Signal transduction by protein tyrosine nitration: competition or cooperation with tyrosine phosphorylation-dependent signaling events?

This review article is an attempt to stimulate a discussion on the significance of protein tyrosine nitration to cellular signaling and its relationships with protein tyrosine phosphorylation. Initially, it provides basic information on growth factor and oxidants as modulators/mediators of tyrosine phosphorylation-dependent signal transduction pathways. The effects of exogenous and endogenous tyrosine nitration on such pathways were examined by reviewing published and unpublished observations. From an initial perspective that tyrosine nitration was a toxic manifestation of nitric oxide, the concept evolved to a protein modification that could also function in cellular signaling events, possibly cooperating with tyrosine phosphorylation.     

Growing larger with domestication: a matter of physiology,morphology or allocation?

• Domestication might affect plant size. We investigated whether herbaceous crops are larger than their wild progenitors, and the traits that influence size variation.
• We grew six crop plants and their wild progenitors under common garden conditions. We measured the aboveground biomass gain by individual plants during the vegetative stage. We then tested whether photosynthesis rate, biomass allocation to leaves, leaf size and specific leaf area (SLA) accounted for variations in whole‐plant photosynthesis, and ultimately in aboveground biomass.
• Despite variations among crops, domestication generally increased the aboveground biomass (average effect +1.38, Cohen's d effect size). Domesticated plants invested less in leaves and more in stems than their wild progenitors. Photosynthesis rates remained similar after domestication. Variations in whole‐plant C gains could not be explained by changes in leaf photosynthesis. Leaves were larger after domestication, which provided the main contribution to increases in leaf area per plant and plant‐level C gain, and ultimately to larger aboveground biomass.
• In general, cultivated plants have become larger since domestication. In our six crops, this occurred despite lower investment in leaves, comparable leaf‐level photosynthesis and similar biomass costs of leaf area (i.e. SLA) than their wild progenitors. Increased leaf size was the main driver of increases in aboveground size. Thus, we suggest that large seeds, which are also typical of crops, might produce individuals with larger organs (i.e. leaves) via cascading effects throughout ontogeny. Larger leaves would then scale into larger whole plants, which might partly explain the increases in size that accompanied domestication.

     

Climbing plants in a temperate rainforest understorey: searching for high light or coping with deep shade?

Background and Aims

While the climbing habit allows vines to reach well-lit canopy areas with a minimum investment in support biomass, many of them have to survive under the dim understorey light during certain stages of their life cycle. But, if the growth/survival trade-off widely reported for trees hold for climbing plants, they cannot maximize both light-interception efficiency and shade avoidance (i.e. escaping from the understorey). The seven most important woody climbers occurring in a Chilean temperate evergreen rainforest were studied with the hypothesis that light-capture efficiency of climbers would be positively associated with their abundance in the understorey.

Methods

Species abundance in the understorey was quantified from their relative frequency and density in field plots, the light environment was quantified by hemispherical photography, the photosynthetic response to light was measured with portable gas-exchange analyser, and the whole shoot light-interception efficiency and carbon gain was estimated with the 3-D computer model Y-plant.

Key Results

Species differed in specific leaf area, leaf mass fraction, above ground leaf area ratio, light-interception efficiency and potential carbon gain. Abundance of species in the understorey was related to whole shoot features but not to leaf level features such as specific leaf area. Potential carbon gain was inversely related to light-interception efficiency. Mutual shading among leaves within a shoot was very low (<20 %).

Conclusions

The abundance of climbing plants in this southern rainforest understorey was directly related to their capacity to intercept light efficiently but not to their potential carbon gain. The most abundant climbers in this ecosystem match well with a shade-tolerance syndrome in contrast to the pioneer-like nature of climbers observed in tropical studies. The climbers studied seem to sacrifice high-light searching for coping with the dim understorey light.     

Intracellular accommodation of microbes by plants: a common developmental program for symbiosis and disease?

Plant cells engage in mutualistic and parasitic endosymbioses with a wide variety of microorganisms, ranging from Gram-negative (Rhizobium, Nostoc) and Gram-positive bacteria (Frankia), to oomycetes (Phytophthora), Chytridiomycetes, Zygomycetes (arbuscular mycorrhizal fungi) and true fungi (Erysiphe, ascomycete; Puccinia, basidiomycete). Endosymbiosis is characterised by the 'symbiosome', a compartment within host cells in which the symbiotic microorganism is either partially or completely enclosed by a host-derived membrane. The analysis of plant mutants indicates that the genetic requirements for the interaction with rhizobia and arbuscular mycorrhiza fungi are partially overlapping. The extent to which plants use similar or identical developmental programs for the intracellular accommodation of different microorganisms is, however, not clear. For example, plant cells actively weaken their cell wall to facilitate bacterial colonisation, whereas penetration by fungal symbionts appears not to be assisted in this manner. Moreover, different transport requirements are imposed on the symbiotic interface of different interactions indicating that additional system-specific components are likely to exist.     

Native or nonnative host plants: What is better for a specialist moth?

The enemy release hypothesis (ERH) predicts that the lack of natural enemies, such as herbivores, contributes to the success of nonnative plants as colonizers. Larvae of the Neotropical specialist moth Utetheisa ornatrix (Erebidae: Arctiinae) can feed on unripe seeds and leaves of both native and nonnative Crotalaria species (Fabaceae). Despite some species being able to eat nonnative plants, such behavior can impair the herbivore, as they are not adapted to the alien plant, and still contribute to the success of the nonnative species via enemy release. We tested the performance of the moth from hatching to adulthood fed on two native (C. micans and C. paulina) and two nonnative (C. pallida, C. juncea) host plants. Utetheisa ornatrix performed better (lower development time, heavier pupae and more eggs) on the native host plants than in the nonnative. However, larva performance in nonnative C. pallida was similar to that in the native host plants. Using the larval weight 7 days after hatching from the eggs as a proxy for performance in twelve Crotalaria species (five Neotropical natives, four nonnatives from Afrotropical region, and three nonnatives from India), we found similar results. Crotalaria nutritional compounds, the defensive pyrrolizidine alkaloids and Crotalaria phylogeny did not explain moth performance. Our results give some support to the ERH. The good moth performance in nonnative C. pallida may be related to its high availability as host plant for U. ornatrix, and its longer time since their introduction in Neotropics which would provide opportunity for the moth to adapt.     

Centrosome competition: a possibility?

A recent theory on the evolution of sexuality, has hypothesized heritable variation in the functional properties of centrosomes, leading to competition for the organization of the mitotic spindle when different centrosomes enter a common cytoplasm. We present here data on polyethylene glycol-induced polykaryocytes of cultured Chinese hamster fibroblasts indirectly supporting centrosome competition. On the assumption that sib centrosomes are similar and variation increases with cell generations, the frequencies of multipolar mitoses were compared in cultures fused under conditions favoring sib cell fusion or fusion of distantly related cells. Multipolar mitoses were considerably more frequent in the former, when the average difference between pairs of centrosomes was assumed to be too small for one centrosome to "outcompete" the other.     

Tree-shrub interactions in a subtropical savanna parkland: competition or facilitation?

Abstract. Prosopis glandulosa var. glandulosa has played a central role in the encroachment of woody plants in southern Texas, grasslands and savannas by acting as a nurse plant for various shrubs that establish in its understory. To test for continued facilitation of established understory shrubs by Prosopis and to determine if established shrubs compete with the Prosopis nucleus, selective removal experiments were conducted and monitored over a 2–5 yr period. Short-term (1–3 days) and long-term (2 yr) growth and physiological activities (midday net photosynthesis and leaf/shoot water potential) of two common understory shrubs, Zanthoxylum fagara and Berberis trifoliolata, growing with Prosopis, were generally comparable to those of individuals occurring in clusters where Prosopis was removed. Shrubs growing with an intact Prosopis occasionally showed significantly higher leaf-[N] and pre-dawn water potentials than those in clusters lacking a live Prosopis, especially under drought conditions; however, these differences did not translate into greater midday leaf gas exchange or shoot growth. By comparison, removal of understory shrubs elicited large increases in Prosopis net photosynthesis, annual trunk growth in each of the 5 yr monitored, and seed pod production in three of the four years monitored. Seven of 26 Prosopis plants in experimental clusters with an intact understory died over a 5-yr period, compared to only two of the 26 plants in clusters with the cleared understory. Results indicate that (1) the founding overstory Prosopis plant may continue to facilitate understory shrubs following their establishment, but these beneficial effects appear to be small and transitory, and (2) the understory shrubs have a pronounced negative effect on Prosopis, such that competition between overstory and understory woody plants is strongly asymmetrical. These findings suggest that understory shrubs will likely persist despite changes in microclimate and soils (potentially) that occur after the Prosopis plant, which facilitated their ingress or establishment, has died. Soil resource depletion by shallow-rooted understory shrubs appears to be a primary factor contributing to the demise of the deeply rooted, overstory Prosopis plants, especially on upland sites with duplex soils where below-ground competition is accentuated.     

Hydrogen peroxide: a metabolic by-product or a common mediator of ageing signals?

The reactive oxygen species that are generated by mitochondrial respiration, including hydrogen peroxide (H2O2), are potent inducers of oxidative damage and mediators of ageing. It is not clear, however, whether oxidative stress is the result of a genetic programme or the by-product of physiological processes. Recent findings demonstrate that a fraction of mitochondrial H2O2, produced by a specialized enzyme as a signalling molecule in the pathway of apoptosis, induces intracellular oxidative stress and accelerates ageing. We propose that genes that control H2O2 production are selected determinants of lifespan.     

Love or fear: Can punishment promote cooperation?

Cooperation is a paradox: Why should one perform a costly behavior only to increase the fitness of another? Human societies, in which individuals cooperate with genetically unrelated individuals on a considerably larger scale than most mammals do, are especially puzzling in this regard. Recently, the threat of punishment has been given substantial attention as one of the mechanisms that could help sustain human cooperation in such situations. Nevertheless, using punishment to explain cooperation only leads to further questions: Why spend precious resources to penalize free‐riders, especially if others can avoid this investment and cheaters can punish you back? Here, it is argued that current evidence supports punishment as an efficient means for the maintenance of cooperation, and that the gravity of proposed limitations of punishment for maintaining cooperation may have been overestimated in previous studies due to the features of experimental design. Most notably, the importance of factors as characteristic of human societies as reputation and language has been greatly neglected. Ironically, it was largely the combination of the two that enabled humans to shape costly punishment into numerous low‐cost and less detrimental strategies that clearly can promote human cooperation.     

Recent fungal diseases of crop plants: is lateral gene transfer a common theme?

A cursory glance at old textbooks of plant pathology reveals that the diseases which are the current scourge of agriculture in many parts of the world are a different set from those that were prominent 50 or 100 years ago. Why have these new diseases arisen? The traditional explanations subscribe to the "nature abhors a vacuum" principle-that control of one disease creates the condition for the emergence of a replacement-but does little to explain why the new pathogen succeeds. The emergence of a new disease requires a series of conditions and steps, including the enhanced fecundity of the new pathogen, enhanced survival from season to season, and spread around the world. Recently, evidence was obtained that wheat tan spot emerged through a lateral gene transfer event some time prior to 1941. Although there have been sporadic and persistent reports of lateral gene transfer between and into fungal plant pathogens, most examples have been dismissed through incomplete evidence. The completion of whole genome sequences of an increasing number of fungal pathogens no longer allows such proposed cases of lateral gene transfer to be dismissed so easily. How frequent are lateral gene transfers involving fungal plant pathogens, and can this process explain the emergence of many of the new diseases of the recent past? Many of the apparently new diseases are dependant on the expression of host-specific toxins. These are enigmatic molecules whose action requires the presence of plant genes with products that specifically encode sensitivity to the toxin and susceptibility to the disease. It is also notable that many new diseases belong to the fungal taxon dothideomycetes. This review explores the coincidence of new diseases, interspecific gene transfer, host-specific toxins, and the dothideomycete class.     

Sexual competition among females: What causes courtship-role reversal?

In most animals, males are the competitive sex whereas females are typically non-competitive and choosy of mates. In a variety of taxa, certain species (or populations within species) show a reversal in these typical courtship roles. Recent research with these organisms supports a central tenet of sexual selection theory: that it is the relative investment of the sexes in offspring that controls the number of males and females available for mating, and thus is the main determinant of the degree of sexual competition in each sex.     

Evolution of cooperation: two for one?

How can cooperation thrive in a selfish world? Recent evolution experiments show how bacteria themselves can generate conditions that make cooperation a winning strategy. At least in the short term.     

Increasing longevity through caloric restriction or rapamycin feeding in mammals: common mechanisms for common outcomes?

Significant extension of lifespan in important mammalian species is bound to attract the attention not only of the aging research community, but also the media and the wider public. Two recent papers published by Harrison et al. (2009) in Nature and by Colman et al. (2009) in Science report increased longevity of mice fed with rapamycin and of rhesus monkeys undergoing caloric restriction, respectively. These papers have generated considerable debate in the aging community. Here we assess what is new about these findings, how they fit with our knowledge of lifespan extension from other studies and what prospects this new work holds out for improvements in human longevity and human health span.     

Fine-scale spatial genetic structure in predominantly selfing plants with limited seed dispersal: a rule or exception?

Gene flow at a fine scale is still poorly understood despite its recognized importance for plant population demographic and genetic processes. We tested the hypothesis that intensity of gene flow will be lower and strength of spatial genetic structure (SGS) will be higher in more peripheral populations because of lower population density. The study was performed on the predominantly selfing Avena sterilis and included: (1) direct measurement of dispersal in a controlled environment; and (2) analyses of SGS in three natural populations, sampled in linear transects at fixed increasing inter plant distances. We found that in A.sterilis major seed dispersal is by gravity in close (less than 2m) vicinity of the mother plant, with a minor additional effect of wind. Analysis of SGS with six nuclear SSRs revealed a significant autocorrelation for the distance class of 1m only in the most peripheral desert population, while in the two core populations with Mediterranean conditions, no genetic structure was found. Our results support the hypothesis that intensity of SGS increases from the species core to periphery as a result of decreased within population gene flow related to low plant density. Our findings also show that predominant self pollination and highly localized seed dispersal lead to SGS at a very fine scale, but only if plant density is not too high.     

Fine-scale spatial genetic structure in predominantly selfing plants with limited seed dispersal:a rule or exception?

Gene flow at a fine scale is still poorly understood despite its recognized importance for plant population demographic and genetic processes.We tested the hypothesis that intensity of gene flow will be lower and strength of spatial genetic structure(SGS) will be higher in more peripheral populations because of lower population density.The study was performed on the predominantly selfing Avena sterilis and included:(1) direct measurement of dispersal in a controlled environment;and(2) analyses of SGS in three natural populations,sampled in linear transects at fixed increasing inter-plant distances.We found that in A.sterilis major seed dispersal is by gravity in close(less than 2 m) vicinity of the mother plant,with a minor additional effect of wind.Analysis of SGS with six nuclear SSRs revealed a significant autocorrelation for the distance class of 1 m only in the most peripheral desert population,while in the two core populations with Mediterranean conditions,no genetic structure was found.Our results support the hypothesis that intensity of SGS increases from the species core to periphery as a result of decreased within-population gene flow related to low plant density.Our findings also show that predominant self-pollination and highly localized seed dispersal lead to SGS at a very fine scale,but only if plant density is not too high.     

Hereditary spastic paraplegia: clues from a rare disorder for a common problem?

Hereditary spastic paraplegia is a rare disorder with gait disturbance due to a degeneration of the corticospinal tract, sometimes accompanied by involvement of other systems. Out of the 20 loci known so far, eight genes have now been identified, allowing the first molecular and cell studies in the pathophysiology of the disorder. These should also help to understand the function of the corticospinal tract at the molecular level and design strategies to prevent and treat spasticity due to more common causes. The proteins encoded by these genes play a role in development, in signal transduction between axons and myelinating cells, in cellular, particularly axonal trafficking or in energy metabolism. Some of them have actions in several areas of cellular function. Here we review the present knowledge about the genes involved in hereditary spastic paraplegia, a field presently undergoing rapid change.