**6.2. Catalase**

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.

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 synthe-

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

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

The enzymatic and non-enzymatic mechanisms are available for scavenging of reactive oxygen species (ROS) in plants. The biochemical adaptive function of osmoprotectants to scavenge these harmful ROS by-products of hyperosmotic and ionic stresses causes membrane dysfunction and cell death ultimately. These active oxygen species are superoxide (O<sup>2</sup>

duced through oxidation phenomenon. Many plants have the ability to eliminate superoxide with the help of superoxide dismutase (SOD), which catalyzes the superoxide into H2

), hydroxyl radical (√OH), and singlet oxygen (1O<sup>2</sup>

. Thylakoid membrane has potential enzyme ascorbate peroxidase (APX) to eliminate

, Ca2+, P, and N contents of root while this effect of proline on shoot N

−),

O2

) which is pro-

sizing this protective solute as droughtstress management.

the shoot and root K<sup>+</sup>

60 Global Wheat Production

under stressed condition.

hydrogen peroxide (H2

and O2

O2

hydrogen peroxide to save cell membrane from severe damage.

**6. Antioxidant enzymes**

Catalase is a common enzyme found in nearly all living organisms exposed to oxygen and catalyzes the decomposition of H2 O2 to water and oxygen. The highest turnover by catalase molecule could convert millions of molecules of H2 O2 to water and oxygen in each second. 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.

#### **6.3. Ascorbate peroxidase**

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 (APX) exists as isoenzymes that play an important role in the metabolism of H<sup>2</sup> O2 in higher 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 higher plants and comprises a family of isoenzymes with different characteristics.

Photosynthetic organisms including higher plants and eukaryotic algae have developed AOS-scavenging systems, including APX isoenzymes. AOS-scavenging system also established in prokaryotic cyanobacteria has an H<sup>2</sup> O2 tolerance system of the Calvin cycle and an H2 O2 diffusion system. The distinct regulatory mechanisms are expressed by APX isoenzymes 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, tetraploid, and diploid) subjected to water stress for 4, 8, and 12 days by withholding water. In most species, the activities of superoxide dismutase and catalase showed an increase in the early phase of drought and then a decrease with further increase in water stress.

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The enzymatic activities partly recovered and malondialdehyde contents decreased with rewatering. Under drought situation, hexaploid wheat had higher peroxidase activities and MDA contents than tetraploid and diploid wheat. The solute potentials and the activity of SOD and CAT were similar among three groups. Conventional breeding techniques have been unsuccessful in transferring the drought tolerance trait to the target species [2]. The basic biotechnology tools can be employed to manage stress tolerance, hence improving yield stability. Different genetic markers were identified as linked to different traits of interest to determine polymorphism among a variety of wheat genotypes. Richard et al. [40] have studied the responses of growth and primary metabolism of waterstressed barley roots to rehydration.

The assessment of the quantity of variety detected with microsatellite exposes additional polymorphism among different genotypes. The individuality and the value of microsatellites started their multiallelic nature, codominant transmission, wide genome treatment, and requirement for a small amount of starting DNA. Genetic diversity among adapted cultivars or elitebreeding materials has a considerable impact on the improvement of crop plants. Molecular markers can determine genetic diversity from pedigree analysis or morphological traits, and they can offer the best estimate of genetic diversity since they are independent of the perplexing effects of environmental factors. Several molecular markers like RAPD and SSRs are available to assess the variability and diversity at a molecular level [41].
