Hydrogen Peroxide in Plants： a Versatile Molecule of the Reactive Oxygen Species Network

Plants often face the challenge of severe environmental conditions, which include various biotic and abiotic stresses that exert adverse effects on plant growth and development. During evolution, plants have evolved complex regulatory mechanisms to adapt to various environmental stressors. One of the consequences of stress is an increase in the cellular concentration of reactive oxygen species （ROS）, which are subsequently converted to hydrogen peroxide （H2O2）. Even under normal conditions, higher plants produce ROS during metabolic processes. Excess concentrations of ROS result in oxidative damage to or the apoptotic death of cells. Development of an antioxidant defense system in plants protects them against oxidative stress damage. These ROS and, more particularly, H2O2, play versatile roles in normal plant physiological processes and in resistance to stresses. Recently, H2O2 has been regarded as a signaling molecule and regulator of the expression of some genes in cells. This review describes various aspects of H2O2 function, generation and scavenging, gene regulation and cross-links with other physiological molecules during plant growth, development and resistance responses.     

Influence of abiotic stress signals on secondary metabolites in plants

Plant secondary metabolites are unique sources for pharmaceuticals, food additives, flavors, and industrially important biochemicals. Accumulation of such metabolites often occurs in plants subjected to stresses including various elicitors or signal molecules. Secondary metabolites play a major role in the adaptation of plants to the environment and in overcoming stress conditions. Environmental factors viz. temperature, humidity, light intensity, the supply of water, minerals, and CO₂ influence the growth of a plant and secondary metabolite production. Drought, high salinity, and freezing temperatures are environmental conditions that cause adverse effects on the growth of plants and the productivity of crops. Plant cell culture technologies have been effective tools for both studying and producing plant secondary metabolites under in vitro conditions and for plant improvement. This brief review summarizes the influence of different abiotic factors include salt, drought, light, heavy metals, frost etc. on secondary metabolites in plants. The focus of the present review is the influence of abiotic factors on secondary metabolite production and some of important plant pharmaceuticals. Also, we describe the results of in vitro cultures and production of some important secondary metabolites obtained in our laboratory.     

Pathogens pulling the strings: Effectors manipulating salicylic acid and phenylpropanoid biosynthesis in plants

During evolution, plants have developed sophisticated ways to cope with different biotic and abiotic stresses. Phytohormones and secondary metabolites are known to play pivotal roles in defence responses against invading pathogens. One of the key hormones involved in plant immunity is salicylic acid (SA), of which the role in plant defence is well established and documented. Plants produce an array of secondary metabolites categorized in different classes, with the phenylpropanoids as major players in plant immunity. Both SA and phenylpropanoids are needed for an effective immune response by the plant. To successfully infect the host, pathogens secrete proteins, called effectors, into the plant tissue to lower defence. Secreted effectors can interfere with several metabolic or signalling pathways in the host to facilitate infection. In this review, we will focus on the different strategies pathogens have developed to affect the levels of SA and phenylpropanoids to increase plant susceptibility.     

Nitric oxide elicitation for secondary metabolite production in cultured plant cells

Nitric oxide (NO) is an important signal molecule in stress responses. Accumulation of secondary metabolites often occurs in plants subjected to stresses including various elicitors or signal molecules. NO has been reported to play important roles in elicitor-induced secondary metabolite production in tissue and cell cultures of medicinal plants. Better understanding of NO role in the biosynthesis of such metabolites is very important for optimizing the commercial production of those pharmaceutically significant secondary metabolites. This paper summarizes progress made on several aspects of NO signal leading to the production of plant secondary metabolites, including various abiotic and biotic elicitors that induce NO production, elicitor-triggered NO generation cascades, the impact of NO on growth development and programmed cell death in medicinal plants, and NO-mediated regulation of the biosynthetic pathways of such metabolites. Cross-talks among NO signaling and reactive oxygen species, salicylic acid, and jasmonic acid are discussed. Some perspectives on the application of NO donors for induction of the secondary metabolite accumulation in plant cultures are also presented.     

Role of plant dehydrins in antioxidation mechanisms

Plant antioxidation system is composed of a series of complex mechanisms, in which many antioxidants including some special proteins are involved. Dehydrins are a family of late embryogenesis abundant (LEA) proteins which usually accumulate in plants during late embryogenesis or in response to environmental stresses. They were suggested to be associated with specific protective functions in plant cells, such as preventing coagulation of macromolecules and maintaining integrity of crucial cell structures. In recent years, many studies implied that dehydrins also play an antioxidative role to alleviate oxidative damage in stressed plants. They were proposed to scavenge radicals directly and sequester metals which are sources for radical generation to avoid the production of reactive oxygen species (ROS). In this paper, we will discuss the novel putative role of dehydrins in plant antioxidation mechanisms and how dehydrins perform their antioxidative activity.     

Revealing on hydrogen sulfide and nitric oxide signals co-ordination for plant growth under stress conditions

In the recent times, plants are facing certain types of environmental stresses, which give rise to formation of reactive oxygen species (ROS) such as hydroxyl radicals, hydrogen peroxides, superoxide anions and so on. These are required by the plants at low concentrations for signal transduction and at high concentrations, they repress plant root growth. Apart from the ROS activities, hydrogen sulfide (H₂S) and nitric oxide (NO) have major contributions in regulating growth and developmental processes in plants, as they also play key roles as signaling molecules and act as chief plant immune defense mechanisms against various biotic as well as abiotic stresses. H₂S and NO are the two pivotal gaseous messengers involved in growth, germination and improved tolerance in plants under stressed and non-stress conditions. H₂S and NO mediate cell signaling in plants as a response to several abiotic stresses like temperature, heavy metal exposure, water and salinity. They alter gene expression levels to induce the synthesis of antioxidant enzymes, osmolytes and also trigger their interactions with each other. However, research has been limited to only cross adaptations and signal transductions. Understanding the change and mechanism of H₂S and NO mediated cell signaling will broaden our knowledge on the various biochemical changes that occur in plant cells related to different stresses. A clear understanding of these molecules in various environmental stresses would help to confer biotechnological applications to protect plants against abiotic stresses and to improve crop productivity.     

Reactive oxygen species, antioxidants and signaling in plants

Several reactive oxygen species (ROS) are continuously produced in plants as byproducts of many metabolic reactions, such as photosynthesis, photo respiration and respiration, Depending on the nature of the ROS species, some are highly toxic and rapidly detoxified by various cellular enzymatic and nonenzymatic mechanisms. Oxidative stress occurs when there is a serious imbalance between the production of ROS and antioxidative defence. ROS participate in signal transduction, but also modify cellular components and cause damage. ROS is highly reactive molecules and can oxidize all types of cellular components. Various enzymes involved in ROS-scavenging have been manipulated and over expressed or down regulated. An overview of the literature is presented in terms of primary antioxidant free radical scavenging and redox signaling in plant cells. Special attention is given to ROS and ROS-anioxidant interaction as a metabolic interface for different types of signals derived from metabolisms and from the changing environment.     

The reactive oxygen species network pathways: an essential prerequisite for perception of pathogen attack and the acquired disease resistance in plants

Availability of complete Arabidopsis(Arabidopsis thaliana) and rice(Oryza sativa) genome sequences, together with molecular recourses of functional genomics and proteomics have revolutionized our understanding of reactive oxygen species (ROS) signalling network mediating disease resistance in plants. So far, ROS have been associated with aging, cellular and molecular alteration in animal and plant cells. Recently, concluding evidences suggest that ROS network is essential to induce disease resistance and even to mediate resistance to multiple stresses in plants. ROS are obligatory by-products emerging as a result of normal metabolic reactions. They have the potential to be both beneficial and harmful to cellular metabolism. Their dual effects on metabolic reactions are dosage specific. In this review we focus our attention on cellular ROS level to trigger beneficial effects on plant cells responding to pathogen attack. By exploring the research related contributions coupled with data of targeted gene disruption, and RNA interference approaches, we show here that ROS are ubiquitous molecules of redox-pathways that play a crucial role in plant defence mechanism. The molecular prerequisites of ROS network to activate plant defence system in response to pathogen infections are here underlined. Bioinformatic tools are now available to scientists for high throughput analysis of cellular metabolisms. These tools are used to illustrate crucial ROS-related genes that are involved in the defence mechanism of plants. The review describes also the emerging findings of ROS network pathways to modulate multiple stress resistance in plants.     

Leaf senescence and abiotic stresses share reactive oxygen species-mediated chloroplast degradation

Leaf senescence is a genetically programmed decline in various cellular processes including photosynthesis and involves the hydrolysis of macromolecules such as proteins, lipids, etc. It is governed by the developmental age and is induced or enhanced by environmental stresses such as drought, heat, salinity and others. Internal factors such as reproductive structures also influence the rate of leaf senescence. Reactive oxygen species (ROS) generation is one of the earliest responses of plant cells under abiotic stresses and senescence. Chloroplasts are the main targets of ROS-linked damage during various environmental stresses and natural senescence as ROS detoxification systems decline with age. Plants adapt to environmental stresses through the process of acclimation, which involves less ROS production coupled with an efficient antioxidant defence. Chloroplasts are a major site of protein degradation, and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is rapidly and selectively degraded during senescence and stress. The process of protein degradation is initiated by ROS and involves the action of proteolytic enzymes such as cysteine and serine proteases. The mechanism of Rubisco degradation still remains to be elucidated. The molecular understanding of leaf senescence was achieved through the characterization of senescence-associated genes and various senescence mutants of Arabidopsis, which is a suitable model plant showing monocarpic senescence. The regulation of senescence involves many regulatory elements composed of positive and negative elements to fine-tune the initiation and progression of senescence. This review gives an overview on chloroplast protein degradation during leaf senescence and abiotic stresses and also highlights the role of ROS management in both processes.     

植物活性氧的产生及其作用和危害

活性氧(ROS)是一类由O2转化而来的自由基或具有高反应活性的离子或分子。植物消耗的O2约有1%在叶绿体、线粒体、过氧化物酶体等多种亚细胞单位中被转化成了ROS。ROS有益或有害取决于它在植物体内的浓度。低浓度的ROS作为第二信使能在植物细胞信号转导途径中介导多种应答反应,高浓度的ROS则引起生物大分子的氧化损伤甚至细胞死亡。植物体内ROS产生和清除之间的平衡十分重要,并由一套有效的酶促和非酶促抗氧化系统来监控。该文主要系统介绍了植物ROS的种类、产生部位、在信号转导中的作用及其对植物细胞造成的主要伤害等方面的研究进展,为利用基因工程手段来提高植物对环境胁迫的抗性提供信息和思路。     

Antimicrobial Phytoprotectants and Fungal Pathogens: A Commentary

Many plants produce antifungal secondary metabolites. These may be preformed compounds which are found in healthy plants and which may represent in-built chemical barriers to infection by potential pathogens (preformed antimicrobial compounds or phytoanticipins). Alternatively they may be synthesized in response to pathogen attack as part of the plant defence response (phytoalexins). If these molecules do play a role in protecting plants against pathogen attack, then successful pathogens are presumably able to circumvent or tolerate these defences. Strategies may include avoidance, enzymatic degradation, and/or nondegradative mechanisms. This review outlines the different ways in which fungal pathogens may counter the antifungal compounds produced by their host plants and summarizes the evidence for and against these compounds as antimicrobial phytoprotectants.     

Signal transduction and biotechnology in response to environmental stresses

Providing sufficient food to burgeoning population from the steadily shrinking arable land seems to be very difficult in near future and is one of the foremost challenges for plant scientists. In addition, there are several biotic and abiotic stresses which frequently encounter crop plants during various stages of life cycle, resulting in considerable yield losses. Environmental stresses, including drought, flooding, salinity, temperature (both low and high), high radiation, and xenobiotics induce toxicity, membrane damage, excessive reactive oxygen species (ROS) production, reduced photosynthesis, and altered nutrient acquisition. Several indigenous defence mechanisms (physiological and molecular) are triggered in plants on exposure to environmental cues. Enhancement of resistance of crop plants to environmental stresses has been the topic of prime interest for agriculturalists and plant scientists since long. Development of water and salinity stress-tolerant crops through genetic engineering provides an avenue towards the reclamation of farmlands that have been lost due to salinity and lack of irrigation water/rainfall. Understanding the complexity of stress tolerance mechanisms in orthodox or model plants at the genetic and molecular levels improves feasibility of enhancing tolerance of sensitive crop plants.     

Metabolomic analysis reveals reliance on secondary plant metabolites to facilitate carnivory in the Cape sundew,Drosera capensis

Background and AimsSecondary metabolites are integral to multiple key plant processes (growth regulation, pollinator attraction and interactions with conspecifics, competitors and symbionts) yet their role in plant adaptation remains an underexplored area of research. Carnivorous plants use secondary metabolites to acquire nutrients from prey, but the extent of the role of secondary metabolites in plant carnivory is not known. We aimed to determine the extent of the role of secondary metabolites in facilitating carnivory of the Cape sundew, Drosera capensis.MethodsWe conducted metabolomic analysis of 72 plants in a time-series experiment before and after simulated prey capture. We used ultra-high-performance liquid chromatography–tandem mass spectrometry (UHPLC–MS/MS) and the retention time index to identify compounds in the leaf trap tissue that changed up to 72 h following simulated prey capture. We identified associated metabolic pathways, and cross-compared these compounds with metabolites previously known to be involved in carnivorous plants across taxa.Key ResultsFor the first time in a carnivorous plant, we have profiled the whole-leaf metabolome response to prey capture. Reliance on secondary plant metabolites was higher than previously thought – 2383 out of 3257 compounds in fed leaves had statistically significant concentration changes in comparison with unfed controls. Of these, ~34 compounds are also associated with carnivory in other species; 11 are unique to Nepenthales. At least 20 compounds had 10-fold changes in concentration, 12 of which had 30-fold changes and are typically associated with defence or attraction in non-carnivorous plants.ConclusionsSecondary plant metabolites are utilized in plant carnivory to an extent greater than previously thought – we found a whole-metabolome response to prey capture. Plant carnivory, at the metabolic level, likely evolved from at least two distinct functions: attraction and defence. Findings of this study support the hypothesis that secondary metabolites play an important role in plant diversification and adaptation to new environments.     

Rethinking the role of many plant phenolics – protection from photodamage not herbivores?

Phenolics have been considered classic defence compounds for protecting plants from herbivores, ever since plant secondary metabolites were suggested to have evolved for that reason. The resource availability and carbon-nutrient balance hypotheses proposed that variation in phenolic levels between and within plant species reflects environmental availability of nutrients and light, and represents a trade-off in allocation by plants to growth and defence against herbivores. In contrast to these concepts, we suggest that (1) the main role of many plant phenolics may be to protect leaves from photodamage, not herbivores; (2) they can achieve this by acting as antioxidants; and (3) their levels may vary with environmental conditions in order to counteract this potential photodamage. We therefore suggest that patterns in phenolic levels, often used to support the concept of trade-off between growth and herbivore defence in relation to resource availability, may actually reflect different risks of photodamage. We suggest that the level of many phenolics is low under some environmental conditions, not because resources to produce them are limited, but simply because the risk of photodamage is low and they are not required. If our photodamage hypothesis is correct, a reassessment of the ecological and evolutionary role of many phenolics in plant defence theory is required.     

Application of LC/MS systems to structural characterization of flavonoid glycoconjugates

Mass spectrometry is currently one of the most versatile and sensitive instrumental methods applied to structural characterization of plant secondary metabolite mixtures isolated from biological material. Plant tissues contain thousands of natural products fulfilling different roles in plant physiology and biochemistry. These natural products have various biological activities in respect to plants synthesizing them, in their responses to different environmental stresses and are also active principles of food supplements and pharmaceuticals of plant origin. Flavonoids constitute a large group of phenolic secondary metabolites and are probably produced by all terrestrial plant species. More than 9000 glycoconjugates of flavonoids are presently known in the plant kingdom and more than 50 of them may be present in a single plant. For this reason methods of identification and analysis of this group of compounds are particularly demanded. Due to a high number of metabolites present in plant extracts, the isolation and purification of most compounds in amounts suitable for unambiguous characterization with NMR methods is often impossible. For these reasons elaboration of strategies for sufficiently precise structural characterization of compounds present in mixture samples is currently a primary task. Mass spectrometry, thanks to application of different physical phenomena for ionization, separation and detection of analyzed molecules, became the method of choice among analytical methods applied for identification, structural characterization and quantitative analysis of the natural products. Methods of analysis of differently substituted flavonoids (O- and C-glycosides, differentiation of various oligosaccharidic substituents, detection of acylated compounds) are presented in the paper. A proper application of mass spectrometric methods in well-defined and strictly controlled technical parameters of analysis permits obtaining important structural information. Among others, recording collision induced dissociation mass spectra allows identification of compounds after comparison of the registered MS spectra with these present in the existing databases.     

Role of plant hormones in plant defence responses

Plant hormones play important roles in regulating developmental processes and signaling networks involved in plant responses to a wide range of biotic and abiotic stresses. Significant progress has been made in identifying the key components and understanding the role of salicylic acid (SA), jasmonates (JA) and ethylene (ET) in plant responses to biotic stresses. Recent studies indicate that other hormones such as abscisic acid (ABA), auxin, gibberellic acid (GA), cytokinin (CK), brassinosteroids (BR) and peptide hormones are also implicated in plant defence signaling pathways but their role in plant defence is less well studied. Here, we review recent advances made in understanding the role of these hormones in modulating plant defence responses against various diseases and pests.     

Proteomics applied on plant abiotic stresses: role of heat shock proteins (HSP)

The most crucial function of plant cell is to respond against stress induced for self-defence. This defence is brought about by alteration in the pattern of gene expression: qualitative and quantitative changes in proteins are the result, leading to modulation of certain metabolic and defensive pathways. Abiotic stresses usually cause protein dysfunction. They have an ability to alter the levels of a number of proteins which may be soluble or structural in nature. Nowadays, in higher plants high-throughput protein identification has been made possible along with improved protein extraction, purification protocols and the development of genomic sequence databases for peptide mass matches. Thus, recent proteome analysis performed in the vegetal Kingdom has provided new dimensions to assess the changes in protein types and their expression levels under abiotic stress. As reported in this review, specific and novel proteins, protein-protein interactions and post-translational modifications have been identified, which play a role in signal transduction, anti-oxidative defence, anti-freezing, heat shock, metal binding etc. However, beside specific proteins production, plants respond to various stresses in a similar manner by producing heat shock proteins (HSPs), indicating a similarity in the plant's adaptive mechanisms; in plants, more than in animals, HSPs protect cells against many stresses. A relationship between ROS and HSP also seems to exist, corroborating the hypothesis that during the course of evolution, plants were able to achieve a high degree of control over ROS toxicity and are now using ROS as signalling molecules to induce HSPs.     

Magnificant role of intracellular reactive oxygen species production and its scavenging encompasses downstream processes

Environmental stresses are often associated with production of certain deleterious chemical entities called reactive oxygen species (ROS), which include hydrogen peroxide (H₂O₂), superoxide radical (O₂^?), hydroxyl radical (OH^?). In plants, ROS are formed by the inevitable leakage of electrons onto O₂ from the electron transport activities of chloroplasts, mitochondria, peroxisomes, vacuole and plasma membranes or as a byproduct of various metabolic pathways. Plants have their own antioxidant defense mechanisms to encounter ROS that is of enzymic and non-enzymic nature. Coordinated activities of these antioxidants regulate ROS detoxification and reduces oxidative load in plants. Though ROS are always regarded to impart negative impact on plants, some reports consider them to be important in regulating key cellular functions; however, such reports in plant are limited. On the other hand, specific ROS function as signaling molecules and activate signal transduction processes in response to various stresses is a matter of investigation.