Is there an inter-organ glutathione redox cycle?

Intracellular glutathione redox status is a function of the flux through glutathione peroxidase-glutathione reductase system. Specific activities of these two enzymes in rat liver cytosol and erythrocyte hemolysates were determined. Relative to glutathione peroxidase levels, glutathione reductase activity was about 15-fold more abundant in the rat liver than in erythrocytes. This is suggestive of greater capacity of the liver to reduce oxidised glutathione (GSSG). Based on these results and from the pattern of glutathione efflux from different cells and tissues [Sies, H. & Akerboom, T.P.M. (1984) Methods Enzymol. 105, 445-451], it is speculated that an interorgan glutathione redox cycle may be operative wherein liver is central to the reduction of GSSG and other disulphides.     

Is nectar reabsorption restricted by the stalk cells of floral and extrafloral nectary trichomes?

Reabsorption is a phase of nectar dynamics that occurs concurrently with secretion; it has been described in floral nectaries that exude nectar through stomata or unicellular trichomes, but has not yet been recorded in extrafloral glands. Apparently, nectar reabsorption does not occur in multicellular secretory trichomes (MST) due to the presence of lipophilic impregnations – which resemble Casparian strips – in the anticlinal walls of the stalk cells. It has been assumed that these impregnations restrict solute movement within MST to occur unidirectionally and exclusively by the symplast, thereby preventing nectar reflux toward the underlying nectary tissues. We hypothesised that reabsorption is absent in nectaries possessing MST. The fluorochrome lucifer yellow (LYCH) was applied to standing nectar of two floral and extrafloral glands of distantly related species, and then emission spectra from nectary sections were systematically analysed using confocal microscopy. Passive uptake of LYCH via the stalk cells to the nectary tissues occurred in all MST examined. Moreover, we present evidence of nectar reabsorption in extrafloral nectaries, demonstrating that LYCH passed the stalk cells of MST, although it did not reach the deepest nectary tissues. Identical (control) experiments performed with neutral red (NR) demonstrated no uptake of this stain by actively secreting MST, whereas diffusion of NR did occur in plasmolysed MST of floral nectaries at the post‐secretory phase, indicating that nectar reabsorption by MST is governed by stalk cell physiology. Interestingly, non‐secretory trichomes failed to reabsorb nectar. The role of various nectary components is discussed in relation to the control of nectar reabsorption by secretory trichomes.     

Does the skeleton of a sponge provide a defense against predatory reef fish?

Sponge tissue often contains two structural components in high concentrations: spicules of silica, and refractory fibers of protein (spongin). Some terrestrial plants contain analogous structures, siliceous inclusions and refractory lignins, that have been demonstrated to deter herbivory. We performed feeding experiments with predatory reef fish to assess the deterrent properties of the structural components of three common Caribbean demosponges, Agelas clathrodes, Ectyoplasia ferox, and Xestospongia muta. The concentrations of spicules and spongin in the tissues varied widely between the three species, but when assayed at their natural volumetric concentrations, neither spicules (all three species assayed) nor the intact spiculated spongin skeleton (A. clathrodes and X. muta assayed) deterred feeding by reef fish in aquarium or field assays using prepared foods of a nutritional quality similar to, or higher than, that of sponge tissue. Spicules deterred feeding in aquarium assays when incorporated into prepared foods of a nutritional quality lower than that of sponge tissue (15–19 times less protein), but spiculated spongin skeleton was still palatable, even in prepared foods devoid of measurable protein, and even though spicules embedded in spongin were oriented in their natural conformation. Based on comparisons of the nutritional qualities of the tissues of the three sponge species and of the prepared foods, sponge tissue would have to be much lower in food value (5 times less protein or lower) for spicules to provide an effective defense, and spicules in combination with the spongin skeleton would be unlikely to provide an effective defense regardless of the nutritional quality of the tissue. Unlike terrestrial plants, marine sponges may use silica and refractory fibers solely for structural purposes.     

Is the citrus-like plumage odorant of crested auklets (Aethia cristatella) a defense against lice?

Plumage odors may function as chemical defenses against ectoparasites in birds. We tested this hypothesis for crested auklets (Aethia cristatella), a species of colonial seabird that emits a very strong citrus-like odor from its plumage. This odorant contains known chemical repellents. We evaluated evidence for chemical defense in this species using two approaches. First, we exposed pigeon lice to the volatiles emitted by freshly plucked plumage and by whole specimens. Louse survivorship was compared between these treatments and two controls. Second, we compared louse abundance on crested auklets versus a closely related congener that nests in close association. Louse survivorship did not differ between crested auklet treatments and controls. Comparison of ectoparasite loads showed that crested auklets had significantly higher louse abundance than least auklets (A. pusilla), even after controlling for body size. Our results failed to support two expectations for chemical defense. Presence of the crested auklet plumage odorant in our experiments did not reduce louse life span. Presence of the aldehyde odorant in nature did not reduce louse abundance on crested auklets. Hence we conclude that the aldehyde odorant is not immediately lethal to lice at natural concentrations in plumage.     

Is the microbial tree of life verificationist?

The field of microbial phylogenetics has questioned the feasibility of using a tree‐like structure to the describe microbial evolution. This debate centres on two main points. First, because microorganisms are able to transfer genes from one to another in zero generations (horizontal gene transfer, or HGT), the use of molecular characters to perform phylogenetic analyses will yield an erroneous topology and HGT clearly makes the evolution of microorganisms non tree‐like. Second, the use of concatenated gene sequences in a total evidence approach to phylogenetic systematics is a verificationist endeavour, the aim of which is to bolster support. However, the goal of the total evidence approach to phylogenetic research is based in the idea of increasing explanatory power over background knowledge through test and corroboration, rather than to bolster support for nodes in a tree. In this context, the testing of phylogenetic data is a falsificationist endeavour that includes the possibility of not rejecting the null hypothesis that there is no tree‐like structure in molecular phylogenetic data. We discuss several tests that aim to test rigorously the hypothesis that a tree of life exists for microorganisms. We also discuss the philosophical ramifications of background knowledge and corroboration in microbial studies that need to be considered when suggesting that HGT confounds the tree of life. © The Willi Hennig Society 2009.     

Consequences of ants and extrafloral nectar for a pollinating seed‐consuming mutualism: ant satiation,floral distraction or plant defense?

Non‐pollinating consumers of floral resources, especially ants, can disrupt pollination and plant reproductive processes. As an alternative food resource to flowers, extrafloral nectar (EFN) may distract and satiate ants from flowers, thereby reducing their antagonistic effects on plants. Yet, EFN may actually attract and increase ant density on plants, thus increasing the disruption of pollination and/or their defense of plants. In this study, we tested the effects of ants and EFN on pollinating seed‐consuming interactions between senita cacti and senita moths in the Sonoran Desert. Prior study of senita showed that EFN can distract ants from flowers, but consequences for plant–pollinator interactions remain unstudied. In our current study, ant exclusion had no effect on pollination or oviposition when moths were abundant (>85% flower visitation). Yet, in an ant by EFN factorial experiment under lower moth abundance (<40% visitation), there was a significant effect of ant exclusion (but not EFN or an ant × EFN) on pollination and oviposition. In contrast with our predictions, ant presence increased rather than decreased pollination (and oviposition) by moths, indicating a beneficial effect of ants on plant reproduction. While ant density on plants showed a saturating response to continuous experimental variation in EFN, in support of ant satiation and distraction, the probability of pollination and oviposition increased and saturated with ant density, again showing a beneficial effect of ants on plant reproduction. Ants showed no significant effect on fruit set, fruit survival, or fruit production of oviposited flowers in the ant exclusion experiment. Ants did not affect the survival of moth larvae, but there was a marginally significant effect of ants in reducing wasp parasitism of moths. We suggest that EFN may not only distract ants from disrupting plant–pollinator interactions, but they may also enhance plant–pollinator interactions by increasing pollination and reducing wasp parasitism. Though often considered dichotomous hypotheses, ant distraction and plant defense may be synergistic, though the mechanism(s) for such positive ant effects on plant–pollinator interactions needs further study.     

Arteriovenous oscillations of the redox potential: Is the redox state influencing blood flow?

Objective: Studies on the regulation of human blood flow revealed several modes of oscillations with frequencies ranging from 0.005 to 1 Hz. Several mechanisms were proposed that might influence these oscillations, such as the activity of vascular endothelium, the neurogenic activity of vessel wall, the intrinsic activity of vascular smooth muscle, respiration, and heartbeat. These studies relied typically on non-invasive techniques, for example, laser Doppler flowmetry. Oscillations of biochemical markers were rarely coupled to blood flow.

Methods: The redox potential difference between the artery and the vein was measured by platinum electrodes placed in the parallel homonymous femoral artery and the femoral vein of ventilated anesthetized pigs.

Results: Continuous measurement at 5 Hz sampling rate using a digital nanovoltmeter revealed fluctuating signals with three basic modes of oscillations: ～ 1, ～ 0.1 and ～ 0.01 Hz. These signals clearly overlap with reported modes of oscillations in blood flow, suggesting coupling of the redox potential and blood flow.

Discussion: The amplitude of the oscillations associated with heart action was significantly smaller than for the other two modes, despite the fact that heart action has the greatest influence on blood flow. This finding suggests that redox potential in blood might be not a derivative but either a mediator or an effector of the blood flow control system.     

Does acoustic priming ‘sweeten the pot’ of floral nectar?

A recent claim that evening primrose flowers adaptively secrete nectar in response to vibrations from hovering bees lacks supporting evidence. The authors fail to demonstrate that bees can access the concealed nectar and that their visits enhance plant fitness. Reanalysis of the authors’ data raises additional concerns about their conclusions.     

Is the microwave irradiation a suitable method for measuring soil microbial biomass?

Soil microbial biomass (SMB), the living part of soil organic matter, is used to quantify the total biomass of microorganisms present in the soil. The importance of studies about SMB has emphasized the need to identify methods which can measure the size of SMB. Among the methods currently available, chloroform-fumigation extraction and incubation are the most commonly used for estimation of SMB. However, several studies have proposed the microwave (MW) irradiation as a quick, simple and safe alternate method. There are different opinions about suitability of this method for measuring SMB. There is a question to do “Is the microwave irradiation a suitable method for measuring soil microbial biomass?” Most of the published papers comparing MW and chloroform-fumigation showed strong relationship between both methods. Therefore, we consider MW a suitable method for measuring SMB; however, it is necessary to calibrate the MW methods in different soils with a range of properties, such as clay content, to find an appropriate conversion factor in order to generate correct values for SMB with MW method.     

Is there a gloxylate cycle in the liver of the fetal guinea pig?

Studies on the effect of the inhibitor of fatty acid oxidation (+)-octanoylcarnitine on the perfused liver of the 48–51 days fetal guinea pig indicate that the oxidation of endogenous fatty acids is a major source of carbon for the citric acid cycle and for synthesis of hexose. Consistent with this the liver can convert isocitrate to glyoxylate and glyoxylate to malate and may therefore operate a glyoxylate cycle allowing the net production of sugars from acetyl-CoA.     

Gamma-aminobutyrate: defense against invertebrate pests?

Gamma-aminobutyrate (GABA) is a ubiquitous four-carbon, non-protein amino acid. In plants, stress-induced GABA accumulation is well documented. However, the role(s) of GABA accumulation is contentious. In this Opinion article, we argue that wounding due to herbivory and crawling by insect larvae causes rapid GABA accumulation via the disruption of cellular compartmentation and the release of the acidic vacuolar contents to the cytosol. The activity of glutamate decarboxylase, the cytosolic enzyme responsible for GABA synthesis, has an acidic pH optimum. Subsequent GABA ingestion has a plant defense function by directly acting on GABA-regulated invertebrate neuromuscular junctions. Plants with an enhanced GABA-producing capacity reduce herbivory by invertebrate pests. These findings suggest that GABA accumulation is a rapidly deployed, local resistance mechanism that constitutes a first line of defense in deterring herbivory.     

Can plants use an entomopathogenic virus as a defense against herbivores?

It is by now well established that plants use various strategies to defend themselves against herbivores. Besides conventional weapons such as spines and stinging hairs and sophisticated chemical defenses, plants can also involve the enemies of the herbivores in their defense. It has been suggested that plants could even use entomopathogens as part of their defense strategies. In this paper, we show that Brassica oleraceae plants that are attacked by Myzus persicae aphids infected with an entomopathogenic parvovirus (M. persicae densovirus) transport the virus through the phloem locally and systematically. Moreover, healthy aphids that fed on the same leaf, but separated from infected aphids were infected via the plant. Hence, this is proof of the principle that plants can be vectors of an insect virus and can possibly use this virus as a defense against herbivores.     

Does a redox cycle provide a mechanism for setting the capacity of neuroglobin to protect cells from apoptosis?

We hypothesize that the various, previously reported, reactivities of neuroglobin with redox partners and oxygen provide for the establishment of a redox cycle within cells, such as neurons and retinal rod cells. Using native cell lysates, from cultured human cells of neuronal origin, we have estimated the rate of reduction of the oxidized form of neuroglobin in vivo. Furthermore we provide evidence that the cytosol of these cells contains factors (presumably enzymes) capable of employing either glutathione or NADH as re-reductants of ferric neuroglobin. Taken in conjunction with previous rate data, for the various redox reactions of neuroglobin, this information allows us to set up a computer model to estimate the steady state cellular level of the antiapoptotic ferrous form of neuroglobin. This model indicates that the steady state level of antiapoptotic neuroglobin is very sensitive to the cellular oxygen tension and moderately sensitive to the redox status of the cell. Further analysis indicates that such a system would be capable of significant modification, on the seconds time scale, following hypoxic transition, as is likely in stroke. We hypothesize that this mechanism might provide a moderately rapid mechanism for adjusting the antiapoptotic status of a cell, whilst the reaction of neuroglobin with mitochondrial cytochrome c provides a very rapid, but limited, capacity to intervene in the apoptotic pathway.     

Is the membrane attack complex of complement an enzyme?

Summary Recent studies on the functional activities of the membrane attack complex of complement, C5b-9, are reviewed. A new speculative hypothesis has been advanced to account for the ability of complement to mediate lysis of various targets. This hypothesis has three major elements: 1) that the membrane attack complex is an enzyme; 2) that the substrate for this putative enzyme is a membrane constituent; 3) that the substrate specificity of the putative enzyme is dependent on the species source of individual complement components within the C5b-9 complex.Abbreviations E = sheep red cells - A = rabbit IgM anti-Forssman antibody - Hu or hu = human - GP or gp = guinea pig     

Why are flowers sweeter than fruits or buds? Variation in extrafloral nectar secretion throughout the floral ontogeny of a myrmecophile

Extrafloral nectar provision during floral ontogeny in ant‐plants has not been widely studied. Extrafloral nectar secretion differed between leaves associated with buds, flowers, and fruits, and peaked during anthesis when pollinators were present. This ontogenetic variation may result from mixed selective pressures involving strategies for defense and mutualist management.     

BVOCs: plant defense against climate warming?

Plants emit a substantial amount of biogenic volatile organic compounds (BVOCs) into the atmosphere. These BVOCs represent a large carbon loss and can be up to approximately 10% of that fixed by photosynthesis under stressful conditions and up to 100gCm(-2) per year in some tropical ecosystems. Among a variety of proven and unproven BVOC functions in plants and roles in atmospheric processes, recent data intriguingly link emission of these compounds to climate. Ongoing research demonstrates that BVOCs could protect plants against high temperatures. BVOC emissions are probably increasing with warming and with other factors associated to global change, including changes in land cover. These increases in BVOC emissions could contribute in a significant way (via negative and positive feedback) to the complex processes associated with global warming.     

Plant chemical defense against herbivores and pathogens: generalized defense or trade-offs?

Plants are often attacked by multiple enemies, including pathogens and herbivores. While many plant secondary metabolites show specific effects toward either pathogens or herbivores, some can affect the performance of both these groups of natural enemies and are considered to be generalized defense compounds. We tested whether aucubin and catalpol, two iridoid glycosides present in ribwort plantain (Plantago lanceolata), confer in vivo resistance to both the generalist insect herbivore Spodoptera exigua and the biotrophic fungal pathogen Diaporthe adunca using plants from P. lanceolata lines that had been selected for high- and low-leaf iridoid glycoside concentrations for four generations. The lines differed approximately three-fold in the levels of these compounds. Plants from the high-selection line showed enhanced resistance to both S. exigua and D. adunca, as evidenced by a smaller lesion size and a lower fungal growth rate and spore production, and a lower larval growth rate and herbivory under both choice and no-choice conditions. Gravimetric analysis revealed that the iridoid glycosides acted as feeding deterrents to S. exigua, thereby reducing its food intake rate, rather than having post-ingestive toxic effects as predicted from in vitro effects of hydrolysis products. We suggest that the bitter taste of iridoid glycosides deters feeding by S. exigua, whereas the hydrolysis products formed after tissue damage following fungal infection mediate pathogen resistance. We conclude that iridoid glycosides in P. lanceolata can serve as broad-spectrum defenses and that selection for pathogen resistance could potentially result in increased resistance to generalist insect herbivores and vice versa, resulting in diffuse rather than pairwise coevolution.