Autophagy,programmed cell death and reactive oxygen species in sexual reproduction in plants

Autophagy is one of the major cellular processes of recycling of proteins, metabolites and intracellular organelles, and plays crucial roles in the regulation of innate immunity, stress responses and programmed cell death (PCD) in many eukaryotes. It is also essential in development and sexual reproduction in many animals. In plants, although autophagy-deficient mutants of Arabidopsis thaliana show phenotypes in abiotic and biotic stress responses, their life cycle seems normal and thus little had been known until recently about the roles of autophagy in development and reproduction. Rice mutants defective in autophagy show sporophytic male sterility and immature pollens, indicating crucial roles of autophagy during pollen maturation. Enzymatic production of reactive oxygen species (ROS) by respiratory burst oxidase homologues (Rbohs) play multiple roles in regulating anther development, pollen tube elongation and fertilization. Significance of autophagy and ROS in the regulation of PCD of transient cells during plant sexual reproduction is discussed in comparison with animals.     

Metabolism of reactive oxygen species and reactive nitrogen species in pepper (Capsicum annuum L.) plants under low temperature stress

Low temperature is an environmental stress that affects crop production and quality and regulates the expression of many genes, and the level of a number of proteins and metabolites. Using leaves from pepper (Capsicum annum L.) plants exposed to low temperature (8 °C) for different time periods (1 to 3 d), several key components of the metabolism of reactive nitrogen and oxygen species (RNS and ROS, respectively) were analysed. After 24 h of exposure at 8 °C, pepper plants exhibited visible symptoms characterized by flaccidity of stems and leaves. This was accompanied by significant changes in the metabolism of RNS and ROS with an increase of both protein tyrosine nitration (NO(2) -Tyr) and lipid peroxidation, indicating that low temperature induces nitrosative and oxidative stress. During the second and third days at low temperature, pepper plants underwent cold acclimation by adjusting their antioxidant metabolism and reverting the observed nitrosative and oxidative stress. In this process, the levels of the soluble non-enzymatic antioxidants ascorbate and glutathione, and the activity of the main NADPH-generating dehydrogenases were significantly induced. This suggests that ascorbate, glutathione and the NADPH-generating dehydrogenases have a role in the process of cold acclimation through their effect on the redox state of the cell.     

Nitric oxide and reactive oxygen species are required for systemic acquired resistance in plants

Systemic acquired resistance (SAR) is a form of broad-spectrum disease resistance that is induced in response to primary infection and that protects uninfected portions of the plant against secondary infections by related or unrelated pathogens. SAR is associated with an increase in chemical signals that operate in a collective manner to confer protection against secondary infections. These include, the phytohormone salicylic acid (SA), glycerol-3-phosphate (G3P), azelaic acid (AzA) and more recently identified signals nitric oxide (NO) and reactive oxygen species (ROS). NO, ROS, AzA and G3P function in the same branch of the SAR pathway, and in parallel to the SA-regulated branch. NO and ROS function upstream of AzA/G3P and different reactive oxygen species functions in an additive manner to mediate chemical cleavage of the C9 double bond on C18 unsaturated fatty acids to generate AzA. The parallel and additive functioning of various chemical signals provides important new insights in the overlapping pathways leading to SAR.     

Stabilization of thylakoid membranes in isoprene-emitting plants reduces formation of reactive oxygen species

Isoprene is emitted by a significant fraction of the world''s vegetation. Isoprene makes leaves more thermotolerant, yet we do not fully understand how. We have recently shown that isoprene stabilizes thylakoid membranes under heat stress. Here we show that heat-stressed, isoprene-emitting transgenic Arabidopsis plants also produce a lower pool of reactive oxygen and reactive nitrogen species, and that this was especially due to a lower accumulation of H₂O₂ in isoprene emitting plants. It remains difficult to disentangle whether in heat stressed plants isoprene also directly reacts with and quenches reactive oxygen species (ROS), or reduces ROS formation by stabilizing thylakoids. We present considerations that make the latter a more likely mechanism, under our experimental circumstances.          

Imaging reactive oxygen species in arthritis

Reactive oxygen species (ROS) have been shown to play a role in the pathogenesis of arthritides. Luminol was used as the primary reporter of ROS and photons resulting from the chemiluminescence reaction were detected using a super-cooled CCD photon counting system. Luminol was injected intravenously into groups of animals with different models of arthritis. Imaging signal correlated well with the severity of arthritis in focal and pan-arthritis as determined by histological measurement of ROS by formazan. Measurements were highly reproducible, sensitive, and repeatable. In vivo chemiluminescence imaging is expected to become a useful modality to elucidate the role of ROS in the pathogenesis of arthritides and in determining therapeutic efficacy of protective therapies.     

Mammalian peroxisomes and reactive oxygen species

The central role of peroxisomes in the generation and scavenging of hydrogen peroxide has been well known ever since their discovery almost four decades ago. Recent studies have revealed their involvement in metabolism of oxygen free radicals and nitric oxide that have important functions in intra- and intercellular signaling. The analysis of the role of mammalian peroxisomes in a variety of physiological and pathological processes involving reactive oxygen species (ROS) is the subject of this review. The general characteristics of peroxisomes and their enzymes involved in the metabolism of ROS are briefly reviewed. An expansion of the peroxisomal compartment with proliferation of tubular peroxisomes is observed in cells exposed to UV irradiation and various oxidants and is apparently accompanied by upregulation of PEX genes. Significant reduction of peroxisomes and their enzymes is observed in inflammatory processes including infections, ischemia-reperfusion injury, and allograft rejection and seems to be related to the suppressive effect of tumor necrosis factor- on peroxisome function and peroxisome proliferator activated receptor-. Xenobiotic-induced proliferation of peroxisomes in rodents is accompanied by the formation of hepatic tumors, and evidently the imbalance in generation and decomposition of ROS plays an important role in this process. In PEX5^–/– knockout mice lacking functional peroxisomes severe alterations of mitochondria in various organs are observed which seem to be due to a generalized increase in oxidative stress confirming the important role of peroxisomes in homeostasis of ROS and the implications of its disturbances for cell pathology.     

Phagocytic leukocytes and reactive oxygen species

Phagocytic leukocytes, when appropriately stimulated, display a respiratory burst in which they consume oxygen and produce superoxide anions. Superoxide is produced by the phagocyte NADPH-oxidase system which is a multiprotein complex that is dissociated in quiescent cells and is assembled into the functional oxidase following stimulation of these cells. Also associated with the respiratory burst is the generation of other reactive oxygen species. The identity of components of the NADPH-oxidase system and their interactions are known in considerable molecular detail. Understanding of the regulation of superoxide production is less well known. This review also points out the important role of microscopy in complementing biochemical studies to understand better the cell biology of the phagocyte respiratory burst. Presented at the 50th Anniversary Symposium of the Society for Histochemistry, Interlaken, Switzerland, October 1–4, 2008.     

Imaging reactive oxygen species in asthma

Inflammatory processes in asthma are characterized by an infiltration of inflammatory cells including mononuclear phagocytes. It has been observed that mononuclear phagocytes, alveolar macrophages and blood monocytes, release higher quantities of reactive oxygen species in asthmatic patients than in healthy subjects. Chemiluminescence assays were developed to measure the superoxide anion and the other reactive oxygen species. The chemiluminescence response was first analysed with a luminometer, which made it possible to study cells in suspension before and after PMA-stimulation. Secondly a video-imaging camera was used in experiments on adherent cells before and after stimulation with PMA and/or specific stimulus IgE/anti-IgE. Both techniques showed that human alveolar macrophages, blood monocytes, PMN and lymphocytes were spontaneously primed in vivo and were more easily stimulated in asthma. Analysis of adherent cells in vitro may provide give information on the physiological condition of adherent cells in vivo.     

Defence against reactive oxygen species

As part of the European Commission Concerted Action on Functional Food which was managed by the International Life Sciences Institute (Europe) a series of Theme Papers was produced which examined the ‘state of the art’ with respect to the subject matter and made recommendations for research. This paper is a summary of the paper concerned with Defence Against Reactive Oxygen species. Having reviewed the scientific literature the authors concluded that certain stringent criteria, which they identified, would need to be satisfied in order to be able to conclude that free radical events are involved in certain human diseases, and that antioxidants are capable of modulating these events and thus reducing the risk of disease. Although there is some evidence that would lead to this conclusion the authors demonstrated that there is at present insufficient evidence available on which to base a firm conclusion that antioxidants are capable of reducing risk of disease, and very little evidence that addresses the important question as to how much of the nutrients concerned are required in the diet to achieve the objective of reducing risk. Research priorities address the need in particular for the development and validation of cellular markers of oxidative damage which are required before there can be new human studies that address the question. There is also a need for more information as to the pharmacokinetics of uptake from diet, distribution and cellular concentration of the antioxidants.     

Defense against reactive oxygen species

As part of the European Commission Concerted Action on Functional Food which was managed by the International Life Sciences Institute (Europe) a series of Theme Papers was produced which examined the 'state of the art' with respect to the subject matter and made recommendations for research. This paper is a summary of the paper concerned with Defence Against Reactive Oxygen species. Having reviewed the scientific literature the authors concluded that certain stringent criteria, which they identified, would need to be satisfied in order to be able to conclude that free radical events are involved in certain human diseases, and that antioxidants are capable of modulating these events and thus reducing the risk of disease. Although there is some evidence that would lead to this conclusion the authors demonstrated that there is at present insufficient evidence available on which to base a firm conclusion that antioxidants are capable of reducing risk of disease, and very little evidence that addresses the important question as to how much of the nutrients concerned are required in the diet to achieve the objective of reducing risk. Research priorities address the need in particular for the development and validation of cellular markers of oxidative damage which are required before there can be new human studies that address the question. There is also a need for more information as to the pharmacokinetics of uptake from diet, distribution and cellular concentration of the antioxidants.     

Oxidation-reduction and reactive oxygen species homeostasis in mutant plants with respiratory chain complex I dysfunction

Mutations in a mitochondrial or nuclear gene encoding respiratory chain complex I subunits lead to decreased or a total absence of complex I activity. Plant mutants with altered or lost complex I activity adapt their respiratory metabolism by inducing alternative pathways of the respiratory chain and changing energy metabolism. Apparently, complex I is a crucial component of the oxidation-reduction (redox) regulatory system in photosynthetic cells, and alternative NAD(P)H dehydrogenases of the mitochondrial electron transport chain (mtETC) cannot fully compensate for its impairment. In most cases, dysfunction of complex I is associated with lowered or unchanged hydrogen peroxide (H(2)O(2)) concentrations, but increased superoxide (O(2)(-)) levels. Higher production of reactive oxygen species (ROS) by mitochondria in the mosaic (MSC16) cucumber mutant may be related to retrograde signalling. Different effects of complex I dysfunction on H(2)O(2) and O(2)(-) levels in described mutants might result from diverse regulation of processes involved in H(2)O(2) and O(2)(-) production. Often, dysfunction of complex I did not lead to oxidative stress, but increased the capacity of the antioxidative system and enhanced stress tolerance. The new cellular homeostasis in mutants with dysfunction of complex I allows growth and development, reflecting the plasticity of plant metabolism.     

Oxygen radicals and reactive oxygen species in reproduction

Free radicals and reactive oxygen species play a number of significant and diverse roles in reproductive biology. In common with other biological systems, mechanisms have evolved to minimize the damaging effects that these highly reactive molecules can have on reproductive integrity. Conversely, however, recent findings illustrate the constructive roles that oxygen radicals and reactive oxygen species play in a number of important junctures in the development of germ cells and the obligate endocrine support they receive for the successful propagation of the species. Specifically addressed in this review are some aspects of sperm development and action, the uterine environment, oocyte maturation and ovulation, and corpus luteum function and regression.