A bacterial cysteine protease effector protein interferes with photosynthesis to suppress plant innate immune responses

The bacterial pathogen Pseudomonas syringae pv tomato DC3000 suppresses plant innate immunity with effector proteins injected by a type III secretion system (T3SS). The cysteine protease effector HopN1, which reduces the ability of DC3000 to elicit programmed cell death in non-host tobacco, was found to also suppress the production of defence-associated reactive oxygen species (ROS) and callose when delivered by Pseudomonas fluorescens heterologously expressing a P. syringae T3SS. Purified His 6 -tagged HopN1 was used to identify tomato PsbQ, a member of the oxygen evolving complex of photosystem II (PSII), as an interacting protein. HopN1 localized to chloroplasts and both degraded PsbQ and inhibited PSII activity in chloroplast preparations, whereas a HopN1 D299A non-catalytic mutant lost these abilities. Gene silencing of NtPsbQ in tobacco compromised ROS production and programmed cell death.

Distinct and conserved transcriptomic changes during nematode-induced giant cell development in tomato compared with Arabidopsis: a functional role for gene repression

Root-knot nematodes (RKNs) induce giant cells (GCs) from root vascular cells inside the galls. Accompanying molecular changes as a function of infection time and across different species, and their functional impact, are still poorly understood. Thus, the transcriptomes of tomato galls and laser capture microdissected (LCM) GCs over the course of parasitism were compared with those of Arabidopsis, and functional analysis of a repressed gene was performed. Microarray hybridization with RNA from galls and LCM GCs, infection-reproduction tests and quantitative reverse transcription-polymerase chain reaction (qRT-PCR) transcriptional profiles in susceptible and resistant (Mi-1) lines were performed in tomato. Tomato GC-induced genes include some possibly contributing to the epigenetic control of GC identity. GC-repressed genes are conserved between tomato and Arabidopsis, notably those involved in lignin deposition. However, genes related to the regulation of gene expression diverge, suggesting that diverse transcriptional regulators mediate common responses leading to GC formation in different plant species. TPX1, a cell wall peroxidase specifically involved in lignification, was strongly repressed in GCs/galls, but induced in a nearly isogenic Mi-1 resistant line on nematode infection. TPX1 overexpression in susceptible plants hindered nematode reproduction and GC expansion. Time-course and cross-species comparisons of gall and GC transcriptomes provide novel insights pointing to the relevance of gene repression during RKN establishment.

The role of the secondary cell wall in plant resistance to pathogens

Plant resistance to pathogens relies on a complex network of constitutive and inducible defensive barriers. The plant cell wall is one of the barriers that pathogens need to overcome to successfully colonize plant tissues. The traditional view of the plant cell wall as a passive barrier has evolved to a concept that considers the wall as a dynamic structure that regulates both constitutive and inducible defense mechanisms, and as a source of signaling molecules that trigger immune responses. The secondary cell walls of plants also represent a carbon-neutral feedstock (lignocellulosic biomass) for the production of biofuels and biomaterials. Therefore, engineering plants with improved secondary cell wall characteristics is an interesting strategy to ease the processing of lignocellulosic biomass in the biorefinery. However, modification of the integrity of the cell wall by impairment of proteins required for its biosynthesis or remodeling may impact the plants resistance to pathogens. This review summarizes our understanding of the role of the plant cell wall in pathogen resistance with a focus on the contribution of lignin to this biological process.

Signwalling: signals derived from arabiopsis cell wall activate specific resistance to pathogens

The cell wall is a dynamic structure that regulates both constitutive and inducible plant defence responses. Different molecules o DAMPs (damage-associated molecular patterns) can be released from plant cell walls upon pathogen infection or wounding and can trigger immune responses. To further characterize the function of cell wall on the regulation of these immune responses, we have performed a biased resistance screening of putative/well-characterized primary/secondary Arabidopsis thaliana cell wall mutants (cwm). In this screening we have identified more than 20 cwm mutants with altered susceptibility/resistance to at least one of the following pathogens: the necrotrophic fungi Plectosphaerella cucumerina, the vascular bacterium Ralstonia solanacearum, the biotrophic oomycete Hyaloperonospora arabidopsidis and the powdery mildew fungus Erisyphe cruciferarum. We found that cell wall extracts from some of these cwm plants contain novel DAMPs that activate immune responses and conferred enhanced resistance to particular pathogens when they were applied to wild-type plants. Using glycomic profiling we have performed an initial characterization of the active carbohydrate structures present in these cwm wall fractions, and we have determined the signalling pathways regulated by thesse fractions. . The data generated with this collection of wall mutants support the existence of specific correlations between cell wall structure/composition, resistance to particular type of pathogens and plant fitness. Remarkably, we have identified specific cwm mutations that uncoupled resistance to pathogens from plant trade-offs, further indicating the plasticity of wall structures in the regulation of plant immune responses.

High-Performance and Traditional Multicrystalline Silicon: Comparing Gettering Responses and Lifetime-Limiting Defects

In recent years, high-performance multicrystalline silicon (HPMC-Si) has emerged as an attractive alternative to traditional ingot-based multicrystalline silicon (mc-Si), with a similar cost structure but improved cell performance. Herein, we evaluate the gettering response of traditional mc-Si and HPMC-Si. Microanalytical techniques demonstrate that HPMC-Si and mc-Si share similar lifetime-limiting defect types but have different relative concentrations and distributions. HPMC-Si shows a substantial lifetime improvement after P-gettering compared with mc-Si, chiefly because of lower area fraction of dislocation-rich clusters. In both materials, the dislocation clusters and grain boundaries were associated with relatively higher interstitial iron point-defect concentrations after diffusion, which is suggestive of dissolving metal-impurity precipitates. The relatively fewer dislocation clusters in HPMC-Si are shown to exhibit similar characteristics to those found in mc-Si. Given similar governing principles, a proxy to determine relative recombination activity of dislocation clusters developed for mc-Si is successfully transferred to HPMC-Si. The lifetime in the remainder of HPMC-Si material is found to be limited by grain-boundary recombination. To reduce the recombination activity of grain boundaries in HPMC-Si, coordinated impurity control during growth, gettering, and passivation must be developed.

The chemokine monocyte chemoattractant protein-1 induces functional responses through dimerization of its receptor CCR2

Cytokines interact with hematopoietin superfamily receptors and stimulate receptor dimerization. We demonstrate that chemoattractant cytokines (chemokines) also trigger biological responses through receptor dimerization. Functional responses are induced after pairwise crosslinking of chemokine receptors by bivalent agonistic antichemokine receptor mAb, but not by their Fab fragments. Monocyte chemoattractant protein (MCP)-1-triggered receptor dimerization was studied in human embryonic kidney (HEK)-293 cells cotransfected with genes coding for the CCR2b receptor tagged with YSK or Myc sequences. After MCP-1 stimulation, immunoprecipitation with Myc-specific antibodies revealed YSK-tagged receptors in immunoblotting. Receptor dimerization also was validated by chemical crosslinking in both HEK-293 cells and the human monocytic cell line Mono Mac 1. Finally, we constructed a loss-of-function CCR2bY139F mutant that acted as a dominant negative, blocking signaling through the CCR2 wild-type receptor. This study provides functional support for a model in which the MCP-1 receptor is activated by ligand-induced homodimerization, allowing discussion of the similarities between bacterial and leukocyte chemotaxis.

A logical analysis of T cell activation and anergy

Interaction of the antigen-specific receptor of T lymphocytes with its antigenic ligand can lead either to cell activation or to a state of profound unresponsiveness (anergy). Although subtle changes in the nature of the ligand or of the antigen-presenting cell have been shown to affect the outcome of T cell receptor ligation, the mechanism by which the same receptor can induce alternative cellular responses is not completely understood. A model for explaining both positive (cell proliferation and cytokine production) and negative (anergy induction) signaling of T lymphocytes is described herein. This model relies on the autophosphorylative properties of the tyrosine kinases associated with the T cell receptor. One of its basic assumptions is that the kinase activity of these receptor-associated enzymes remains above background level after ligand removal and is responsible for cellular unresponsiveness. Using a simple Boolean formalism, we show how the timing of the binding and intracellular signal-transduction events can affect the properties of receptor signaling and determine the type of cellular response. The present approach integrates into a common framework a large body of experimental observations and allows specification of conditions leading to cellular activation or to anergy.

Copresentation of natural HIV-1 agonist and antagonist ligands fails to induce the T cell receptor signaling cascade

It is not known how human immunodeficiency virus type 1 (HIV-1)-derived antagonist peptides interfere with intracellular activation of cytotoxic T lymphocytes (CTL). We identified Gag epitope variants in HIV-1-infected patients that act as antagonists of CTL responses to unmutated epitopes. We then investigated the effect that presentation of each variant has on the early events of T cell receptor (TCR) signal transduction. We found that altered peptide ligands (APL) failed to induce phosphorylation of pp36, a crucial adaptor protein involved in TCR signal transduction. We further investigated the effect that simultaneous presentation of APL and native antigen at low, physiological, peptide concentrations (1 nM) has on TCR signal transduction, and we found that the presence of APL can completely inhibit induction of the protein tyrosine phosphorylation events of the TCR signal transduction cascade.

T cell vaccination induces T cell receptor Vβ-specific Qa-1-restricted regulatory CD8+ T cells

Vaccination of mice with activated autoantigen-reactive CD4+ T cells (T cell vaccination, TCV) has been shown to induce protection from the subsequent induction of a variety of experimental autoimmune diseases, including experimental allergic encephalomyelitis (EAE). Although the mechanisms involved in TCV-mediated protection are not completely known, there is some evidence that TCV induces CD8+ regulatory T cells that are specific for pathogenic CD4+ T cells. Previously, we demonstrated that, after superantigen administration in vivo, CD8+ T cells emerge that preferentially lyse and regulate activated autologous CD4+ T cells in a T cell receptor (TCR) Vβ-specific manner. This TCR Vβ-specific regulation is not observed in β2-microglobulin-deficient mice and is inhibited, in vitro, by antibody to Qa-1. We now show that similar Vβ8-specific Qa-1-restricted CD8+ T cells are also induced by TCV with activated CD4+ Vβ8+ T cells. These CD8+ T cells specifically lyse murine or human transfectants coexpressing Qa-1 and murine TCR Vβ8. Further, CD8+ T cell hybridoma clones generated from B10.PL mice vaccinated with a myelin basic protein-specific CD4+Vβ8+ T cell clone specifically recognize other CD4+ T cells and T cell tumors that express Vβ8 and the syngeneic Qa-1a but not the allogeneic Qa-1b molecule. Thus, Vβ-specific Qa-1-restricted CD8+ T cells are induced by activated CD4+ T cells. We suggest that these CD8+ T cells may function to specifically regulate activated CD4+ T cells during immune responses.