Oryza sativa class III peroxidase (OsPRX38) overexpression in Arabidopsis thaliana reduces arsenic accumulation due to apoplastic lignification

Antioxidants as modulators of arsenic-induced oxidative stress tolerance in plants: An overview

Arsenic (As) is a highly toxic contaminant in the environment. Although both inorganic and organic types of arsenic exist in the environment, the most common inorganic forms of As that adversely affect plants are arsenite (As III) and arsenate (As V). Despite no evidence for As being essential for plant growth, exposure of roots to this element can cause its uptake primarily via transporters responsible for the transport of essential mineral nutrients. Arsenic exposure even at low concentrations disturbs the plant normal functioning via excessive generation of reactive oxygen species, a condition known as oxidative stress leading to an imbalance in the redox system of the plant. This is associated with considerable damage to the cell components thereby impairing normal cellular functions and activation of several cell survival and cell death pathways. To counteract this oxidative disorder, plants possess natural defense mechanisms such as chemical species and enzymatic antioxidants. This review considers how different types of antioxidants participate in the oxidative defense mechanism to alleviate As stress in plants. Since the underlying phenomena of oxidative stress tolerance are not yet fully elucidated, the potential for "Omics" technologies to uncover molecular mechanisms are discussed. Various strategies to improve As-induced oxidative tolerance in plants such as exogenous supplementation of effective growth regulators, protectant chemicals, transgenic approaches, and genome editing are also discussed thoroughly in this review.

Arsenic-Induced Oxidative Stress and Antioxidant Defense in Plants

The non-essential metalloid arsenic (As) is widely distributed in soil and underground water of many countries. Arsenic contamination is a concern because it creates threat to food security in terms of crop productivity and food safety. Plants exposed to As show morpho-physiological, growth and developmental disorder which altogether result in loss of productivity. At physiological level, As-induced altered biochemistry in chloroplast, mitochondria, peroxisome, endoplasmic reticulum, cell wall, plasma membrane causes reactive oxygen species (ROS) overgeneration which damage cell through disintegrating the structure of lipids, proteins, and DNA. Therefore, plants tolerance to ROS-induced oxidative stress is a vital strategy for enhancing As tolerance in plants. Plants having enhanced antioxidant defense system show greater tolerance to As toxicity. Depending upon plant diversity (As hyperaccumulator/non-hyperaccumulator or As tolerant/susceptible) the mechanisms of As accumulation, absorption or toxicity response may differ. There can be various crop management practices such as exogenous application of nutrients, hormones, antioxidants, osmolytes, signaling molecules, different chelating agents, microbial inoculants, organic amendments etc. can be effective against As toxicity in plants. There is information gap in understanding the mechanism of As-induced response (damage or tolerance response) in plants. This review presents the mechanism of As uptake and accumulation in plants, physiological responses under As stress, As-induced ROS generation and antioxidant defense system response, various approaches for enhancing As tolerance in plants from the available literatures which will make understanding the to date knowledge, knowledge gap and future guideline to be worked out for the development of As tolerant plant cultivars.

A tau class GST, OsGSTU5, interacts with VirE2 and modulates the Agrobacterium-mediated transformation in rice

OsGSTU5 interacts and glutathionylates the VirE2 protein of Agrobacterium and its (OsGSTU5) overexpression and downregulation showed a low and high AMT efficiency in rice, respectively. During Agrobacterium-mediated transformation (AMT), T-DNA along with several virulence proteins such as VirD2, VirE2, VirE3, VirD5, and VirF enter the plant cytoplasm. VirE2 serves as a single-stranded DNA binding (SSB) protein that assists the cytoplasmic trafficking of T-DNA inside the host cell. Though the regulatory roles of VirE2 have been established, the cellular reaction of their host, especially in monocots, has not been characterized in detail. This study identified a cellular interactor of VirE2 from the cDNA library of rice. The identified plant protein encoded by the gene cloned from rice was designated OsGSTU5, it interacted specifically with VirE2 in the host cytoplasm. OsGSTU5 was upregulated during Agrobacterium infection and involved in the post-translational glutathionylation of VirE2 (gVirE2). Interestingly, the in silico analysis showed that the 'gVirE2 + ssDNA' complex was structurally less stable than the 'VirE2 + ssDNA' complex. The gel shift assay also confirmed the attenuated SSB property of gVirE2 over VirE2. Moreover, knock-down and overexpression of OsGSTU5 in rice showed increased and decreased T-DNA expression, respectively after Agrobacterium infection. The present finding establishes the role of OsGSTU5 as an important target for modulation of AMT efficiency in rice.

Nitric oxide-mediated alleviation of arsenic stress involving metalloid detoxification and physiological responses in rice (Oryza sativa L.)

Rice is a staple crop, and food chain contamination of arsenic in rice grain possesses a serious health risk to billions of population. Arsenic stress negatively affects the rice growth, yield and quality of the grains. Nitric oxide (NO) is a major signaling molecule that may trigger various cellular responses in plants. The protective role of NO during arsenite (AsIII) stress and its relationship with plant physiological and metabolic responses is not explored in detail. Exogenous NO, supplemented through the roots in the form of sodium nitroprusside, has been shown to provide protection vis-à-vis AsIII toxicity. The NO-mediated variation in physiological traits such as stomatal density, size, chlorophyll content and photosynthetic rate maintained the growth of the rice plant during AsIII stress. Besides, NO exposure also enhanced the lignin content in the root, decreased total arsenic content and maintained the activities of antioxidant isoenzymes to reduce the ROS level essential for protecting from AsIII mediated oxidative damage in rice plants. Further, NO supplementation enhanced the GSH/GSSG ratio and PC/As molar ratio by modulating PC content to reduce arsenic toxicity. Further, NO-mediated modulation of the level of GA, IAA, SA, JA, amino acids and phenolic metabolites during AsIII stress appears to play a central role to cope up with AsIII toxicity. The study highlighted the role of NO in AsIII stress tolerance involving modulation of metalloid detoxification and physiological pathways in rice plants.

A tau class glutathione-S-transferase (OsGSTU5) confers tolerance against arsenic toxicity in rice by accumulating more arsenic in root

Arsenic (As) considered as one of the hazardous metalloid that hampers various physiological activities in rice. To study the mechanism of As tolerance in rice, one differentially expressed tau class glutathione-S-transferase (OsGSTU5) has been selected and transgenic rice plants with knockdown (KD) and overexpressing (OE) OsGSTU5 were generated. Our results suggested that KD lines became less tolerant to As stress than WT plants, while OE lines showed enhanced tolerance to As. Under As toxicity, OE and KD lines showed enhanced and reduced antioxidant activities such as, SOD, PRX and catalase, respectively indicating its role in ROS homeostasis. In addition, higher malondialdehyde content, poor photosynthetic parameters and higher reactive oxygen species (ROS) in KD plant, suggests that knockdown of OsGSTU5 renders KD plants more susceptible to oxidative damage. Also, the relative expression profile of various transporters such as OsABCC1 (As sequestration), Lsi2 and Lsi6 (As translocaters) and GSH dependent activity of GSTU5 suggests that GSTU5 might help in chelation of As with GSH and sequester it into the root vacuole using OsABCC1 transporter and thus limits the upward translocation of As towards shoot. This study suggests the importance of GSTU5 as a good target to improve the As tolerance in rice.

The IbBBX24-IbTOE3-IbPRX17 module enhances abiotic stress tolerance by scavenging reactive oxygen species in sweet potato

Soil salinity and drought limit sweet potato yield. Scavenging of reactive oxygen species (ROS) by peroxidases (PRXs) is essential during plant stress responses, but how PRX expression is regulated under abiotic stress is not well understood. Here, we report that the B-box (BBX) family transcription factor IbBBX24 activates the expression of the class III peroxidase gene IbPRX17 by binding to its promoter. Overexpression of IbBBX24 and IbPRX17 significantly improved the tolerance of sweet potato to salt and drought stresses, whereas reducing IbBBX24 expression increased their susceptibility. Under abiotic stress, IbBBX24- and IbPRX17-overexpression lines showed higher peroxidase activity and lower H₂ O₂ accumulation compared with the wild-type. RNA sequencing analysis revealed that IbBBX24 modulates the expression of genes encoding ROS scavenging enzymes, including PRXs. Moreover, interaction between IbBBX24 and the APETALA2 (AP2) protein IbTOE3 enhances the ability of IbBBX24 to activate IbPRX17 transcription. Overexpression of IbTOE3 improved the tolerance of tobacco plants to salt and drought stresses by scavenging ROS. Together, our findings elucidate the mechanism underlying the IbBBX24-IbTOE3-IbPRX17 module in response to abiotic stress in sweet potato and identify candidate genes for developing elite crop varieties with enhanced abiotic stress tolerance.

Genome-wide characterization and functional analysis of class III peroxidase gene family in soybean reveal regulatory roles of GsPOD40 in drought tolerance

Class III peroxidases (PODs) are plant-specific glycoproteins, that play essential roles in various plant physiological processes and defence responses. To date, scarce information is available about the POD gene family in soybean. Hence, the present study is the first comprehensive report about the genome-wide characterization of GmPOD gene family in soybean (Glycine max L.). Here, we identified a total of 124 GmPOD genes in soybean, that are unevenly distributed across the genome. Phylogenetic analysis classified them into six distinct sub-groups (A-F), with one soybean specific subgroup. Exon-intron and motif analysis suggested the existence of structural and functional diversity among the sub-groups. Duplication analysis identified 58 paralogous gene pairs; segmental duplication and positive/Darwinian selection were observed as the major factors involved in the evolution of GmPODs. Furthermore, RNA-seq analysis revealed that 23 out of a total 124 GmPODs showed differential expression between drought-tolerant and drought-sensitive genotypes under stress conditions; however, two of them (GmPOD40 and GmPOD42) revealed the maximum deregulation in all contrasting genotypes. Overexpression (OE) lines of GsPOD40 showed considerably higher drought tolerance compared to wild type (WT) plants under stress treatment. Moreover, the OE lines showed enhanced photosynthesis and enzymatic antioxidant activities under drought stress, resulting in alleviation of ROS induced oxidative damage. Hence, the GsPOD40 enhanced drought tolerance in soybean by regulating the key physiological and biochemical pathways involved in the defence response. Lastly, the results of our study will greatly assist in further functional characterization of GsPODs in plant growth and stress tolerance in soybean.

Transcriptome analysis and physiological indicators reveal the role of sulfur in cadmium accumulation and transportation in water spinach (Ipomoea aquatica Forsk.)

Cadmium (Cd) contamination of croplands has become a threat to crop food safety and human health. In this study, we investigated the effect of sulfur on the growth of water spinach under Cd stress and the amount of Cd accumulation by increasing the soil sulfate content. We found that the biomass of water spinach significantly increased after the application of sulfur while the shoot Cd concentration was considerably reduced (by 31%). The results revealed that sulfur could promote the expression of PME and LAC genes, accompanied by an increase in PME activity and lignin content. Also, the cell wall Cd content of water spinach roots was significantly increased under sulfur treatment. This finding suggests that sulfur could enhance the adsorption capacity of Cd by promoting the generation of cell wall components, thereby inhibiting the transportation of Cd via the apoplastic pathway. In addition, the higher expression of Nramp5 under the Cd1S0 (concentration of Cd and sulfur are 2.58 and 101.31 mg/kg respectively) treatment led to increased Cd uptake. The CAX3 and ABC transporters and GST were expressed at higher levels along with a higher cysteine content and GSH/GSSR value under Cd1S1 (concentration of Cd and sulfur are 2.60 and 198.36 mg/kg respectively) treatment, which contribute to the Cd detoxification and promotion of Cd compartmentalization in root vacuoles, thereby reducing the translocation of Cd to the shoot via the symplastic pathway.

Salicylic acid reduces cadmium (Cd) accumulation in rice (Oryza sativa L.) by regulating root cell wall composition via nitric oxide signaling

The effects of salicylic acid (SA) on cadmium (Cd) accumulation, Cd subcellular distribution, cell wall composition and Cd adsorption in Cd-stressed rice seedlings were examined. The interaction between SA and nitric oxide (NO) signaling in regulating cell wall composition under Cd exposure was also investigated. Our results showed that 5 μmol·L^-1 Cd treatment significantly decreased plant height, root length and plant dry weight by 40.1%, 46.1% and 21.3% (p < 0.05), respectively, and the inhibitory effects of Cd on the growth parameters were alleviated by exogenous SA. Application of SA remarkably decreased Cd concentrations in roots and shoots of rice seedlings by 48.0% and 19.6%, respectively, and increased the distribution ratio of Cd in the root cell wall fraction (from 35.7% to 40.6%) compared with Cd treatment alone. The reduced Cd accumulation in rice plants could be attributed to that SA application promoted pectin synthesis and demethylesterification, thereby increasing Cd deposition in the root cell wall. Moreover, SA application promoted lignin biosynthesis to strengthen the cell wall and prevent Cd from entering the root cells. In addition, NO might be involved in SA-induced pectin synthesis, pectin demethylesterification and lignin biosynthesis as a downstream signaling molecule, contributing to reduced Cd accumulation in Cd-stressed rice seedlings. The results provide deep insights into the mechanisms of exogenous SA action in reducing Cd accumulation in rice plants.

Arsenic acquisition, toxicity and tolerance in plants - From physiology to remediation: A review

Globally, environmental contamination by potentially noxious metalloids like arsenic is becoming a critical concern to the living organisms. Arsenic is a non-essential metalloid for plants and can be acclimatised in plants to toxic levels. Arsenic acquisition by plants poses serious health risks in human due to its entry in the food chain. High arsenic regimes disturb plant water relations, promote the generation of reactive oxygen species (ROS) and induce oxidative outburst in plants. This review evidences a conceivable tie-up among arsenic levels, speciation, its availability, uptake, acquisition, transport, phytotoxicity and arsenic detoxification in plants. The role of different antioxidant enzymes to confer plant tolerance towards the enhanced arsenic distress has also been summed up. Additionally, the mechanisms involved in the modulation of different genes coupled with arsenic tolerance have been thoroughly discussed. This review is intended to present an overview to rationalise the contemporary progressions on the recent advances in phytoremediation approaches to overcome ecosystem contamination by arsenic.

Phenoloxidases in Plants-How Structural Diversity Enables Functional Specificity

The metabolism of polyphenolic polymers is essential to the development and response to environmental changes of organisms from all kingdoms of life, but shows particular diversity in plants. In contrast to other biopolymers, whose polymerisation is catalysed by homologous gene families, polyphenolic metabolism depends on phenoloxidases, a group of heterogeneous oxidases that share little beyond the eponymous common substrate. In this review, we provide an overview of the differences and similarities between phenoloxidases in their protein structure, reaction mechanism, substrate specificity, and functional roles. Using the example of laccases (LACs), we also performed a meta-analysis of enzyme kinetics, a comprehensive phylogenetic analysis and machine-learning based protein structure modelling to link functions, evolution, and structures in this group of phenoloxidases. With these approaches, we generated a framework to explain the reported functional differences between paralogs, while also hinting at the likely diversity of yet undescribed LAC functions. Altogether, this review provides a basis to better understand the functional overlaps and specificities between and within the three major families of phenoloxidases, their evolutionary trajectories, and their importance for plant primary and secondary metabolism.

Multistage Defense System Activated by Tetrachlorobiphenyl and its Hydroxylated and Methoxylated Derivatives in Oryza sativa

Crops can initiate various defense responses to environmental stresses. The process is often accompanied by extensive transcriptional and metabolic changes to reallocate metabolites. However, it remains unclear how organic pollutants activate the defense systems to reallocate metabolites in crops. The current study demonstrates that three defense systems, including the cytochrome P450s (CYP450s), glutathione S-transferases (GSTs), and phenylpropanoid biosynthesis, were sequentially activated after Oryza sativa was exposed to 2,3,4,5-tetrachlorobipheny l (PCB 61) and its derivatives 4'-hydroxy-2,3,4,5-tetrachlorobiphenyl (OH-PCB 61) and 4'-methoxy-2,3,4,5-tetrachlorobiphenyl (MeO-PCB 61), respectively. Genes encoding CYP76Ms and CYP72As were significantly upregulated after 0.5 h of exposure, followed by the GST-coding gene GSTU48, suggesting that the biotransformation and detoxification of PCB 61, OH-PCB 61, and MeO-PCB 61 occurred. Subsequently, CCR1 and CCR10 involved in phenylpropanoid biosynthesis were activated after 12 h, potentially reducing the oxidative stress induced by PCB 61 and its derivatives. Furthermore, β-d-glucan exohydrolase involved in both phenylpropanoid biosynthesis and starch and sucrose metabolism was significantly downregulated by 7.04-fold in the OH-PCB 61-treated group, potentially contributing to the inhibition of sugar hydrolysis. These findings provide insights into increasing rice adaptability to organic pollutants by reinforcing the enzyme-mediated defense systems, characterizing a novel and critical strategy that enables augmented crop outputs and quality in environments stressed by organic contaminants.

Inorganic arsenic toxicity and alleviation strategies in rice

Direct or indirect exposure to inorganic arsenic (iAs) in the forms of AsIII (arsenite) and AsV (arsenate) through consumption of As-contaminated food materials and drinking water leads to arsenic poisoning. Rice (Oryza sativa L.) plant potentially accumulates a high amount of iAs from paddy fields than any other cereal crops. This makes it to be a major source of iAs especially among the population that uses it as their dominant source of diet. The accumulation of As in human bodies poses a serious global health risk to the human population. Various conventional methods have been applied to reduce the arsenic accumulation in rice plant. However, the success rate of these techniques is low. Therefore, the development of efficient and effective methods aimed at lowering iAs toxicity is a very crucial public concern. With the current advancement in technology, new strategies aimed at addressing this concern are being developed and utilized in various parts of the world. In this review, we discuss the recent advances in the management of iAs in rice plants emphasizing the use of nanotechnology and biotechnology approaches. Also, the prospects and challenges facing these approaches are described.

Coordination between root cell wall thickening and pectin modification is involved in cadmium accumulation in Sedum alfredii

Root cell wall (RCW) modification is a widespread important defense strategy of plant to cope with trace metals. However, mechanisms underlying its remolding in cadmium (Cd) accumulation are still lacking in hyperaccumulators. In this study, changes of RCW structures and components between nonhyperaccumulating ecotype (NHE) and hyperaccumulating ecotype (HE) of Sedum alfredii were investigated simultaneously. Under 25 μM Cd treatment, RCW thickness of NHE is nearly 2 folds than that of HE and the thickened cell wall of NHE was enriched in low-methylated pectin, leading to more Cd trapped in roots tightly. In the opposite, large amounts of high-methylated pectin were assembled around RCW of HE with Cd supply, in this way, HE S. alfredii decreased its root fixation of Cd and enhanced Cd migration into xylem. TEM and AFM results further confirmed that thickened cell wall was caused by the increased amounts of cellulose and lignin while root tip lignification was resulted from variations of sinapyl (S) and guaiacyl (G) monomers. Overall, thickened cell wall and methylated pectin have synchronicity in spatial location of roots, and their coordination contributed to Cd accumulation in S. alfredii.

Carbon dot induces tolerance to arsenic by regulating arsenic uptake, reactive oxygen species detoxification and defense-related gene expression in Cicer arietinum L

The scientific and technological applications of one of the nanomaterials viz.; carbon dot (C-dots), having extraordinary properties, is becoming an emerging and ongoing research area in recent times. In the present study, we have evaluated the effectiveness of C-dots in reducing arsenic (As) toxicity by analyzing physiological, biochemical and molecular parameters in Cicer arietinum L. The results revealed that As decreased the germination rate, growth, biomass, and membrane stability of the cell to a significant extent. Further, As was taken up by the growing seeds which eventually caused cell death. Levels of reactive oxygen species (ROS), stress markers (malondialdehyde), activities of defensive enzymes (glutathione-S-transferase and pyrroline-5-carboxylate synthetase) and non-enzymatic antioxidant contents (proline and glutathione) were increased under As stress. Moreover, As treatment resulted in the up-regulation of expressions of NADPH oxidase and defense-related genes in Cicer arietinum L. However, application of C-dots along with As improved the germination and growth of Cicer arietinum L. Exogenous application of C-dots, enhanced the expressions of defense-related genes and, contents of proline and glutathione, thereby causing considerable reductions in ROS, and malondialdehyde levels. Overall, this study suggests the possible involvement of C-dots in lowering the toxic effects of As on biomass by reducing As uptake and, inducing the activities/gene expressions and contents of enzymatic and non-enzymatic antioxidants.

Class III peroxidase: an indispensable enzyme for biotic/abiotic stress tolerance and a potent candidate for crop improvement

Class III peroxidases are secretory enzymes which belong to a ubiquitous multigene family in higher plants and have been identified to play role in a broad range of physiological and developmental processes. Potentially, it is involved in generation and detoxification of hydrogen peroxide (H2O2), and their subcellular localization reflects through three different cycles, namely peroxidative cycle, oxidative and hydroxylic cycles to maintain the ROS level inside the cell. Being an antioxidant, class III peroxidases are an important initial defence adapted by plants to cope with biotic and abiotic stresses. Both these stresses have become a major concern in the field of agriculture due to their devastating effect on plant growth and development. Despite numerous studies on plant defence against both the stresses, only a handful role of class III peroxidases have been uncovered by its functional characterization. This review will cover our current understanding on class III peroxidases and the signalling involved in their regulation under both types of stresses. The review will give a view of class III peroxidases and highlights their indispensable role under stress conditions. Its future application will be discussed to showcase their importance in crop improvement by genetic manipulation and by transcriptome analysis.

Overexpression of jasmonate-responsive OsbHLH034 in rice results in the induction of bacterial blight resistance via an increase in lignin biosynthesis

OsbHLH034 acts as a positive regulator in jasmonate signaling in rice. Jasmonic acid (JA) is a plant hormone under strict regulation by various transcription factors (TFs) that acts as a signaling compound in the regulation of plant defense responses and development. Here, we report that a basic helix-loop-helix (bHLH)-type TF, OsbHLH034, plays an important role in the JA-mediated resistance response against rice bacterial blight caused by Xanthomonas oryzae pv. oryzae. The expression of OsbHLH034 was upregulated at a late phase after JA treatment. OsbHLH034 interacted with a Jasmonate ZIM-domain (JAZ) protein, OsJAZ9, in both plant and yeast cells. Transgenic rice plants overexpressing OsbHLH034 exhibited a JA-hypersensitive phenotype and increased resistance against rice bacterial blight. Conversely, OsbHLH034-overexpressing plants exhibited high sensitivity to salt stress. The expression of some JA-responsive secretory-type peroxidase genes was upregulated in the OsbHLH034-overexpressing rice plants. Concomitantly, the lignin content significantly increased in these transgenic plants compared to that in the wild-type. These results indicate that OsbHLH034 acts as a positive regulator of the JA-mediated defense response in rice.

A member of wheat class III peroxidase gene family, TaPRX-2A, enhanced the tolerance of salt stress

BACKGROUND

Salt and drought are the main abiotic stresses that restrict the yield of crops. Peroxidases (PRXs) are involved in various abiotic stress responses. Furthermore, only few wheat PRXs have been characterized in the mechanism of the abiotic stress response.

RESULTS

In this study, a novel wheat peroxidase (PRX) gene named TaPRX-2A, a member of wheat class III PRX gene family, was cloned and its response to salt stress was characterized. Based on the identification and evolutionary analysis of class III PRXs in 12 plants, we proposed an evolutionary model for TaPRX-2A, suggesting that occurrence of some exon fusion events during evolution. We also detected the positive selection of PRX domain in 13 PRXs involving our evolutionary model, and found 2 or 6 positively selected sites during TaPRX-2A evolution. Quantitative reverse transcription-polymerase chain reaction (qRT-PCR) results showed that TaPRX-2A exhibited relatively higher expression levels in root tissue than those exhibited in leaf and stem tissues. TaPRX-2A expression was also induced by abiotic stresses and hormone treatments such as polyethylene glycol 6000, NaCl, hydrogen peroxide (H2O2), salicylic acid (SA), methyljasmonic acid (MeJA) and abscisic acid (ABA). Transgenic wheat plants with overexpression of TaPRX-2A showed higher tolerance to salt stress than wild-type (WT) plants. Confocal microscopy revealed that TaPRX-2A-eGFP was mainly localized in cell nuclei. Survival rate, relative water content, and shoot length were higher in TaPRX-2A-overexpressing wheat than in the WT wheat, whereas root length was not significantly different. The activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) were enhanced in TaPRX-2A-overexpressing wheat compared with those in the WT wheat, resulting in the reduction of reactive oxygen species (ROS) accumulation and malondialdehyde (MDA) content. The expression levels of downstream stress-related genes showed that RD22, TLP4, ABAI, GST22, FeSOD, and CAT exhibited higher expressions in TaPRX-2A-overexpressing wheat than in WT under salt stress.

CONCLUSIONS

The results show that TaPRX-2A plays a positive role in the response to salt stress by scavenging ROS and regulating stress-related genes.

Reactive Oxygen Species (ROS) Metabolism and Nitric Oxide (NO) Content in Roots and Shoots of Rice (Oryza sativa L.) Plants under Arsenic-Induced Stress

Arsenic (As) is a highly toxic metalloid for all forms of life including plants. Rice is the main food source for different countries worldwide, although it can take up high amounts of As in comparison with other crops, showing toxic profiles such as decreases in plant growth and yield. The induction of oxidative stress is the main process underlying arsenic toxicity in plants, including rice, due to an alteration of the reactive oxygen species (ROS) metabolism. The aim of this work was to gain better knowledge on how the ROS metabolism and its interaction with nitric oxide (NO) operate under As stress conditions in rice plants. Thus, physiological and ROS-related biochemical parameters in roots and shoots from rice (Oryza sativa L.) were studied under 50 μM arsenate (AsV) stress, and the involvement of the main antioxidative systems and NO in the response of plants to those conditions was investigated. A decrease of 51% in root length and 27% in plant biomass was observed with 50 μM AsV treatment, as compared to control plants. The results of the activity of superoxide dismutase (SOD) isozymes, catalase, peroxidase (POD: total and isoenzymatic), and the enzymes of the ascorbate–glutathione cycle, besides the ascorbate and glutathione contents, showed that As accumulation provoked an overall significant increase of most of them, but with different profiles depending on the plant organ, either root or shoot. Among the seven identified POD isozymes, the induction of the POD-3 in shoots under As stress could help to maintain the hydrogen peroxide (H2O2) redox homeostasis and compensate the loss of the ascorbate peroxidase (APX) activity in both roots and shoots. Lipid peroxidation was slightly increased in roots and shoots from As-treated plants. The H2O2 and NO contents were enhanced in roots and shoots against arsenic stress. In spite of the increase of most antioxidative systems, a mild oxidative stress situation appears to be consolidated overall, since the growth parameters and those from the oxidative damage could not be totally counteracted. In these conditions, the higher levels of H2O2 and NO suggest that signaling events are simultaneously occurring in the whole plant.

Influence of Aquatic pH on chemical speciation, phytochelation and vacuolar compartmentalization of arsenic in Vallisneria denseserrulata (Makino)

Arsenic (As) pollution of fresh water has become a major concern worldwide. The present study reports the As accumulation potential and detoxification mechanism in a native plant, Vallisneria denseserrulata (Makino), under different aquatic acidity conditions (pH). V. denseserrulata showed maximum growth at pH ∼7.0 and accumulated ∼1700 mg/kg of As. The increase in pH from 3.5 to 7 significantly (p ≤ 0.05) increased As accumulation, thiol and total protein contents while malondialdehyde (MDA) content, soluble sugar content and percentage electrolytic leakage (%EL) of V. denseserrulata were decreased. The reduction of arsenate [As(V)] to arsenite As(III) was observed as a key step (81% reduction) of the As detoxification in V. denseserrulata. Majority of accumulated As was found in vacuoles (56-72%), while >80% of As in vacuoles was in the form of As(III). FT-IR spectra indicated the complexsation of As with carboxyl, amide, thiol, and hydroxyl groups. Our findings showed the presence of As detoxification mechanism in V. denseserrulata. Vacuolar As compartmentalization and formation of As-Phytochelatins/thiol complexes can be a part of As detoxification mechanisms in V. denseserrulata.

Functional characterization of tau class glutathione-S-transferase in rice to provide tolerance against sheath blight disease

Glutathione-S-transferase (GST) is an important defense gene that confers resistance against several abiotic and biotic stresses. The present study identifies a tau class GST in rice (Oryza sativa L.), OsGSTU5 (Os09g20220), which provided tolerance against sheath blight (SB) disease, caused by a necrotrophic fungus, Rhizoctonia solani (RS). Overexpression and knockdown rice transgenic lines of OsGSTU5 were generated and tested for the severity of infection during sheath blight disease. The results obtained after RS infection showed that the lesion cover area and hyphal penetration were more in knockdown line and lesser in the overexpression line. Analysis of reactive oxygen species (ROS) accumulation showed more spots of H2O2 and O2- in knockdown lines compared to overexpressed lines. Later, RS transcript level was analyzed in RS-infected transgenic lines, which manifested that the knockdown line had higher RS transcripts in comparison to the control line and least RS transcripts were observed in the overexpressed line. In conclusion, rice transgenic lines overexpressing OsGSTU5 were found to be more tolerant, while the knockdown lines were more prone to Rhizoctonia infection compared to control lines.

Indigenous strain Bacillus XZM assisted phytoremediation and detoxification of arsenic in Vallisneria denseserrulata

The symbiosis between Vallisneria denseserrulata and indigenous Bacillus sp. XZM was investigated for arsenic removal for the first time. It was found that the native bacterium was able to reduce arsenic toxicity to the plant by producing higher amount of extra cellular polymeric substances (EPS), indole-3-acetic acid (IAA) and siderosphore. Interestingly, V. denseserrulata-Bacillus sp. XZM partnership showed significantly higher arsenic uptake and removal efficiency. The shift in FT-IR spectra indicated the involvement of amide, carboxyl, hydroxyl and thiol groups in detoxification of arsenic, and the existence of an arsenic metabolizing process in V. denseserrulata leaves. The scanning electron microscopy (SEM) images further confirmed that the bacterium colonized on plant roots and facilitated arsenic uptake by plant under inoculation condition. In plant, most of the arsenic existed as As(III) (85%) and was massively (>77%) found in vacuole of particularly leaves cells. Thus, these findings are highly suggested for arsenic remediation in the constructed wetlands.

Genome-wide and evolutionary analysis of the class III peroxidase gene family in wheat and Aegilops tauschii reveals that some members are involved in stress responses

Background

The class III peroxidase (PRX) gene family is a plant-specific member of the PRX superfamily that is closely related to various physiological processes, such as cell wall loosening, lignification, and abiotic and biotic stress responses. However, its classification, evolutionary history and gene expression patterns are unclear in wheat and Aegilops tauschii.

Results

Here, we identified 374, 159 and 169 PRXs in Triticum aestivum, Triticum urartu and Ae. tauschii, respectively. Together with PRXs detected from eight other plants, they were classified into 18 subfamilies. Among subfamilies V to XVIII, a conserved exon-intron structure within the “001” exon phases was detected in the PRX domain. Based on the analysis, we proposed a phylogenetic model to infer the evolutionary history of the exon-intron structures of PRX subfamilies. A comparative genomics analysis showed that subfamily VII could be the ancient subfamily that originated from green algae (Chlamydomonas reinhardtii). Further integrated analysis of chromosome locations and collinearity events of PRX genes suggested that both whole genome duplication (WGD) and tandem duplication (TD) events contributed to the expansion of T. aestivum PRXs (TaePRXs) during wheat evolution. To validate functions of these genes in the regulation of various physiological processes, the expression patterns of PRXs in different tissues and under various stresses were studied using public microarray datasets. The results suggested that there were distinct expression patterns among different tissues and PRXs could be involved in biotic and abiotic responses in wheat. qRT-PCR was performed on samples exposed to drought, phytohormone treatments and Fusarium graminearum infection to validate the microarray predictions. The predicted subcellular localizations of some TaePRXs were consistent with the confocal microscopy results. We predicted that some TaePRXs had hormone-responsive cis-elements in their promoter regions and validated these predicted cis-acting elements by sequencing promoters.

Conclusion

In this study, identification, classification, evolution, and expression patterns of PRXs in wheat and relative plants were performed. Our results will provide information for further studies on the evolution and molecular mechanisms of wheat PRXs.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-6006-5) contains supplementary material, which is available to authorized users.

Plant Cell Wall Proteomics: A Focus on Monocot Species, Brachypodium distachyon, Saccharum spp. and Oryza sativa

Plant cell walls mostly comprise polysaccharides and proteins. The composition of monocots' primary cell walls differs from that of dicots walls with respect to the type of hemicelluloses, the reduction of pectin abundance and the presence of aromatic molecules. Cell wall proteins (CWPs) differ among plant species, and their distribution within functional classes varies according to cell types, organs, developmental stages and/or environmental conditions. In this review, we go deeper into the findings of cell wall proteomics in monocot species and make a comparative analysis of the CWPs identified, considering their predicted functions, the organs analyzed, the plant developmental stage and their possible use as targets for biofuel production. Arabidopsis thaliana CWPs were considered as a reference to allow comparisons among different monocots, i.e., Brachypodium distachyon, Saccharum spp. and Oryza sativa. Altogether, 1159 CWPs have been acknowledged, and specificities and similarities are discussed. In particular, a search for A. thaliana homologs of CWPs identified so far in monocots allows the definition of monocot CWPs characteristics. Finally, the analysis of monocot CWPs appears to be a powerful tool for identifying candidate proteins of interest for tailoring cell walls to increase biomass yield of transformation for second-generation biofuels production.

Recent advances in arsenic metabolism in plants: current status, challenges and highlighted biotechnological intervention to reduce grain arsenic in rice

Arsenic (As), classified as a "metalloid" element, is well known for its carcinogenicity and other toxic effects to humans. Arsenic exposure in plants results in the alteration of the physiochemical and biological properties and consequently, loss of crop yield. Being a staple food for half of the world's population, the consumption of As-contaminated rice grain by humans may pose serious health issues and risks for food security. In this study, we have described the principal understanding of the molecular basis of arsenic toxicity and accumulation in plant parts. We described the measures for decreasing As accumulation in rice and understanding the mechanism and transport of As uptake, its transport from root to shoot to rice grain, its metabolism, detoxification, as well as the mechanisms lying behind its accumulation in rice grains. There are various checkpoints, such as the tuning of AsV/Pi specific Pi transporters, arsenate reductase, transporters that are involved in the efflux of As to either the vacuole or outside the cell, xylem loading, loading and unloading to the phloem, and transporters involved in the loading of As to grain, that can be targeted to reduce As accumulation in rice grain. Genes/proteins involved in As detoxification, particularly the glutathione (GSH) biosynthesis pathway, phytochelatin (PC) synthesis, and arsenic methyltransferase, also provide a great pool of pathways that can also be castellated for the low As in rice grains. Paddy rice is also used as fodder for animals, enhancing vacuolar sequestration and using constitutive promoters, which may be of concern for animal health. Therefore, using a root-specific promoter and/or converting inorganic arsenic into volatile organic arsenic might be a better strategy for low As in grain. Furthermore, in this review, the other specific approaches, such as bio-remediation, bio-augmentation practices, and molecular breeding, which have great potential to reduce As uptake from soil to rice grains, have also been highlighted.

Genotype Variation in Rice (Oryza sativa L.) Tolerance to Fe Toxicity Might Be Linked to Root Cell Wall Lignification

Iron (Fe) is an essential element to plants, but can be harmful if accumulated to toxic concentrations. Fe toxicity can be a major nutritional disorder in rice (Oryza sativa) when cultivated under waterlogged conditions, as a result of excessive Fe solubilization of in the soil. However, little is known about the basis of Fe toxicity and tolerance at both physiological and molecular level. To identify mechanisms and potential candidate genes for Fe tolerance in rice, we comparatively analyzed the effects of excess Fe on two cultivars with distinct tolerance to Fe toxicity, EPAGRI 108 (tolerant) and BR-IRGA 409 (susceptible). After excess Fe treatment, BR-IRGA 409 plants showed reduced biomass and photosynthetic parameters, compared to EPAGRI 108. EPAGRI 108 plants accumulated lower amounts of Fe in both shoots and roots compared to BR-IRGA 409. We conducted transcriptomic analyses of roots from susceptible and tolerant plants under control and excess Fe conditions. We found 423 up-regulated and 92 down-regulated genes in the susceptible cultivar, and 42 up-regulated and 305 down-regulated genes in the tolerant one. We observed striking differences in root gene expression profiles following exposure to excess Fe: the two cultivars showed no genes regulated in the same way (up or down in both), and 264 genes were oppositely regulated in both cultivars. Plants from the susceptible cultivar showed down-regulation of known Fe uptake-related genes, indicating that plants are actively decreasing Fe acquisition. On the other hand, plants from the tolerant cultivar showed up-regulation of genes involved in root cell wall biosynthesis and lignification. We confirmed that the tolerant cultivar has increased lignification in the outer layers of the cortex and in the vascular bundle compared to the susceptible cultivar, suggesting that the capacity to avoid excessive Fe uptake could rely in root cell wall remodeling. Moreover, we showed that increased lignin concentrations in roots might be linked to Fe tolerance in other rice cultivars, suggesting that a similar mechanism might operate in multiple genotypes. Our results indicate that changes in root cell wall and Fe permeability might be related to Fe toxicity tolerance in rice natural variation.