David J. Burritt

Hydrogen peroxide priming modulates abiotic oxidative stress tolerance: insights from ROS detoxification and scavenging

Plants are constantly challenged by various abiotic stresses that negatively affect growth and productivity worldwide. During the course of their evolution, plants have developed sophisticated mechanisms to recognize external signals allowing them to respond appropriately to environmental conditions, although the degree of adjustability or tolerance to specific stresses differs from species to species. Overproduction of reactive oxygen species (ROS; hydrogen peroxide, H2O2; superoxide, O2⋅-; hydroxyl radical, OH⋅ and singlet oxygen, 1O2) is enhanced under abiotic and/or biotic stresses, which can cause oxidative damage to plant macromolecules and cell structures, leading to inhibition of plant growth and development, or to death. Among the various ROS, freely diffusible and relatively long-lived H2O2 acts as a central player in stress signal transduction pathways. These pathways can then activate multiple acclamatory responses that reinforce resistance to various abiotic and biotic stressors. To utilize H2O2 as a signaling molecule, non-toxic levels must be maintained in a delicate balancing act between H2O2 production and scavenging. Several recent studies have demonstrated that the H2O2-priming can enhance abiotic stress tolerance by modulating ROS detoxification and by regulating multiple stress-responsive pathways and gene expression. Despite the importance of the H2O2-priming, little is known about how this process improves the tolerance of plants to stress. Understanding the mechanisms of H2O2-priming-induced abiotic stress tolerance will be valuable for identifying biotechnological strategies to improve abiotic stress tolerance in crop plants. This review is an overview of our current knowledge of the possible mechanisms associated with H2O2-induced abiotic oxidative stress tolerance in plants, with special reference to antioxidant metabolism.

Celery (Apium graveolens L.) parenchyma cell walls examined by atomic force microscopy: effect of dehydration on cellulose microfibrils.

Abstract. Atomic force microscopy (AFM) was used to image celery (Apium graveolens L.) parenchyma cell walls in situ. Cellulose microfibrils could clearly be distinguished in topographic images of the cell wall. The microfibrils of the hydrated walls appeared smaller, more uniformly distributed, and less enmeshed than those of dried peels. In material that was kept hydrated at all times and imaged under water, the microfibril diameter was mainly in the range 6–25 nm. The cellulose microfibril diameters were highly dependent on the water content of the specimen. As the water content was decreased, by mixing ethanol with the bathing solution, the microfibril diameters increased. Upon complete dehydration of the specimen we observed a significant increase in microfibril diameter. The procedure used to dehydrate the parenchyma cells also influenced the size of cellulose microfibrils with freeze-dried material having larger diameters than air-dried material.

The polycyclic aromatic hydrocarbon phenanthrene causes oxidative stress and alters polyamine metabolism in the aquatic liverwort Riccia fluitans L.

The polycyclic aromatic hydrocarbon (PAH) phenanthrene (PHEN) is a highly toxic pollutant, commonly found in aquatic environments, the effects of which on aquatic plants have not been studied in depth. As PAHs are known to induce oxidative stress and recent studies have shown that polyamines (PAs) participate in the defence reactions protecting plants against environmental stresses, PA metabolism and oxidative damage were investigated in the aquatic form of the liverwort Riccia fluitans L. exposed to PHEN. Exposure of Riccia fluitans plants to PHEN at concentrations of 0.5 microm or less induced oxidative stress, but at a level from which plants could recover. Despite increased levels of enzymatic and non-enzymatic antioxidants, recovery appeared, at least in part, due to increased synthesis of PAs, achieved via increased activities of the enzymes arginine decarboxylase (ADC) and S-adenosylmethionine decarboxylase (SAMDC). Chemical inhibition of these enzymes inhibited plant recovery, while treatment with PAs aided recovery. Finally, as chloroplasts and the plasma membrane appeared to be key targets for PHEN-induced damage, the potential roles of PAs in protecting these cellular components were considered. How PAs could protect plant cells from serious environmental pollutants such as PHEN and could prevent oxidative stress is discussed.

Sea ice protects the embryos of the Antarctic sea urchin Sterechinus neumayeri from oxidative damage due to naturally enhanced levels of UV-B radiation.

SUMMARY The ‘ozone hole’ has caused an increase in ultraviolet B radiation (UV-B, 280–320 nm) penetrating Antarctic coastal marine ecosystems, however the direct effect of this enhanced UV-B on pelagic organisms remains unclear. Oxidative stress, the in vivo production of reactive oxygen species to levels high enough to overcome anti-oxidant defences, is a key outcome of exposure to solar radiation, yet to date few studies have examined this physiological response in Antarctic marine species in situ or in direct relation to the ozone hole. To assess the biological effects of UV-B, in situ experiments were conducted at Cape Armitage in McMurdo Sound, Antarctica (77.06°S, 164.42°E) on the common Antarctic sea urchin Sterechinus neumayeri Meissner (Echinoidea) over two consecutive 4-day periods in the spring of 2008 (26–30 October and 1–5 November). The presence of the ozone hole, and a corresponding increase in UV-B exposure, resulted in unequivocal increases in oxidative damage to lipids and proteins, and developmental abnormality in embryos of S. neumayeri growing in open waters. Results also indicate that embryos have only a limited capacity to increase the activities of protective antioxidant enzymes, but not to levels sufficient to prevent severe oxidative damage from occurring. Importantly, results show that the effect of the ozone hole is largely mitigated by sea ice coverage. The present findings suggest that the coincidence of reduced stratospheric ozone and a reduction in sea ice coverage may produce a situation in which significant damage to Antarctic marine ecosystems may occur.

Production and decomposition dynamics of hydrogen peroxide in freshwater

Environmental context. Hydrogen peroxide (H2O2) is the most stable reactive oxygen species (ROS) formed through irradiation of chromophoric dissolved organic matter (CDOM) in freshwater. It can act as a reductant or as an oxidant and decays largely through interaction with microorganisms via unknown mechanisms. In this way it can affect biological and chemical processes in natural waters and thus shape the ecosystem biogeochemistry. Abstract. Hydrogen peroxide (H2O2) is widely recognised as the most stable of the reactive oxygen species produced by solar radiation-driven photochemical reactions in natural waters. H2O2 concentrations were determined in a shallow fresh water system (water of Leith, Dunedin, New Zealand) by flow-injection analysis (FIA) using an acridinium ester chemiluminescent reaction system. Daytime measurements of H2O2 concentration showed a rapid increase from early morning (15 nM) to 1300 hours (491 nM), consistent with photochemical formation, lagging maximum solar irradiance by ~1.5 h. The wavelength dependency of H2O2 formation was studied and it was shown that UV-B, UV-A and PAR contributed 40, 33 and 27%, respectively. The average formation rate was 339 nM h–1 during springtime. The influence of biotic communities on the rate of H2O2 decomposition was also studied and the majority of decomposition was due to particles smaller than 0.22 μm. The overall first order decay rate constant was of the order of 7.1 h–1. The bacterial and algal communities in the water column and on the riverbed were primarily responsible for the decomposition of H2O2.

Formulation of oil-in-water β-carotene microemulsions: Effect of oil type and fatty acid chain length

The impact of oil type and fatty acid chain length on the development of food-grade microemulsions for the entrapment of β-carotene was investigated. The microemulsion region of a ternary phase diagram containing short chain monoglycerides was larger than for di- and triglycerides when Tween 80 was used as surfactant. The cytotoxicity of microemulsions composed of a 30% monoglyceride oil, 20% Tween 80 and 50% aqueous buffer were evaluated using an in vitro cell culture model (human epithelial colorectal adenocarcinoma, Caco-2). The cytotoxicity test showed that the viability of Caco-2 cells against β-carotene microemulsions at concentrations of 0.03125% (v/v) was higher than 90%. This study suggests that short chain monoglycerides could be used with Tween 80 to prepare transparent β-carotene-encapsulated O/W microemulsions in the particle size range of 12-100 nm.

LIGHT QUALITY INFLUENCES ADVENTITIOUS SHOOT PRODUCTION FROM COTYLEDON EXPLANTS OF LETTUCE (LACTUCA SATIVA L.)

SummaryThe influence of light quality on competence and determination for organogenesis was investigated using lettuce cotyledon explants. Lettuce seedlings from four genotypes were germinated in the dark or under white, red, or blue light. Cotyledon explants were excised and cultured on a shoot-inducing medium for 28 d under white light. Germination in the dark reduced shoot numbers, suggesting that light improves the competence of explants for organogenesis. When explants from seedlings germinated under white light were cultured under different light qualities, blue was found to inhibit shoot production while red light either promoted production or had no effect on shoot number compared to controls. Treatment with blue plus red light failed to overcome the inhibition by blue light. To ascertain the temporal responses of explants to light quality, they were cultured under red or blue light prior to transfer to the alternate treatment. Exposure to blue light within 7 d of excision permanently reduced explant competence for organogenesis. Exposure after this time had a minimal effect. These results suggest that both phytochrome and cryptochrome can regulate shoot production from lettuce cotyledon explants and blue light can only inhibit organogenesis, in lettuce, during a relatively small developmental window.

The frost resistance of juvenile and adult forms of some heteroblastic New Zealand plants

Abstract Many New Zea1and trees are heteroblastic, with juvenile forms that differ markedly from the fom Of the adu1t tree. Their phase change ‘Orresponds with the transition from cold air near the ground to warmer air above. Heteroblastic species, therefore, might be expected to have juvenile forms that are more frost resistant than their mature forms. Juveni1e forms of Streblus heteroPhYllus and so‐phora microphylla were more frost resistant than their mature forms and the juvenile and adult forms of the homoblastic Pittosporum eugenioides showed no consistent differences in their frost resistance. Juvenile forms of Carpodetus serratus, Hoheria angustifolia, P1agianthus regius, and Pseudopanax crassifolius were less frost resistant than their mature counte∼ms. We conc1ude that juvenile foms are not inherently more frost resistant than their equivalent mature forms and note that populations of the same species from different environments readily acquire the frost resistance appropriate to their habitat.

Pectins influence microfibril aggregation in celery cell walls: An atomic force microscopy study.

Atomic force microscopy (AFM) was used to image the microfibrilar network of celery parenchyma cell wall material (CWM), before and after each step in the selective extraction of pectins and hemicelluloses. The images collected were subjected to image analysis and the diameters of the cellulose microfibrils were measured following each step in the extraction process. Not only was an increase in the mean size of the microfibrils observed as the pectins were selectively removed, but an increase in the proportion of large to small microfibrils was also observed. This suggests that removal of the pectic matrix not only results in the swelling of existing microfibrils, but also removal of pectins would enable the microfibrils to move closer together within the cell wall, and hence have a greater tendency to self-associate and form aggregates.

Methylglyoxal: An Emerging Signaling Molecule in Plant Abiotic Stress Responses and Tolerance

The oxygenated short aldehyde methylglyoxal (MG) is produced in plants as a by-product of a number of metabolic reactions, including elimination of phosphate groups from glycolysis intermediates dihydroxyacetone phosphate and glyceraldehyde 3-phosphate. MG is mostly detoxified by the combined actions of the enzymes glyoxalase I and glyoxalase II that together with glutathione make up the glyoxalase system. Under normal growth conditions, basal levels of MG remain low in plants; however, when plants are exposed to abiotic stress, MG can accumulate to much higher levels. Stress-induced MG functions as a toxic molecule, inhibiting different developmental processes, including seed germination, photosynthesis and root growth, whereas MG, at low levels, acts as an important signaling molecule, involved in regulating diverse events, such as cell proliferation and survival, control of the redox status of cells, and many other aspects of general metabolism and cellular homeostases. MG can modulate plant stress responses by regulating stomatal opening and closure, the production of reactive oxygen species, cytosolic calcium ion concentrations, the activation of inward rectifying potassium channels and the expression of many stress-responsive genes. MG appears to play important roles in signal transduction by transmitting and amplifying cellular signals and functions that promote adaptation of plants growing under adverse environmental conditions. Thus, MG is now considered as a potential biochemical marker for plant abiotic stress tolerance, and is receiving considerable attention by the scientific community. In this review, we will summarize recent findings regarding MG metabolism in plants under abiotic stress, and evaluate the concept of MG signaling. In addition, we will demonstrate the importance of giving consideration to MG metabolism and the glyoxalase system, when investigating plant adaptation and responses to various environmental stresses.