mechanism of salt tolerance

Salt tolerance

Introduction:

Owing to their sessile lifestyle, plants are continuously exposed to a broad range of environmental stresses. The main abiotic stresses that affect plants and crops in the field are being extensively studied. They include drought, salinity, heat, cold, chilling, freezing, nutrient, high light intensity, ozone (O3) and anaerobic stresses. Under natural conditions, combinations of two or more stresses, such as drought and salinity, salinity and heat, and combinations of drought with extreme temperature or high light intensity are common to many agricultural areas around the world and could impact crop productivity. A major challenge towards world agriculture involves production of 70% more food crop for an additional 2.3 billion people by 2050 worldwide. Salinity is a major stress limiting the increase in the demand for food crops. More than 20% of cultivated land worldwide (about 45 hectares) is affected by salt stress and the amount is increasing day by day. Plants on the basis of adaptive evolution can be classified roughly into two major types: the halophytes (that can withstand salinity) and the glycophytes (that cannot withstand salinity and eventually die).Majority of major crop species belong to this second category. Thus salinity is one of the most brutal environmental stresses that hamper crop productivity worldwide.

Salinity stress involves changes in various physiological and metabolic processes, depending on severity and duration of the stress, and ultimately inhibits crop production. Initially soil salinity is known to represses plant growth in the form of osmotic stress which is then followed by ion toxicity. During the initial phases of salinity stress, water absorption capacity of root systems decreases and water loss from leaves is accelerated due to osmotic stress of high salt accumulation in soil and plants, and therefore salinity stress is also considered as hyperosmotic stress.

Physiological changes Includes; interruption of membranes, nutrient imbalance, impairs the ability to detoxify reactive oxygen species (ROS), differences in the antioxidant enzymes and decreased photosynthetic activity, and decrease in stomatal aperture. Salinity stress is also considered as a hyperionic stress. One of the most detrimental effects of salinity stress is the accumulation of Na+ and Cl− ions in tissues of plants exposed to soils with high NaCl concentrations. Entry of both Na+ and Cl− into the cells causes severe ion imbalance and excess

uptake might cause significant physiological disorder(s). High Na+ concentration inhibits uptake of K+ ions which is an essential element for growth and development that results into lower productivity and may even lead to death. Response to salinity stress, the production of ROS,

such as singlet oxygen, superoxide, hydroxyl radical, and hydrogen peroxide, is enhanced . Salinity-induced ROS formation can lead to oxidative damages in various cellular components such as proteins, lipids, and DNA, interrupting vital cellular functions of plants.

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,

(1) ion homeostasis and compartmentalization, (2) ion transport and uptake,  (3) biosynthesis of osmoprotectants and compatible solutes, (4) activation of antioxidant enzyme and synthesis of antioxidant compounds, (5) synthesis of polyamines, (6) generation of nitric oxide (NO), and (7)

hormone modulation.

Salinity tolerance involves a complex of responses at molecular, cellular, metabolic, physiological, and whole-plant levels. Extensive research through cellular, metabolic, and physiological analysis has elucidated that among various salinity responses, mechanisms or strategies controlling ion uptake, transport and balance, osmotic regulation, hormone metabolism, antioxidant metabolism, and stress signaling play critical roles in plant adaptation to salinity stress. In addition, in spite of the significant progress in the understanding of plant stress responses, there is still a large gap in our knowledge of transmembrane ion transport, sensor and receptor in the signalling transduction, molecules in long distance signalling, and metabolites in energy supply. The future focus should be on the study of intercellular and intracellular molecular interaction involved in salinity stress response.

Review of literature

The plant responses to abiotic stress condition are believed to be complex in nature as these are the reflections of integration of stress effects and responses at various levels of plant organization. To provide tolerance against stresses, plants are equipped with several inbuilt physiological and biochemical mechanisms occurring at cellular level. An understanding of processes linked to these mechanisms is vital in optimizing the crop growth and productivity under stress conditions. One of the important and widely discussed aspects in abiotic stress tolerance is the regulatory roles of plant growth regulators (PGR). PGR are chemical substances that profoundly influence the growth and differentiation of plant cells. The functions of ABA in plants are multiple. High cellular ABA facilitate modifications in stomatal functioning, root hydraulic conductivity, photosynthesis, biomass allocation between roots and shoots, plant water relations, osmolyte production, and synthesis of stress-responsive proteins and genes to confer stress tolerance (Finkelstein et al. 2008;).

Vankova et al. (2011) in radish reported that the higher stress sensitivity of radish is associated with higher decline in bioactive cytokinin levels, as a consequence of stimulation in cytokinin regulatory enzymes, cytokinin oxidase. Maggio et al. (2010) reported that GA3 treatment in tomato reduced stomatal resistance and enhanced plant water use at low salinity. Likewise, GA3-priming increases grain yield due to the GA3-priming-induced modulation of ion uptake and partitioning (within the shoots and roots) as well as hormone homeostasis under saline conditions.  Brassinosteroids are also found effective in modulating salinity stress as evident from improvements in plant tolerance by epibrassinolide treatment. The effect is due to 2 Role of Plant Growth Regulators in Abiotic Stress Tolerance 31 protective action against stress-induced oxidative damage of membrane lipids and induction in antioxidant enzymes (Hayat et al. 2010). Spraying the vegetative parts of the five tested plants  includes Maize, Cotton , Wheat , Broad bean and parsley plants   with 200 ppm of either GA3 or kinetin completely ameliorated the deleterious effect of salinity in fresh, dry matter, leaf area and pigment contents (Abd El-Samad and Shaddad,2014).

(Ashref et al, 2010) reported that addition of  K and Si either alone or in combination significantly inhibited the uptake and transport of Na+ from root to shoots and improved dry matter yield under NaCl condition in sugarcane genotypes. (A.Byborti,2013) reported that foliar application of potassium nitrate (2 mmol L-1) and silicon (4 mmol L-1 ) at the wheat booting stage might be a promising approach to obtain higher grain yield on saline lands. Foliar spray with micronutrient (Fe, Mn, Zn in ratios 1:1:1) may have a potential role for increasing wheat tolerance to salinity stress (Mohamed et al 2011). Application of 10mM foliar KNO3 and Ca(NO3) 2 application, resulting in increase in plant root dry weight (50%), shoot dry weight (50%), LRWC (8.2%) and MP decrease (27.4%) at 40mM NaCl in strawberry (Yildirim et al 2009).