[Show abstract][Hide abstract] ABSTRACT: Anthocerotophyta (hornworts) belong to a group of ancient nonvascular plants and originate from a common ancestor with contemporary vascular plants. Hornworts represent a unique model for investigating mechanisms of formation of stress resistance in higher plants due to their high tolerance to the action of adverse environmental factors. In this work, we demonstrate that the thallus of Anthoceros natalensis exhibits high redox activity changing under stress. Dehydration of the thallus is accompanied by the decrease in activities of intracellular peroxidases, DOPA-peroxidases, and tyrosinases, while catalase activity increases. Subsequent rehydration results in the increase in peroxidase and catalase activities. Kinetic features of peroxidases and tyrosinases were characterized as well as the peroxidase isoenzyme composition of different fractions of the hornwort cell wall proteins. It was shown that the hornwort peroxidases are functionally similar to peroxidases of higher vascular plants including their ability to form superoxide anion-radical. The biochemical mechanism was elucidated, supporting the possible participation of peroxidases in the formation of reactive oxygen species (ROS) via substrate–substrate interactions in the hornwort thallus. It has been suggested that the ROS formation by peroxidases is an evolutionarily ancient process that emerged as a protective mechanism for enhancing adaptive responses of higher land plants and their adaptation to changing environmental conditions and successful colonization of various ecological niches.
[Show abstract][Hide abstract] ABSTRACT: Lichens produce various oxidoreductases including heme-containing peroxidases and the copper-containing phenol oxidases tyrosinase and laccase. Our earlier findings suggested that significant oxidoreductase activity occurs mainly in lichens from the order Peltigerales. Here we show that the non-Peltigeralean lichen Usnea can display significant activities of peroxidases and laccases. Strong evidence for the involvement of peroxidases and laccases in saprotrophic activities comes from the observation that their activities are induced by “starvation” due to prolonged dark storage, and also by treatment with soluble cellulose and lignin breakdown products. We also show that, given a quinone and chelated Fe, Usnea can produce hydroxyl radicals; these radicals contribute to the break down of carbohydrates or lignin. However, hydroxyl radical production is independent of laccase and peroxidase activity. Laccases and peroxidases are involved in other aspects of lichen biology; here we show that peroxidases, but not laccases, can break down lichen substances. Reduction in the amounts of lichen substances will reduce photoprotection, which will increase the photosynthetic capacity of thalli during winter when light intensities are low.
[Show abstract][Hide abstract] ABSTRACT: Apoplastic class III peroxidases (EC 220.127.116.11) play key roles in the response of plants to pathogen infection and abiotic stresses, including wounding. Wounding is a common stress for plants that can be caused by insect or animal grazing or trampling, or result from agricultural practices. Typically, mechanical damage to a plant immediately induces a rapid release and activation of apoplastic peroxidases, and an oxidative burst of reactive oxygen species (ROS), followed by the upregulation of peroxidase genes. We discuss how plants control the expression of peroxidases genes upon wounding, and also the sparse information on peroxidase-mediated signal transduction pathways. Evidence reviewed here suggests that in many plants production of the ROS that comprise the initial oxidative burst results from a complex interplay of peroxidases with other apoplastic enzymes. Later responses following wounding include various forms of tissue healing, for example through peroxidase-dependent suberinization, or cell death. Limited data suggest that ROS-mediated death signalling during the wound response may involve the peroxidase network, together with other redox molecules. In conclusion, the ability of peroxidases to both generate and scavenge ROS plays a key role in the involvement of these enigmatic enzymes in plant stress tolerance.
[Show abstract][Hide abstract] ABSTRACT: Plant surfaces form the barrier between a plant and its environment. Upon damage, the wound healing process begins immediately and is accompanied by a rapid production of extracellular reactive oxygen species (ROS), essential in deterring pathogens, signalling responses and cell wall restructuring. Although many enzymes produce extracellular ROS, it is unclear if ROS-producing enzymes act synergistically. We characterised the oxidative burst of superoxide (O2−) and hydrogen peroxide (H2O2) that follows wounding in pea (Pisum sativum L.) seedlings. Rates of ROS production were manipulated by exogenous application of enzyme substrates and inhibitors. The results indicate significant roles for di-amine oxidases (DAO) and peroxidases (Prx) rather than NADPH oxidase. The burst of O2− was strongly dependent on the presence of H2O2 produced by DAO. Potential substrates released from wounded seedlings included linoleic acid that, upon exogenous application, strongly stimulated catalase-sensitive O2− production. Moreover, a 65 kD plasma membrane (PM) guaiacol Prx was found in the secretome of wounded seedlings and showed dependence on linoleic acid for O2− production. Lipoxygenases are suggested to modulate O2− production by consuming polyunsaturated fatty acids in the apoplast. Overall, a O2−-producing mechanism involving H2O2-derived from DAO, linoleic acid and a PM-associated Prx is proposed.
[Show abstract][Hide abstract] ABSTRACT: The dependence of membrane functioning on its sterol component has been intensively studied using model lipids and isolated animal membranes, but to a much lesser extent with plant membranes. Depleting membrane sterols could be predicted to have a strong effect on membrane activity and cause harmful physiological consequences. In this report we characterize membrane lipid composition, membrane permeability for ions and some physiological parameters, such as H2 O2 accumulation, formation of autophagosomal vacuoles, expression of peroxidase and autophagic genes, and cell viability in the roots of wheat (Triticum aestivum L.) seedlings in the presence of two agents that specifically bind with endogenous sterols. The polyene antibiotic nystatin binds with endogenous sterols forming so-called "nystatin pores" or "channels" in the membrane, while methyl-β-cyclodextrin has the capacity to sequester sterols in its hydrophobic core. Unexpectedly, although application of both methyl-β-cyclodextrin and nystatin reduced in the sterol content, their effects on membrane permeability, oxidative status and autophagosome formation in roots differed dramatically. For comparison, we also tested the effects of the antibiotic gramicidin S, which does not bind with sterols but forms non-specific channels in the membrane. Gramicidin S considerably increased membrane permeability, caused oxidative stress and reduced cell viability. Our results suggest that a decrease in the sterol content in itself is not sufficient to cause deleterious consequences for a cell. The disturbance of membrane integrity rather than the decrease in the sterol content is responsible for the toxicity of sterol-binding compounds. This article is protected by copyright. All rights reserved.
[Show abstract][Hide abstract] ABSTRACT: We made use of pot experiments and soil mineralisation assays to test the effect of temperature on the soil nitrogen (N) economy of the Drakensberg Alpine Centre ('mountain site'). The approach was enhanced by the inclusion of a contrasting warm, subtropical environment on the east coast of southern Africa ('coast site') which presented an opportunity to test plant growth in mountain soil outside of the mountain site's natural climatic envelope. This study was further augmented by two greenhouse experiments that helped isolate the factors responsible for the growth responses in the experiments above. Plant morphology, plant nutrients and soil nutrients were used as the basis for comparing treatment effects. The primary pot experiment showed that plant growth was uniform in the mountain site regardless of whether the test species was grown in intrinsically N-rich mountain soil or intrinsically N-poor coast soil. However, we noted significant growth differences at the coast site using the aforementioned soil nutrient regimes. In terms of the soil mineralisation assay, coast soil, derived from intrinsically N-poor sandstone, predictably mineralised little soil inorganic N at the mean spring temperature of 19 degrees C. However against expectations, the intrinsically N-rich mountain soil mineralised <1% of its total soil N budget into inorganic N at 12 degrees C, most probably because the microbes responsible for the conversion of organic soil N to inorganic soil N were severely inhibited at this mean spring temperature. However, the potential to mineralise far more N in mountain soil was apparent when using an elevated experimental temperature of 30 degrees C, with 369% more soil N being available under the latter regime. Our results suggest that the cooler temperatures associated with high elevations in the mountain site constrain the activity of soil microbes in mountain soil, resulting in a functionally N-poor soil economy particularly deficient in inorganic N. This also explains the similar growth responses regardless of the soil being intrinsically N-rich or N-poor. We speculate whether or not more soil inorganic N may become available under a regime of warming due to accelerated N mineralisation, to the detriment of plant taxa adapted to low soil N availability.
No preview · Article · Sep 2013 · South African Journal of Botany
[Show abstract][Hide abstract] ABSTRACT: Lichens are symbiotic associations of a fungus (usually an Ascomycete) with green algae and/or a cyanobacterium. They dominate on 8 % of the world's land surface, mainly in Arctic and Antarctic regions, tundra, high mountain elevations and as components of dryland crusts. In many ecosystems, lichens are the pioneers on the bare rock or soil following disturbance, presumably because of their tolerance to desiccation and high temperature. Lichens have long been recognized as agents of mineral weathering and fine-earth stabilization. Being dominant biomass producers in extreme environments they contribute to primary accumulation of soil organic matter. However, biochemical role of lichens in soil processes is unknown. Our recent research has demonstrated that Peltigeralean lichens contain redox enzymes which in free-living fungi participate in lignocellulose degradation and humification. Thus lichen enzymes may catalyse formation and degradation of soil organic matter, particularly in high-stress communities dominated by lower plants. In the present review we synthesize recently published data on lichen phenol oxidases, peroxidases, and cellulases and discuss their possible roles in lichen physiology and soil organic matter transformations.
[Show abstract][Hide abstract] ABSTRACT: In our earlier work, we demonstrated that the oxidases tyrosinase and laccase occur widely in lichens from the Peltigerales. Recently, we discovered the occurrence of another oxidoreductase, a heme peroxidase, in the Peltigeralean ‘jelly lichens’ Leptogium and Collema. Here we present the results of a survey of peroxidase activity in a range of lichens. In addition to the jelly lichens, strong peroxidase activity also occurs within the Peltigeralean genera Lobaria, Pseudocyphellaria and Sticta. Significant activity occurs in the cell wall, and, unlike laccase activity, peroxidase activity increases considerably following the rehydration of dry thalli. However, activity is absent from Peltigera and from the non-Peltigeralean species tested here. Electrophoretic investigation showed that lichen peroxidases are oligomeric. Possible roles for peroxidases in lichen biology are discussed.
Full-text · Article · Mar 2013 · The Lichenologist
[Show abstract][Hide abstract] ABSTRACT: In our earlier work, we demonstrated the presence of the multicopper oxidases tyrosinase and laccase in the cell walls of lichens from the Peltigerales, while these enzymes appeared to be absent in lichens from other orders. Likely roles for tyrosinase in lichens include melanin synthesis, the generation of quinones needed for laccase-mediated redox cycling, and the removal of harmful reactive molecules formed by this cycling. Non-Peltigeralean lichens will not need tyrosinase to detoxify laccase-generated radicals. However, many non-Peltigeralean lichens are often heavily melanized. Apparent absence of tyrosinase activity in these species prompted us to suggest that, in these lichens, melanins are probably synthesized by the polyketide pathway, which does not involve tyrosinase. Here, we surveyed intracellular tyrosinase activity in thallus homogenates from a range of lichens. Results showed that Peltigeralean species generally have much higher activities than species from other orders. However, the non-Peltigeralean lichen Dermatocarpon miniatum displays significant tyrosinase activity. In D. miniatum, tyrosinase differs from the corresponding enzyme from Peltigeralean lichens with respect to cellular location, substratum specificity, stability and pH optimum. Furthermore, unlike Peltigeralean lichens, in D. miniatum tyrosinase activity increased strongly following the rehydration of dry thalli. These differences are possibly a consequence of the role of tyrosinase in melanin synthesis rather than laccase-mediated redox cycling.
Full-text · Article · Nov 2012 · The Lichenologist
[Show abstract][Hide abstract] ABSTRACT: Lichens belonging to the order Peltigerales display strong activity of multi-copper oxidases (e.g. tyrosinase) as well as heme-containing peroxidases. The lichen peroxidase was purified to homogeneity from the thallus of Leptogium saturninum (LsaPOX) by fast protein liquid chromatography and then partially characterized. The oligomeric protein occurs as both 79 kDa dimeric and 42 kDa monomeric forms, and displayed broad substrate specificity. In addition to an ability to oxidize classic peroxidase substrates (e.g. 2,6-dimethoxyphenol), the enzyme could convert recalcitrant compounds such as synthetic dyes (e.g. Azure B and Reactive Blue 5), 4-nitrophenol and non-phenolic methoxylated aromatics (e.g. veratryl alcohol). Comparing LsaPOX with a basidiomycete dye-decolorizing (DyP)-type peroxidase from Auricularia auricula-judae showed that the lichen enzyme has a high-redox potential, with oxidation capabilities ranging between those of known plant and fungal peroxidases. Internal peptide fragments show homology (up to 60%) with putative proteins from free-living ascomycetes (e.g. Penicillium marneffei and Neosartorya fischeri), but not to sequences of algal or cyanobacterial peptides or to known fungal, bacterial or plant peroxidases. LsaPOX is the first heme peroxidase purified from an ascomyceteous lichen that may help the organism to successfully exploit the extreme micro-environments in which they often grow.
Full-text · Article · Oct 2011 · Fungal Genetics and Biology
[Show abstract][Hide abstract] ABSTRACT: Ex situ storage of recalcitrant (desiccation-sensitive) seeds is possible only in the short term; long-term storage for germplasm conservation requires cryopreservation. The most suitable explant for cryopreservation is the embryo or embryonic axis, which needs to be excised and partially dried prior to cryogenic cooling. In many cases, particularly for seeds with fleshy cotyledons, these treatments lead to the failure of shoot development. The act of cutting the cotyledons induces a wounding response that leads to a burst of reactive oxygen species (ROS). If the site of excision is close to the shoot meristem, oxidative damage to the meristem will occur, leading to failure of the shoot to develop. In seeds of the family Amaryllidaceae (and possibly others) whole embryos can be removed from the seed, avoiding excision damage, and in many species successful cryopreservation can be achieved. Thus the differences in embryo morphology can have an influence on the response to manipulations for cryopreservation.
No preview · Article · Sep 2011 · Acta horticulturae
[Show abstract][Hide abstract] ABSTRACT: 'Stresses' that impact upon seeds can affect plant reproduction and productivity, and, hence, agriculture and biodiversity. In the absence of a clear definition of plant stress, we relate concepts from physics, medicine and psychology to stresses that are specific to seeds. Potential 'eustresses' that enhance function and 'distresses' that have harmful effects are considered in relation to the seed life cycle. Taking a triphasic biomedical stress concept published in 1936, the 'General Adaptation Syndrome', to the molecular level, the 'alarm' response is defined by post-translational modifications and stress signalling through cross-talk between reactive oxygen and nitrogen species, and seed hormones, that result in modifications to the transcriptome. Protection, repair, acclimation and adaptation are viewed as the 'building blocks' of the 'resistance' response, which, in seeds, are the basis for their longevity over centuries. When protection and repair mechanisms eventually fail, depending on dose and time of exposure to stress, cell death and, ultimately, seed death are the result, corresponding to 'exhaustion'. This proposed seed stress concept may have wider applicability to plants in general.
[Show abstract][Hide abstract] ABSTRACT: Bidens pilosa L. is a weedy species in the Asteraceae producing dimorphic one-seeded fruits, with longer black seeds centrally situated in the capitulum, and shorter dormant brown peripheral seeds. While the outer seeds germinate readily after seed shedding, the shorter seeds possess various dormancy mechanisms, including requirements for after-ripening and red light. Here we show that applications of reactive oxygen species (ROS)-generating reagents can remove dormancy in the short seeds. In B. pilosa, reagents that generate hydroxyl radicals (OH) and superoxide (O2¯) partially replaced the requirement for after-ripening, while O2¯ generation replaced the requirement for red light. Hence, ROS appear to be implicated in the alleviation of dormancy in the seeds of B. pilosa.
Full-text · Article · Aug 2010 · South African Journal of Botany