Stress Responses in Plants: Mechanisms of Toxicity and Tolerance
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Abstract: Plants respond to various environmental hazards like heavy metal s by expressing genes that encode proteins involved in stress response. Toxic metal ions intensely affect the cellular protein homeostasis by interfering the folding and aggregation of native and non-native proteins leading to decreased cell viability. Though, plants contain a range of ubiquitous cellular surveillance systems that facilitate them to detoxify heavy metals toward enhanced tolerance to metal stress.
As proteins comprise the key workhorses of living cells, the chelation of metal ions in cytosol with phytochelatins and metallothioneins followed by compartmentalization of metals in the vacuoles as well as the repair of stress-damaged proteins or elimination and degradation of proteins that fail to achieve their natural conformations are critical for plant to tolerate heavy metal stress. In this review, we present a wide overview of current advances in cellular protein research with regards to heavy metal tolerance in plants.
We also discuss how plants uphold functional and healthy proteomes for survival under such unreliable environmental conditions. Keywords: Heavy metals , Phytochelatins , Metallothioneins , Ubiquition proteasome system. Proteins are categorized as macromolecules which play various roles in living organisms.
They are involved in various cellular functions such as regulation, catalysis, cellular signaling and movement of nutrients and also provide structural support and protection to the cell Amm et al.
The structure of a protein determines its function which is obtained at the time of its synthesis. Additionally, the conformation of a protein depending upon the chemical and physical environment, which is largely affected by external environments such as reactive molecules, heavy metal HM ions and other stresses Goldberg, ; Amm et al. Rapid urbanization, industrialization and excessive application of insecticides and pesticides lead to increase in emission of pollutants into the environment. Heavy metals bind to proteins, alter its native confirmation and thus, barricade its functionality Hossain and Komatsu, Various heavy metals like methyl-mercury MeHg inhibits L-glutamine: D-fructosephosphate aminotransferase, and overexpression of this enzyme confers tolerance to MeHg Naganuma et al.
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Correspondingly, thiol transferase activity is inhibited by cadmium Cd which binds to cysteine residues leads to oxidative damage. Cd disrupts the stabilizing interactions of tertiary structure of a protein which leads to loss of functions of that protein Chrestensen et al. Synthesis of peptides rich in metal binding properties take part in immobilizing and detoxification of metal ions Clemens, This result into detoxification of HMs Viehweger, However, in extreme conditions, cellular protein homeostasis is largely affected by metal ions which interfere with the folding process of proteins and accelerate aggregation of native or non-native proteins.
This leads to endoplasmic reticulum ER stress and less cell viability. In order to prevent the aggregation of these proteins, cells posses various quality control systems. A special kind of protein known as heat shock proteins plays a key role in this system which mainly synthesized under stress conditions and maintain the cellular functionality Amm et al.
The damaged proteins which lost their native conformation and do not work properly undergo ubiquitin mediated degradation called as ER-associated degradation ERAD. Another mechanism to eliminate these proteins occurs through autophagy Liu and Howell, Although, plants contain a very strong mechanism to eliminate such unfolded proteins, still a very little information is available that how plants tolerate HM stress.
In view of the above, we aim to explore a better understanding about the protein quality control system in plants with respect to tolerance of HM. We also try to uncover the various mechanisms adopted by plants to ensure functional and healthy proteomes under HM stress. Toxic metal ions generate reactive oxygen species at cellular level which creates oxidative stress to the cell ROS; Li et al. Alternatively, plant cells are adopted to control excess metal ions and utilize various detoxification mechanisms to avert their participations in redundant toxic reactions.
In the first line of defense, plants utilize strategies that prevent or reduce uptake by restricting metal ions to the apoplast through binding them to the cell wall or to cellular exudates, or by preventing long distance transport Manara, ; Hasan et al. In contrast, when present at elevated concentrations, cells activate a complex network of storage and detoxification strategies, such as chelation of metal ions with phytochelatins and metallothioneins in the cytosol, trafficking, and sequestration into the vacuole by vacuolar transporters Zhao and Chengcai, To avoid the damage caused by HMs, plants usually synthesize small cysteine-rich oligomers, called Phytochelatins PCs at the very beginning of metal stress Ashraf et al.
Phytochelatins syntheses play a crucial role in tolerance to HMs in plants Clemens, ; Emamverdian et al. It is observed that the biosynthesis of PCs can be regulated at post-translational level by metal oid s in many plant species. Though, their over expression in plants not always gives rise to enhancement in tolerance to HM stress.
As a matter of fact, excessive PCs levels in mutant plants accelerate accumulation of HMs like Cd without improving plant tolerance Pomponi et al. This observable fact perhaps indicates few supplementary roles of PCs in plant cells, like their involvement in essential for sulfur metabolism, metal ion homeostasis and antioxidant mechanisms Furini, Thus, prevention of toxic metals free movement inside the cytosol exhibits a latent mechanism for dealing with HM-induced toxicity Hasan et al.
HMs detoxification mechanism is not only restricted to the chelation, but also do accumulate and stabilize HM in the vacuole of cells via formation of high molecular weight HMW complexes with PCs Jabeen et al. Normally, metal ions sequestration is an adopted mechanism by organisms to ameliorate toxicity. The immobilized metal ions are transported to vacuole for sequestration through transporters.
Vacuolar sequestration is very crucial machinery to HM homeostasis in plants, which is mediated by ATP-dependent vacuolar pumps V-ATPase as well as via tonoplast transporters Sharma et al. Example of such kind of protein involved in uptake of Cd in A. Another transporter viz. HMA2 present in the plasma membrane of pericycle cells is responsible for transport of Cd to the symplast from the apoplast to facilitate translocation via the phloem in rice, while another transporter i.
HMs primarily targets the proteins. In response to such environmental conditions, a set of genes, known as stress genes, are induced to synthesize a group of proteins called HSPs Gupta et al. Cellular stress created by HM often cause interruption to the cellular homeostasis because they inactivate essential enzymes and also suppress the functioning of proteins Hossain et al.
Therefore, the induction of HSPs proteins is thought-out as a critical protective, ecophysiologically adaptive and genetically conserved response of organisms in response to the environmental anxiety. Thus, they achieved crucial function in the antagonism of stress by re-establishing conformation of normal protein structure and cellular homeostasis Rhee et al. Functional studies of HSP70 revealed that HSP70 is accumulated in response to environmental stressors in a broad range of plant species Gupta et al.
The specific members of this family are localized into the cytosol, mitochondria and endoplasmic reticulum ER and are constitutively expressed as well as regulated to maintain cellular homeostasis. For example, a member of HSP family, the KDa heat shock cognates HSC70 are constitutively synthesized in cells and frequently assist the folding of newly synthesized polypeptides and also take part in translocations of precursor proteins Wang et al.
The current advancements in proteomics research have explained about the functional genes or proteins which involved in the responses of plants to HM stress at molecular levels Ahsan et al. Analysis of different transcripts in many plant species concluded that HSP70 is highly expressed under a variety of metal stress. Even though, various researchers concluded that the overexpression of HSP70 genes is positively correlated with the acquisition of tolerance to various stresses, including HMs.
But the cellular mechanisms of HSP70 function under stress situation are not completely understood Wang et al. HSP70 chaperones, together with their co-chaperones like DnaJ, make a set of prominent cellular strategies to check accumulation of de novo synthesized polypeptides as aggregates and ensure the proper folding of protein during their transfer at their appropriate locations Al- Whaibi, ; Park and Seo, In transportation of precursor protein, the HSC70 is essential for cell-to-cell transport through interaction with the plasmodesmatal translocation pathway Aoki et al.
The induction of HSP70 not only limits the proteotoxic symptoms of metalions, but also helps the sequestration and detoxification of these ions through MTs Haap et al. Whereas the entire mechanism of HSPs-induced metal detoxification via MT has yet to be explored, only few studies pointed out that HSP60 might participate in protein folding and aggregation of various other proteins that are transported to organelles like mitochondria and chloroplasts Al-Whaibi, Through our rising understanding of the proteome, it is clear that HSP60 is very crucial for cellular functions both at normal or stress environments, including metal stress.
Interestingly, proteomics studies also revealed that the induction HSP60 chaperones prevents the denaturation of proteins even in the presence of metal ions in the cytoplasm Sarry et al.
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The currently synthesized and preexisting polypeptides present in the cell are always at a risk of misfolding and agreegation. It is observed that almost one-third of currently synthesized proteins are misfolded Schubert et al. Additionally, cells constantly countenance various environmental challenges such as mutations, active oxygen radicals, heat and HM ions, which disrupt protein folding and also cause the misfolding of proteins which are already folded Amm et al.
Various researchers have documented that HMs and metalloids inhibit refolding of chemically denatured proteins in vitro , creat hindrance in folding of proteins in vivo and also promote the aggregation of nascent proteins inside the cells Sharma et al. It has been reported that in yeast, Cr can trigger oxidative protein damage and protein aggregation by enhancing mistranslation of mRNA Sumner et al.
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Similarly, Cd has also been shown to cause the widespread aggregation of nascent protein and ER stress in yeast Gardarin et al. However, it could be related to metal-induced structural alteration of ER. Recently, Karmous and co-workers concluded that treatment of Cu in embryonic cells of Phaseolus vulgaris leads to prevalently swollen cisternae of smooth ER and vesicles with electrondense contents. Even though, this incident is often not clearly recognizable, it strongly checks the cellular homeostasis.
Under these conditions, a synchronized adaptive program known as unfolded protein response UPR is commonly initiated. The UPR is a homeostatic response to tolerate ER stress via transcriptional and translational mechanisms. The stimulation of UPR is required for; primarily to restore the normal functioning of cell by inhibiting the production of secreted and membrane proteins, elimination of incorrectly folded proteins via ER-associated degradation ERAD mechanism and activation of the signaling pathways which are responsible to increased production of molecular chaperones involved in protein folding.
If these cell sparing activities are not achieved within a certain time span or the disturbance is prolonged, the UPR aims toward programmed cell death PCD which is called apoptosis Deng et al.
The maintenance of cellular homeostatis, involves selective degradation of unnecessary misfolded or damaged protein which is essential for cellular functions, growth, development and viability. Degradation of protein or proteolysis can occur through various mechanisms such as UPS ubiquitin proteasome system or autophagosome induction Liu and Howell, UPS is a regulated mechanism which involve in the regulation of cellular protein homeostatis through various enzymatic cascade.
Damaged protein degradation occurs rapidly , but sometime it occurs only in response to various cellular signallings Pines and Lindon, The current studies concluded that UPS components controls the different processes in plants Sandanandom et al. In all eukaryotes , targeted proteins are marked for degradation by the attachment of ubiquitin. Such ubiquitylated protein recognized and then degraded into small peptides by the large proteolytic complex , the 26S proteasome Goldberg, Ubiquitination is a highly regulated process which required energy in the form of ATP.
It requires the action of three enzymes namely E1 Ub -activating enzyme , E2 Ub — conjugating enzyme and E3 Ub- ligase which work sequentially in a cascade Maupin — Furlow , The first step in ubiquitinylation , Ub covalently linked with E1 in an energy dependant reaction, the ubiquitin is first transferred from E1 to E2 through transesterification. Finally transfer of ubiquitin to the target protein from E2 is mediated by a enzyme E3.
This step is repeated to form polyubiquitinated chains and then designated for degradation which is mediated by 26S proteasome Ruschak et al. Finally, polyub iquitinated chain is removed by deubiquitinating enzyme before import and proteolysis of proteins Hartmann- Petersen et al.
Protein quality affected by increasing free radicals due to harsh environmental conditions such as HM stress which create misfolding , denaturing and damage to protein. During stress , plant require a defense machinery to repair the damaged proteins or removed if damage is irreparable. In such situations, UPS plays a key role in plant response and adaptation to improve environmental conditions Stone, The UPS function in nucleus as well as in cytoplasm which is responsible for removing most of the abnormal peptides and balanced the level of regulatory proteins and remove the temporary cellular regulators which may aggregate successive exposure to abiotic stress Lyzenga and stone, The significance of UPS in cell has been recognized several years ago and manifested due to increased expression of polyubiquitin genes Genschik et al.
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Expression of polyubiquitin genes under stressed environment indicates that UPS is involved in tolerance of heavy metal stress in plants Sun and Callis , ; Chai and Zhang, Genetic analysis of rice plant exhibited that treatment with low concentrations of Cd induces polyubiquitin expression in root and shoot Oono et al. Whereas, increased metal concentration causes the disturbance in proteasomal activity, which results in the deposition of abnormal proteins in the cytoplasm , which changes the cellular protein homeostasis and thus activate apoptosis Yu et al.
In response to a wide range of harsh environmental stresses such as heat stress, drought, salt, nutrient starvation, oxidative stress and pathogen invasion plants have involved a defense mechanism called autophagy Han et al.