Intraspecific competition reveals conditional fitness effects of single gene polymorphism at the Arabidopsis root growth regulator BRX.

Intraspecific genetic variation for morphological traits is observed in many organisms. In Arabidopsis thaliana, alleles responsible for intraspecific morphological variation are increasingly being identified. However, the fitness consequences remain unclear in most cases. Here, the fitness effects of alleles of the BRX gene are investigated. A brx loss-of-function allele, which was found in a natural accession, results in a highly branched but poorly elongated root system. Comparison between the control accession Sav-0 and an introgression of brx into this background (brxS) indicated that, surprisingly, brx loss of function did not negatively affect fitness in pure stands. However, in mixed, well-watered stands brxS performance and reproductive output decreased significantly, as the proportion of Sav-0 neighbors increased. Additional comparisons between brxS and a brxS line that was complemented by a BRX transgene confirmed a direct effect of the loss-of-function allele on plant performance, as indicated by restored competitive ability of the transgenic genotype. Further, because plant height was very similar across genotypes and because the experimental setup largely excluded shading effects, the impaired competitiveness of the brx loss-of-function genotype likely reflects below-ground competition. In summary, these data reveal conditional fitness effects of a single gene polymorphism in response to intraspecific competition in Arabidopsis.

Transcriptome analysis of intraspecific competition in Arabidopsis thaliana reveals organ-specific signatures related to nutrient acquisition and general stress response pathways.

ABSTRACT: BACKGROUND: Plants are sessile and therefore have to perceive and adjust to changes in their environment. The presence of neighbours leads to a competitive situation where resources and space will be limited. Complex adaptive responses to such situation are poorly understood at the molecular level. RESULTS: Using microarrays, we analysed whole-genome expression changes in Arabidopsis thaliana plants subjected to intraspecific competition. The leaf and root transcriptome was strongly altered by competition. Differentially expressed genes were enriched in genes involved in nutrient deficiency (mainly N, P, K), perception of light quality, and responses to abiotic and biotic stresses. Interestingly, performance of the generalist insect Spodoptera littoralis on densely grown plants was significantly reduced, suggesting that plants under competition display enhanced resistance to herbivory. CONCLUSIONS: This study provides a comprehensive list of genes whose expression is affected by intraspecific competition in Arabidopsis. The outcome is a unique response that involves genes related to light, nutrient deficiency, abiotic stress, and defence responses.

Promise for plant pest control: root-associated pseudomonads with insecticidal activities.

Insects are an important and probably the most challenging pest to control in agriculture, in particular when they feed on belowground parts of plants. The application of synthetic pesticides is problematic owing to side effects on the environment, concerns for public health and the rapid development of resistance. Entomopathogenic bacteria, notably Bacillus thuringiensis and Photorhabdus/Xenorhabdus species, are promising alternatives to chemical insecticides, for they are able to efficiently kill insects and are considered to be environmentally sound and harmless to mammals. However, they have the handicap of showing limited environmental persistence or of depending on a nematode vector for insect infection. Intriguingly, certain strains of plant root-colonizing Pseudomonas bacteria display insect pathogenicity and thus could be formulated to extend the present range of bioinsecticides for protection of plants against root-feeding insects. These entomopathogenic pseudomonads belong to a group of plant-beneficial rhizobacteria that have the remarkable ability to suppress soil-borne plant pathogens, promote plant growth, and induce systemic plant defenses. Here we review for the first time the current knowledge about the occurrence and the molecular basis of insecticidal activity in pseudomonads with an emphasis on plant-beneficial and prominent pathogenic species. We discuss how this fascinating Pseudomonas trait may be exploited for novel root-based approaches to insect control in an integrated pest management framework.

Dirigent domain-containing protein is part of the machinery required for formation of the lignin-based Casparian strip in the root.

The endodermis acts as a "second skin" in plant roots by providing the cellular control necessary for the selective entry of water and solutes into the vascular system. To enable such control, Casparian strips span the cell wall of adjacent endodermal cells to form a tight junction that blocks extracellular diffusion across the endodermis. This junction is composed of lignin that is polymerized by oxidative coupling of monolignols through the action of a NADPH oxidase and peroxidases. Casparian strip domain proteins (CASPs) correctly position this biosynthetic machinery by forming a protein scaffold in the plasma membrane at the site where the Casparian strip forms. Here, we show that the dirigent-domain containing protein, enhanced suberin1 (ESB1), is part of this machinery, playing an essential role in the correct formation of Casparian strips. ESB1 is localized to Casparian strips in a CASP-dependent manner, and in the absence of ESB1, disordered and defective Casparian strips are formed. In addition, loss of ESB1 disrupts the localization of the CASP1 protein at the casparian strip domain, suggesting a reciprocal requirement for both ESB1 and CASPs in forming the casparian strip domain.

Glomus intraradices induces changes in root system architecture of rice independently of common symbiosis signaling

? Arbuscular mycorrhizal fungi colonize the roots of most monocotyledons and dicotyledons despite their different root architecture and cell patterning. Among the cereal hosts of arbuscular mycorrhizal fungi, Oryza sativa (rice) possesses a peculiar root system composed of three different types of roots: crown roots; large lateral roots; and fine lateral roots. Characteristic is the constitutive formation of aerenchyma in crown roots and large lateral roots and the absence of cortex from fine lateral roots. Here, we assessed the distribution of colonization by Glomus intraradices within this root system and determined its effect on root system architecture. ? Large lateral roots are preferentially colonized, and fine lateral roots are immune to arbuscular mycorrhizal colonization. Fungal preference for large lateral roots also occurred in sym mutants that block colonization of the root beyond rhizodermal penetration. ? Initiation of large lateral roots is significantly induced by G. intraradices colonization and does not require a functional common symbiosis signaling pathway from which some components are known to be needed for symbiosis-mediated lateral root induction in Medicago truncatula. ? Our results suggest variation of symbiotic properties among the different rice root-types and induction of the preferred tissue by arbuscular mycorrhizal fungi. Furthermore, signaling for arbuscular mycorrhizal-elicited alterations of the root system differs between rice and M. truncatula.

A stepwise protocol to coat aAPC beads prevents out-competition of anti-CD3 mAb and consequent experimental artefacts.

Artificial antigen-presenting cells (aAPC) are widely used for both clinical and basic research applications, as cell-based or bead-based scaffolds, combining immune synapse components of interest. Adequate and controlled preparation of aAPCs is crucial for subsequent immunoassays. We reveal that certain proteins such as activatory anti-CD3 antibody can be out-competed by other proteins (e.g. inhibitory receptor ligands such as PDL1:Fc) during the coating of aAPC beads, under the usually performed coating procedures. This may be misleading, as we found that decreased CD8 T cell activity was not due to inhibitory receptor triggering but rather because of unexpectedly low anti-CD3 antibody density on the beads upon co-incubation with inhibitory receptor ligands. We propose an optimized protocol, and emphasize the need to quality-control the coating of proteins on aAPC beads prior to their use in immunoassays.

Oral insecticidal activity of root colonizing Pseudomonas fluorescens CHA0: a biocontrol agent with potential to control plant pests and diseases

The application of microbial biocontrol agents for the control of fungal plant diseases and plant insect pests is a promising approach in the development of environmentally benign pest management strategies. The ideal biocontrol organism would be a bacterium or a fungus with activity against both, insect pests and fungal pathogens. Here we demonstrate the oral insecticidal activity of the root colonizing Pseudomonas fluorescens CHA0, which is so far known for its capacity to efficiently suppress fungal plant pathogens. Feeding assays with CHA0-sprayed leaves showed that this strain displays oral insecticidal activity and is able to efficiently kill larvae of three important insect pests. We further show data indicating that the Fit insect toxin produced by CHA0 and also metabolites controlled by the global regulator GacA contribute to oral insect toxicity.

Radiocarbon dating of modern groundwater: the role of the unsaturated zone

Biological and physical processes occurring in soils may lead to significant isotopic changes between the isotopic compositions of atmospheric CO2 and of soil CO2. Also, during water and gas transport from the soil surface to the water table, isotopic changes likely occur due to numerous physical processes such as gas production and diffusion, water advection, and gas-water-rock interactions. In most cases, these changes are not included in the correction models developed for groundwater dating, whereas they can significantly impact the calculation of the 14C age. We explore the role of these processes using: i) experimental data from two aquifer sites (Fontainebleau sands and Astian sands, France), ii) a distributed model to simulate the 14C activities of soil CO2, and iii) numerical simulations in order to highlight the role of the physical processes.¦The 13C content in soil CO2 showed seasonal variations and highlighted the competition between CO2 production and CO2 diffusion. Their respective contributions played a significant role in defining the isotopic composition of CO2 at the water table. On both study sites, variations of the 14C activity in soil CO2 reflect the competition between the fluxes of root derived-CO2 and organic matter derived-CO2. Since the nuclear weapon tests in the fifties and sixties, soil CO2 became significantly depleted in 14C compared to modern atmospheric CO2. Models that take into account this 14C depletion in soil CO2 for dating modern groundwater would lead to apparent younger 14C ages than models that only consider the 14C activity in atmospheric CO2. Moreover, since 2000-2005, the inverse effect is observed as soil CO2 is enriched in 14C compared to atmospheric CO2.¦Therefore, we conclude that the isotopic composition of CO2 at the water table have to be taken into account for the dating of modern groundwater. This requires a systematic sampling of soil CO2 and the measurement of its 13C and 14C contents. We used this information in a numerical simulation to calculate the evolution of isotopic composition of CO2 from the soil surface to the water table. This simulation integrated physical processes in the unsaturated zone (e.g. CO2 production and diffusion, water advection, etc.) and gas-water-rock interactions.

Social rules govern vocal competition in the barn owl

To resolve the share of limited resources, animals often compete through exchange of signals about their relative motivation to compete. When two competitors are similarly motivated, the resolution of conflicts may be achieved in the course of an interactive process. In barn owls, Tyto alba, in which siblings vocally compete during the prolonged absence of parents over access to the next delivered food item, we investigated what governs the decision to leave or enter a contest, and at which level. Siblings alternated periods during which one of the two individuals vocalized more than the other. Individuals followed turn-taking rules to interrupt each other and momentarily dominate the vocal competition. These social rules were weakly sensitive to hunger level and age hierarchy. Hence, the investment in a conflict is determined not only by need and resource-holding potential, but also by social interactions. The use of turn-taking rules governing individual vocal investment has rarely been shown in a competitive context. We hypothesized that these rules would allow individuals to remain alert to one another's motivation while maintaining the cost of vocalizing at the lowest level.

No experimental evidence that sibling competition induces young to switch nests in the colonial Alpine swift, Apus melba

Adoption is frequent in colonial animals where opportunities for dependent young to receive care from nonbiological parents are high. The departure of dependent young from their original family to seek adoption in neighbouring families is thought to be induced by sibling competition for access to limited resources provided by poor-quality parents. We tested this hypothesis in the colonial Alpine swift by manipulating the number of young reared per brood, with the prediction that offspring from enlarged broods switch nests more frequently than those from reduced broods. Although nestling swifts hatch with little locomotor activity, from 20 days until their first flight at 50-70 days they frequently move out of their nests to seek adoption in neighbouring families. Although nestlings reared in experimentally enlarged broods were lighter and their body mass at day 20 after hatching was more variable than in nestlings reared in reduced broods, there was no difference between the two treatments in the frequency of nests switching and in the age when nestlings switched nests for the first time. However, consistent with other evidence that nest switching by nestling swifts evolved as a strategy to reduce ectoparasite load, young from broods with naturally high numbers of the ectoparasitic louse fly Crataerina melbae were more prone to switch nests. This shows that ectoparasitism rather than sibling competition is a key proximate factor promoting the evolution of nest switching in the colonial Alpine swift. (c) 2006 The Association for the Study of Animal Behaviour Published by Elsevier Ltd. All rights reserved.