correct function in the face of the new elevated external Na+, but also for them to signal to the shoot that shoot function must also be altered.The first recorded response to an increase in Na+ around roots is an increase in cytosolic free Ca2+ ([Ca2+]cyt); the extracellular addition of Na+ is apparently able to activate the flux of Ca2+ into the cytosol across the plasma membrane and also, interestingly, the tonoplast (Kiegle et al. 2000 ; Knight et al. 1997; Moore et al. 2004 ; Tracy et al. 2008).

2.1.1.2. Na+ exclusion

In the majority of plant species grown under salinity, Na+ appears to reach a toxic concentration before Cl− does, and so most studies have concentrated on Na+ exclusion and the control of Na+ transport within the plant ( Munns and Tester, 2008). Therefore, another essential mechanism of tolerance involves the ability to reduce the ionic stress on the plant by minimizing the amount of Na+ that accumulates in the cytosol of cells, particularly those in the transpiring leaves. This process, as well as tissue tolerance, involves up- and down regulation of the expression of specific ion channels and transporters, allowing the control of Na+ transport throughout the plant ( Munns and Tester, 2008; Rajendran, et al. 2009).

Exclusion of Na+ from the leaves is due to low net Na+ uptake by cells in the root cortex and the tight control of net loading of the xylem by parenchyma cells in the stele (Davenport et al. , 2005). Na+ exclusion by roots ensures that Na+ does not accumulate to toxic concentrations within leaf blades. A failure in Na+ exclusion manifests its toxic effect after days or weeks, depending on the species, and causes premature death of older leaves ( Munns and Tester, 2008).

An efficient cytosolic Na+ exclusion is also got through operation of vacuolar Na+/H+ antiports that move potentially harmful ions from cytosol into large, internally acidic, tonoplast-bound vacuoles. These ions, in turn, act as an osmoticum within the vacuole, which then maintain water flow into the cell, thus allowing plants to grow in soils containing high salinity (Carillo, et al. 2011).

The mechanism of Na+ exclusion allows the plant to avoid or postpone the problem related to ion toxicity, but if Na+ exclusion is not compensated for by the uptake of K+, it determines a greater demand for organic solutes for osmotic adjustment. The synthesis of organic solutes jeopardizes the energy balance of the plant. Thus, the plant must cope ion toxicity on the one hand, and turgor loss on the other (Munns and Tester, 2008).

The net delivery of Na+ to the xylem can be divided into four distinct components (Tester and Davenport , 2003): 1. Influx into cells in the outer half of the root; 2. Efflux back out from these cells to the soil solution; 3. Efflux from cells in the inner half of the root to the xylem; and 4. Influx back into these cells from the xylem before the transpiration stream delivers the Na+ to the leaf blade.

2.1.1.3. Tissue tolerance of sodium ions

Osmotolerance or tissue tolerance entails an increase of survival of old leaves. It requires compartmentalization of Na+ and Cl− at the cellular and intracellular level to avoid toxic concentrations within the cytoplasm, especially in mesophyll cells in the leaf ( Munns and Tester, 2008). One of the main consequences of NaCl stress is the loss of intracellular water. Plants accumulate many metabolites that are also known as “compatible (organic) solutes” in the cytoplasm to increase their hyperosmotic tolerance against salt stress-induced water loss from the cells. This process is also required to balance the osmotic potential of Na+ and Cl− being sequestered into the vacuole (Wyn Jones et al., 1977).

Various compounds such as sugars (chiefly fructose, sucrose and glucose), sugar alcohols (mannitol, glycerol and methylated inositols), complex sugars (trehalose, raffinose and fructans), quaternary amino acid derivatives (proline, glycinebetaine,_-alaninebetaine, prolinebetaine), tertiary amines (1,4,5,6-tetrahydro-2-mehyl-4-carboxyl pyrimidine) and sulfonium compounds (choline-O-sulfate), dimethylsulfoniopropionate (DMSP) have been suggested to accomplish this function in halophytes (Flowers and Colmer, 2008).

The high concentration of compatible solutes exists primarily in the cytosol, to balance the high concentration of salt outside the cell on one side, and on the other, to counteract the high concentrations of sodium and chloride ions in the vacuole. In many halophyte plants, proline (Pro) or glycinebetaine (GB) accumulate at suitably high concentrations to create osmotic pressure over 0.1MPa (Flowers et al., 1977).

However, attempts to improve yield under stress conditions by plant improvement have been largely unsuccessful, primarily due to the multigenic origin of the adaptive responses. Therefore, a well-focused approach combining the molecular, physiological, biochemical and metabolic aspects of salt tolerance is essential to develop salt-tolerant crop varieties. Exploring suitable ameliorants or stress alleviant is one of the tasks of plant biologists. In recent decades exogenous protectants such as osmoprotectants (proline, glycinebetaine, trehalose, etc.), plant hormone (gibberellic acids, jasmonic acids, brassinosterioids, salicylic acid, etc.), antioxidants (ascorbic acid, glutathione, tocopherol, citric acid etc.), signaling molecules (nitric oxide, hydrogen peroxide, etc.), polyamines (spermidine, spermine, putrescine),trace elements (selenium, silicon, etc.) essential macronutrients ( Ca, K ,S) and soil conditioners (Humic acid) have been found effective in mitigating the salt induced damage in plant These protectants showed the capacity to enhance the plant’s growth, yield as well as stress tolerance under salinity (Ma,2000; Ibrahim et al 2013; Sun and Hong, 2011; Hoque et al. 2007 ; Ahmad et al. 2010a, 2012 ; Azzedine et al. 2011 ; Hasanuzzaman et al. 2011a, b ; Hayat and Ahmad 2011 ; Hossain et al. 2011 ; Poor et al. 2011 ; Ioannidis et al. 2012 ; Nounjan et al. 2012 ; Rawia et al. 2011 ; Iqbal et al. 2012 ; Ahmad and Jabeen, 2005; Tahir et al. 2012 ; Yusuf et al. 2012 ). Various causes of salinity over globe and how plants response to their suboptimal and toxic doses along with tolerance strategies has illustrated in( Fig. 1.)

I need your Opinions please

Looks quite comprehensive to me. If you are writing a review, I would still include the mechanisms of regulation on chromatin level and potential transgenerational effects, as well as mechanisms of adaptation/acclimation.

Dr.Kovalchuk

thank you so much , your are right ,did you suggest about alter in DNA and gene expression .

Mechanism of salt tolerance:

I. Avoidance: Avoidance is the process of keeping the salt ions away from the parts of the plant where they are harmful, by-

• Salt exclusion: The ability to exclude salts occurs through filtration at the surface of the root. Root membranes prevent salt from entering while allowing the water to pass through. The red mangrove is an example of a salt-excluding species.

• Salt excretion/extrusion: Salt excreters remove salt through glands or bladders or cuticle located on each leaf.

 Salt bladders - eg) Atriplex , Mesembryanthemum crystallinum L.

 Salt glands - active process, selective for sodium and chloride. eg. Black and white mangroves. Salt glands are dump sites for the excess salt absorbed in water from the soil; help plants adapt to life in saline environments.

 Secretion through cuticle – eg. Tamarix

• Salt Dilution: By dilution of ions in the tissue of the plant by maintaining succulence. Plants achieve this by increasing their storage volume by developing thick, fleshy, succulent structures Succulence is mainly a result of vacuoles of mesophyll cells filling with water and increasing in size. This mechanism is limited by the dilution capacity of plant tissues

• Compartmentation of ions:

 Organ level - high salts only in roots compared to shoots especially leaves.

 Cellular level- high salts in vacuoles than cytoplasm thus protecting enzymes.

II. Tolerance

• Osmotic adjustment- a biochemical mechanism that helps plants acclimates to dry and saline conditions.

• Hormone synthesis - ABA stress hormone hardens plants against excess salts.

Dr.Mohammed Mohi-Ud-Din

thank you so much for your review

You are welcome Dr. Hussein Shareef.

Salinity could be define as  the concentrations of mineral salts that are found in soil or dissolved in irrigation water, with a considerable harmful effects to plants. Soil salinity problems and using saline water for irrigation affect approximately about one-third of the entire world's irrigated lands in humid as well as in arid and semi-arid regions.

Many strategies could be followed by the plant cells to live with salinity, starting with recognition of salt in soil solution, and respond to these kind of stimuli by Mitogen Activated Protein Kinase (MAPK) cascade, and then the response on molecular levels.

Generally there are  two main types of plant tolerance mechanisms, first, salt exclusion by minimizing the salt entry into the roots of plant, Secondly, tissue tolerance by minimizing the concentration of salt in the cytoplasm.

 I do recommend further reading the link below for more explanation.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4041829/

Dear, Dr.Mohammed abbas

Thank you so much for your answer

Dear Hussein Shareef

Plants develop various physiological and biochemical mechanisms in order to survive in soils with high salt concentration. Principle mechanisms include, but are not limited to, ion homeostasis and compartmentalization, ion transport and uptake, biosynthesis of osmoprotectants and compatible solutes, activation of antioxidant enzyme and synthesis of antioxidant compounds, synthesis of polyamines, generation of nitric oxide (NO), and hormone modulation.

For more details about discuss research advances on the complex physiological and molecular mechanisms that are involved in plant salinity tolerance. Please see link:

https://www.hindawi.com/journals/ijg/2014/701596/

Good luck

For study the mechanism of telorence salts is very difficult because there are many mechanism like physiological or biochemical and biological ... this following article is explained more details of mechanism . http://www.google.dz/url?sa=t&source=web&cd=2&ved=0ahUKEwiBmpnvuJnRAhXMVxQKHWG4BvQQFggkMAE&url=http%3A%2F%2Fwww.cnd.mcgill.ca%2F~ivan%2Fannurev%25252Earplant%25252E59%25252E032607%25252E092911.pdf&usg=AFQjCNHcwk9dp3QDfCaEZisIyOi6PiTBBg&sig2=_9N46CsMBTM2Zi4lZmc43w

Dear, Dr. Ahmed and Dr. Nasrine 

Thank you so much for your answer.

You are  welcome Dr hussein 

You will find my answer in next paper.