5. Transcriptomic analysis

and natural alleles. A large number of molecular markers including single nucleotide polymorphisms (SNPs), simple sequence repeats (SSRs) and insertions/deletions (InDels) are available for Arabidopsis and rice plants. Map-based cloning approach that uses these various molecular markers have been used to identify a large number of abiotic stress-related genes such as the salt overly sensitive (SOS1, SOS2, SOS3, SOS4 and SOS5) genes, and other stressresponsive genes [10]. For generation of mutant lines, ethyl methane sulfonate and irradiations have been extensively used so far. In addition, the recent development of new techniques such as stress-associated genes (SAGs) and TILLING have added new dimensions in identifying mutations in stress-related genes and variant alleles [12]. In the near future, these techniques will be available for a number of crop plants such as Arabidopsis, wheat, maize, rice and

Map-based cloning strategy has also been exploited to unravel abiotic stress-related QTLs in plants. As abiotic stress tolerance trait is polygenic in nature, the QTLs studies have received immense importance in understanding stress responses [14]. Recently, using map-based cloning, a large number of drought and salt stress-related QTLs have been reported in crop plants. QTLs were mapped in Oryza sativa for abiotic stress tolerance [15, 16], Brassica napus for salt tolerance [17], maize for salt tolerance [18], wheat for drought tolerance [19] and cotton for salt tolerance [20]. Gene stacking approach through marker-assisted selection was successfully used in an elite rice cultivar for stacked QTLs related to biotic and abiotic stresses (submergence and salinity tolerance) [21, 22]. Two out of 10 pyramid lines showed adequate tolerance to all tested stresses including abiotic stresses. Similar studies using abiotic stress tolerance

The use of mutant populations of plants, developed through insertional mutagenesis is an important tool to dissect the functions of abiotic stress-related genes [23]. Insertional mutagenesis is accomplished through T-DNA or transposable elements. Such mutant populations are available for Arabidopsis and rice plants. These saturation mutant populations of Arabidopsis and rice cover more than 90% of their genes that could be employed for characterization of abiotic stress tolerance genes [24]. Development of high throughput genomic platforms such as serial analysis of gene expression (SAGE), HRM (differential display, high resolution melt) analysis, TILLING, microarray, etc. have made rapid analysis of these mutation events. A large number of abiotic stress-related genes have been identified using Arabidopsis and rice knockout populations. In a 250,000 independent T-DNA insertional Arabidopsis population, more than 200 mutants were found with altered stress responses. Some of these include mutations in genes encoding transcription factors, ABA biosynthetic enzymes and sodium transporter high affinity K+ transporter (HKT1) [25]. Recent progress on the generation of T-DNA insertion

Along with T-DNA and transposable elements based mutant populations; the need for alternative means of studying gene function is growing day by day. This is mainly because of the

genes/QTLs need to be extended to other crop plants.

78 Transgenic Crops - Emerging Trends and Future Perspectives

4. Development of mutant populations

lines have been reviewed in several articles [26, 27].

brassica [13].

Progress in transcriptomic analysis tools has revealed massive genomic sequence information in many plants. Identification of the partial or complete cDNAs sequences provide a holistic picture of the transcriptomes. The available ESTs are organized in three main databases, that is, NCBI, TIGR and Sputnik, which organize these ESTs with fully characterized gene sequences. Abiotic stress-related ESTs have contributed a great deal in exploring gene expression profiles of stress tolerance-related traits in in Arabidopsis and rice [31].

In recent years, different functional and molecular tools were used to identify abiotic stressresponsive genes in plants. These included genome wide physical and genetic mapping of chromosomes, isolation and sequencing of genes, ESTs, proteomics techniques and cDNA microarray analysis [32]. Particularly, the cDNA and microarrays were widely used to study gene expression profiles in Arabidopsis, potato, rice, sorghum, maize and wheat under abiotic stresses. The identified genes/proteins include late embryogenesis abundance (LEA) proteins, compatible osmolytes, ROS scavengers and proteins involved in signal transduction.

The genomic approaches related to abiotic stress tolerance in plants are summarized (Table 1). In one study, Oono et al. [33] used a full-length cDNA microarray containing 7000 Arabidopsis full-length cDNAs and identified 152 rehydration-inducible genes. Among the 152 rehydration-inducible genes, 58 genes showed proline- and hypoosmolarity-inducible gene expression. Similar study was conducted in Arabidopsis under drought stress [34]. Transcriptomic analysis of M. sativa and M. esculenta revealed expression of several genes responsive to salt and drought, respectively [35, 36]. In rice plants, the pioneering work came from Rabbani et al. [37]. They used cDNA and gel microarray analysis to identify cold, drought, salinity and ABA inducible genes. They identified 73 stress inducible genes, among which 15 genes were highly responsive to all four treatments. Lan et al. [38] determined and compared the drought and wounding stress-related gene expression profiles. Drought stress regulated many of the pollination/fertilization-related genes. Similarly, the drought stress-related transcriptomic analysis was conducted in some other studies in rice [39]. Using a cDNA microarray, 486 salt responsive ESTs were determined in shoots of rice plants under salt stress [40]. Moreover, Hmida-Sayari et al. [41] used the cDNA amplified fragment length polymorphism (AFLP) technique to investigate the expression profile of potato under salt stress. The expression profile showed 5000 bands, of which 154 were up-regulated, while 120 were down-regulated. Most of these ESTs were found to have a role in biotic and abiotic stresses. Sequence comparison of some of these fragments revealed close homologies with proteins, involved in cell wall structure, stress proteins such as glyceraldehyde dehydrogenase and proteins related to hypersensitive response to pathogens. Approximately 20,000 ESTs were generated from a cDNA library constructed


from potato leaves and roots, which were subjected to salt, heat, cold and drought stresses [42, 43]. Some of these ESTs were found to have sequence similarities with abiotic stress-responsive genes in other plant species. Similar transcriptomic studies were conducted in some other plants

Findings Reference

Understanding Plant Responses to Drought and Salt Stresses: Advances and Challenges in "Omics" Approaches

Zheng et al. [46]

81

http://dx.doi.org/10.5772/intechopen.81041

Song et al. [50]

Recently, transcriptomic analysis through RNA sequencing has been proved to be a powerful tool for analysis of drought and salt stress-responsive genes. RNA-Seq uses next generation sequencing to reveal quantities of RNA in a given sample in real time. Examples of transcriptomic analysis through RNA-Seq have been reported in several crop plants subjected to drought and salt stresses. Shankar et al. [47] studied comparative transcriptomic analysis in drought sensitive and tolerant rice cultivars. A total of 801 and 507 transcripts were found differentially expressed in drought-tolerant (N22) and salt-tolerant (Pokkali) rice cultivars, respectively, under stress conditions. Overall, the study identified common and cultivarspecific stress-responsive transcripts. Ma et al. [48] conducted RNA-Seq analysis in wheat to study the drought-responsive transcriptomic changes during reproductive stages under field conditions. A total of 115,656 genes were detected and among these, 309 genes were found differentially expressed under drought at various developmental stages. Fracasso et al. [49] conducted transcriptomic analysis to study responses of drought sensitive and tolerant sorghum genotypes subjected to drought stress. Several genes such as those involved in photosynthesis, carbon fixation and antioxidants were found differentially expressed in the two genotypes under drought stress. Correlation in maize flowering time and drought stress was studied through RNA-seq and bioinformatics tools [50]. A total of 619 genes were identified, among which the expression of 126 transcripts was altered by drought stress. Among droughtresponsive genes, the important transcripts included zinc finger and NAC domains. The study also identified 20 genes such as transcription factor HY5, PRR37 and CONSTANS involved in

The above-mentioned transcriptomic studies revealed that RNA-Seq analysis could be used as a very powerful tool not only to study stress-specific gene expression analysis but also to

The study and characterization of the complete set of proteins in a cell, organ or organism at a given time is termed as proteomics [51]. Along transcriptomic studies, proteome analysis has

explore differences between stress sensitive and tolerant genotypes of crop plants.

such as sorghum [44], wheat [45], and maize [46] subjected to drought and salt stresses.

Zea mays Drought Differential expression levels of cell-wall related and transporter

Zea mays Drought A total of 619 genes and 126 transcripts were identified whose expression was altered by drought stress

Table 1. Drought and salinity stress-responsive transcriptomic studies in various plant species.

genes were found to contribute to drought tolerance

flowering times.

Species Stress

type

6. Proteomic analysis

Understanding Plant Responses to Drought and Salt Stresses: Advances and Challenges in "Omics" Approaches http://dx.doi.org/10.5772/intechopen.81041 81


Table 1. Drought and salinity stress-responsive transcriptomic studies in various plant species.

from potato leaves and roots, which were subjected to salt, heat, cold and drought stresses [42, 43]. Some of these ESTs were found to have sequence similarities with abiotic stress-responsive genes in other plant species. Similar transcriptomic studies were conducted in some other plants such as sorghum [44], wheat [45], and maize [46] subjected to drought and salt stresses.

Recently, transcriptomic analysis through RNA sequencing has been proved to be a powerful tool for analysis of drought and salt stress-responsive genes. RNA-Seq uses next generation sequencing to reveal quantities of RNA in a given sample in real time. Examples of transcriptomic analysis through RNA-Seq have been reported in several crop plants subjected to drought and salt stresses. Shankar et al. [47] studied comparative transcriptomic analysis in drought sensitive and tolerant rice cultivars. A total of 801 and 507 transcripts were found differentially expressed in drought-tolerant (N22) and salt-tolerant (Pokkali) rice cultivars, respectively, under stress conditions. Overall, the study identified common and cultivarspecific stress-responsive transcripts. Ma et al. [48] conducted RNA-Seq analysis in wheat to study the drought-responsive transcriptomic changes during reproductive stages under field conditions. A total of 115,656 genes were detected and among these, 309 genes were found differentially expressed under drought at various developmental stages. Fracasso et al. [49] conducted transcriptomic analysis to study responses of drought sensitive and tolerant sorghum genotypes subjected to drought stress. Several genes such as those involved in photosynthesis, carbon fixation and antioxidants were found differentially expressed in the two genotypes under drought stress. Correlation in maize flowering time and drought stress was studied through RNA-seq and bioinformatics tools [50]. A total of 619 genes were identified, among which the expression of 126 transcripts was altered by drought stress. Among droughtresponsive genes, the important transcripts included zinc finger and NAC domains. The study also identified 20 genes such as transcription factor HY5, PRR37 and CONSTANS involved in flowering times.

The above-mentioned transcriptomic studies revealed that RNA-Seq analysis could be used as a very powerful tool not only to study stress-specific gene expression analysis but also to explore differences between stress sensitive and tolerant genotypes of crop plants.

### 6. Proteomic analysis

Species Stress

Arabidopsis thaliana

Medicago sativa

Manihot esculenta

Oryza sativa

Oryza sativa

Oryza sativa

Oryza sativa

Oryza sativa

Oryza sativa

Oryza sativa

Solanum tuberosum

Solanum tuberosum

Solanum tuberosum

Solanum tuberosum

Sorghum bicolor

Triticum aestivum

Triticum aestivum

Triticum aestivum

Zea mays Water

stress

type

80 Transgenic Crops - Emerging Trends and Future Perspectives

Salt, drought

Drought, salt

Salt, heat, drought

Findings Reference

Postnikova et al. [35]

Rabbani et al. [37]

Moumeni et al. [39]

Shankar et al. [47]

Hmida-Sayari et al. [41]

Pratt et al. [44]

Li et al. [45]

Ma et al. [48]

—

Fracasso et al. [49]

Lan et al. [38]

Drought Total of 152 rehydration-inducible genes were identified. Oono et al. [33]

Drought Up-regulation of 1300 drought-responsive genes Utsumi et al. [36]

Drought 589 genes were found responsive to drought Gorantla et al. [14]

Salt 486 salt responsive ESTs were determined in shoots Chao et al. [40]

Salt Six ADP-ribosylation factors like proteins were identified. Kim et al. [110]

Salt, heat 3314 clones were identified as up- or down regulated Rensink et al. [43]

1476 stress-related ESTs were found Rensink et al. [42]

A. thaliana Drought Translational regulation of 2000 genes was evaluated Kawaguchi et al. [34]

73 stress inducible genes were identified, among which 15 genes were highly responsive to salt, drought and cold stresses

were regulated by dehydration and wounding, respectively

Drought — —

Drought About 55% of genes differentially expressed in roots of rice under

Differential expression of large number of genes encoding transcription factors in stress sensitive and tolerant genotypes

Drought 333 genes responded to ABA, NaCl or osmotic stress —

carbon fixation, antioxidants in sensitive and tolerant genotypes

Salt Gene expression of 1811 genes was changed in response to salt stress —

79 genes in placenta and 56 genes in endosperm, were up- and

Salt Expression profile showed 5000 ESTs, of which 154 were up-regulated, and 120 were down-regulated

S. bicolor Drought 775 genes were found differentially expressed in response to

S. bicolor Drought Differential expression of genes involved in photosynthesis,

drought-tolerant genotype

down regulated, simultaneously

Drought 3831 transcripts showed changes in expression in the

Drought Large number of genes including 309 differentially expressed genes, responsive to drought stress were up-regulated

drought stress

Salt Expression of large number of genes including 86 transcription

Drought 53.8% and 21% of the pollination/fertilization-related genes

factors was altered significantly

drought stress

The study and characterization of the complete set of proteins in a cell, organ or organism at a given time is termed as proteomics [51]. Along transcriptomic studies, proteome analysis has contributed much to our understanding of the expression of stress-related genes in plants under abiotic stress. Proteomic studies on plant responses to salinity and drought stresses are being explored at large scale. Proteomic approaches have been applied at whole plant, organ and at subcellular levels to unravel the stress-response mechanism in plants. The prominent proteomic studies in plant species facing drought and salinity stresses are summarized (Table 2). Proteomic studies on sugar beet under drought stress identified that heat-shock proteins, nucleoside diphosphate kinase, RuBisCO, Cu-Zn superoxide dismutase (SOD) and 2-Cys-peroxiredoxin were highly induced [52]. Kim et al. [53] conducted proteomic analysis of maize subjected to drought stress and identified proteins involved in metabolism, photosynthesis and stress responses. Proteomic analysis of Arabidopsis under drought stress revealed that branched-chain amino acid amino transferase 3 protein and zinc finger transcription factor oxidative stress 2 proteins had a significant role in drought stress responses in the plants that over-expressed ethylene response factor AtERF019 [54].

Species Stress Proteomic changes Plant

stress response, photosynthesis, and protein

seedlings recovering from drought stress. Majority of these proteins belonged to stress, hormone metabolism,

Understanding Plant Responses to Drought and Salt Stresses: Advances and Challenges in "Omics" Approaches

46 proteins were increased while 22 were decreased. Asparagine synthetase, alpha-galactosidase, fatty acid desaturase and plastid proteins were among the highly

Drought Abundance of 138 proteins was differentially changed. Drought-responsive differentially abundant proteins were involved in signal transduction, photosynthesis

and glutathione-ascorbate metabolism.

Drought A total of 31 proteins were differentially changed in

chloroplast to nucleus signaling pathway

Drought Abundance of HSP-70 protein was highly changed. Protein synthesis, proteolysis and folding-related

Drought Among the 79 significant identified proteins, nitrogen assimilation, and ATP and redox Homeostasis were up-regulated in water savers cultivars; while photosynthesis, carbohydrate, RNA processing and stress related proteins were increased in water spender

proteins increased in abundance

cultivars during water stress

down-regulated

regulated

salt stress

Glycine max Salt Under 100 mM salt stress, seven proteins were found to

regulated in the sensitive one

Salt ROS scavenging proteins were up-regulated in the

Salt Total 18 proteins were differentially expressed under salt stress. Photosynthesis related proteins were upregulated while defense-related proteins were down-

Salt Total 23 salt stress-responsive proteins belonging to six

functional groups were identified

Glycine max Salt Metabolism-related proteins were found up- and

abundance under drought and 54 were changed during recovery phase. ABA accumulation pointed activation of

be up- or down-regulated. LEA, b-conglycinin, elicitor peptide three precursor, and basic/helix–loop–helix protein were up-regulated. While protease inhibitor, lectin, and stem 31-kDa glycoprotein precursor were

tolerant genotype, while iron uptake proteins were up-

down-regulated in leaves, hypocotyls and roots under

Zea mays Drought Identified proteins were involved metabolism,

Glycine max Drought 643 proteins were significantly changed in soybean

glycolysis and redox categories.

Zea mays Drought Abundance of 68 proteins was changed. Out of these,

changed proteins

Brassica napus

Solanum lycopersicum

Phaseolus vulgaris

Brassica napus

Hordeum vulgare

Nicotiana tabaccum

Solanum lycopersicum modification

organ/ organelle

Root including hypocotyl Reference

Khan and Komatsu [64]

83

Leaves Kim et al. [15]

http://dx.doi.org/10.5772/intechopen.81041

Leaf Zhao et al. [118]

Leaf Wang et al. [67]

Leaf Tamburino et al. [65]

Stem Zadražnik et al. [66]

Leaf Urban et al. [68]

Root Witzel et al. [73]

Leaves —

Root, Hypocotyl

Root, Hypocotyl Aghaei et al. [71]

Chen et al. [119]

Sobhanian et al. [75]

Root Hypocotyl


contributed much to our understanding of the expression of stress-related genes in plants under abiotic stress. Proteomic studies on plant responses to salinity and drought stresses are being explored at large scale. Proteomic approaches have been applied at whole plant, organ and at subcellular levels to unravel the stress-response mechanism in plants. The prominent proteomic studies in plant species facing drought and salinity stresses are summarized (Table 2). Proteomic studies on sugar beet under drought stress identified that heat-shock proteins, nucleoside diphosphate kinase, RuBisCO, Cu-Zn superoxide dismutase (SOD) and 2-Cys-peroxiredoxin were highly induced [52]. Kim et al. [53] conducted proteomic analysis of maize subjected to drought stress and identified proteins involved in metabolism, photosynthesis and stress responses. Proteomic analysis of Arabidopsis under drought stress revealed that branched-chain amino acid amino transferase 3 protein and zinc finger transcription factor oxidative stress 2 proteins had a significant role in drought stress responses in the plants

> organ/ organelle

Reference

Leaf Hajheidari et al. [52]

Leaf sheath Ali and Komatsu [116]

Mohammadi et al. [59]

Root Mohammadi et al. [60]

Root Mirzaei et al. [61]

Shoot Cramer et al. [117]

Leaf Caruso et al. [58]

Leaf —

Root Hypocotyl Leaf

that over-expressed ethylene response factor AtERF019 [54].

82 Transgenic Crops - Emerging Trends and Future Perspectives

and chaperone activities

transduction proteins

kinase were increased

Glycine max Drought 32 proteins changed in root. HSP 70, actin B and

Oryza sativa Drought Out of 900 identified proteins, 38% were changed in

Vitis vinifera Drought Early responding proteins included photosynthesis,

carbohydrate metabolism

organs

decreased.

Oryza sativa Drought Out of 12 proteins, 10 were up-regulated and 2 were

Beta Vulgaris

Triticum durum

Helianthus annuus

Brassica napus

Species Stress Proteomic changes Plant

Drought 79 proteins showed significant changes under drought. Important were RuBisCO and 11 others involved in redox regulation, oxidative stress, signal transduction

energy, metabolism, cell structure and signal

Drought Out of 36 significantly changed proteins, 12 were increased in abundance while 24 were decreased. RuBisCO large subunit, triose phosphate isomerase, thiol-specific antioxidant protein, phosphoglycerate

Drought Six proteins related to stress and carbon metabolism were found significantly up-regulated in leaves of

Drought 35 proteins in sensitive and 32 in tolerant line were

common to sensitive and tolerant lines

responding proteins included transport, photorespiration, antioxidants, amino acid and

drought stressed sunflower leaves.

down-regulated. These were mainly grouped as defense,

methionine synthase were differentially changed in the 3

differentially expressed. Six proteins in F1 hybrid were

abundance compared to non-treated. Pathogenesisrelated, chitinases and redox proteins were increased while tubulins and transport-related proteins were

glycolysis, translation, antioxidant defense, while late-



photosynthesis and glutathione metabolism. Phosphorylation of β carbonic anhydrase 1 imparted adaptation to drought stress in Brassica napus [67]. Proteomic analysis of rapeseeds under drought stress indicated that nitrogen assimilation, oxidative phosphorylation, redox homeostasis, energy, photosynthesis and stress-related proteins were raised in abundance in

Understanding Plant Responses to Drought and Salt Stresses: Advances and Challenges in "Omics" Approaches

http://dx.doi.org/10.5772/intechopen.81041

85

Salinization of arable lands may result in up to 50% land loss by the year 2050 [69]. Proteomic techniques have been employed for analyzing salt stress responses in plants. In salt-tolerant and -sensitive potato cultivars, photosynthesis-related proteins were down-regulated; whereas osmotin-like proteins, heat-shock proteins and protein inhibitors were up-regulated [70, 71]. In soybean, β-conglycinin, elicitor peptide three precursor, late embryogenesis-abundant protein, and basic/helix-loop-helix protein, were up-regulated, suggesting soybean adaptation to salt stress; whereas protease inhibitor, lectin and stem, 31-kDa glycoprotein precursor were downregulated, suggesting the weakening of plant defense system under the salinity stress [72]. Differentiation of salt stress-related proteins was evaluated in tolerant and sensitive barley genotypes [73]. Another study conducted on barley found expression of germin-like and pathogenesis-related proteins important for salt stress responses [74]. ATP production-related glyceraldehyde-3-phosphate was down-regulated in soybean under salt stress [75]. Cupin domain protein 3.1 was revealed in enhancing seed germination in rice under salt stress [76]. In barley, salt stress increased the abundance of proteins related to anti-oxidation, signal transduction, protein biosynthesis, ATP generation and photosynthesis [77]. Proteomic analysis of oat leaves under salt stress indicated decrease in abundance of calvin cycle-related and adenosine-triphosphate regulation-related proteins; whereas antioxidant enzymes level was increased [78]. Alterations in proteomic profiles were recorded in wheat cultivars under salt stress [63]. Kamal et al. [79] reported a decrease in ATP synthase and V-type proton ATPase subunits; whereas cytochrome b6-f, germin-like-protein, glutamine synthetase, fructose-bisphosphatealdolase, S-adenosylmethionine synthase and carbonic anhydrase were gradually increased. Damaris et al. [80] reported induction of actin-7, tubulin alpha, V-type proton ATPase, SOD and pyruvate decarboxylase in salt-stressed wheat cultivars. Proteomic analysis of wheat roots indicated differential expression of a number of proteins such as transcription factors, proteins related to ubiquitination pathogenesis and antioxidant enzymes under salt stress [81]. All the above discussed studies show the importance of proteomics in unraveling the vital information about the plants responses to abiotic stresses such as drought and salinity

Metabolomics is one of the most important "Omics" technologies that can be applied to different organisms with little or no modification. The term metabolomics was introduced by Nicholson et al. [82], and since then it has been utilized extensively in agricultural research [83, 84]. The metabolite profiling provides valuable information on the stress tolerance mechanisms and may be applied to bioengineer plants with improved stress tolerance. Metabolomics studies reveal information about compounds involved in acclimation to the stress, those which

different cultivars [68].

stress responses.

7. Metabolomic analysis

Table 2. Drought and salinity stress-related proteomic studies in various plant species.

In addition to the above-mentioned studies of proteomic analysis on the whole plant level, some notable studies have also focused the impact of drought and salinity stresses on organspecific proteomic constituents. The metabolism-related proteins such as the isoflavone reductase, were observed as down-regulated which possibly played an important role in plant defense against various stresses [55]. Leaf-specific protein analysis in other plants identified drought-responsive proteins. These studies were conducted in rice [56], sunflower [57], wheat [58] and soybean [59, 60]. Root-specific proteome analysis was conducted in a number of crops under various drought stress, which identified a wide range of proteins including those involved in pathogenesis, transport and oxidation-reduction reactions. Prominent studies were conducted incanola (Brassica napus) [60], soybean [59] and rice [61]. Similar studies were conducted in rice [62] and wheat [63] subjected to salt stress, which identified changes more prominently in metabolism-related gene expression. Khan and Komatsu [64] performed proteomic analysis of soybean root including hypocotyl during recovery from drought stress and concluded that peroxidase and aldehyde dehydrogenase scavenge toxic reactive oxygen species and reduce the load of harmful aldehydes for helping the plant to recover. In tomato facing drought stress, chloroplast to nucleus signaling pathway in connection to abscisic acid (ABA) signaling network was activated [65]. In common bean stem, heat-shock protein 70 was highly increased in abundance suggesting its role in restoration of normal conformations of proteins for cellular homeostasis [66]. Proteomic analysis of maize leaves under drought stress revealed that ABA regulates the signaling pathways pertaining to oxidative phosphorylation, photosynthesis and glutathione metabolism. Phosphorylation of β carbonic anhydrase 1 imparted adaptation to drought stress in Brassica napus [67]. Proteomic analysis of rapeseeds under drought stress indicated that nitrogen assimilation, oxidative phosphorylation, redox homeostasis, energy, photosynthesis and stress-related proteins were raised in abundance in different cultivars [68].

Salinization of arable lands may result in up to 50% land loss by the year 2050 [69]. Proteomic techniques have been employed for analyzing salt stress responses in plants. In salt-tolerant and -sensitive potato cultivars, photosynthesis-related proteins were down-regulated; whereas osmotin-like proteins, heat-shock proteins and protein inhibitors were up-regulated [70, 71]. In soybean, β-conglycinin, elicitor peptide three precursor, late embryogenesis-abundant protein, and basic/helix-loop-helix protein, were up-regulated, suggesting soybean adaptation to salt stress; whereas protease inhibitor, lectin and stem, 31-kDa glycoprotein precursor were downregulated, suggesting the weakening of plant defense system under the salinity stress [72]. Differentiation of salt stress-related proteins was evaluated in tolerant and sensitive barley genotypes [73]. Another study conducted on barley found expression of germin-like and pathogenesis-related proteins important for salt stress responses [74]. ATP production-related glyceraldehyde-3-phosphate was down-regulated in soybean under salt stress [75]. Cupin domain protein 3.1 was revealed in enhancing seed germination in rice under salt stress [76]. In barley, salt stress increased the abundance of proteins related to anti-oxidation, signal transduction, protein biosynthesis, ATP generation and photosynthesis [77]. Proteomic analysis of oat leaves under salt stress indicated decrease in abundance of calvin cycle-related and adenosine-triphosphate regulation-related proteins; whereas antioxidant enzymes level was increased [78]. Alterations in proteomic profiles were recorded in wheat cultivars under salt stress [63]. Kamal et al. [79] reported a decrease in ATP synthase and V-type proton ATPase subunits; whereas cytochrome b6-f, germin-like-protein, glutamine synthetase, fructose-bisphosphatealdolase, S-adenosylmethionine synthase and carbonic anhydrase were gradually increased. Damaris et al. [80] reported induction of actin-7, tubulin alpha, V-type proton ATPase, SOD and pyruvate decarboxylase in salt-stressed wheat cultivars. Proteomic analysis of wheat roots indicated differential expression of a number of proteins such as transcription factors, proteins related to ubiquitination pathogenesis and antioxidant enzymes under salt stress [81]. All the above discussed studies show the importance of proteomics in unraveling the vital information about the plants responses to abiotic stresses such as drought and salinity stress responses.
