Peroxisome dynamics in Arabidopsis plants under oxidative stress induced by cadmium

Cadmium exposure induced light/dark- and time-dependent redox changes at subcellular level in Arabidopsis plants

Cadmium (Cd) is one of the most toxic heavy metals for plants and humans. Reactive oxygen species (ROS) are some of the primary signaling molecules produced after Cd treatment in plants but the contribution of different organelles and specific cell types, together with the impact of light is unknown. We used Arabidopsis lines expressing GRX1-roGFP2 (glutaredoxin1-roGFP) targeted to different cell compartments and analysed changes in redox state over 24 h light/dark cycle in Cd-treated leaf discs. We imaged redox state changes in peroxisomes and chloroplasts in leaf tissue. Chloroplasts and peroxisomes were the most affected organelles in the dark and blocking the photosynthetic electron transport chain (pETC) by DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea) promotes higher Cd-dependent oxidation in all organelles. Peroxisomes underwent the most rapid changes in redox state in response to Cd and DCMU and silencing chloroplastic NTRC (NADPH thioredoxin reductase C) considerably increases peroxisome oxidation. Total NAD(P)H and cytosolic NADH decreased during exposure to Cd, while Ca+2 content in chloroplasts and cytosol increased in the dark period. Our results demonstrate a Cd-, time- and light-dependent increase of oxidation of all organelles analysed, that could be in part triggered by disturbances in pETC and photorespiration, the decrease of NAD(P)H availability, and differential antioxidants expression at subcellular level.

Evaluation of Resistance Resources and Analysis of Resistance Mechanisms of Maize to Stalk Rot Caused by Fusarium graminearum

Stalk rot is one of the most destructive and widely distributed diseases in maize plants worldwide. Research on the performance and resistance mechanisms of maize against stem rot is constantly improving. In this study, among 120 inbred maize lines infected by Fusarium graminearum using the injection method, 4 lines (3.33%) were highly resistant to stalk rot, 28 lines (23.33%) were resistant, 57 lines (47.50%) were susceptible, and 31 lines (25.84%) were highly susceptible. The inbred lines 18N10118 and 18N10370 were the most resistant and susceptible with disease indices of 7.5 and 75.6, respectively. Treatment of resistant and susceptible maize inbred seedlings with F. graminearum showed that root hair growth of the susceptible inbred lines was significantly inhibited, and a large number of hyphae attached and adsorbed multiple conidia near the root system. However, the resistant inbred lines were delayed and inconspicuous, with only a few hyphae and spores appearing near the root system. Compared with susceptible inbred lines, resistant maize inbred line seedlings treated with F. graminearum exhibited elevated activities of catalase, phenylalanine ammonia-lyase, polyphenol oxidase, and superoxide dismutase. We identified 153 genes related to disease resistance by transcriptome analysis. The mitogen-activated protein kinase signaling and peroxisome pathways mainly regulated the resistance mechanism of maize inbred lines to F. graminearum infection. These two pathways might play an important role in the disease resistance mechanism, and the function of genes in the two pathways must be further studied, which might provide a theoretical basis for further understanding the molecular resistance mechanism of stalk rot and resistance gene mining.

Visualizing the dynamics of plant energy organelles

Plant organelles predominantly rely on the actin cytoskeleton and the myosin motors for long-distance trafficking, while using microtubules and the kinesin motors mostly for short-range movement. The distribution and motility of organelles in the plant cell are fundamentally important to robust plant growth and defense. Chloroplasts, mitochondria, and peroxisomes are essential organelles in plants that function independently and coordinately during energy metabolism and other key metabolic processes. In response to developmental and environmental stimuli, these energy organelles modulate their metabolism, morphology, abundance, distribution and motility in the cell to meet the need of the plant. Consistent with their metabolic links in processes like photorespiration and fatty acid mobilization is the frequently observed inter-organellar physical interaction, sometimes through organelle membranous protrusions. The development of various organelle-specific fluorescent protein tags has allowed the simultaneous visualization of organelle movement in living plant cells by confocal microscopy. These energy organelles display an array of morphology and movement patterns and redistribute within the cell in response to changes such as varying light conditions, temperature fluctuations, ROS-inducible treatments, and during pollen tube development and immune response, independently or in association with one another. Although there are more reports on the mechanism of chloroplast movement than that of peroxisomes and mitochondria, our knowledge of how and why these three energy organelles move and distribute in the plant cell is still scarce at the functional and mechanistic level. It is critical to identify factors that control organelle motility coupled with plant growth, development, and stress response.

RNA-Seq Analysis of Aboveground and Underground Parts of Biomass Sorghum Was Performed to Evaluate Its Suitability for Environmental Remediation

"Alto2" is a new biomass sorghum variety, which has the characteristics of fast growth, high growth, and strong cadmium (Cd) resistance, so it has the application prospect of soil remediation plants. In order to reveal the Cd resistance mechanism of this plant and pave the way for genetic breeding and cultivation of efficient remediation plants in the future, in this research, through the determination of Cd content in various tissues of sorghum under Cd stress and the physicochemical response combined with RNA-Seq analysis, the mechanism of Cd resistance of "Alto2" was initially revealed. The results show biomass sorghum "Alto2" was mainly connected with aboveground and underground parts through the MAPK signaling pathway and plant hormone signaling pathway, and transmit stress signal in response to Cd stress. Chelase and metal-binding proteins may be the functional genes mainly responsible for Cd enrichment and transport and regulated by stress signals. However, the expression of aboveground transporters was not significant. This may be because Cd in biomass sorghum is mainly concentrated in the underground part and is enriched by the chelation of secondary metabolites from plant roots by the cell wall leading to inhibition of aboveground transporter expression. The results of this study indicate that the biomass sorghum "Alto2" on Cd has high resistance, but the lack of the aboveground enrichment of transportability requires further research to improve the Cd transportability of this plant.

Genome-Wide Identification of Polyamine Oxidase (PAO) Family Genes: Roles of CaPAO2 and CaPAO4 in the Cold Tolerance of Pepper (Capsicum annuum L.)

Polyamine oxidases (PAOs), which are flavin adenine dinucleotide-dependent enzymes, catalyze polyamine (PA) catabolism, producing hydrogen peroxide (H₂O₂). Several PAO family members have been identified in plants, but their expression in pepper plants remains unclear. Here, six PAO genes were identified in the ‘Zunla-1’ pepper genome (named CaPAO1–CaPAO6 according to their chromosomal positions). The PAO proteins were divided into four subfamilies according to phylogenetics: CaPAO1 belongs to subfamily I; CaPAO3 and CaPAO5 belong to subfamily III; and CaPAO2, CaPAO4, and CaPAO6 belong to subfamily IV (none belong to subfamily II). CaPAO2, CaPAO4, and CaPAO6 were ubiquitously and highly expressed in all tissues, CaPAO1 was mainly expressed in flowers, whereas CaPAO3 and CaPAO5 were expressed at very low levels in all tissues. RNA-seq analysis revealed that CaPAO2 and CaPAO4 were notably upregulated by cold stress. CaPAO2 and CaPAO4 were localized in the peroxisome, and spermine was the preferred substrate for PA catabolism. CaPAO2 and CaPAO4 overexpression in Arabidopsis thaliana significantly enhanced freezing-stress tolerance by increasing antioxidant enzyme activity and decreasing malondialdehyde, H₂O₂, and superoxide accumulation, accompanied by the upregulation of cold-responsive genes (AtCOR15A, AtRD29A, AtCOR47, and AtKIN1). Thus, we identified candidate PAO genes for breeding cold-stress-tolerant transgenic pepper cultivars.

Plant peroxisome proteostasis-establishing, renovating, and dismantling the peroxisomal proteome

Plant peroxisomes host critical metabolic reactions and insulate the rest of the cell from reactive byproducts. The specialization of peroxisomal reactions is rooted in how the organelle modulates its proteome to be suitable for the tissue, environment, and developmental stage of the organism. The story of plant peroxisomal proteostasis begins with transcriptional regulation of peroxisomal protein genes and the synthesis, trafficking, import, and folding of peroxisomal proteins. The saga continues with assembly and disaggregation by chaperones and degradation via proteases or the proteasome. The story concludes with organelle recycling via autophagy. Some of these processes as well as the proteins that facilitate them are peroxisome-specific, while others are shared among organelles. Our understanding of translational regulation of plant peroxisomal protein transcripts and proteins necessary for pexophagy remain based in findings from other models. Recent strides to elucidate transcriptional control, membrane dynamics, protein trafficking, and conditions that induce peroxisome turnover have expanded our knowledge of plant peroxisomal proteostasis. Here we review our current understanding of the processes and proteins necessary for plant peroxisome proteostasis-the emergence, maintenance, and clearance of the peroxisomal proteome.

Insights into ROS-dependent signalling underlying transcriptomic plant responses to the herbicide 2,4-D

The synthetic auxin 2,4-dichlorophenoxyacetic acid (2,4-D) functions as an agronomic weed control herbicide. High concentrations of 2,4-D induce plant growth defects, particularly leaf epinasty and stem curvature. Although the 2,4-D triggered reactive oxygen species (ROS) production, little is known about its signalling. In this study, by using a null mutant in peroxisomal acyl CoA oxidase 1 (acx1-2), we identified acyl-coenzyme A oxidase 1 (ACX1) as one of the main sources of ROS production and, in part, also causing the epinastic phenotype following 2,4-D application. Transcriptomic analyses of wild type (WT) plants after treatment with 2,4-D revealed a ROS-related peroxisomal footprint in early plant responses, while other organelles, such as mitochondria and chloroplasts, are involved in later responses. Interestingly, a group of 2,4-D-responsive ACX1-dependent transcripts previously associated with epinasty is related to auxin biosynthesis, metabolism, and signalling. We found that the auxin receptor auxin signalling F-box 3 (AFB3), a component of Skp, Cullin, F-box containing complex (SCF) (ASK-cullin-F-box) E3 ubiquitin ligase complexes, which mediates auxin/indole acetic acid (AUX/IAA) degradation by the 26S proteasome, acts downstream of ACX1 and is involved in the epinastic phenotype induced by 2,4-D. We also found that protein degradation associated with ubiquitin E3-RING and E3-SCF-FBOX in ACX1-dependent signalling in plant responses to 2,4-D is significantly regulated over longer treatment periods.

An update on redox signals in plant responses to biotic and abiotic stress crosstalk: insights from cadmium and fungal pathogen interactions

Complex signalling pathways are involved in plant protection against single and combined stresses. Plants are able to coordinate genome-wide transcriptional reprogramming and display a unique programme of transcriptional responses to a combination of stresses that differs from the response to single stresses. However, a significant overlap between pathways and some defence genes in the form of shared and general stress-responsive genes appears to be commonly involved in responses to multiple biotic and abiotic stresses. Reactive oxygen and nitrogen species, as well as redox signals, are key molecules involved at the crossroads of the perception of different stress factors and the regulation of both specific and general plant responses to biotic and abiotic stresses. In this review, we focus on crosstalk between plant responses to biotic and abiotic stresses, in addition to possible plant protection against pathogens caused by previous abiotic stress. Bioinformatic analyses of transcriptome data from cadmium- and fungal pathogen-treated plants focusing on redox gene ontology categories were carried out to gain a better understanding of common plant responses to abiotic and biotic stresses. The role of reactive oxygen and nitrogen species in the complex network involved in plant responses to changes in their environment is also discussed.

The influence of application of biochar and metal-tolerant bacteria in polluted soil on morpho-physiological and anatomical parameters of spring barley

The paper presents the results of the model experiment on spring barley (Hordeum vulgare L.) grown in polluted soil. The influence of separate and combined application of wood biochar and heavy metal-tolerant bacteria on morpho-physiological, anatomical and ultrastructural parameters of H. vulgare L. has been studied. The joint application of biochar and bacteria increased the shoot length by 2.1-fold, root length by 1.7-fold, leaf length by 2.3-fold and dry weight by threefold compared to polluted variant, bringing the plant parameters to the control level. The maximal quantum yield of photosystem II decreased by 8.3% in H. vulgare L. grown in contaminated soil, whereas this decrease was less in biochar (7%), bacteria (6%) and in combined application of bacteria and biochar (5%). As for the transpiration rate, the H. vulgare L. grown in polluted soil has shown a decrease in transpiration rate by 26%. At the same time, the simultaneous application of biochar and bacteria has led to a significant improvement in the transpiration rate (14%). The H. vulgare L. also showed anatomical (integrity of epidermal, vascular bundles, parenchymal and chlorenchymal cells) and ultrastructural (chloroplasts, thylakoid system, plastoglobules, starch grains, mitochondria, peroxisomes, ribosomes, endoplasmic reticulum, vacuoles) changes, revealed by light-optical and transmission electron microscopy of leaf sections. The effects were most prominent in H. vulgare L., grown in polluted soil but gradually improved with application of biochar, bacteria and their combination. The use of biochar in combination with metal-tolerant bacteria is an efficient tool for remediation of soils, contaminated with heavy metals. The positive changes caused by the treatment can be consistently traced at all levels of plant organization.

Silencing MdGH3-2/12 in apple reduces cadmium resistance via the regulation of AM colonization

Arbuscular mycorrhizal fungi (AMF) can form a symbiotic relationship with most terrestrial plant roots, promote plant growth, and heavy metal (HM) tolerance and thus plays a crucial role in phytoremediation. However, research on the relationship between colonization level and HM tolerance is limited. In this study, apple (Malus domestica) Gretchen Hagen3 genes MdGH3-2/12 silencing plants were treated with four AMF and Cd combination treatments to determine AMF colonization levels, biomass, Cd accumulation, photosynthesis, fluorescence, reactive oxygen species (ROS) and antioxidant substance accumulation, and Cd uptake, transport and detoxification gene expression levels. Results indicate the greater sensitivity of transgenic plants under AMF inoculation and Cd treatment compared with wild type (WT) via lower AMF colonization levels, biomass accumulation, photosynthetic parameters, and the accumulation and clearance homeostasis of ROS, as well as lower detoxification expression levels and higher Cd uptake and transport expression levels. Our study essentially demonstrates that MdGH3-2/12 plays an important role in Cd stress tolerance by regulating AM colonization in apple.

Salicylic acid mediated reduction in grain cadmium accumulation and amelioration of toxicity in Oryza sativa L. cv Bandana

Contamination of agricultural fields with Cadmium (Cd) due to several agricultural practices is increasing worldwide. The rice plants can easily take up Cd and accumulate it into different parts, including the grains, posing a threat to human health even at low concentration exposure. Several phytohormones, including Salicylic acid (SA) have been investigated since long for its alleviating properties under various biotic and abiotic stress conditions. In the present study, 100 μM SA application to ameliorate 25 μM Cd stress was studied for 72 h in hydroponics in Oryza sativa cv. Bandana seedlings. Pot experiments were done with same treatment condition and plants were grown till maturity. SA application to Cd exposed rice seedlings alleviated the stress condition, which was established by several physiological, biochemical, histochemical and gene expression analysis. SA treatment to Cd stressed seedlings showed elevated photosynthetic pigment content, on-protein thiol content and relieved the Cd induced growth inhibition considerably. It lowered the accumulation of ROS like, O2^- and H₂O₂ with a regulated antioxidative enzymatic activity. SA application in Cd exposed rice seedlings had upregulated expression of OsHMA3 and OsPCS1 whereasOsNRAMP2 gene was downregulated. Co-application of SA and Cd led to higher yield and improved agronomic traits in comparison to only Cd exposed plants under pot experimentation. Daily intake of Cd and Carcinogenic risk were also reduced by 99.75% and 99.99% respectively in the SA treated Cd stressed plants. SA positively affected the growth and tolerance of rice seedlings to Cd stress. Hence, SA addition to Cd contaminated soil can ensure rice cultivation without posing health risk to consumers.

Metabolic rearrangements in imbibed maize (Zea mays L) embryos in the presence of oxidative stressors

Cadmium (Cd) is a metal known to generate oxidative stress in plants and may be particularly harmful during germination. Herein, the growth and metabolic rearrangements of maize embryo axes subjected during the imbibition stage to Cd ions and other two well-known oxidative stressors, methyl viologen (MV) and hydrogen peroxide (H₂O₂), were assessed for 48 h. Similar decreases in embryo's length were detected for all stressed axes up to 48 h of imbibition. By this time, treated embryos revealed greater accumulation of reactive oxygen species (ROS) and increased levels of carbonylated and ubiquitinated proteins. The proteolytic activities were intensely enhanced in the treated axes, particularly at 48 h of imbibition, and several antioxidant enzymes were induced in most cases. NMR spectroscopy followed by principal component analysis (PCA) and hierarchical cluster analysis (HCA) showed that a large proportion of polar metabolites, mainly amino acids and organic acids, were decreased under stress conditions, while carbohydrates were increased at 48 h of imbibition, with significant increases in glucose and raffinose for treated embryos relatively to controls. We demonstrated that maize embryo axes were capable of shifting their metabolism to improve their antioxidant defense system, at the expense of their growth. Under these adverse conditions, proteolysis seems to play a key role by providing free amino acids needed for the de novo synthesis of defense-related proteins.

Nitric oxide is essential for cadmium-induced peroxule formation and peroxisome proliferation

Nitric oxide (NO) and nitrosylated derivatives are produced in peroxisomes, but the impact of NO metabolism on organelle functions remains largely uncharacterised. Double and triple NO-related mutants expressing cyan florescent protein (CFP)-SKL (nox1 × px-ck and nia1 nia2 × px-ck) were generated to determine whether NO regulates peroxisomal dynamics in response to cadmium (Cd) stress using confocal microscopy. Peroxule production was compromised in the nia1 nia2 mutants, which had lower NO levels than the wild-type plants. These findings show that NO is produced early in the response to Cd stress and was involved in peroxule production. Cd-induced peroxisomal proliferation was analysed using electron microscopy and by the accumulation of the peroxisomal marker PEX14. Peroxisomal proliferation was inhibited in the nia1 nia2 mutants. However, the phenotype was recovered by exogenous NO treatment. The number of peroxisomes and oxidative metabolism were changed in the NO-related mutant cells. Furthermore, the pattern of oxidative modification and S-nitrosylation of the catalase (CAT) protein was changed in the NO-related mutants in both the absence and presence of Cd stress. Peroxisome-dependent signalling was also affected in the NO-related mutants. Taken together, these results show that NO metabolism plays an important role in peroxisome functions and signalling.

Effects of cadmium on two wheat cultivars depending on different nitrogen supply

Heavy metal pollution as well as improper fertilization management represent serious threats to a clean environment and healthy food. This study was conducted to investigate how nitrogen supply influences a plant's ability to cope with cadmium stress in the two wheat cultivars - the modern cv. Katya (carrier of the semi-dwarfing gene Rht8) and the old cv. Slomer. Here we examined the effects of 100 μM CdCl₂ on both wheat genotypes grown hydroponically under three different nutrition regimes of 5.5, 10 and 20 mM NO₃^- by investigating plant growth, pigment content and the functional activity of the photosynthetic apparatus through a combination of PAM chlorophyll fluorescence, P700 photooxidation, oxygen evolution and oxidative stress markers. Data showed that the different genetic background affects the different strategies for metal uptake and allocation, as well as abilities to deal with oxidative stress. The modern cv. Katya restricts the entry of the metal to the roots, but allows its translocation to the shoots. Nevertheless, the photosynthetic performance indicated better protection, possibly mediated by the Rht8 allele. In contrast, the old cv. Slomer tolerates higher cadmium levels in roots and possesses efficient barriers against its transfer to the shoots, but still showed more impaired photosynthetic activity. In general, the impact of cadmium on the photosynthetic apparatus was most deleterious under the lowest nitrogen concentration which was applied, while the highest nitrogen supply alleviated the negative effects of cadmium. The data suggest that the modern breeding allele (Rht8), as well as a better nutrition might contribute to the tolerance to heavy metal stress in the wheat.

Subcellular compartmentalization of the plant antioxidant system: an integrated overview

The antioxidant system (AOS) maintains the optimal concentration of reactive oxygen species (ROS) in a cell and protects it against oxidative stress. In plants, the AOS consists of seven main classes of antioxidant enzymes, low-molecular antioxidants (e.g., ascorbate, glutathione, and their oxidized forms) and thioredoxin/glutaredoxin systems which can serve as reducing agents for antioxidant enzymes. The number of genes encoding AOS enzymes varies between classes, and same class enzymes encoded by different gene copies may have different subcellular localizations, functional loads and modes of evolution. These facts hereafter reinforce the complex nature of AOS regulation and functioning. Further studies can describe new trends in the behavior and functioning of systems components, and provide new fundamental knowledge about systems regulation. The system is revealed to have a lot of interactions and interplay pathways between its components at the subcellular level (antioxidants, enzymes, ROS level, and hormonal and transcriptional regulation). These facts should be taken into account in further studies during the AOS modeling by describing the main pathways of generating and utilizing ROS, as well as the associated signaling processes and regulation of the system on cellular and organelle levels, which is a complicated and ambitious task. Another objective for studying the phenomenon of the AOS is related to the influence of cell dynamics and circadian rhythms on it. Therefore, the AOS requires an integrated and multi-level approach to study. We focused this review on the existing scientific background and experimental data used for the systems biology research of the plant AOS.

Cadmium and arsenic-induced-stress differentially modulates Arabidopsis root architecture, peroxisome distribution, enzymatic activities and their nitric oxide content

In plant cells, cadmium (Cd) and arsenic (As) exert toxicity mainly by inducing oxidative stress through an imbalance between the production of reactive oxygen species (ROS) and reactive nitrogen species (RNS), and their detoxification. Nitric oxide (NO) is a RNS acting as signalling molecule coordinating plant development and stress responses, but also as oxidative stress inducer, depending on its cellular concentration. Peroxisomes are versatile organelles involved in plant metabolism and signalling, with a role in cellular redox balance thanks to their antioxidant enzymes, and their RNS (mainly NO) and ROS. This study analysed Cd or As effects on peroxisomes, and NO production and distribution in the root system, including primary root (PR) and lateral roots (LRs). Arabidopsis thaliana wild-type and transgenic plants enabling peroxisomes to be visualized in vivo, through the expression of the 35S-cyan fluorescent protein fused to the peroxisomal targeting signal1 (PTS1) were used. Peroxisomal enzymatic activities including the antioxidant catalase, the H₂O₂-generating glycolate oxidase, and the hydroxypyruvate reductase, and root system morphology were also evaluated under Cd/As exposure. Results showed that Cd and As differently modulate these activities, however, catalase activity was inhibited by both. Moreover, Arabidopsis root system was altered, with the pollutants differently affecting PR growth, but similarly enhancing LR formation. Only in the PR apex, and not in LR one, Cd more than As caused significant changes in peroxisome distribution, size, and in peroxisomal NO content. By contrast, neither pollutant caused significant changes in peroxisomes size and peroxisomal NO content in the LR apex.

On the move: redox-dependent protein relocation in plants

Compartmentation of proteins and processes is a defining feature of eukaryotic cells. The growth and development of organisms is critically dependent on the accurate sorting of proteins within cells. The mechanisms by which cytosol-synthesized proteins are delivered to the membranes and membrane compartments have been extensively characterized. However, the protein complement of any given compartment is not precisely fixed and some proteins can move between compartments in response to metabolic or environmental triggers. The mechanisms and processes that mediate such relocation events are largely uncharacterized. Many proteins can in addition perform multiple functions, catalysing alternative reactions or performing structural, non-enzymatic functions. These alternative functions can be equally important functions in each cellular compartment. Such proteins are generally not dual-targeted proteins in the classic sense of having targeting sequences that direct de novo synthesized proteins to specific cellular locations. We propose that redox post-translational modifications (PTMs) can control the compartmentation of many such proteins, including antioxidant and/or redox-associated enzymes.

Multilevel Regulation of Peroxisomal Proteome by Post-Translational Modifications

Peroxisomes, which are ubiquitous organelles in all eukaryotes, are highly dynamic organelles that are essential for development and stress responses. Plant peroxisomes are involved in major metabolic pathways, such as fatty acid β-oxidation, photorespiration, ureide and polyamine metabolism, in the biosynthesis of jasmonic, indolacetic, and salicylic acid hormones, as well as in signaling molecules such as reactive oxygen and nitrogen species (ROS/RNS). Peroxisomes are involved in the perception of environmental changes, which is a complex process involving the regulation of gene expression and protein functionality by protein post-translational modifications (PTMs). Although there has been a growing interest in individual PTMs in peroxisomes over the last ten years, their role and cross-talk in the whole peroxisomal proteome remain unclear. This review provides up-to-date information on the function and crosstalk of the main peroxisomal PTMs. Analysis of whole peroxisomal proteomes shows that a very large number of peroxisomal proteins are targeted by multiple PTMs, which affect redox balance, photorespiration, the glyoxylate cycle, and lipid metabolism. This multilevel PTM regulation could boost the plasticity of peroxisomes and their capacity to regulate metabolism in response to environmental changes.

Cadmium induces reactive oxygen species-dependent pexophagy in Arabidopsis leaves

Cadmium treatment induces transient peroxisome proliferation in Arabidopsis leaves. To determine whether this process is regulated by pexophagy and to identify the mechanisms involved, we analysed time course-dependent changes in ATG8, an autophagy marker, and the accumulation of peroxisomal marker PEX14a. After 3 hr of Cd exposure, the transcript levels of ATG8h, ATG8c, a, and i were slightly up-regulated and then returned to normal. ATG8 protein levels also increased after 3 hr of Cd treatment, although an opposite pattern was observed in PEX14. Arabidopsis lines expressing GFP-ATG8a and CFP-SKL enabled us to demonstrate the presence of pexophagic processes in leaves. The Cd-dependent induction of pexophagy was demonstrated by the accumulation of peroxisomes in autophagy gene (ATG)-related Arabidopsis knockout mutants atg5 and atg7. We show that ATG8a colocalizes with catalase and NBR1 in the electron-dense peroxisomal core, thus suggesting that NBR1 may be an autophagic receptor for peroxisomes, with catalase being possibly involved in targeting pexophagy. Protein carbonylation and peroxisomal redox state suggest that protein oxidation may trigger pexophagy. Cathepsine B, legumain, and caspase 6 may also be involved in the regulation of pexophagy. Our results suggest that pexophagy could be an important step in rapid cell responses to cadmium.

Re-evaluation of physical interaction between plant peroxisomes and other organelles using live-cell imaging techniques

The dynamic behavior of organelles is essential for plant survival under various environmental conditions. Plant organelles, with various functions, migrate along actin filaments and contact other types of organelles, leading to physical interactions at a specific site called the membrane contact site. Recent studies have revealed the importance of physical interactions in maintaining efficient metabolite flow between organelles. In this review, we first summarize peroxisome function under different environmental conditions and growth stages to understand organelle interactions. We then discuss current knowledge regarding the interactions between peroxisome and other organelles, i.e., the oil bodies, chloroplast, and mitochondria from the perspective of metabolic and physiological regulation, with reference to various organelle interactions and techniques for estimating organelle interactions occurring in plant cells.

Transcriptomic Analysis Reveals a Comprehensive Calcium- and Phytohormone-Dominated Signaling Response in Leymus chinensis Self-Incompatibility

Sheepgrass (Leymus chinensis (Trin.) Tzvel.) is an economically and ecologically important forage in the grass family. Self-incompatibility (SI) limits its seed production due to the low seed-setting rate after self-pollination. However, investigations into the molecular mechanisms of sheepgrass SI are lacking. Therefore, microscopic observation of pollen germination and pollen tube growth, as well as transcriptomic analyses of pistils after self- and cross-pollination, were performed. The results indicated that pollen tube growth was rapidly inhibited from 10 to 30 min after self-pollination and subsequently stopped but preceded normally after cross-pollination. Time course comparative transcriptomics revealed different transcriptome dynamics between self- and cross-pollination. A pool of SI-related signaling genes and pathways was generated, including genes related to calcium (Ca2+) signaling, protein phosphorylation, plant hormone, reactive oxygen species (ROS), nitric oxide (NO), cytoskeleton, and programmed cell death (PCD). A putative SI response molecular model in sheepgrass was presented. The model shows that SI may trigger a comprehensive calcium- and phytohormone-dominated signaling cascade and activate PCD, which may explain the rapid inhibition of self-pollen tube growth as observed by cytological analyses. These results provided new insight into the molecular mechanisms of sheepgrass (grass family) SI.

Foliar spraying of melatonin confers cadmium tolerance in Nicotiana tabacum L

Melatonin is a multifunctional signaling molecule that regulates broad aspects of responses to environmental stresses in plants. Cadmium (Cd) is a persistent soil contaminant that is toxic to all living organisms. Recent reports have uncovered the protective role of melatonin in alleviating Cd phytotoxicity, but little is known about its regulatory mechanisms in plants. In this study, we found that foliar application of melatonin (in particular 100 μmol L-1) remarkably enhanced Cd tolerance of tobacco (Nicotiana tabacum L.) leaves, as evidenced by less Cd accumulation and alleviation of growth inhibition and photoinhibition, compared with nontreated Cd-stressed plants. The addition of melatonin also controlled oxidative damage of Cd on tobacco through direct scavenging and by enhancing the activities of antioxidative enzymes. Melatonin application promoted Cd sequestration in the cell wall and vacuoles based on the analysis of subcellular distribution of Cd in tobacco cells. Structural equation modeling (SEM) analysis revealed that melatonin-induced Cd tolerance in tobacco leaves was modulated by the expression of Cd-transport genes. Molecular evidence illustrated that modulation of IRT1, Nramp1, HMA2, HMA4, and HMA3 genes caused by melatonin could be responsible for weakening Cd uptake, Cd transportation to xylem, and intensifying Cd sequestration into the root vacuoles.

Dynamics of Peroxisome Homeostasis and Its Role in Stress Response and Signaling in Plants

Peroxisomes play vital roles in plant growth, development, and environmental stress response. During plant development and in response to environmental stresses, the number and morphology of peroxisomes are dynamically regulated to maintain peroxisome homeostasis in cells. To execute their various functions in the cell, peroxisomes associate and communicate with other organelles. Under stress conditions, reactive oxygen species (ROS) produced in peroxisomes and other organelles activate signal transduction pathways, in a process known as retrograde signaling, to synergistically regulate defense systems. In this review, we focus on the recent advances in the plant peroxisome field to provide an overview of peroxisome biogenesis, degradation, crosstalk with other organelles, and their role in response to environmental stresses.

The effect of technogenic emissions on the heavy metals accumulation by herbaceous plants

The effect of technogenic emissions on the input of Pb, Zn, Cd, Cu, Mn, Cr, and Ni into plants from the Poaceae and Asteraceae families has been studied. Soil and plant contamination by anthropogenic emissions from industrial enterprises leads the decreasing of crop quality; therefore, the monitoring investigation of plants and soils acquires special importance. The herbaceous plants may be used as bioindicators for main environmental changes. It was found that the high level of anthropogenic load related to atmospheric emissions from the power plant favors the heavy metal (HM) accumulation in herbaceous plants. Contamination with Pb, Cd, Cr, and Ni was revealed in plants growing near the power plant. Heavy metals arrive to plants from the soil in the form of mobile compounds. Plant family is one of the main factors affecting the HM distribution in the above- and underground parts of plants. Plants from the Poaceae family accumulate less chemical elements in their aboveground parts than the Asteraceae plants. Ambrosia artemisiifolia and Artemisia austriaca are HM accumulators. For assessing the stability of plants under contamination with HMs, metal accumulation by plants from soil (the bioconcentration factor) and metal phytoavailability from plants above- and underground parts (the acropetal coefficient) were calculated. According to the bioconcentration factor and translocation factor values, Poaceae species are most resistant to technogenic contamination with HMs. The translocation factor highest values were found for Tanacetum vulgare; the lowest bioconcentration factor values were typical for Poa pratensis.

They Can Handle the Stress: MPK17 and PMD1 act in a salt-specific pathway

Arabidopsis MAP KINASE17 (MPK17) was recently identified as a novel regulator of peroxisome division in response to salt stress. Further, the known peroxisome division factor PEROXISOME AND MITOCHONDRIAL DIVISION FACTOR1 (PMD1) genetically acts downstream of MPK17. We previously showed that mutants defective in either MPK17 or PMD1 fail to proliferate peroxisomes in response to NaCl stress. Here, we show that, unlike their abnormal NaCl responses, mpk17 and pmd1 mutants display wild type responses to other stresses known to alter peroxisome proliferation, suggesting that plants distinguish among peroxisome division-inducing stresses and alter the peroxisome division pathway based on the stress applied.

Peroxisome Mitochondria Inter-relations in Plants

A large amount of ultrastructural, biochemical and molecular analysis indicates that peroxisomes and mitochondria not only share the same subcellular space but also maintain considerable overlap in their proteins, responses and functions. Recent approaches using imaging of fluorescent proteins targeted to both organelles in living plant cells are beginning to show the dynamic nature of their interactivity. Based on the observations of living cells, mitochondria respond rapidly to stress by undergoing fission. Mitochondrial fission is suggested to release key membrane-interacting members of the FISSION1 and DYNAMIN RELATED PROTEIN3 families and appears to be followed by the formation of thin peroxisomal extensions called peroxules. In a model we present the peroxules as an intermediate state prior to the formation of tubular peroxisomes, which, in turn are acted upon by the constriction-related proteins released by mitochondria and undergo rapid constriction and fission to increase the number of peroxisomes in a cell. The fluorescent protein aided imaging of peroxisome-mitochondria interaction provides visual evidence for their cooperation in maintenance of cellular homeostasis in plants.

Hydrogen-rich water alleviates the toxicities of different stresses to mycelial growth in Hypsizygus marmoreus

In plants, hydrogen gas (H₂) enhances tolerance to several abiotic stresses, including salinity and heavy metals. However, the effect of H₂ on fungal growth under different stresses remains largely unclear. In this study, hydrogen-rich water (HRW) was employed to characterize physiological roles and molecular mechanisms of H₂ in the alleviation of three different stresses in basidiomycete Hypsizygus marmoreus. Our results showed that HRW treatment, of which the H₂ concentration was 0.8 mM, significantly reduced the toxicities of CdCl₂, NaCl and H₂O₂, leading to significantly improved mycelial growth and biomass. These beneficial effects could be attributed to a significantly decreased formation of malondialdehyde (MDA). Besides, HRW treatment significantly increased the activities of antioxidants (SOD, CAT and GR) as well as the gene expressions of these antioxidants (SOD, CAT, and GR) at the mRNA level. In vivo detection of reactive oxygen species (ROS), including H₂O₂ and O₂⁻, as well as lipid peroxidation provided further evidence that HRW could significantly improve tolerances of CdCl₂, NaCl and H₂O₂. Furthermore, pyruvate kinase was activated in the mycelia treated with HRW, along with its induced gene expression, suggesting that HRW treatment enhanced the glucose metabolism. Taken together, our findings suggested that the usage of HRW could be an effective approach for contaminant detoxification in H. marmoreus, which was similar with the effects of HRW in plants, and such effects could be also beneficial in entire agricultural system.

Hydrogen-rich water induces aluminum tolerance in maize seedlings by enhancing antioxidant capacities and nutrient homeostasis

The ameliorative effect of H₂ on aluminum (Al)-induced stress remains poorly understood. We treated maize seedlings with Al and hydrogen-rich water (HRW) to determine the roles of H₂ in the alleviation of Al toxicity. Our results demonstrated that Al stress triggered damage to the photosynthetic apparatus, plant growth inhibition, and reactive oxygen species (ROS) production, and boosted lipid peroxidation. However, the addition of HRW at 75% saturation markedly alleviated Al toxicity symptoms through the promotion of root elongation. These responses were related to the significantly increased activities of typical antioxidant enzymes (CAT, APX, SOD, and POD). In vivo imaging of plasma membrane integrity, lipid peroxidation, and the level of ROS provided further evidence that HRW could improve Al tolerance. Our results also indicate that 100% HRW mitigated Al toxicity less than 75% HRW. Moreover, different concentrations of HRW significantly improved photosynthesis and increased nutrient uptake. We conclude that exogenous H₂ supplementation could enhance Al tolerance by reestablishing redox homeostasis and maintaining nutrient homeostasis.

Spatial organization of organelles in fungi: Insights from mathematical modelling

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Modelling of dynein motility reveals a stochastic role in dynein comet formation.

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Modelling helps to elucidate mechanisms in spatial organization of early endosomes.

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A combination of diffusion and directed motion distributes ribosomes and peroxisomes.

Mathematical modelling in cellular systems aims to describe biological processes in a quantitative manner. Most accurate modelling is based on robust experimental data. Here we review recent progress in the theoretical description of motor behaviour, early endosome motility, ribosome distribution and peroxisome transport in the fungal model system Ustilago maydis and illustrate the power of modelling in our quest to understand molecular details and cellular roles of membrane trafficking in filamentous fungi.

Peroxisome Motility Measurement and Quantification Assay

Organelle movement, distribution and interaction contribute to the organisation of the eukaryotic cell. Peroxisomes are multifunctional organelles which contribute to cellular lipid metabolism and ROS homeostasis. They distribute uniformly in mammalian cells and move along microtubules via kinesin and dynein motors. Their metabolic cooperation with mitochondria and the endoplasmic reticulum (ER) is essential for the β-oxidation of fatty acids and the synthesis of myelin lipids and polyunsaturated fatty acids. A key assay to assess peroxisome motility in mammalian cells is the expression of a fluorescent fusion protein with a peroxisomal targeting signal (e.g., GFP-PTS1), which targets the peroxisomal matrix and allows live-cell imaging of peroxisomes. Here, we first present a protocol for the transfection of cultured mammalian cells with the peroxisomal marker EGFP-SKL to observe peroxisomes in living cells. This approach has revealed different motile behaviour of peroxisomes and novel insight into peroxisomal membrane dynamics (Rapp et al., 1996; Wiemer et al., 1997; Schrader et al., 2000). We then present a protocol which combines the live-cell approach with peroxisome motility measurements and quantification of peroxisome dynamics in mammalian cells. More recently, we used this approach to demonstrate that peroxisome motility and displacement is increased when a molecular tether, which associates peroxisomes with the ER, is lost (Costello et al., 2017b). Silencing of the peroxisomal acyl-CoA binding domain protein ACBD5, which interacts with ER-localised VAPB, increased peroxisome movement in skin fibroblasts, indicating that membrane contact sites can modulate organelle distribution and motility. The protocols described can be adapted to other cell types and organelles to measure and quantify organelle movement under different experimental conditions.

ROS Generation in Peroxisomes and its Role in Cell Signaling

In plant cells, as in most eukaryotic organisms, peroxisomes are probably the major sites of intracellular H₂O₂ production, as a result of their essentially oxidative type of metabolism. In recent years, it has become increasingly clear that peroxisomes carry out essential functions in eukaryotic cells. The generation of the important messenger molecule hydrogen peroxide (H₂O₂) by animal and plant peroxisomes and the presence of catalase in these organelles has been known for many years, but the generation of superoxide radicals (O₂^·- ) and the occurrence of the metalloenzyme superoxide dismutase was reported for the first time in peroxisomes from plant origin. Further research showed the presence in plant peroxisomes of a complex battery of antioxidant systems apart from catalase. The evidence available of reactive oxygen species (ROS) production in peroxisomes is presented, and the different antioxidant systems characterized in these organelles and their possible functions are described. Peroxisomes appear to have a ROS-mediated role in abiotic stress situations induced by the heavy metal cadmium (Cd) and the xenobiotic 2,4-D, and also in the oxidative reactions of leaf senescence. The toxicity of Cd and 2,4-D has an effect on the ROS metabolism and speed of movement (dynamics) of peroxisomes. The regulation of ROS production in peroxisomes can take place by post-translational modifications of those proteins involved in their production and/or scavenging. In recent years, different studies have been carried out on the proteome of ROS metabolism in peroxisomes. Diverse evidence obtained indicates that peroxisomes are an important cellular source of different signaling molecules, including ROS, involved in distinct processes of high physiological importance, and might play an important role in the maintenance of cellular redox homeostasis.

Active diffusion and microtubule-based transport oppose myosin forces to position organelles in cells

Even distribution of peroxisomes (POs) and lipid droplets (LDs) is critical to their role in lipid and reactive oxygen species homeostasis. How even distribution is achieved remains elusive, but diffusive motion and directed motility may play a role. Here we show that in the fungus Ustilago maydis ∼95% of POs and LDs undergo diffusive motions. These movements require ATP and involve bidirectional early endosome motility, indicating that microtubule-associated membrane trafficking enhances diffusion of organelles. When early endosome transport is abolished, POs and LDs drift slowly towards the growing cell end. This pole-ward drift is facilitated by anterograde delivery of secretory cargo to the cell tip by myosin-5. Modelling reveals that microtubule-based directed transport and active diffusion support distribution, mobility and mixing of POs. In mammalian COS-7 cells, microtubules and F-actin also counteract each other to distribute POs. This highlights the importance of opposing cytoskeletal forces in organelle positioning in eukaryotes.

The mechanisms underlying the positioning of eukaryotic organelles remain elusive. Here Lin et al. use imaging and a mathematical model to show that microtubule-based transport and active diffusion and actin-based polar drift act together to facilitate even distribution of peroxisomes in filamentous fungi.

RNA-Seq Analysis of Rice Roots Reveals the Involvement of Post-Transcriptional Regulation in Response to Cadmium Stress

Widely-spread cadmium (Cd) pollution in the soil threatens both crop production and human health. How plants deal with the excess Cd are largely unknown. To evaluate the molecular mechanism by which plants respond to Cd stress, rice seedlings were treated with two concentrations of Cd and subjected to deep RNA sequencing. Comprehensive RNA-Seq analysis of rice roots under two gradients of Cd treatment revealed 1169 Cd toxicity-responsive genes. These genes were involved in the reactive oxygen species scavenging system, stress response, cell wall formation, ion transport, and signal transduction. Nine out of 93 predicted long non-coding RNAs (lncRNAs) were detected as Cd-responsive lncRNAs due to their high correlation with the Cd stress response. In addition, we analyzed alternative splicing (AS) events under different Cd concentrations. Four hundred and seventy-six differential alternatively spliced genes with 542 aberrant splicing events were identified. GO analysis indicated that these genes were highly enriched in oxidation reduction and cellular response to chemical stimulus. Real-time qRT-PCR validation analysis strengthened the reliability of our RNA-Seq results. The results suggest that post-transcriptional AS regulation may also be involved in plant responses to high Cd stress.

Peroxisomes sense and respond to environmental cues by regulating ROS and RNS signalling networks

BACKGROUND

Peroxisomes are highly dynamic, metabolically active organelles that used to be regarded as a sink for H2O2 generated in different organelles. However, peroxisomes are now considered to have a more complex function, containing different metabolic pathways, and they are an important source of reactive oxygen species (ROS), nitric oxide (NO) and reactive nitrogen species (RNS). Over-accumulation of ROS and RNS can give rise oxidative and nitrosative stress, but when produced at low concentrations they can act as signalling molecules.

SCOPE

This review focuses on the production of ROS and RNS in peroxisomes and their regulation by antioxidants. ROS production is associated with metabolic pathways such as photorespiration and fatty acid β-oxidation, and disturbances in any of these processes can be perceived by the cell as an alarm that triggers defence responses. Genetic and pharmacological studies have shown that photorespiratory H2O2 can affect nuclear gene expression, regulating the response to pathogen infection and light intensity. Proteomic studies have shown that peroxisomal proteins are targets for oxidative modification, S-nitrosylation and nitration and have highlighted the importance of these modifications in regulating peroxisomal metabolism and signalling networks. The morphology, size, number and speed of movement of peroxisomes can also change in response to oxidative stress, meaning that an ROS/redox receptor is required. Information available on the production and detection of NO/RNS in peroxisomes is more limited. Peroxisomal homeostasis is critical for maintaining the cellular redox balance and is regulated by ROS, peroxisomal proteases and autophagic processes.

CONCLUSIONS

Peroxisomes play a key role in many aspects of plant development and acclimation to stress conditions. These organelles can sense ROS/redox changes in the cell and thus trigger rapid and specific responses to environmental cues involving changes in peroxisomal dynamics as well as ROS- and NO-dependent signalling networks, although the mechanisms involved have not yet been established. Peroxisomes can therefore be regarded as a highly important decision-making platform in the cell, where ROS and RNS play a determining role.

The role of reactive oxygen species and nitric oxide in programmed cell death associated with self-incompatibility

Successful sexual reproduction often relies on the ability of plants to recognize self- or genetically-related pollen and prevent pollen tube growth soon after germination in order to avoid self-fertilization. Angiosperms have developed different reproductive barriers, one of the most extended being self-incompatibility (SI). With SI, pistils are able to reject self or genetically-related pollen thus promoting genetic variability. There are basically two distinct systems of SI: gametophytic (GSI) and sporophytic (SSI) based on their different molecular and genetic control mechanisms. In both types of SI, programmed cell death (PCD) has been found to play an important role in the rejection of self-incompatible pollen. Although reactive oxygen species (ROS) were initially recognized as toxic metabolic products, in recent years, a new role for ROS has become apparent: the control and regulation of biological processes such as growth, development, response to biotic and abiotic environmental stimuli, and PCD. Together with ROS, nitric oxide (NO) has become recognized as a key regulator of PCD. PCD is an important mechanism for the controlled elimination of targeted cells in both animals and plants. The major focus of this review is to discuss how ROS and NO control male-female cross-talk during fertilization in order to trigger PCD in self-incompatible pollen, providing a highly effective way to prevent self-fertilization.

Contribution of glutathione to the control of cellular redox homeostasis under toxic metal and metalloid stress

The accumulation of toxic metals and metalloids, such as cadmium (Cd), mercury (Hg), or arsenic (As), as a consequence of various anthropogenic activities, poses a serious threat to the environment and human health. The ability of plants to take up mineral nutrients from the soil can be exploited to develop phytoremediation technologies able to alleviate the negative impact of toxic elements in terrestrial ecosystems. However, we must select plant species or populations capable of tolerating exposure to hazardous elements. The tolerance of plant cells to toxic elements is highly dependent on glutathione (GSH) metabolism. GSH is a biothiol tripeptide that plays a fundamental dual role: first, as an antioxidant to mitigate the redox imbalance caused by toxic metal(loid) accumulation, and second as a precursor of phytochelatins (PCs), ligand peptides that limit the free ion cellular concentration of those pollutants. The sulphur assimilation pathway, synthesis of GSH, and production of PCs are tightly regulated in order to alleviate the phytotoxicity of different hazardous elements, which might induce specific stress signatures. This review provides an update on mechanisms of tolerance that depend on biothiols in plant cells exposed to toxic elements, with a particular emphasis on the Hg-triggered responses, and considering the contribution of hormones to their regulation.

Hydrogen-rich water enhances cadmium tolerance in Chinese cabbage by reducing cadmium uptake and increasing antioxidant capacities

The aim of the present paper was to understand the specific mechanism of hydrogen-rich water (HRW) in alleviating cadmium (Cd) toxicity in Chinese cabbage (Brassica campestris spp. chinensis L.). Our results showed that the addition of 50% saturation HRW significantly alleviated the Cd toxic symptoms, including the improvement of both root elongation and seedling growth inhibition. These responses were consistent with a significant decrease of Cd accumulation in roots and shoots, which was further confirmed by the histochemical staining. Molecular evidence illustrated that Cd-induced up-regulations of IRT1 and Nramp1 genes, responsible for Cd absorption, were blocked by HRW. By contrast, Cd-induced up-regulation of the HMA3 gene, which regulates Cd sequestration into the root vacuoles, was substantially strengthened by HRW. Furthermore, compared with those in Cd stress alone, the expressions of HMA2 and HMA4, which function in the transportation of Cd to xylem, were repressed by co-treatment with HRW. HRW enhanced the activities of antioxidant enzymes, including superoxide dismutase, guaiacol peroxidase, catalase and ascorbate peroxidase. These results were further confirmed by the alleviation of oxidative damage, as indicated by the decrease of thiobarbituric acid reactive substances (TBARS) and reactive oxygen species (ROS) production. Taken together, these results suggest that the improvement of Cd tolerance by HRW was associated with reduced Cd uptake and increased antioxidant defense capacities. Therefore, the application of HRW may be a promising strategy to improve Cd tolerance of Chinese cabbage.

2,4-Dichlorophenoxyacetic acid promotes S-nitrosylation and oxidation of actin affecting cytoskeleton and peroxisomal dynamics

2,4-Dichlorophenoxyacetic acid (2,4-D) is a synthetic auxin used as a herbicide to control weeds in agriculture. A high concentration of 2,4-D promotes leaf epinasty and cell death. In this work, the molecular mechanisms involved in the toxicity of this herbicide are studied by analysing in Arabidopsis plants the accumulation of reactive oxygen species (ROS) and nitric oxide (NO), and their effect on cytoskeleton structure and peroxisome dynamics. 2,4-D (23 mM) promotes leaf epinasty, whereas this process was prevented by EDTA, which can reduce ·OH accumulation. The analysis of ROS accumulation by confocal microscopy showed a 2,4-D-dependent increase in both H2O2 and O2·(-), whereas total NO was not affected by the treatment. The herbicide promotes disturbances on the actin cytoskeleton structure as a result of post-translational modification of actin by oxidation and S-nitrosylation, which could disturb actin polymerization, as suggested by the reduction of the F-actin/G-actin ratio. These effects were reduced by EDTA, and the reduction of ROS production in Arabidopsis mutants deficient in xanthine dehydrogenase (Atxdh) gave rise to a reduction in actin oxidation. Also, 2,4-D alters the dynamics of the peroxisome, slowing the speed and shortening the distances by which these organelles are displaced. It is concluded that 2,4-D promotes oxidative and nitrosative stress, causing disturbances in the actin cytoskeleton, thereby affecting the dynamics of peroxisomes and some other organelles such as the mitochondria, with xanthine dehydrogenase being involved in ROS production under these conditions. These structural changes in turn appear to be responsible for the leaf epinasty.

ER network dynamics are differentially controlled by myosins XI-K, XI-C, XI-E, XI-I, XI-1, and XI-2

The endoplasmic reticulum (ER) of higher plants is a complex network of tubules and cisternae. Some of the tubules and cisternae are relatively persistent, while others are dynamically moving and remodeling through growth and shrinkage, cycles of tubule elongation and retraction, and cisternal expansion and diminution. Previous work showed that transient expression in tobacco leaves of the motor-less, truncated tail of myosin XI-K increases the relative area of both persistent cisternae and tubules in the ER. Likewise, transient expression of XI-K tail diminishes the movement of organelles such as Golgi and peroxisomes. To examine whether other class XI myosins are involved in the remodeling and movement of the ER, other myosin XIs implicated in organelle movement, XI-1 (MYA1),XI-2 (MYA2), XI-C, XI-E, XI-I, and one not, XI-A, were expressed as motor-less tail constructs and their effect on ER persistent structures determined. Here, we indicate a differential effect on ER dynamics whereby certain class XI myosins may have more influence over controlling cisternalization rather than tubulation.

Heavy-metal-induced reactive oxygen species: phytotoxicity and physicochemical changes in plants

As a result of the industrial revolution, anthropogenic activities have enhanced there distribution of many toxic heavy metals from the earth's crust to different environmental compartments. Environmental pollution by toxic heavy metals is increasing worldwide, and poses a rising threat to both the environment and to human health.Plants are exposed to heavy metals from various sources: mining and refining of ores, fertilizer and pesticide applications, battery chemicals, disposal of solid wastes(including sewage sludge), irrigation with wastewater, vehicular exhaust emissions and adjacent industrial activity.Heavy metals induce various morphological, physiological, and biochemical dysfunctions in plants, either directly or indirectly, and cause various damaging effects. The most frequently documented and earliest consequence of heavy metal toxicity in plants cells is the overproduction of ROS. Unlike redox-active metals such as iron and copper, heavy metals (e.g, Pb, Cd, Ni, AI, Mn and Zn) cannot generate ROS directly by participating in biological redox reactions such as Haber Weiss/Fenton reactions. However, these metals induce ROS generation via different indirect mechanisms, such as stimulating the activity of NADPH oxidases, displacing essential cations from specific binding sites of enzymes and inhibiting enzymatic activities from their affinity for -SH groups on the enzyme.Under normal conditions, ROS play several essential roles in regulating the expression of different genes. Reactive oxygen species control numerous processes like the cell cycle, plant growth, abiotic stress responses, systemic signalling, programmed cell death, pathogen defence and development. Enhanced generation of these species from heavy metal toxicity deteriorates the intrinsic antioxidant defense system of cells, and causes oxidative stress. Cells with oxidative stress display various chemical,biological and physiological toxic symptoms as a result of the interaction between ROS and biomolecules. Heavy-metal-induced ROS cause lipid peroxidation, membrane dismantling and damage to DNA, protein and carbohydrates. Plants have very well-organized defense systems, consisting of enzymatic and non-enzymatic antioxidation processes. The primary defense mechanism for heavy metal detoxification is the reduced absorption of these metals into plants or their sequestration in root cells.Secondary heavy metal tolerance mechanisms include activation of antioxidant enzymes and the binding of heavy metals by phytochelatins, glutathione and amino acids. These defense systems work in combination to manage the cascades of oxidative stress and to defend plant cells from the toxic effects of ROS.In this review, we summarized the biochemiCal processes involved in the over production of ROS as an aftermath to heavy metal exposure. We also described the ROS scavenging process that is associated with the antioxidant defense machinery.Despite considerable progress in understanding the biochemistry of ROS overproduction and scavenging, we still lack in-depth studies on the parameters associated with heavy metal exclusion and tolerance capacity of plants. For example, data about the role of glutathione-glutaredoxin-thioredoxin system in ROS detoxification in plant cells are scarce. Moreover, how ROS mediate glutathionylation (redox signalling)is still not completely understood. Similarly, induction of glutathione and phytochelatins under oxidative stress is very well reported, but it is still unexplained that some studied compounds are not involved in the detoxification mechanisms. Moreover,although the role of metal transporters and gene expression is well established for a few metals and plants, much more research is needed. Eventually, when results for more metals and plants are available, the mechanism of the biochemical and genetic basis of heavy metal detoxification in plants will be better understood. Moreover, by using recently developed genetic and biotechnological tools it may be possible to produce plants that have traits desirable for imparting heavy metal tolerance.

Alleviation of cadmium toxicity in Medicago sativa by hydrogen-rich water

Hydrogen gas (H₂) induces plant tolerance to several abiotic stresses, including salinity and paraquat exposure. However, the role of H₂ in cadmium (Cd)-induced stress amelioration is largely unknown. Here, pretreatment with hydrogen-rich water (HRW) was used to characterize physiological roles and molecular mechanisms of H₂ in the alleviation of Cd toxicity in alfalfa plants. Our results showed that the addition of HRW at 10% saturation significantly decreased contents of thiobarbituric acid reactive substances (TBARS) caused by Cd, and inhibited the appearance of Cd toxicity symptoms, including the improvement of root elongation and seedling growth. These responses were related to a significant increase in the total or isozymatic activities of representative antioxidant enzymes, or their corresponding transcripts. In vivo imaging of reactive oxygen species (ROS), and the detection of lipid peroxidation and the loss of plasma membrane integrity provided further evidence for the ability of HRW to improve Cd tolerance significantly, which was consistent with a significant enhancement of the ratio of reduced/oxidized (homo)glutathione ((h)GSH). Additionally, plants pretreated with HRW accumulated less amounts of Cd. Together, this study suggested that the usage of HRW could be an effective approach for Cd detoxification and could be explored in agricultural production systems.

Simultaneous live-imaging of peroxisomes and the ER in plant cells suggests contiguity but no luminal continuity between the two organelles

Transmission electron micrographs of peroxisomes in diverse organisms, including plants, suggest their close association and even luminal connectivity with the endoplasmic reticulum (ER). After several decades of debate de novo peroxisome biogenesis from the ER is strongly favored in yeasts and mammals. Unfortunately many of the proteins whose transit through the ER constitutes a major evidence for peroxisome biogenesis from the ER do not exhibit a similar localization in plants. Consequently, at best the ER acts as a membrane source for peroxisome in plants. However, in addition to their de novo biogenesis from the ER an increase in peroxisome numbers also occurs through fission of existing peroxisomes. In recent years live-imaging has been used to visualize peroxisomes and the ER but the precise spatio-temporal relationship between the two organelles has not been well-explored. Here we present our assessment of the peroxisome-ER relationship through imaging of living Arabidopsis thaliana plants simultaneously expressing different color combinations of fluorescent proteins targeted to both organelles. Our observations on double transgenic wild type and a drp3a/apm1 mutant Arabidopsis plants suggest strong correlations between the dynamic behavior of peroxisomes and the neighboring ER. Although peroxisomes and ER are closely aligned there appears to be no luminal continuity between the two. Similarly, differentially colored elongated peroxisomes of a drp3a mutant expressing a photoconvertible peroxisomal matrix protein are unable to fuse and share luminal protein despite considerable intermingling. Substantiation of our observations is suggested through 3D iso-surface rendering of image stacks, which shows closed ended peroxisomes enmeshed among ER tubules possibly through membrane contact sites (MCS). Our observations support the idea that increase in peroxisome numbers in a plant cell occurs mainly through the fission of existing peroxisomes in an ER aided manner.

Plant sugars are crucial players in the oxidative challenge during abiotic stress: extending the traditional concept

Plants suffering from abiotic stress are commonly facing an enhanced accumulation of reactive oxygen species (ROS) with damaging as well as signalling effects at organellar and cellular levels. The outcome of an environmental challenge highly depends on the delicate balance between ROS production and scavenging by both enzymatic and metabolic antioxidants. However, this traditional classification is in need of renewal and reform, as it is becoming increasingly clear that soluble sugars such as disaccharides, raffinose family oligosaccharides and fructans--next to their associated metabolic enzymes--are strongly related to stress-induced ROS accumulation in plants. Therefore, this review aims at extending the current concept of antioxidants functioning during abiotic stress, with special focus on the emanate role of sugars as true ROS scavengers. Examples are given based on their cellular location, as different organelles seem to exploit distinct mechanisms. Moreover, the vacuole comes into the picture as important player in the ROS signalling network of plants. Elucidating the interplay between the mechanisms controlling ROS signalling during abiotic stress will facilitate the development of strategies to enhance crop tolerance to stressful environmental conditions.

Nitric oxide (NO) in alleviation of heavy metal induced phytotoxicity and its role in protein nitration

Nitric oxide (NO) is recognized as a biological messenger in various tissues to regulate diverse range of physiological process including growth, development and response to abiotic and biotic factors. The NO emission from plants is known since the 1970s, and there is copious information on the multiple effects of exogenously applied NO on different physiological and biochemical processes of plants. Heavy metal toxicity is one of the major abiotic stresses leading to hazardous effects in plants and its toxicity is based on chemical and physical property. A common consequence of heavy metal toxicity is the uncontrolled and excessive accumulation of reactive oxygen species (ROS) which leads to peroxidation of lipids, oxidation of protein, inactivation of enzymes, DNA damage and/or interact with other vital constituents of plant cells. Recently, an increasing number of articles have reported the effects of exogenous NO on alleviating heavy metal toxicity in plants but knowledge of physiological mechanisms of NO in alleviating heavy metal toxicity is quite limited, and some results contradict one another. Therefore, to help clarify the roles of NO in heavy metal tolerance, it is important to review and discuss the recent advances on this area of research. NO can provoke both beneficial and harmful effects, which depend on the concentration and location of NO in the plant cells. NO alleviates the harmfulness of the ROS, and reacts with other target molecules, and regulates the expression of stress responsive genes under various stress conditions. This manuscript includes, the latest advances in understanding the effects of endogenous NO on heavy metal toxicity and the mechanisms and role of NO as an antioxidant as well as in protein nitration are highlighted.

Role of plant peroxisomes in the production of jasmonic acid-based signals

Jasmonates are a family of oxylipins derived from linolenic acid that control plant responses to biotic and abiotic stress factors and also regulate plant growth and development. Jasmonic acid (JA) is synthesized through the octadecanoid pathway that involves the translocation of lipid intermediates from the chloroplast membranes to the cytoplasm and later on into peroxisomes. The peroxisomal steps of the pathway involve the reduction of cis-(+)-12-oxophytodienoic acid (12-OPDA) and dinor-OPDA, which are the final products of the choroplastic phase of the biosynthetic pathway acting on 18:3 and 16:3 fatty acids, respectively. Further shortening of the carbon side-chain by successive rounds of β-oxidation reactions are required to complete JA biosynthesis. After peroxisomal reactions are completed, (+)-7-iso-JA is synthesized and then transported to the cytoplasm where is conjugated to the amino acid isoleucine to form the bioactive form of the hormone (+)-7-iso-JA-Ile (JA-Ile). Further regulatory activity of JA-Ile triggering gene activation in the jasmonate-dependent signaling cascades is exerted through a process mediated by the perception via the E3 ubiquitin ligase COI1 and further ligand-activated interaction with the family of JAZ repressor proteins. Upon interaction, JAZ are ubiquitinated and degraded by the proteasome, thus releasing transcription factors such as MYC2 from repression and allowing the activation of JA-responsive genes.