**5.3. Trehalose**

Trehalose is a vital soluble sugar osmolyte frequently used by cells to accommodate osmotic pressure within the effected cells to avoid cellular injury due to oxidation phenomenon. According to recent research, sugarsignaling mechanism plays a vital role in accelerating the photosynthetic performance of plants to its maximum rate in association with trehalose metabolism. These positive effects of trehalose on gas exchange parameters are due to its role in osmoregulation which may affect the stomatal opening. It can be concluded that improvement in growth in wheat cultivars under waterstressed condition with trehalose application may have been due to the role of trehalose in osmotic adjustment. Different plant species respond differently on exogenous application of trehalose and proline. The plant development may be hampered by the external application of these compounds resulting in growth inhibition or yield reduction. The beneficial applications of these osmolytes on crop stress tolerance must carefully be determined for appropriate plant developmental stages.

In plants, trehalose increased the biomass production in shoots and roots in all wheat cultivars under waterstressed conditions as an osmoprotectant under adverse environmental conditions. Exogenous applications of trehalose and proline to plants during or after stress exposure and the increase in the internal levels of these compounds generally enhance plant growth and final crop yield under stress conditions [30].

#### **5.4. Glycine-betaine**

Among the many quaternary ammonium compounds known in plants, glycine-betaine (GB) occurs most abundantly in response to dehydration stress. GB is abundant mainly in chloroplast where it plays a vital role in adjustment and protection of thylakoid membrane, thereby maintaining a photosynthetic efficiency. In higher plants, GB is synthesized in chloroplast from serine via ethanolamine, choline, and betaine aldehyde [34]. The accumulation of two valuable osmolytes like glycine-betaine and proline in different plant species in response to environmental stresses such as drought, salinity, extreme temperatures, UV radiations, and some heavy metals. The role of these compounds has positive effects on enzymes and membrane integrity along with adaptive ways for osmotic adjustment in plants grown under stress conditions.

**6.1. Superoxide dismutase**

**6.2. Catalase**

an H2 O2

might be due to the reasons discussed earlier.

molecule could convert millions of molecules of H2

lished in prokaryotic cyanobacteria has an H<sup>2</sup>

catalyzes the decomposition of H2

**6.3. Ascorbate peroxidase**

SOD concentrations typically increase with the degree of stress conditions as the compartmentalization of different forms of SOD throughout the plant makes them counteract stress very effectively. There are three classes of SOD metallic coenzymes that exist in plants that act to control increased levels of oxidative stress. SOD's role as a free radical scavenger is established, and those genotypes have higher levels indicating a higher level of stress tolerance in wheat. The availability of different forms of SOD in plants show a maximum stress tolerance in affected crops, giving protection to the plant. The trends observed in the present research

Catalase is a common enzyme found in nearly all living organisms exposed to oxygen and

Hydrogen peroxide is a toxic byproduct of many regular metabolic processes. It must be quickly converted into less toxic substances to prevent most cellular damage and tissue injuries.

Ascorbate peroxidases (APXs) are the enzymes that detoxify hydrogen peroxide using ascorbate as a substrate. It is frequently used by cells to rapidly catalyze the decomposition of hydrogen peroxide into less reactive gaseous oxygen and water molecules. Higher plants produce active oxygen species during metabolic processes including mitochondrial, chloroplastic, and plasma membrane-linked electron transport system. Due to biotic and abiotic stress, conditions can give rise to excess concentration of these active oxygen species resulting in oxidative damage at a cellular level. At this stage, the antioxidant enzymes have to function to interrupt the cascades of uncontrolled oxidation in each organelle. Ascorbate peroxidase

plants. APX activities generally increase along with activities of other antioxidant enzymes like catalase, SOD, and GSH reductase in response to various environmental stress factors regulating the components of ROS-scavenging systems. APX has been identified in many

Photosynthetic organisms including higher plants and eukaryotic algae have developed AOS-scavenging systems, including APX isoenzymes. AOS-scavenging system also estab-

enzymes in response to various environmental stresses or cell conditions and play a cooperative role to protect each organelle and minimize tissue injury. The action of antioxidant systems under drought has been investigated by many authors in several crops, such as spinach, pea, sorghum and sunflower, and wheat [39]. Richard et al. [40] have studied the responses to abiotic stresses and activities of superoxide dismutase, catalase, and peroxidase, as well as malondialdehyde (MDA) contents and solute potentials in seedlings of seven wheat (*Triticum*) species (nine genotypes representing three ploidy levels: hexaploid,

O2

diffusion system. The distinct regulatory mechanisms are expressed by APX iso-

(APX) exists as isoenzymes that play an important role in the metabolism of H<sup>2</sup>

higher plants and comprises a family of isoenzymes with different characteristics.

O2

to water and oxygen. The highest turnover by catalase

Role of Osmolytes and Antioxidant Enzymes for Drought Tolerance in Wheat

to water and oxygen in each second.

http://dx.doi.org/10.5772/intechopen.75926

61

O2

tolerance system of the Calvin cycle and

in higher

O2

Cellular responses to stress include changes in the cell cycle and cell division, changes in the endomembrane system and vacuolization of cells, and changes in cell wall architecture, all leading to enhanced stress tolerance of cells. Plants alter metabolism in various ways to accommodate environmental stresses at a biochemical level by producing osmoregulatory compounds such as proline and glycine-betaine. The molecular events linking the perception of a stress signal with the genomic responses leading to tolerance have been intensively investigated in recent years. Certain plants accumulate significant amounts of glycine-betaine [35] in response to high salinity, cold, and drought stress. This quaternary amine has protective functions for macrocomponents of plant cells such as protein complexes and membranes under stress. GB is known to accumulate in response to stress in many crop plants, including sugar beet (*Beta vulgaris*), spinach (*Spinacia oleracea*), barley (*Hordeum vulgare*), wheat (*T. aestivum*), and sorghum (*Sorghum bicolor*). In these species, tolerant genotypes normally accumulate more GB than sensitive genotypes in response to stress. The relationship between GB accumulation and stress tolerance is species or even genotype specific [36]. The increased biosynthesis of GB from choline in stress-sensitive plants is capable of synthesizing this protective solute as droughtstress management.

All plant species are not equally capable of natural production or accumulation of osmolytes in response to stress. Tolerance to abiotic stresses is very complex at the whole plant and cellular levels. The complexity of interactions between stress factors and various molecular, biochemical, and physiological phenomena affects plant growth and development eventually [24]. Exogenous application of proline as pre-sowing seed treatments significantly affected the shoot and root K<sup>+</sup> , Ca2+, P, and N contents of root while this effect of proline on shoot N contents was inconsistent [37]. Similarly, Cuin et al. [38] reported that compatible solutes such as glycine-betaine, proline, and trehalose have explanatory effects on K+ efflux in Arabidopsis under stressed condition.
