*2.2.1 Late embryogenesis abundant (LEA) proteins*

A relatively well-known family of drought-responsive genes is the late embryogenesis abundant (LEA) gene family. LEAs are proteins that accumulate at late stage of development of many plant seeds. LEA proteins also accumulate in vegetative parts of plants as a response to water and osmotic stresses and ABA application [55]. It has been proposed that LEA proteins have an important role protecting cellular structures from water deficit [56]. LEA proteins have been grouped into at least eight different groups according to their amino acid sequence similarities and repeated sequence motifs (LEA1 to LEA6, dehydrins, and seed maturation protein). Most LEA proteins are highly hydrophilic, glycine-rich, and low-complexity proteins. They have a strongly disordered conformation in the hydrated state [57]. LEA genes are highly diverse and have been found in a wide range of plant species [58]. Up to 242 LEA genes have been found in *Gossypium hirsutum* [57]. The LEA genes have several stress-responsive cis-acting regulatory elements in the promoter region such as ABRE, DRE/CRT, MYBS, and LTRE [59]. The expression of LEA proteins is associated with acquisition of drought stress tolerance [60, 61]. The specific cellular role of LEA proteins is not well known. However, different studies have shown that LEA proteins may function in scavenging free radicals and ions; stabilization of enzymes, proteins, and membranes; interactions with RNA and DNA; and water retention during drought and other abiotic stresses [61].

Several novel LEA genes have been found in plants native to arid lands, and they have been successfully applied to transform plants for drought stress tolerance. For example, [62] cloned a LEA gene from *Tamarix androssowii*, a shrub that grows in arid or saline environments. The cloned LEA gene was transformed into tobacco. The transgenic plants showed less ion leakage and MDA content under drought than nontransgenic plants. Also, transgenic plants had a greater growth and lower number of wilted leaves. Also [63] use a *Tamarix androssowii* LEA gene (TaLEA) to obtain transgenic lines of *Populus simonii* × *P. nigra* which were compared to nontransgenic plants under salt and drought stress. They found that the constitutive expression of TaLEA in transgenic poplars improved salt and drought tolerance, which was attributed to the protection of cell membranes from damage. Moreover, [64] transformed a dehydrin protein from the desert grass *Cleistogenes songorica* (CsLEA) into alfalfa (*Medicago sativa* L.). Transgenic plants grew more than wildtype plants under drought stress. Moreover, transgenic plants were able to return to normal after rewatering. Identification of novel LEA genes in plants adapted to arid lands may be useful for improving drought tolerance of cultivated plants.

**65**

*Native Plants to Arid Areas: A Genetic Reservoir for Drought-Tolerant Crops*

Some plants respond to drought stress via organic and/or inorganic solute accumulation. Osmolytes or compatible solutes are produced in plants under drought or saline stress. They are small and neutral and do not affect plant metabolism even at relatively high concentrations [21]. These compounds are also called osmoprotectants and are included into three major chemical groups: amino acids (e.g., proline), polyols (e.g., mannitol, trehalose, fructans), and quaternary amines (e.g., glycine betaine and polyamines) [65]. Active accumulation of osmoprotectants may help plants to endure water stress by maintaining the fluidity of cell membranes, protecting and stabilizing proteins and macromolecular structures, detoxification of free radicals, and osmotic adjustment. Osmotic adjustment is a mechanism that allows plants to keep a flow of water to the cells. A greater concentration of osmolytes reduces the osmotic potential of cells, which in turn produces water movement to the cells, which allows maintenance of turgor. Osmotic adjustment also involves

sion [66]. Therefore, cell activities take place at approximately normal speed for

Drought as other kinds of abiotic and biotic stresses may produce oxidative stress. The oxidative stress is caused by at least two processes: (i) an imbalance of production and detoxification of reactive oxygen species (ROS) and (ii) de novo ROS biosynthesis as a response for defense and adaptation to environment. These processes produce significant variations in the general cellular redox state [53, 73]. The term ROS includes substances with one or more activated atoms of oxygen that can be radicals. Some free radicals do not contain oxygen atoms. The key ROS are triplet oxygen, singlet oxygen, superoxide anion radical, hydrogen peroxide, and hydroxyl radical [53]. Oxidative stress causes lipid peroxidation that impairs membranes and induces loss of their barrier function, and consequently a breakdown of organelles occurs [74]. Plants have evolved defense systems against excess of ROS. Those systems include nonenzymatic and enzymatic responses. The enzymatic system consists of several enzymes such as superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), glutathione reductase (GR), and oth-

Several genes associated with synthesis of osmoprotectants have been identified, isolated, and clonated from a variety of plant species including those native to arid lands [68]. For example, the pyrroline-5-carboxylate synthase (P5CS) gene involved in proline synthesis was characterized in *Calotropis procera* from de novo assembled transcriptome contigs of a high-throughput sequencing dataset [69]. Also, [70] transformed wheat plants with a P5CS gene from *Vigna aconitifolia* a legume grown in arid and semiarid regions of India. They found that transgenic wheat acquired drought tolerance by proline accumulation, which may have protected plants against oxidative stress. A betaine aldehyde dehydrogenase (BADH) gene from *Atriplex canescens*, a perennial bush native to arid lands of Northern America, was introduced into a soybean cultivar. The expression of AcBADH increased after drought treatment of transgenic plants. Besides glycine betaine, proline content also increased, and transgenic soybean lines yielded up to 8.8% more than control plants under drought treatments [71]. A plasma membrane intrinsic protein (PIP) gene (ScPIP1) from *Simmondsia chinensis,* a typical desert shrub, was cloned and overexpressed in *Arabidopsis thaliana.* ScPIP1 conferred drought and salt tolerance probably by reducing membrane damage and increasing osmotic adjustment [72].

by limiting influx, sequestration, or exclu-

*DOI: http://dx.doi.org/10.5772/intechopen.86485*

lowering of toxic concentrations of Na+

*2.2.3 Detoxifying enzymes*

ers that eliminate or scavenge ROS [75].

better growth and development of plants [67].

*2.2.2 Osmoprotectants*

## *2.2.2 Osmoprotectants*

*Drought - Detection and Solutions*

and various proteases [25, 54].

*2.2.1 Late embryogenesis abundant (LEA) proteins*

retention during drought and other abiotic stresses [61].

of cellular turgor, changes in membrane fluidity and composition, changes in osmotic potential, and protein-protein interaction [10]. Cell turgor loss is perhaps the most evident indicator of water stress which affects integrity of cells, metabolism, and whole plant performance [50]. Maintaining cell turgor is critical for surviving and growth of plants. The changes in osmotic potential play a relevant role for that purpose. Loss of cell turgor, among other effects, may cause stomata closure and limitations of gas exchange which in turn decreases CO2 supply for RuBisCo. Photosynthesis decreases, and the reducing power production exceeds the rate of its use by the Calvin cycle [51]. Consequently, overproduction

and accumulation of reactive oxygen species (ROS) alter the redox status [52, 53]. ROS damage all major cell biomolecules impairing their function [53]. Plants may respond to these effects by activating several defense mechanisms that involve participation of numerous proteins such as late embryogenesis abundant (LEA) proteins, osmoprotectants, chaperons, detoxifying enzymes,

A relatively well-known family of drought-responsive genes is the late embryogenesis abundant (LEA) gene family. LEAs are proteins that accumulate at late stage of development of many plant seeds. LEA proteins also accumulate in vegetative parts of plants as a response to water and osmotic stresses and ABA application [55]. It has been proposed that LEA proteins have an important role protecting cellular structures from water deficit [56]. LEA proteins have been grouped into at least eight different groups according to their amino acid sequence similarities and repeated sequence motifs (LEA1 to LEA6, dehydrins, and seed maturation protein). Most LEA proteins are highly hydrophilic, glycine-rich, and low-complexity proteins. They have a strongly disordered conformation in the hydrated state [57]. LEA genes are highly diverse and have been found in a wide range of plant species [58]. Up to 242 LEA genes have been found in *Gossypium hirsutum* [57]. The LEA genes have several stress-responsive cis-acting regulatory elements in the promoter region such as ABRE, DRE/CRT, MYBS, and LTRE [59]. The expression of LEA proteins is associated with acquisition of drought stress tolerance [60, 61]. The specific cellular role of LEA proteins is not well known. However, different studies have shown that LEA proteins may function in scavenging free radicals and ions; stabilization of enzymes, proteins, and membranes; interactions with RNA and DNA; and water

Several novel LEA genes have been found in plants native to arid lands, and they have been successfully applied to transform plants for drought stress tolerance. For example, [62] cloned a LEA gene from *Tamarix androssowii*, a shrub that grows in arid or saline environments. The cloned LEA gene was transformed into tobacco. The transgenic plants showed less ion leakage and MDA content under drought than nontransgenic plants. Also, transgenic plants had a greater growth and lower number of wilted leaves. Also [63] use a *Tamarix androssowii* LEA gene (TaLEA) to obtain transgenic lines of *Populus simonii* × *P. nigra* which were compared to nontransgenic plants under salt and drought stress. They found that the constitutive expression of TaLEA in transgenic poplars improved salt and drought tolerance, which was attributed to the protection of cell membranes from damage. Moreover, [64] transformed a dehydrin protein from the desert grass *Cleistogenes songorica* (CsLEA) into alfalfa (*Medicago sativa* L.). Transgenic plants grew more than wildtype plants under drought stress. Moreover, transgenic plants were able to return to normal after rewatering. Identification of novel LEA genes in plants adapted to arid

lands may be useful for improving drought tolerance of cultivated plants.

**64**

Some plants respond to drought stress via organic and/or inorganic solute accumulation. Osmolytes or compatible solutes are produced in plants under drought or saline stress. They are small and neutral and do not affect plant metabolism even at relatively high concentrations [21]. These compounds are also called osmoprotectants and are included into three major chemical groups: amino acids (e.g., proline), polyols (e.g., mannitol, trehalose, fructans), and quaternary amines (e.g., glycine betaine and polyamines) [65]. Active accumulation of osmoprotectants may help plants to endure water stress by maintaining the fluidity of cell membranes, protecting and stabilizing proteins and macromolecular structures, detoxification of free radicals, and osmotic adjustment. Osmotic adjustment is a mechanism that allows plants to keep a flow of water to the cells. A greater concentration of osmolytes reduces the osmotic potential of cells, which in turn produces water movement to the cells, which allows maintenance of turgor. Osmotic adjustment also involves lowering of toxic concentrations of Na+ by limiting influx, sequestration, or exclusion [66]. Therefore, cell activities take place at approximately normal speed for better growth and development of plants [67].

Several genes associated with synthesis of osmoprotectants have been identified, isolated, and clonated from a variety of plant species including those native to arid lands [68]. For example, the pyrroline-5-carboxylate synthase (P5CS) gene involved in proline synthesis was characterized in *Calotropis procera* from de novo assembled transcriptome contigs of a high-throughput sequencing dataset [69]. Also, [70] transformed wheat plants with a P5CS gene from *Vigna aconitifolia* a legume grown in arid and semiarid regions of India. They found that transgenic wheat acquired drought tolerance by proline accumulation, which may have protected plants against oxidative stress. A betaine aldehyde dehydrogenase (BADH) gene from *Atriplex canescens*, a perennial bush native to arid lands of Northern America, was introduced into a soybean cultivar. The expression of AcBADH increased after drought treatment of transgenic plants. Besides glycine betaine, proline content also increased, and transgenic soybean lines yielded up to 8.8% more than control plants under drought treatments [71]. A plasma membrane intrinsic protein (PIP) gene (ScPIP1) from *Simmondsia chinensis,* a typical desert shrub, was cloned and overexpressed in *Arabidopsis thaliana.* ScPIP1 conferred drought and salt tolerance probably by reducing membrane damage and increasing osmotic adjustment [72].
