The role of phytohormone signaling in ozone-induced cell death in plants

Ozone is the main photochemical oxidant that causes leaf damage in many plant species, and can thereby significantly decrease the productivity of crops and forests. When ozone is incorporated into plants, it produces reactive oxygen species (ROS), such as superoxide radicals and hydrogen peroxide. These ROS induce the synthesis of several plant hormones, such as ethylene, salicylic acid, and jasmonic acid. These phytohormones are required for plant growth, development, and defense responses, and regulate the extent of leaf injury in ozone-fumigated plants. Recently, responses to ozone have been studied using genetically modified plants and mutants with altered hormone levels or signaling pathways. These researches have clarified the roles of phytohormones and the complexity of their signaling pathways. The present paper reviews the biosynthesis of the phytohormones ethylene, salicylic acid, and jasmonic acid, their roles in plant responses to ozone, and multiple interactions between these phytohormones in ozone-exposed plants.Key words: cross-talk, ethylene, jasmonic acid, ozone, phytohormones, programmed cell death, salicylic acid, signaling pathways     

Physiological and biochemical responses to acute ozone-induced oxidative stress in Medicago truncatula.

Oxidative signaling mediated by reactive oxygen species (ROS) is a central component of biotic and abiotic stresses in plants. Acute ozone (O(3)) fumigation is a useful non-invasive treatment for eliciting endogenous ROS in planta. In this study, 38 different accessions of the model legume, Medicago truncatula, from various geographical regions were fumigated with 300 nmol mol(-1) of O(3) for a period of six hours. Phenotypic symptoms were evaluated 24 and 48 h after the end of treatment. A majority of the accessions showed distinct visible damage. Eight accessions showing varying sensitivities to ozone were subjected to biochemical analysis to evaluate correlations between ozone damage and levels of ROS, antioxidants, and lipid peroxidation. Two-way analysis of variance indicated highly significant interactions between O(3) damage and levels of ROS, ascorbate, glutathione and lipid peroxidation. There were significant differences among the accessions for these traits before and after the end of O(3) fumigation, as indicated by equal variance Student's t-test. This study suggests that multiple physiological and biochemical mechanisms may govern O(3) tolerance or sensitivity. Surveying a large collection of germplasm led to identification of multiple resistant and sensitive lines for investigating molecular basis of O(3) phytotoxicity. The most resistant JE154 accession also showed enhanced tolerance to chronic O(3) and dehydration stress, suggesting germplasm with increased tolerance to acute O(3) can be a useful resource for improving resistance to multiple abiotic stressors.     

Drought acclimation reduces O2*- accumulation and lipid peroxidation in wheat seedlings

Abiotic stresses cause ROS accumulation, which is detrimental to plant growth. It is well known that acclimation of plants under mild or sub-lethal stress condition leads to development of resistance in plants to severe or lethal stress condition. The generation of ROS and subsequent oxidative damage during drought stress is well documented in the crop plants. However, the effect of drought acclimation treatment on ROS accumulation and lipid peroxidation has not been examined so far. In this study, the effect of water stress acclimation treatment on superoxide radical (O(2)(-z.rad;)) accumulation and membrane lipid peroxidation was studied in leaves and roots of wheat (Triticum aestivum) cv. C306. EPR quantification of superoxide radicals revealed that drought acclimation treatment led to 2-fold increase in superoxide radical accumulation in leaf and roots with no apparent membrane damage. However under subsequent severe water stress condition, the leaf and roots of non-acclimated plants accumulated significantly higher amount of superoxide radicals and showed higher membrane damage than that of acclimated plants. Thus, acclimation-induced restriction of superoxide radical accumulation is one of the cellular processes that confers enhanced water stress tolerance to the acclimated wheat seedlings.     

Redox proteomics: basic principles and future perspectives for the detection of protein oxidation in plants

The production and scavenging of chemically reactive species, such as ROS/RNS, are central to a broad range of biotic and abiotic stress and physiological responses in plants. Among the techniques developed for the identification of oxidative stress-induced modifications on proteins, the so-called 'redox proteome', proteomics appears to be the best-suited approach. Oxidative or nitrosative stress leaves different footprints in the cell in the form of different oxidatively modified components and, using the redox proteome, it will be possible to decipher the potential roles played by ROS/RNS-induced modifications in stressed cells. The purpose of this review is to present an overview of the latest research endeavours in the field of plant redox proteomics to identify the role of post-translational modifications of proteins in developmental cell stress. All the strategies set up to analyse the different oxidized/nitrosated amino acids, as well as the different reactivities of ROS and RNS for different amino acids are revised and discussed. A growing body of evidence indicates that ROS/RNS-induced protein modifications may be of physiological significance, and that in some cellular stresses they may act causatively and not arise as a secondary consequence of cell damage. Thus, although previously the oxidative modification of proteins was thought to represent a detrimental process in which the modified proteins were irreversibly inactivated, it is now clear that, in plants, oxidatively/nitrosatively modified proteins can be specific and reversible, playing a key role in normal cell physiology. In this sense, redox proteomics will have a central role in the definition of redox molecular mechanisms associated with cellular stresses.     

Role of plant respiratory burst oxidase homologs in stress responses

Plant respiratory burst oxidase homologs (Rbohs), which are also named nicotinamide adenine dinucleotide phosphate (NADPH) oxidases (NOXs), are the homologs of mammalian phagocyte gp91^phox. As a unique among other reactive oxygen species (ROS) production mechanisms in plants, NADPH oxidases can integrate different signal transduction pathways, such as calcium, protein phosphorylation catalysed by protein kinases, nitric oxide, and lipid messengers. Coupling with genetic studies, the ability of plant NADPH oxidases to integrate different signal transduction pathways with ROS production demonstrates their involvement in many important biological processes in cells, such as morphogenesis and development, and stress responses. Here, we focus on several current studies concerning the role of plant NADPH oxidases in stress responses.     

化感胁迫诱导植物细胞损伤研究进展

化感胁迫(allelochemical stress)是指一种植物通过淋溶、挥发、根系分泌和残株腐解等途径释放化学物质,对另一种植物(包括微生物)产生直接或间接的伤害作用。有害化感物质对受体植物具有显著的细胞毒性,影响根边缘细胞的形成过程和活性,改变细胞壁和细胞膜的特性,破坏细胞内部结构,干扰细胞有丝分裂过程和基因表达模式;此外,化感胁迫往往伴随着氧化胁迫,受体植物细胞活性氧(ROS)水平升高,膜脂过氧化程度加剧,抗氧化系统被破坏,ROS影响与凋亡相关的信号调控过程,引起细胞大量死亡。因此,化感胁迫诱导的氧化胁迫可能是引起细胞凋亡的原因之一。阐明化感胁迫介导的氧化损伤和细胞损伤的相互关系以及根边缘细胞对化感胁迫的响应机制,是今后研究化感作用机制的一个方向。     

Alterations of the biochemical pathways of plants by the air pollutant ozone: which are the true gauges of injury?

Plant strategies to survive ozone stress include exclusion or tolerance of ozone. If these processes fail, past observations of ozone injury have indicated many physiological and metabolic changes then occur; most of these changes are likely to have been initiated at the level of gene expression, suggesting signal transduction. In the last decade considerable understanding of the biochemical process within plants has been developed. Currently there are several hypotheses regarding a response of plants to ozone fumigation: [1] membrane dysfunction and alteration of purpose; [2] stress ethylene interactions; [3] impairment of photosynthesis via changes in Rubisco levels and the guard cells so that the stomata do not track correctly the environment; [4] antioxidant protection through metabolites and enzyme systems to reduce the oxidant load; and [5] general impairment or disruption of metabolic pathways. Many believe that free radicals and other oxidative products, formed in plant leaves under ozone exposure, are responsible for much of the spread of the biochemical alterations. There are obvious chemicals that may account for the changes that are observed, such as hydrogen peroxide. Once the ozone enters the tissue, evidence suggests the first line of defense is a range of antioxidants, such as ascorbate, glutathione peroxidase, superoxide dismutase, and catalase. If overwhelmed, subsequent events occur which are highly suggestive of systemic acquired resistance. Furthermore, other defensive indicators, such as salicylic acid and jasmonic acid, tend to increase, but more slowly than ethylene, and spread their signaling effects more widely in the plant. The primary set of metabolic reactions that ozone triggers is thought to be "wounding" responses with a secondary response of senescence. The dramatic strides in understanding the genetic make-up of plants, gene control, and signal transduction/control over the last few years will only accelerate in the future. We need now to have an understanding of those events that can be translated into more detailed schemes of how ozone alters much of the basic metabolism of plants and how plants counteract or cope with ozone. What is now known about how varied biochemicals and their pathways are changed upon ozone exposure will be discussed.     

NO是植物应激反应的信号分子

根据NO的性质和可能的产生途径,略述了生物胁迫(病原菌侵害)和干旱胁迫、盐胁迫、极端温度、机械损伤、臭氧和紫外辐射等各种非生物胁迫信号与NO信号分子的偶联及其信号的级联途径,概括了NO可能介导的生物过程,讨论了NO通过其下游信号过程对与细胞的生理影响以及该下游信号过程所涉及到的cGMP、cADPR的产生和NO与其它信号分子(ROS、SA、ABA等)的协同作用,表明胁迫诱导的NO爆发是激发、启动和装备植物细胞的重要信号级联环节,这个环节能使植物细胞处于应激状态,并迅速作出反应,形成一系列适应机制。     

Ozone Quenching Properties of Isoprene and Its Antioxidant Role in Leaves

Isoprene is formed in and emitted by plants and the reason for this apparent carbon waste is still unclear. It has been proposed that isoprene stabilizes cell and particularly chloroplast thylakoid membranes. We tested if membrane stabilization or isoprene reactivity with ozone induces protection against acute ozone exposures. The reduction of visible, physiological, anatomical, and ultrastructural (chloroplast) damage shows that clones of plants sensitive to ozone and unable to emit isoprene become resistant to acute and short exposure to ozone if they are fumigated with exogenous isoprene, and that isoprene-emitting plants that are sensitive to ozone do not suffer damage when exposed to ozone. Isoprene-induced ozone resistance is associated with the maintenance of photochemical efficiency and with a low energy dissipation, as indicated by fluorescence quenching. This suggests that isoprene effectively stabilizes thylakoid membranes. However, when isoprene reacts with ozone within the leaves or in a humid atmosphere, it quenches the ozone concentration to levels that are less or non-toxic for plants. Thus, protection from ozone in plants fumigated with isoprene may be due to a direct ozone quenching rather than to an induced resistance at membrane level. Irrespective of the mechanism, isoprene is one of the most effective antioxidants in plants.     

Flow cytometric analysis of the oxidative status of normal and thalassemic red blood cells.

BACKGROUND: The oxidative status of cells has been shown to modulate various cell functions and be involved in physiological and pathological conditions, including hereditary chronic anemias, such as thalassemia. It is maintained by the balance between oxidants, such as reactive oxygen species (ROS), and antioxidants, such as reduced glutathione (GSH). METHODS: We studied peripheral RBC derived from normal and thalassemic donors. Flow cytometric methods were used to measure (1) generation of ROS; (2) the content of reduced GSH; and (3) peroxidation of membrane lipids as an indication of membrane damage. RESULTS: ROS and lipid peroxidation were found to be higher, and GSH lower, in thalassemic RBC compared with normal RBC, both at baseline as well as following oxidative stress, such as exposure to hydrogen peroxide. To simulate a state of iron overload, normal RBC were exposed to extracellular ferric ammonium citrate or hemin, or their Hb was denatured by phenylhydrazine. All these treatments increased ROS and lipid peroxidation and decreased GSH. These effects were reversed by N-acetyl cysteine, a known ROS scavenger. CONCLUSIONS: Flow cytometry can be useful for measuring oxidative stress and its effects on RBC in various diseases and for studying various chemical agents as antioxidants.     

Discovery of oxidative burst in the field of plant immunity: Looking back at the early pioneering works and towards the future development

This article is introductory to the series of works presented in this special issue on the homeostasis and the signaling roles of reactive oxygen species (ROS) in plants. Upper half of this article briefly describes the history of the ROS study in the field of plant immunity research initiated by the observation that the attacks by pathogenic microorganisms possibly stimulate the burst of ROS production in the plant tissues. The topics covered in the series of works presented here include the plants'' responses to abiotic oxidative stress (atmospheric ozone), regulation of seed germination, chemical interaction between parasitic and host plants and the draught tolerance, all controlled through homeostasis of ROS at biochemical and molecular biological levels. Lastly a discussion forum was proposed to further deepen our understanding of ROS behaviors in plants.Key words: hypersensitive response, NADPH oxidase, oxidative burst, plant immunity, reactive oxygen species     

An antagonist of lipid A action in mammals has complex effects on lipid A induction of defence responses in the model plant Arabidopsis thaliana

Lipopolysaccharides, the ubiquitous part of the outer membrane of Gram-negative bacteria, and their derivatives are recognised by plants to trigger or potentiate particular defence responses such as induction of genes encoding pathogenesis-related proteins. The molecular mechanisms of LPS perception that underpin these effects in plants are, however, unknown. Here, lipid A from Halomonas magadiensis, which is an antagonist of lipid A action in human cells, was used to investigate lipid A action in plants. Our findings offer an insight into the different structural requirements for direct induction and potentiation of plant defences by lipid A.     

Signalling and cell death in ozone-exposed plants

Experiments with Arabidopsis mutants and sensitive and tolerant pairs in several other species have elucidated the molecular basis of plant ozone sensitivity and ozone lesion development. They have indicated an important role for hormonal signalling in determining the outcome of ozone challenge at the cellular level. The reactive oxygen species (ROS) from ozone degradation can cause either direct necrotic damage or induce the process of programmed cell death. Perception of ozone or ROS from its degradation in the apoplast activates several signal transduction pathways that regulate the responses of the cells to the increased oxidative load. Plant hormones salicylic acid, jasmonic acid, ethylene and abscisic acid are involved in determining the duration and extent of ozone-induced cell death and its propagation. Salicylic acid is required for the programmed cell death, ethylene promotes endogenous ROS formation and lesion propagation, and jasmonic acid is involved in limiting the lesion spreading. Abscisic acid is most likely involved through the regulation of stomata and thus is expected to affect lesion initiation. The roles and interactions of perception of ozone, the immediate downstream responses, hormone biosynthesis and signalling during ozone lesion initiation and formation are reviewed.     

Repair and tolerance of oxidative DNA damage in plants

DNA damage caused by exposure to reactive oxygen species is one of the primary causes of DNA decay in most organisms. In plants, endogenous reactive oxygen species (ROS) are generated not only by respiration and photosynthesis, but also by active responses to certain environmental challenges, such as pathogen attack. Significant extracellular sources of activated oxygen include air pollutants such as ozone and oxidative effects of UV light and low-level ionizing radiation. Plants are well equipped to cope with oxidative damage to cellular macromolecules, including DNA. Oxidative attack on DNA generates both altered bases and damaged sugar residues that undergo fragmentation and lead to strand breaks. Recent advances in the study of DNA repair in higher plants show that they use mechanisms similar to those present in other eukaryotes to remove and/or tolerate oxidized bases and other oxidative DNA lesions. Therefore, plants represent a valuable model system for the study of DNA oxidative repair processes in eukaryotic cells.     

UV radiation-responsive proteins in rice leaves: a proteomic analysis

Depletion of stratospheric ozone has led to increased UV radiation reaching the surface of the Earth. This may damage plants. Using physiological, proteomic and quantitative real-time PCR (qPCR) methods, we systematically studied the response of 16-day-old rice seedlings to UV [0.67 W m(-2) biologically effective UVB (UVB(BE)) and 0.28 W m(-2) UVA] exposure for 6, 12 and 24 h. UV exposure resulted in the appearance of light brown patches on leaves, a decrease in the net photosynthetic rate (Pn), lipid peroxidation, accumulation of UV-absorbing compounds (including flavonoids and other phenolic pigments) and differential expression of 22 proteins. Both physiological and molecular responses became stronger with increasing UV exposure time, indicating the effects of UV accumulation on plants. UV-induced responses included (i) phytohormone-regulative responses (up-regulation of proteins related to phytohormone synthesis such as IAA and ethylene); (ii) injurious responses (photosynthesis suppression, lipid peroxidation and visible injury); and (iii) protective responses (accumulation of UV-absorbing compounds and differential expression of proteins involved in detoxification/antioxidation, defense, protein processing, RNA processing, carbohydrate metabolism and secondary metabolism). The identification of UV-responsive proteins provided a better understanding of the molecular mechanism of plant responses to UV stress. Proteomic and qPCR analysis identified one up-regulated and two induced proteins with important functions: tryptophan synthase α chain (production of radical oxygen species), glyoxalase I (detoxification/antioxidation) and a Bet v I family protein (defense). These results will contribute to future research into their roles in UV stress responses in plants.     

A novel device detects a rapid ozone-induced transient stomatal closure in intact Arabidopsis and its absence in abi2 mutant

To follow stomatal responses to ozone (O₃) in different Arabidopsis lines, we constructed a rapid-response O₃ exposure/gas-exchange measurement device consisting of eight through-flow whole-rosette cuvettes. To separate rosette from roots and growth substrate, plant is grown through an agar-filled hole in a polished glass plate fixed on the pot. Following insertion of the plant, the plate forms air-tight bottom surface of the cuvette; thus the rosette is enclosed without touching it during any phase of the insertion of the plant to the cuvette. The device allows monitoring rapid responses in the stomatal function. For example, an acute exposure to 150 ppb O₃ decreased stomatal conductance to 60–70% of its initial value within 9–12 min. Thereafter, the conductance regained its preexposure value within further 30–40 min in spite of the continuing O₃ exposure. The transient decrease was absent in the abscisic acid-insensitive mutant abi2 defective in a class 2C protein phosphatase. This provides an in vivo confirmation that the early transient decrease in stomatal conductance is not a result of physical damage by the reactive oxygen species (ROS) formed from O₃ breakdown but reflects the biological action of ROS, transduced through a signalling cascade. Thus, the apparatus will be helpful in specifying complex molecular and genetic interactions in rapid responses in guard cells in vivo.