Terrestrial plants rely on stomata, small pores in the leaf surface, for photosynthetic gas exchange and transpiration of water. The stomata, formed by a pair of guard cells, dynamically increase and decrease their volume to control the pore size in response to environmental cues. Stresses can trigger similar or opposing movements: for example, drought induces closure of stomata, whereas many pathogens exploit stomata and cause them to open to facilitate entry into plant tissues. The latter is an active process as stomatal closure is part of the plant's immune response. Stomatal research has contributed much to clarify the signalling pathways of abiotic stress, but guard cell signalling in response to microbes is a relatively new area of research. In this article, we discuss present knowledge of stomatal regulation in response to microbes and highlight common points of convergence, and differences, compared to stomatal regulation by abiotic stresses. We also expand on the mechanisms by which pathogens manipulate these processes to promote disease, for example by delivering effectors to inhibit closure or trigger opening of stomata. The study of pathogen effectors in stomatal manipulation will aid our understanding of guard cell signalling.
Deirdre H. McLachlan, Michaela Kopischke and Silke Robatzek
Les chercheurs du Laboratoire des Interactions Plantes-Microorganismes (LIPM) du centre INRA de Toulouse ont déterminé comment le xylane, un composé majeur de la paroi cellulaire végétale, était décomposé par la bactérie phytopathogène Xanthomonas campestris pv campestris (Xcc). Leurs résultats ont mis en évidence que cette dégradation dépendait d’un système spécifique composé de transporteurs et d’enzymes. Plus intéressant encore, ce même système de dégradation est utilisé par les bactéries symbiotiques de l’intestin humain ! Cette découverte a fait l’objet d’une publication dans la revue scientifique New Phytologist. (Déjean et al. 2013, 198(3):899-915).
Plants sense invasion of potential microbial pathogens using various receptors and launch cascades of innate immune responses that are critical for survival and fecundity. Recognition of pathogens occurs through detection of pathogen-associated patterns (PAMPs) or pathogen effectors, setting off a cascade of signaling events that triggers early cellular and molecular responses. Plant innate immunity is constituted by an elaborate, multilayered system involving two intertwined lines of defense: a first level of immunity termed PAMP–triggered immunity (PTI) or basal resistance, and a second layer of plant defense, mediated by resistance (R) proteins, leading to a complete resistance response often accompanied by the hypersensitive cell death (HR), and called effector-triggered immunity (ETI; Jones & Dangl, 2006). Another form of resistance, that confers partial resistance to pathogens and usually referred as quantitative disease resistance (QDR), has been extensively observed in crops and natural plant populations (Kover & Cheverud, 2007; Poland et al., 2009; Roux et al., 2014). However, there is still very limited information about the molecular mechanisms underlying this form of immunity. More generally, protein kinases play critical roles during immunity in signaling through phosphorylation of target proteins and as modulators of cell metabolism and gene expression (Romeis, 2001; Meng & Zhang, 2013). Pseudokinases are topologically related to protein kinases but lack catalytic residue(s) classically required for phosphotransfer. Interestingly, recent reports identified two Arabidopsis pseudokinases, RKS1 and ZED1, that belong to a gene cluster within the receptor-like cytoplasmic kinase (RLCK)-XII-2 subfamily and confer different forms of immunity....
Fabrice Roux, Laurent Noël, Susana Rivas and Dominique Roby
Background - The white mold fungus Sclerotinia sclerotiorum is a devastating necrotrophic plant pathogen with a remarkably broad host range. The interaction of necrotrophs with their hosts is more complex than initially thought, and still poorly understood.
Results - We combined bioinformatics approaches to determine the repertoire of S. sclerotiorum effector candidates and conducted detailed sequence and expression analyses on selected candidates. We identified 486 S. sclerotiorum secreted protein genes expressed in planta, many of which have no predicted enzymatic activity and may be involved in the interaction between the fungus and its hosts. We focused on those showing (i) protein domains and motifs found in known fungal effectors, (ii) signatures of positive selection, (iii) recent gene duplication, or (iv) being S. sclerotiorum-specific. We identified 78 effector candidates based on these properties. We analyzed the expression pattern of 16 representative effector candidate genes on four host plants and revealed diverse expression patterns.
Conclusions - These results reveal diverse predicted functions and expression patterns in the repertoire of S. sclerotiorum effector candidates. They will facilitate the functional analysis of fungal pathogenicity determinants and should prove useful in the search for plant quantitative disease resistance components active against the white mold.
The tandem repeats of transcription activator like effectors (TALEs) mediate sequence-specific DNA binding using a simple code. Naturally, TALEs are injected by Xanthomonas bacteria into plant cells to manipulate the host transcriptome. In the laboratory TALE DNA binding domains are reprogrammed and used to target a fused functional domain to a genomic locus of choice. Research into the natural diversity of TALE-like proteins may provide resources for the further improvement of current TALE technology. Here we describe TALE-like proteins from the endosymbiotic bacterium Burkholderia rhizoxinica, termed Bat proteins. Bat repeat domains mediate sequence-specific DNA binding with the same code as TALEs, despite less than 40% sequence identity. We show that Bat proteins can be adapted for use as transcription factors and nucleases and that sequence preferences can be reprogrammed. Unlike TALEs, the core repeats of each Bat protein are highly polymorphic. This feature allowed us to explore alternative strategies for the design of custom Bat repeat arrays, providing novel insights into the functional relevance of non-RVD residues. The Bat proteins offer fertile grounds for research into the creation of improved programmable DNA-binding proteins and comparative insights into TALE-like evolution.
Cytoplasmic plant immune receptors recognize specific pathogen effector proteins and initiate effector-triggered immunity. In Arabidopsis, the immune receptors RPS4 and RRS1 are both required to activate defense to three different pathogens. We show that RPS4 and RRS1 physically associate. Crystal structures of the N-terminal Toll–interleukin-1 receptor/resistance (TIR) domains of RPS4 and RRS1, individually and as a heterodimeric complex (respectively at 2.05, 1.75, and 2.65 angstrom resolution), reveal a conserved TIR/TIR interaction interface. We show that TIR domain heterodimerization is required to form a functional RRS1/RPS4 effector recognition complex. The RPS4 TIR domain activates effector-independent defense, which is inhibited by the RRS1 TIR domain through the heterodimerization interface. Thus, RPS4 and RRS1 function as a receptor complex in which the two components play distinct roles in recognition and signaling.
Simon J. Williams, Kee Hoon Sohn, Li Wan, Maud Bernoux, Panagiotis F. Sarris, Cecile Segonzac, Thomas Ve, Yan Ma, Simon B. Saucet, Daniel J. Ericsson, Lachlan W. Casey, Thierry Lonhienne, Donald J. Winzor, Xiaoxiao Zhang, Anne Coerdt, Jane E. Parker, Peter N. Dodds, Bostjan Kobe, Jonathan D. G. Jones
This Summer School in Integrative Ecology and Biology in english is part of the TULIP Labex (Laboratory of Excellence) training program of the and will take place in the French Pyrenees in a grandiose scenery at the foot of the well-known “Pic du Midi”, a 2800m-high mountain with one of the last functional astronomy observatories in Europe.
Some international researchers will participate to this event. Below the list of those who have already confirmed their participation:
Julio Salinas (Spain - Departamento de Biología Medioambiental)
Patricia Beldade (Portugal - Evolutionary Biology, Instituto Gulbenkian de Ciencia)
Through plenary lectures and workshops, the program will cover many features of organism-organism interactions from both ecological and biological points of view. The programme will also include visit of unique place like the Experimental Ecology Center of CNRS in Moulis... At a glance :
Actively participate in elaborating research projects during workshops,Meet experts from Toulouse and other internationally renowned laboratories,Enjoy the inspiring Pyrenees location chosen for the Summer School.
Free accommodation is provided for all participants, limited in number to a total of 25 (undergraduate, graduate and post-docs)
Plants protect themselves from the harmful effects of pathogens by resistance and tolerance. Disease resistance, which eliminates pathogens, can be modulated by bacterial type III effectors. Little is known about whether disease tolerance, which sustains host fitness with a given pathogen burden, is regulated by effectors. Here, we examined the effects of the Xanthomonas effector protein XopDXcc8004 on plant disease defenses by constructing knockout and complemented Xanthomonas strains, and performing inoculation studies in radish (Raphanus sativus L. var. radiculus XiaoJinZhong) and Arabidopsis plants. XopDXcc8004 suppresses disease symptoms without changing bacterial titers in infected leaves. In Arabidopsis, XopDXcc8004 delays the hormone gibberellin (GA)-mediated degradation of RGA (repressor of ga1-3), one of five DELLA proteins that repress GA signaling and promote plant tolerance under biotic and abiotic stresses. The ERF-associated amphiphilic repression (EAR) motif-containing region of XopDXcc8004 interacts with the DELLA domain of RGA and might interfere with the GA-induced binding of GID1, a GA receptor, to RGA. The EAR motif was found to be present in a number of plant transcriptional regulators. Thus, our data suggest that bacterial pathogens might have evolved effectors, which probably mimic host components, to initiate disease tolerance and enhance their survival.
Leitao Tan, Wei Rong, Hongli Luo, Yinhua Chen, and Chaozu He
Recent advances in DNA targeting allow unprecedented control over gene function and expression. Targeting based on TAL effectors is arguably the most promising for systems biology and metabolic engineering. Multiple, orthogonal TAL-effector reagents of different types can be used in the same cell. Furthermore, variation in base preferences of the individual structural repeats that make up the TAL effector DNA recognition domain makes targeting stringency tunable. Realized applications range from genome editing to epigenome modification to targeted gene regulation to chromatin labeling and capture. The principles that govern TAL effector DNA recognition make TAL effectors well suited for applications relevant to plant physiology and metabolism. TAL effector targeting has merits that are distinct from those of the RNA-based DNA targeting CRISPR/Cas9 system.
In this paper, we show how a single web resource can engage a wide student audience with plant science. Developed by the University of Leeds, UK, the Plant Science TREE (Tool for Research Engaged Education (www.tree.leeds.ac.uk)) is an online teaching tool giving access to online research lectures, downloadable lecture slides, practicals, movies and other material on topical plant science to support lecturers in their teaching. The Plant Science TREE complements the annual Gatsby Plant Science Summer School, which has already succeeded in engaging undergraduates with plant science (Levesley et al., 2012). Both initiatives were instigated to address the decline in student numbers in plant science (Sundberg, 2004; Stagg et al., 2009; Jones, 2010; Drea, 2011) at a time when there is concern that future demand for plant scientists will not be met (The Royal Society, 2009). The causes of this decline are unproven but may be the result of a combination of factors including, greater preference by students for animal and medically-based degrees, disengagement from plant science at school, and narrowing of plant-based undergraduate curricula. Where the summer school aims, by face-to-face contact, to inspire relatively small numbers of high-achieving students to consider plant science as a career option, the Plant Science TREE aims to reach a much larger, more diverse global audience through the use of web technologies.
Aurora Levesley, Steve Paxton, Richard Collins, Alison Baker and Celia Knight
Scott Jackson, Jeremy Schmutz, Phillip McClean and colleagues report the genome sequence of the common bean (Phaseolus vulgaris) and resequenced wild individuals and landraces from Mesoamerican and Andean gene pools, showing that common bean underwent two independent domestications.
Next generation sequencing (NGS) technologies have impressively accelerated research in biological science during the last years by enabling the production of large volumes of sequence data to a drastically lower price per base, compared to traditional sequencing methods. The recent and ongoing developments in the field allow addressing research questions in plant-microbe biology that were not conceivable just a few years ago. The present review provides an overview of NGS technologies and their usefulness for the analysis of microorganisms that live in association with plants. Possible limitations of the different sequencing systems, in particular sources of errors and bias, are critically discussed and methods are disclosed that help to overcome these shortcomings. A focus will be on the application of NGS methods in metagenomic studies, including the analysis of microbial communities by amplicon sequencing, which can be considered as a targeted metagenomic approach. Different applications of NGS technologies are exemplified by selected research articles that address the biology of the plant associated microbiota to demonstrate the worth of the new methods.
Both type III effector proteins and non-ribosomal peptide toxins play important roles for Pseudomonas syringae pathogenicity in host plants, but whether and how these pathways interact to promote infection remains unclear. Genomic evidence from one clade of P. syringae suggests a tradeoff between the total number of type III effector proteins and presence of syringomycin, syringopeptin, and syringolin A toxins. Here we report the complete genome sequence from P. syringae CC1557, which contains the lowest number of known type III effectors to date and has also acquired genes similar to sequences encoding syringomycin pathways from other strains. We demonstrate that this strain is pathogenic on Nicotiana benthamiana and that both the type III secretion system and a new type III effector family, hopBJ1, contribute to pathogenicity. We further demonstrate that activity of HopBJ1 is dependent on residues structurally similar to the catalytic site of E. coli CNF1 toxin. Taken together, our results provide additional support for a negative correlation between type III effector repertoires and the potential to produce syringomycin-like toxins while also highlighting how genomic synteny and bioinformatics can be used to identify and characterize novel virulence proteins.
AvrBs3, the founding member of the Xanthomonas transcription-activator-like effectors (TALEs), is translocated into the plant cell where it localizes to the nucleus and acts as transcription factor. The DNA-binding domain of AvrBs3 consists of 17.5 nearly-identical 34 amino acid-repeats. Each repeat specifies binding to one base in the target DNA via amino acid residues 12 and 13 termed repeat variable diresidue (RVD). Natural target sequences of TALEs are generally preceded by a thymine (T0), which is coordinated by a tryptophan residue (W232) in a degenerated repeat upstream of the canonical repeats. To investigate the necessity of T0 and the conserved tryptophan for AvrBs3-mediated gene activation we tested TALE mutant derivatives on target sequences preceded by all possible four bases. In addition, we performed domain swaps with TalC from a rice pathogenic Xanthomonas because TalC lacks the tryptophan residue, and the TalC target sequence is preceded by cytosine. We show that T0 works best and that T0 specificity depends on the repeat number and overall RVD-composition. T0 and W232 appear to be particularly important if the RVD of the first repeat is HD (‘rep1 effect’). Our findings provide novel insights into the mechanism of T0 recognition by TALE proteins and are important for TALE-based biotechnological applications.
Plants are constantly interpreting microbial signals from potential pathogens and potential commensals or mutualists. Because plants have no circulating cells dedicated to this task, every plant cell must, in principle, recognize any microbe as friend, foe, or irrelevant bystander. That tall order is mediated by an array of innate immune system receptors: pattern-recognition receptors outside the plant cell and nucleotide-binding oligomerization domain (NOD)–like receptors (NLRs) inside the cell. Despite their importance for plant health, how NLRs function mechanistically has remained obscure. On page 299 of this issue, Williams et al. (1) reveal a role for heterodimerization between NLRs and show how the rather limited NLR repertoire of any plant genome might be enhanced by combinatorial diversity.
Xanthomonas is one of the most widespread phytobacteria, causing diseases on a variety of agricultural plants. To develop novel control techniques, knowledge of bacterial behavior inside plant cells is essential. Xanthomonas campestris pv. campestris, a vascular pathogen, is the causal agent of black rot on leaves of Brassicaceae, including Arabidopsis thaliana. Among the X. campestris pv. campestris stocks in the MAFF collection, we selected XccMAFF106712 as a model compatible pathogen for the A. thaliana reference ecotype Columbia (Col-0). Using modified green fluorescent protein (AcGFP) as a reporter, we observed real time XccMAFF106712 colonization in planta with confocal microscopy. AcGFP-expressing bacteria colonized the inside of epidermal cells and the apoplast, as well as the xylem vessels of the vasculature. In the case of the type III mutant, bacteria colonization was never detected in the xylem vessel or apoplast, though they freely enter the xylem vessel through the wound. After 9 days post inoculation with XccMAFF106712, the xylem vessel became filled with bacterial aggregates. This suggests that Xcc colonization can be divided into main four steps, (1) movement in the xylem vessel, (2) movement to the next cell, (3) adhesion to the host plant cells, and (4) formation of bacterial aggregates. The type III mutant abolished at least steps (1) and (2). Better understanding of Xcc colonization is essential for development of novel control techniques for black rot.
This program will provide the students with some insights into new developments in plant science research. The purpose is to give students the opportunity to be exposed to multidisciplinary approaches in the field of sustainable management of plant health and quality.
Workshops: Visits of international research institutes – GEVES lab (the French Group for the Study and Inspection of Varieties and Seeds) – LSV-ANSES (Plant Health Laboratory) – CIRM-CFBP (French Collection of Bacteria associated to Plants) – ANAN (Nucleic acid analysis) Platform – ITEPMAI (Institute for medicinal and aromatic plants) – Visits of a botanical garden, a rose garden, and an organic vineyard
The program relies on interactive classes and hands-on ativities. Several workshops are organized, among them
"Experiments in chemical ecology and pest management" "In Silico design of plant pathogen identification tests" "Field observations of Black spot disease on rose genotypes" "Extraction-identification and biological activity testing of plant polyphenols" "Macroscopic and microscopic observations of medicinal plants" "Rose phenotyping and genotyping" "Analytical methods for assessing fruit quality"
Sessions: Plenary conference by Prof Jean Weissenbach Session 1: Chemical ecology and Pest management Session 2: Genomic and Bacterial diagnostic Session 3: Fungal foliar disease on ornamentals Session 4: Weed control in intercropping systems Session 5: Metabolomics and medicinal plants Session 6: Ornamentals Interaction genotype X environment Session 7: Fruit production, Fruit development and self-thinning Session 8: Fruit production and quality
Invited speakers: – J. Weissenbach – P. Anderson, Alnarp, Sweden – M M. López, Valencia, Spain – T. Debener, Hannover, Germany – JL. Wolfender, Genève, Switzerland – MC. van Labeke, Gent, Belgium – Al. Botton, Padova, Italy
Criteria: The program is open to undergraduate students in the fields of agriculture, plant science and biological science who may be interested in pursuing Ph.D. programs and envision a career in plant science/agriculture.