Various abiotic stresses in the environment such as high salt concentration, scarcity of water, and high temperatures cause an overall decrease in the productivity and development of plants leading to death in some cases. Abiotic stresses in agricultural crops result in dwindling food supplies due to a reduction of crop yield potential by a margin greater than 50% 1011. Plants in semi-arid regions have increased drought and salinity stresses due to inadequate rains making irrigation their only source for survival. It is inferred from previous studies that as the severity of the drought stress increases, the number of affected genes and proteins rapidly scale 1213. 

Analyzing the gene at the protein level quantitatively enables expression profiling thus determining their response to the plant stress which is a major step. There is a need to understand more about plant tolerance mechanisms. As there are variations in these stress responses by different cops. For instance, sorghum is much more resistant to these stresses than maize. Similarly, there is a change in the transcription profile of rice and Arabidopsis thaliana under various drought and salt stresses 1415. An outline of internal tolerance or resistance mechanisms by plants is presented in Figure 2.

Plants react differently to the salt stress conditions by removing excess salt from the cells thus reducing its overall concentration and thereby entirely preventing its entry into the plant. Lowering the concentration of various organic acids compensate for the reduced salt levels in the cells. 1617. It was found that the drought and salt stress affects a large number of genes involved in the process of photosynthesis with the salt stress playing a greater role in affecting those genes in comparison to drought stress. When the plants face salt stress, the ion concentrations of sodium, potassium, and chloride ions burgeon rapidly enforcing the plants to reduce their concentration. Developing food crops adapted to such stresses is concurrent with changes in the climate and population sizes 181920. This knowledge leads to improved food security and stress tolerance of agricultural crops by carrying out various genetic advancements.

Electrical conductivity tests decipher the concentration of salts in a solution which is measured at 25 degrees Celsius with the least concentrated saline soils having an electrical conductivity value of 4 Deci Siemens/m 21. The age of the plant and the type of the species determine the impact of these abiotic stresses. As described in the recent experimental studies that plants in its germination stage is much more resistant than the latter stages of growth to salt stress and are thus classified into halophytes or glycophytes based on their sensitivity 2223.

Disruption of the ionic equilibrium resulting in the reactive oxygen species production which in turn reduces the protein content of seeds and overall dry weight of the plant shoot under high salt stress severely affecting the human food and animal feeds 2425. The proteomic studies carried out during salt stress in rice, wheat, grapevine, soybean, and potato showed a surge in the expression of oxidative stress-related proteins. Reproduction in plants like Arabidopsis was affected as the flowers being highly sensitive to ROS could not produce the minimal quantity of seeds essential for its propagation. But artificially introducing an osmolyte like glycine betaine into Arabidopsis prevents the production of malformed flowers by hindering the formation of surplus ROS 262728. Mitochondria the powerhouse of the cell largely accounts for the production of ROS apart from undergoing substantial changes in its proteome during salt stress 2930. As previously reported in rice, apoptosis occurred in those cells subjected to salt stress. Glycoside hydrolase alongside other proteins was reported to play a role in initializing programmed cell death and ROS production 31.

Tremendous progress has been made in analyzing the proteome of various subcellular components such as plasma membranes, Golgi membranes, mitochondria, chloroplasts, etc, and advancement in the methods of their isolation. Among these, plasma membranes are pivotal as they act as pathways for transducing the generated stress signals 3233. In a study, the plasma membrane of IR651, a variety of rice having a high tolerance to salt stress was subjected to proteomic analysis which resulted in the identification of 24 proteins involved in the formation of signaling centers through efficient PPIs and their significant role in establishing a K+/Na+ balance under such conditions 34. Hence, proteomics has paved the way for understanding and elucidating the intricate expression mechanisms of proteins when subjected to salt and drought stresses.

The plant cells exploit various cellular, molecular, and biochemical mechanisms to keep up and maintain their normal physiological functions under water stress due to drought. One of the major challenges is to improve crop yield under drought conditions. The cellular processes in plants such as photosynthesis get largely affected leading to a collapse in its carbon dioxide assimilation rate, ultimately causing a stunted growth of the plant and a reduced crop yield. During such conditions, cells tend to lose their turgor pressure along with facing a considerable increase in the concentration of solutes. In order to cope up with such losses of turgor pressure, plants osmotically adjust it with compatible solutes like glycine betaine and sugars as can be seen in some sorghum varieties 3553.

Vegetative tissues of plants facing water stress have been observed to accumulate late embryogenesis associated (LEA) proteins which help in guarding cell structures against dehydration by confiscating ions, sustaining the structures of membrane proteins, renaturing the unfolded proteins, and also operating as a hydrating buffer 5455. Under such conditions, a substantial rise in the antioxidant compounds and enzymes can be seen as they prevent the damage of proteins and DNA caused by oxidative stress. Along with this, protease production takes place for protein degradation and recycling to prevent the aggregation of damaged cellular structures and for renewing the synthesis of proteins 5657. 

One major feature that can be seen in plants facing drought stress is the closure of stomatal pores aided by guard cells. Abscisic acid (ABA), a plant growth hormone controls this process along with producing many transcriptional factors. A guard cell proteome that was subjected to in-silico based analysis revealed 336 proteins previously unknown along with 52 signaling proteins involved in the mechanism. One such identified protein is known as myrosinase TGG1, involved in limiting the ABA-based signaling process that causes the stomata to open under drought conditions 5859. Proteomic studies prove to be useful for uncovering the proteins involved in novel signaling mechanisms under such drought conditions. Further, it is also understood that though there is no change in the gene expression during drought and salt stress, there may be some alterations in the protein concentration 60.

Apart from pathways that depend on ABA for acquiring tolerance during drought stress, certain genes like DREB follow an ABA independent pathway. DREB2A, a member of the DREB family and vital for multiple signaling pathways, when over-expressed, advanced the drought tolerance levels in Arabidopsis 61. Drought resistant plants could be developed by altering the pathways of ABA signaling utilizing molecules such as Sphingosine-1-phosphate as observed in Arabidopsis. Proteome research could be seen to be extremely beneficial for plant improvement if various stress-related genes and other resources could be employed in such studies 62.

Fundamentally, plant response to stresses is relatively complicated stemming from the perception of a signal to its transduction and gene expression thus conferring tolerance to the stress. Advancements in proteomic technologies have led researchers to identify the presence of shared sensory mechanisms during stress commonly called cross-talk signaling systems. Such signaling systems are evident in plants during salt and drought stress in the form of osmotic adjustments 61. An array of experimental designs utilizing a variety of plant species, organs, and organelles, response levels to stresses, and variable sampling times has been incorporated for perceiving the spatial and temporal expression proteomics during stress and elucidated below in Figure 363.

Chourey et.al. Identified six stress proteins that arrested the growth of the seedlings among which four of them were classified as LEA proteins that were degraded during salt stress recovery studies conducted on rice 64. Zang et. al., reported the expression of 327 protein spots when the leaf sheath of 14 days old rice seedlings were subjected to osmotic stress 65. Among these, around 12 spots were found to be upregulated which constituted GST and glyoxalase. The 3 other downregulated proteins were found to be chaperone proteins. But the reason, why only those 3 proteins were downregulated could not be understood 65. In a similar study reported by Chitteti et. al., the emphasis was made on the changes in the expression of phosphoproteomes in rice roots during salt stress utilizing a fluorescent Pro-Q diamond stain. They identified 17 out of the 20 proteins that were upregulated such as GST and Hsp70. The 11 out of 18 downregulated proteins included ATP Synthase, protein kinase, etc 66. As previously discussed, subjecting plants to salt stress results in both ionic and osmotic effects characterized by a growth phase governed by 2 phases. In the first phase, the plant growth is stunted caused by osmotic stress, and if the salinity level advances, ion toxicity, and death occur due to the concentration of ions in the plant shoots 10.

According to the study performed by Salekdeh et. al., two cultivars of rice plants, one resistant and the other sensitive to stress were propagated through hydroponics where concentrations of 50mM and 100mM NaCl was introduced for a period of 2 and 3 weeks respectively 67. The root tissues and their proteins were extracted after 4 weeks and further subjected to two-dimension electrophoresis (2DE) for identifying protein expression changes. Very few spots were found to exhibit qualitative changes whereas the remaining 5 spots showed quantitative changes and the 3 proteins identified and can be seen in Figure 4.

Upon subsequent analysis with MALDI-TOF MS and ESI-Q MS/MS among them were CCOMT, ascorbate peroxidase, an ROS scavenging enzyme, and a highly stress-responsive DNA protecting ASR1-like protein were identified. They found both the cultivars had similar mechanisms for dealing with oxidative stresses except for Caffeoyl- CoA O-methyltransferase, a protein crucial for hindering the movement of sodium-rich water from the surrounding medium to the plant cell, a phenomenon termed as lignification, in the salt-resistant rice cultivar. By genetically engineering the salt stress proteins, they could be utilized as valuable molecular markers for developing highly efficient salt-tolerant crops 67.

Stress-induced adaptive responses are reported efficient way in the plants that depend on intricate coordination among the multiple signaling pathways. This has act in coordinate manners but in some cases, it is found in antagonistic manner. Modification in protein post-translation process is reported to regulate the protein activity and it is reported to localize with protein- interactions in various cellular processes. That can lead to elaborate the regulation of plant responses due to various external stimuli 73. Via understanding the responses among the crop plants under various field condition and that is crucial to design novel stress-tolerant cultivars and it has maintained in robust ways homeostasis under extreme conditions or factors. The proteomic studies or reports on protein post-translation modification (PTM) for different crops were discussed. Role of crop PTM is found in regulation of stress response mechanism and it is difficult to notice due to several novel factor or insights. The technique for detection of PTM in different plants under abiotic stress conditions and PTM control function is reported un representative proteins. Response of PTMs under different abiotic condition is reported soybean crops under submergence conditions. Current findings for PTMs in soybean protein is reported under flooding stresses and helped in providing information on advanced form in PTM processes in relation to plant adaptation due to abiotic stresses. The important of PTM study can provide the adequate solution in agriculture production in coming periods 7374. The many crop plant under natural conditions or factors is reported to various abiotic components of environment stresses and these are reported as heat wave or drought conditions via becoming more prevalent in coming years or decades. Plant acclimation as well as tolerance approaches to th abiotics stresses are reported to associate with modified PTM processes with specific proteins and is reported to important for regulation of protein functions, sub-cellular locations, stability as well as protein activity in many crops including soybean 75. The reports on plant responses to various abiotics stresses at PTMs level is reported to crucial for plant phenotypes for crops improvements. The identification and quantification of ability of PTM on a large scale can contribute to details protein functional characterization approach that can improve our understanding the processes of various crops under stress acclimation and stress tolerance conditions. There are hundreds of PTMs that can help to determine the possible protein modification mechanisms. Various types of PTMs is reported to characterize with detection approaches that can help in advances in PTMs processes with crop proteomics information. This specific PTMs in crop response to abiotics stresses is found to helpful solutions 76. The plant responses due to delicate force signal such as light tough, and are found to very similar to animal or human neural system. The demonstration of thigmotropism, thigmonastic movement or thgmomorphogenesis is reported in various crops plants. The approaches of force-signaling network can be understand via application stable isotope labeling in Arabidopsis (SILIA) tool and it can be applied for proteomic of quantitative post-translation modification that can access the protein phosphorylation medications in Arabidopsis species 77. This plant species is subjected to nearly forty second cotton-swab toruc experiments and these have identified a 4895 non-redundant phosphopeptides and 579 unreported phosphosites, 509 phosphoproteins as well as 24 touch regulated phosphoprotein (TREPH) groups. Further, molecular biological, genetics or bioinformatics analyses can also report the uncharacterized TREPH1 protein; require for bottling-delay regards in Arabidopsis species due to touch response. Protein phosphorylation and TREPH1 protein are found for mechanotransduction pathway due to plant thgmomorphogenesis 78.

Various types of stress-signaling pathways are reported to play crucial role in the maintenance of homoeostasis in plant cells and these pathways is shown important role adaptation of new cellular adaptation situations. Plant organelles such as ER (endoplasmic reticulum) are reported with unfolded protein responses (UPR) with activation of biosynthetic stresses that leads to compensatory increase in ER activity. Further, The JNK (Jun N-terminal kinase) and p38 MAPK (mitogen-actvated protein kinase) signal pathway are reported to control the adaptive responses to intracellular or extracellular stresses 79. Various environmental changes i.e. U.V. light, heat or hyperosmotics conditions as well as exposure to inflammatory cytokines are reported. Metabolic stress due to high-fat diet can be good stimulus with coordination of activities from UPR and JNK/38 signaling pathways. Chronic activation due to stress responses pathways can cause metabolic changes with association with obesity and altered insulin sensitive capability 12. ER homoeostasis is regulated by network of signal pathways due to stearoyl-coA desaturase (SCD), p-38 MAPK and also UPR responses. All these pathways are found to locate at the interface of cell cycle control and cell stress. Cross-regulation can also help to various plant stresses. Interference with SCD-1 with small interfering (si) RNA or the specific SCD-1 inhibitors can induced phosphorylation with activation of p38 MARK in NEH-313 mouse fibroblast 80.

There are various stresses including salt or drought signal transduction that are reported to consist of ionic or osmotic homeostasis signal pathways. Further, detoxification processes (such as damage control and repair mechanism) pathways is reported for growth regulation. Salt stress due to ionic concentration is reported to signaling via SOS (as global response to plant DNA damaging pathways) pathway and also calcium-responsive SOS3-SOS2 protein kinases complexes that can control the expression and activity of ion transporter such as SOS1. Next, osmotic stress is found to activate the several protein kinases (i.e. MAK-mitogen-actvated kinase) with mediation of osmotic homeostasis and or detoxification responses 81. There are numbers of phospholipids system that can be activated via osmotic stresses with generation of diverse arrays of messenger molecules. There is discussion on functional upstream of the osmotic stress-activated protein kinases. Abscisic acid (ABA) biosynthesis regulation is reported by osmotic stress at multiples steps. Further ABA dependent or independent osmotic stress signaling can show the modification of constitutive manner expressed transcription factors via leading to the expression of early response transcriptional activators with activation of downstream stress tolerance effectors genes 8182. Salt stress is reported as major environmental factors that limit plant growth and productivity. Mechanism mediating salt resistance can be understand with design ways for improving crops performance under adverse environmental condition such as salt or drought stresses. Salt stress can generate ionic stress, osmotic or secondary stresses with also oxidative stresses in various plants. Adaptation due salt stresses, plant can rely on signals and pathways via re-establishing cellular ionic, osmotic or reactive oxygen species (ROS) homeostasis. Genetic and biochemical analyses can reveal for several core stress signal pathways with participation in salt resistance 83. Salt sensitive signaling pathways can play key role in maintaining the ionic homeostasis via extruding sodium ions into the apoplast. Further, mitogen activated protein kinase (MAPK) cascade system is reported for mediation of ionic, osmotic as well as ROR homeostasis and SnRK2 (responsible for encoding 1-related protein kinase 2) protein is involved in maintaining of osmotic homeostasis. Next, identification of components and pathways, involved in plants responses to salt stress with their regulatory mechanism via also identification of sensors that involved in salt induced stress signaling pathways in plants 8384.