7. Metabolomic analysis

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,

Species Stress Proteomic changes Plant

significantly changed under both stresses

induced by drought, included chaperonin,

among the observed salt-induced proteins

enzymes abundance were increased.

Salt Out of total of 121 proteins, ubiquitination-related

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

and anti-oxidant enzymes were increased for

Oryza sativa Salt In the two different cultivars, 104 and 102 proteins were

Avena sativa Salt From 30 differential protein spots, protein related to

homeostasis

The levels of ATP synthase alpha and beta subunits, RuBisCO, photosynthesis and ROS-related proteins were

Of the total 124 stress responsive proteins, 26.61% were

cys-peroxiredoxin, ethylene response, and elongation factor; while 23.38% were induced by salinity stress, included bowman-birk type protease inhibitor, calcineurin B-like protein, cyclophilin and RNA binding proteins

significantly altered. Actin-7, tubulin alpha, V-type proton ATPase, SOD and pyruvate decarboxylase were

calvin cycle, adenosine-triphosphate regulation-related and 50S ribosomal proteins decreased while antioxidant

proteins, transcription factors, pathogen-related proteins

Phoenix dactylifera

Triticum aestivum

Triticum aestivum Salt, drought

84 Transgenic Crops - Emerging Trends and Future Perspectives

Salt, Drought organ/ organelle Reference

Leaves El Rabey et al. [120]

Seed Kamal et al. [121]

Root Damaris et al. [80]

Leaf Bai et al. [78]

Root Jiang et al. [122]

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 are by-products as a result of disruption of normal homeostasis and those involved in signal transduction in response to the stresses [85]. Due to involvement of metabolites in important life processes, the field of metabolic profiling could contribute significantly to the study of stress biology in plants. Both primary and secondary metabolites have been shown to play important roles in responses of plants to drought and salinity stresses. Primary metabolites such as sugars, amino acids and intermediates of Krebs cycle were found with important roles in photosynthetic dysfunction and osmotic readjustment. While, the secondary metabolites such as antioxidant scavengers, coenzymes and regulatory molecules responded to specific stress conditions. Both qualitative and quantitative studies of metabolites in response to abiotic stress are helpful in not only determining the phenotypic response of the plant and screening for stress tolerant lines but also reveal the genetic and biochemical mechanisms underlying the stress condition [86].

in Arabidopsis under salinity stress. Further, metabolite changes were found positively correlated with growth potential and salt tolerance in rice genotypes for allantoin and glutamine [100]. Meulebroek et al. [101] carried out metabolomic profiling of tomato carotenoid content under salt stress. The results revealed that metabolites had several roles at the fruit level in salinity response; however, 46 metabolites had ascribed a noticeable role in carotenoid metabolism as well. In barley, concentrations of most amino acids such as 4-hydroxy-proline, arginine, citrulline, glutamine, phenylalanine, proline and amines increased significantly in roots facing salinity stress [102]. Behr et al. [103] carried out metabolomics analysis in Suaeda maritima exposed to salinity stress. Results revealed increase in metabolites associated with osmotic stress and photorespiration; furthermore, alanine fermentation was enhanced. Oxidative stress produced by salinity in roots of Salicornia herbacea induced defense metabolites such as shikimic acid, vitamin K1 and indole-3-carboxylic acid that are generated as a result of defense mechanisms, to protect against ROS [104]. Metabolomic profiling studies revealed that sugars, sugar alcohols, proline, TCA cycle intermediates, histidine, glutathione and GABA were accumulated in Arabidopsis thaliana under salt stress [105, 106]. Production of signaling molecules such as serotonin and gentisic acid increased in salt-tolerant varieties indicating their importance as biomarker. Ferulic acid and vanillic acid were also produced in high levels. In the salt sensitive varieties, elevated levels of 4-hydroxycinnamic acid and 4-hydroxybenzoic acid were found in the leaves [19]. Epidermal bladder cells help in salt dumping, improved potassium retention in leaf mesophyll and space provision for storage of metabolites [107]. The above discussion revealed that metabolomics is very important tool in investigating abiotic

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

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

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stress-response mechanisms such as those observed in drought and salt stresses.

RNA-Seq and genome sequencing and proteomic techniques/technologies (2D, iTRAQ, MALDI, gel-free, label-free, LC-MS/MS-based technologies) have widened the dimensions of analyzing plant responses to abiotic stresses such as drought and salinity. Recent advances in the omics technologies have contributed considerably to our understanding of the plant abiotic stress-responsive mechanisms. In addition to advancing research in other related areas, emphasis has been on the proteomic analysis specific to whole plants, individual organs, tissues and cells [55]. These technologies are helping to characterize individual proteins specific to different organs, tissues and cells subjected to various abiotic stresses. Advanced proteomic information, coupled with other omics approaches would further strengthen the efforts to develop breeding programs based on identification of novel proteins/genes and their integration through marker-assisted selection. However, further efforts are required to focus on individual target points associated with "Omics" technologies and their application to dissect stress-responsive mechanisms. Research needs to be focused on several fronts such as more studies that target post translational modifications (PTMs), cell type-specific proteome analysis, advanced mapping populations in crop plants and comparative proteomic studies. PTMs of proteins may change their stability, subcellular localization, interactions with other proteins and ultimately proteins functioning. A number of studies revealed the important role of PTMs in protein functioning. Studies have been conducted to analyses protein phosphorylation in maize

8. The way forward

Drought and salt stresses affect the process of photosynthesis, affecting CO2 diffusion leading to photorespiration and hydrogen peroxide production, causing cell damage [87]. Most recently, Rabara et al. [88] analyzed the metabolomics profile of tobacco and soybean roots and leaves facing dehydration stress. The study revealed highest tissue specific accumulation of 4-hydroxy-2-oxoglutaric acid in tobacco roots and coumestrol in soybean roots; indicating 4-hydroxy-2 oxoglutaric acid and coumestrol can be used as markers for drought stress. Metabolomic analysis of intense drought-stressed grapevine leaves was conducted to reveal induction of several metabolites [89]. Metabolomic profiling of Arabidopsis exposed to drought and heat stresses in combination revealed accumulation of sucrose, maltose and glucose [90]. In tolerant and sensitive thyme facing water stress, metabolomics analysis revealed differential changes in carbohydrates, amino acids, fatty acids and organic acids profiles [91]. Metabolites related to the mechanisms of osmotic adjustment, ROS scavenging, cellular components protection and membrane lipid showed significant changes. Metabolomic and proteomic analysis of xylem sap in maize under drought stress revealed a higher abundance of cationic peroxidases, which with the increase in phenylpropanoids may lead to a reduction in lignin biosynthesis in the xylem vessels and could induce cell wall stiffening [92]. Catola et al. [93] reported that trans-2-hexenal showed a significant increase in water-stressed and recovered leaves respect to the well-watered ones in pomegranate plants. This indicated a possible role of the oxylipin pathway in the response to water stress. Metabolites changes in rice grains during water-stressed and recovery indicated involvement in stress signaling pathways such as gamma-amino butyric acid (GABA) biosynthesis, sucrose metabolism and antioxidant defense [94]. Zhang et al. [95] reported that myo-inositol and proline had striking regulatory profiles in Medicago indicating involvement in drought tolerance. Metabolite profiling of hybrid poplar genotypes revealed that amino acids, the antioxidant phenolic compounds catechin and kaempferol, as well as the osmolytes raffinose and galactinol exhibited increased abundance under drought stress, whereas metabolites involved in photorespiration, redox regulation and carbon fixation showed decreased abundance under drought stress [96]. Concentrations of flavonoids, glycosides of kaempferol, quercetin and cyanidin were found in Arabidopsis during drought stress [97].

Salinity stress has been investigated at metabolite level to reveal the response mechanism. In salinity-stressed barley plants, cell division and root elongation was found associated with accumulation of amino acids, sugars and organic acids [98]. Chen and Hoehenwarter [99] reported that sucrose, fructose, glycolysis intermediates and amino acids levels were altered in Arabidopsis under salinity stress. Further, metabolite changes were found positively correlated with growth potential and salt tolerance in rice genotypes for allantoin and glutamine [100]. Meulebroek et al. [101] carried out metabolomic profiling of tomato carotenoid content under salt stress. The results revealed that metabolites had several roles at the fruit level in salinity response; however, 46 metabolites had ascribed a noticeable role in carotenoid metabolism as well. In barley, concentrations of most amino acids such as 4-hydroxy-proline, arginine, citrulline, glutamine, phenylalanine, proline and amines increased significantly in roots facing salinity stress [102]. Behr et al. [103] carried out metabolomics analysis in Suaeda maritima exposed to salinity stress. Results revealed increase in metabolites associated with osmotic stress and photorespiration; furthermore, alanine fermentation was enhanced. Oxidative stress produced by salinity in roots of Salicornia herbacea induced defense metabolites such as shikimic acid, vitamin K1 and indole-3-carboxylic acid that are generated as a result of defense mechanisms, to protect against ROS [104]. Metabolomic profiling studies revealed that sugars, sugar alcohols, proline, TCA cycle intermediates, histidine, glutathione and GABA were accumulated in Arabidopsis thaliana under salt stress [105, 106]. Production of signaling molecules such as serotonin and gentisic acid increased in salt-tolerant varieties indicating their importance as biomarker. Ferulic acid and vanillic acid were also produced in high levels. In the salt sensitive varieties, elevated levels of 4-hydroxycinnamic acid and 4-hydroxybenzoic acid were found in the leaves [19]. Epidermal bladder cells help in salt dumping, improved potassium retention in leaf mesophyll and space provision for storage of metabolites [107]. The above discussion revealed that metabolomics is very important tool in investigating abiotic stress-response mechanisms such as those observed in drought and salt stresses.
