Physiological and Proteomics Studies in Bread Wheat for Salt Tolerance

Triticum aestivum, commonly called as bread wheat, is an annual grass in the Poaceae (grass family) native to the Mediterranean region and southwest Asia, which is one of several species of cultivated wheat, now grown in temperate climates worldwide. Wheat is one of the top two cereal crops grown in the world for human consumption, along with rice (Oryza sativa). About 2600 million tonnes is estimated production globally (FAO, 2017).

Next to rice, wheat is the most important food-grain of India and is the staple food of millions of Indians, particularly in the northern and north-western parts of the country.

It is rich in proteins, vitamins and carbohydrates and provides balanced food. India is the 4thlargest producer of wheat in the world after Russia, USA and China accounts for 8.7 per cent of the world's total production of wheat.

A major challenge towards world agriculture involves production of 70% more food crop for an additional 2.3 billion people by 2050 worldwide (FAO, 2009). Salinity is a major stress limiting the increase in the demand for food crops. It has been estimated that more than 20% of cultivated land worldwide is affected by salt stress and this amount is increasing day by day.In India, about 6.73 Mha of land is affected by salinity and sodicity problems.

Out of the total 6.73 million ha of salt-affected soils, 2.96 million ha is saline and the rest 3.77 million ha is sodic (Sharma and Singh, 2015). Out of the total 2.347 million ha salt-affected soils in the Indo-Gangetic Plain (wheat bowl of India), 0.56 million ha is saline and 1.787 million ha is sodic. Among all the crops wheat suffer the higher production loss of 4.06 Mtonn along with monetary loss of Rs. 56.49 billion due to salt stress (Sharma et al., 2015).In India, salinization or soil degradation caused by incorrect irrigation management or intrusion of sea water into coastal soils arising from overabstraction of groundwater (Rao et al. 2014).

Salinity in soil interferes with water uptake by the plant. It also leads to ion toxicity in the plants with the time. It negatively affects different physiological activities of the plant. Thenegative effect of salinity on plant physiological and biochemical mechanisms are shown. Principle mechanisms involved in plant resistance towards salinity are (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.

The genome of common wheat is large (17 Gb) (Safer et al., 2010) and complex due to numerous polyploidy events that occurred between 8000 and 10,000 years ago (Gupta et al., 2008; Brenchley et al., 2012). The wheat genome is essentially comprised of the DNA of three different primitive species, which may explain the great capacity of wheat plants to adapt to various ecological conditions (Brenchley et al., 2012). The sequencing of the wheat genome is enabling a more effective and focused approach to the breeding of high-yielding varieties with increased tolerance to environmental stresses.

The International Wheat Genome Sequencing Consortium recently published a chromosome-based draft sequence of the bread wheat genome (Brenchley et al., 2012), an accomplishment that is expected to facilitate the breeding of varieties that are tolerant to the biotic and abiotic stresses that cause yield losses. However, because knowledge of a genomic sequence alone does not indicate how a plant interacts with the environment, and not all open reading frames correspond to a functional gene (Ribeiro et al., 2013), proteomics approaches are critical for understanding plant mechanisms of stress tolerance.

Plant stress response is a dynamic process aimed at an enhancement of plant stress tolerance and an establishment of a novel homeostasis between plant and environment. Several phases of plant stress response could be distinguished including an alarm phase, an acclimation phase, a resistance phase, an exhaustion phase when stress is too severe or lasts too long, and a recovery phase after a cessation of the given stress factor (Levitt, 1980; Larcher, 2003; Kosov?? et al., 2011).

At proteome level, profound alterations in protein relative abundance were found between stressed and control plants as well as between differential genotypes. During an alarm phase, stress induces profound alterations in proteins involved in cell signaling although these proteins are detected scarcely on 2DE gels due to their low abundance.Salinity regulates the expression of many plant genes at the transcriptional and post-translational levels.

The molecular mechanism of plant salt tolerance is very complex (Zhu, 2001, 2002; Munns and Tester, 2008).The root is the primary tissue involved in salinity perception and is one of the first to be injured following exposure to several types of stresses. The sensitivity of the root to stress often limits the productivity of the entire plant (Steppuhn et al., 2010). Therefore, a comprehensive understanding of root molecular responses to salt stress is necessary for researchers to be able to increase crop tolerance to salt stress.

Review of Literature

Salinity is increasingly affecting the growth and productivity of wheat as it is sensitive to high concentration of salt.Salinity prone areas found in the arid and semiarid zones are usually accounted to the accumulation of salts over ages.Different agricultural practices and continuous use of surface and ground water has led to soil salinity and sodicity problems (Bannari et al., 2008). Saline soils have a high concentration of soluble salts. They are classed as saline when the ECe is ?‰? 4 dS m?€’1 (USSL, 2005).A saline soil is usually the reservoir of a number of soluble salts such as Ca2+, Mg2+, Na+ and anions SO42-, Cl-, HCO3- with exceptional amounts of K+, CO32-, and NO3-.

Of the current 230 million ha of irrigated land, 45 million ha (20 %) are salt-affected(Mickelbart et al. 2015).Wheat (Triticumaestivum) is a moderately salt-tolerant crop (Maas and Hoffman, 1977). Wheat can withstand an amount of 100 mMNaCl (about 10 dS m-1) in field.Salinity stress involves changes in growth, various physiological andmetabolicprocesses,dependingonseverityandduration of the stress (James et al., 2011).Salt stress shows a negative correlation with the plant growth by affecting numerous physiological and biochemical processes, including photosynthesis, antioxidant capacity and ion homeostasis (Ashraf and Harris 2009).

Changes in the plants metabolism in response to salinity could be responsible for the diminished growth of plants under high concentrations of NaCl (Erdal et al., 2011). Rahnama (2010) reported that soil salinity represses plant growth initially in the form of osmotic stress later by leading to ion toxicity.Salt stress also negatively a?¬?ects lipid and energy metabolism, photosynthesis, and protein synthesis (Parida and Das, 2005). This further leads to reduction in transpiration, chlorophyll content, tiller number, and biomass (Hassanein, 1999; Chartzoulakis and Klapaki, 2000).

The altered water status and unbalanced ion homeostasis resulting from saline conditions induce several mechanisms to reducedamageintheplant.Osmotictolerancecanbeachievedby adapting water uptake properties of the roots, plant hydraulics, and by adjusting the plant's osmotic potential. Many osmolytes have been found to be produced in salinity stress including proline (Khatkar and Kuhad, 2000), glycine betaine (Khan et al., 2000; Wang and Nii, 2000), sugars (Kerepesi and Galiba, 2000), and polyols (Bohnert et al., 1995; Zhifang and Loescher, 2003) which provides osmotic protection to plants.

Although, there are several strategies to increase wheat production in the salt-affected areas (such as leaching and drainage), the cultivation of tolerant genotypes is recognized as the most effective way to overcome this limitations. The prerequisite is the identi?¬?cation of wheat genotypes with proven wide adaptation under saline conditions.

The cultivar, Kharchia 65, is one of the very few reputed donors of salt tolerance (ST) in wheat and has been extensively studied for ion exclusion – the net exclusion of toxic ions from the shoot; tissue tolerance – the compartmentalization of toxic ions into specific tissues, cells and subcellular organelles; and shoot ion-independent tolerance – the maintenance of growth and water uptake independent of theused in breeding for ST cultivars globally (Chatrath et al. 2007). KRL 1-4, KRL 19, KRL 210, KRL 213, KRL 283 are few cultivars developed by CSSRI, Karnal which are salt tolerant.

Salinity regulates the expression of many plant genes at the transcriptional and post-translational levels. The molecular mechanism of plant salt tolerance is very complex. To investigate this mechanism, several studies have been conducted in many plant models. Published analyses have helped characterize the expression profiles of many genes and proteins involved in salt stress responses in Arabidopsis thaliana, rice, wheat, soybean, tobacco, barrel medic (Medicagotruncatula), and other plant species. In this study, we explored new potential regulatory proteins of salt stress tolerance in wheat roots. High salt concentrations surrounding plant roots can induce rapid changes to cell growth and associated metabolic activities.

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