Plants use pattern recognition receptors (PRRs) to perceive pathogen-associated molecular pattern (PAMPs) and initiate defence responses. PAMP-triggered immunity (PTI) plays an important role in ...general resistance, and constrains the growth of most microbes on plants. Despite the importance of PRRs in plant immunity, the vast majority of them remain to be identified. We recently showed that the Arabidopsis LysM receptor kinase CERK1 is required not only for chitin signalling and fungal resistance, but plays an essential role in restricting bacterial growth on plants. We proposed that CERK1 may mediate the perception of a bacterial PAMP, or an endogenous plant cell wall component released during infection, through its extracellular carbohydrate-binding LysM-motifs. Here we report reduced activation of a PAMP-induced defence response on plants lacking the CERK1 gene after treatment with crude bacterial extracts. This demonstrates that CERK1 mediates perception of an unknown bacterial PAMP in Arabidopsis.
In plants, potential pathogenic bacteria do not enter the host cell. Therefore, a large portion of the molecular interaction between microbial pathogen and host occurs in the extracellular space. To ...investigate potential mechanisms of disease resistance and susceptibility, we analyzed changes in the extracellular proteome, or secretome, using the Arabidopsis-Pseudomonas syringae pathosystem. This system provides the possibility to directly compare interactions resulting in basal resistance, susceptibility, and gene-specific resistance by using different genotypes of Pseudomonas on the same host. After infecting suspension-cultured cells of Arabidopsis with the Pseudomonas strain of interest, we isolated protein from the cell culture medium representing the secretome. After one-dimensional gel separation and in-gel digestion of proteins, we used iTRAQ (isobaric tags for relative and absolute quantitation) labeling in conjunction with LC-MS/MS to perform relative quantitative comparisons of the secretomes from each of these interactions. We obtained quantitative information from 45 Arabidopsis proteins that were present in all three biological experiments. We observed complex patterns of accumulation, ranging from proteins that decreased in abundance in the presence of all three bacterial strains to proteins that specifically increased or decreased during only one of the interactions. A particularly intriguing result was that the virulent bacteria (e.g. a susceptible interaction) caused the extracellular accumulation of a specific subset of host proteins lacking traditional signal peptides. These results indicate that the pathogen may manipulate host secretion to promote the successful invasion of plants.
Incredible progress has been made over the last 20 years in understanding the components and mechanisms governing plant innate immunity. The most important discoveries concern pathogen recognition ...mechanisms, which divide perception of conserved elicitors at the cell periphery, and recognition of variable elicitors within the host cytoplasm. The underlying mechanisms of immunity post elicitation are complex and poorly defined. This review highlights emergent themes in plant–microbe interactions with a particular focus on the plant immune responses against infection by the bacterium
Pseudomonas syringae.
Plant Toll/interleukin-1 receptor/resistance protein (TIR) type nucleotide-binding and leucine-rich repeat immune receptors (NLRs) require enhanced disease susceptibility 1 (EDS1) family proteins and ...the helper NLRs NRG1 and ADR1 for immune activation. We show that the NbEDS1-NbSAG101b-NbNRG1 signaling pathway in N. benthamiana is necessary for cell death signaling by TIR-NLRs from a range of plant species, suggesting a universal requirement for this module in TIR-NLR-mediated cell death in N. benthamiana. We also find that TIR domains physically associate with NbEDS1, NbPAD4, and NbSAG101 in planta, independently of each other. Furthermore, NbNRG1 associates with NbSAG101b, but not with other EDS1 family members, via its C-terminal EP domain. Physical interaction between activated TIRs and EDS1 signaling complexes may facilitate the transfer of low abundance products of TIR catalytic activity or alter TIR catalytic activity to favor the production of EDS1 heterodimer ligands.
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•Diverse TIR-NLRs require NbEDS1-NbSAG101b-NbNRG1 for cell death signaling•TIR domains associate with NbEDS1, NbPAD4, and NbSAG101 in planta•NbNRG1 specifically interacts with NbSAG101b via its C-terminal EP domain
Biochemistry; Molecular interaction; Molecular plant pathology
The plant pathogen
injects about 30 different virulence proteins, so-called effectors, via a type III secretion system into plant cells to promote disease. Although some of these effectors are known ...to suppress either pattern-triggered immunity (PTI) or effector-triggered immunity (ETI), the mode of action of most of them remains unknown. Here, we used transient expression in
, to test the abilities of type III effectors of
pv. tomato (
) DC3000 and
pv.
(
) 11528 to interfere with plant immunity. We monitored the sequential and rapid bursts of cytoplasmic Ca
and reactive oxygen species (ROS), the subsequent induction of defense gene expression, and promotion of cell death. We found that several effector proteins caused cell death, but independently of the known plant immune regulator
, a gene essential for ETI. Furthermore, many effectors delayed or blocked the cell death-promoting activity of other effectors, thereby potentially contributing to pathogenesis. Secondly, a large number of effectors were able to suppress PAMP-induced defense responses. In the majority of cases, this resulted in suppression of all studied PAMP responses, suggesting that these effectors target common elements of PTI. However, effectors also targeted different steps within defense pathways and could be divided into three major groups based on their suppressive activities. Finally, the abilities of effectors of both
DC3000 and
11528 to suppress plant immunity was conserved in most but not all cases. Overall, our data present a comprehensive picture of the mode of action of these effectors and indicate that most of them suppress plant defenses in various ways.
Bacterial pathogens must suppress host defences to cause disease. New research shows that the Pseudomonas effector protein AvrPto does so by directly targeting plant transmembrane receptor kinases ...involved in bacterial perception.
Plant pathogenic bacteria secrete effector proteins that attack the host signaling machinery to suppress immunity. Effectors can be recognized by hosts leading to immunity. One such effector is ...AvrPtoB of Pseudomonas syringae, which degrades host protein kinases, such as tomato Fen, through an E3 ligase domain. Pto kinase, which is highly related to Fen, recognizes AvrPtoB in conjunction with the resistance protein Prf. Here we show that Pto is resistant to AvrPtoB-mediated degradation because it inactivates the E3 ligase domain. AvrPtoB ubiquitinated Fen within the catalytic cleft, leading to its breakdown and loss of the associated Prf protein. Pto avoids this by phosphorylating and inactivating the AvrPtoB E3 domain. Thus, inactivation of a pathogen virulence molecule is one mechanism by which plants resist disease.
Abstract
Stripe rust of wheat, caused by the obligate biotrophic fungus Puccinia striiformis f.sp. tritici, is a major threat to wheat production worldwide with an estimated yearly loss of US $1 ...billion. The recent advances in long-read sequencing technologies and tailored-assembly algorithms enabled us to disentangle the two haploid genomes of Pst. This provides us with haplotype-specific information at a whole-genome level. Exploiting this novel information, we perform whole-genome comparative genomics of two P. striiformis f.sp. tritici isolates with contrasting life histories. We compare one isolate of the old European lineage (PstS0), which has been asexual for over 50 years, and a Warrior isolate (PstS7 lineage) from a novel incursion into Europe in 2011 from a sexual population in the Himalayan region. This comparison provides evidence that long-term asexual evolution leads to genome expansion, accumulation of transposable elements, and increased heterozygosity at the single nucleotide, structural, and allele levels. At the whole-genome level, candidate effectors are not compartmentalized and do not exhibit reduced levels of synteny. Yet we were able to identify two subsets of candidate effector populations. About 70% of candidate effectors are invariant between the two isolates, whereas 30% are hypervariable. The latter might be involved in host adaptation on wheat and explain the different phenotypes of the two isolates. Overall, this detailed comparative analysis of two haplotype-aware assemblies of P. striiformis f.sp. tritici is the first step in understanding the evolution of dikaryotic rust fungi at a whole-genome level.
The enlargement of receptacle cells during strawberry (Fragaria × ananassa) fruit development is a critical factor determining fruit size, with the increase in cell expansion being one of the most ...important physiological processes regulated by the phytohormone gibberellin (GA). Here, we studied the role of GA during strawberry fruit development by analyzing the endogenous content of bioactive GAs and the expression of key components of GA signalling and metabolism. Bioactive GA₁, GA₃ and GA₄ were monitored during fruit development, with the content of GA₄ being extremely high in the receptacle, peaking at the white stage of development. Genes with high homology to genes encoding GA pathway components, including receptors (FaGID1(GIBBERELLIN-INSENSITIVE DWARF1)b and FaGID1c), DELLA (FaRGA(REPRESSOR OF GA) and FaGAI(GA-INSENSITIVE)), and enzymes involved in GA biosynthesis (FaGA3ox) and catabolism (FaGA2ox), were identified, and their expression in different tissues and developmental stages of strawberry fruit was studied in detail. The expression of all of these genes showed a stage-specific pattern during fruit development and was highest in the receptacle. FaGID1c bound GA in vitro, interacted with FaRGA in vitro and in vivo, and increased GA responses when ectopically expressed in Arabidopsis. This study thus reveals key elements of GA responses in strawberry and points to a critical role for GA in the development of the receptacle.
Immunity in tomato (Solanum lycopersicum) to Pseudomonas syringae bacteria expressing the effector proteins AvrPto and AvrPtoB requires both Pto kinase and the NBARC-LRR (for nucleotide binding ...domain shared by Apaf-1, certain R gene products, and CED-4 fused to C-terminal leucine-rich repeats) protein Prf. Pto plays a direct role in effector recognition within the host cytoplasm, but the role of Prf is unknown. We show that Pto and Prf are coincident in the signal transduction pathway that controls ligand-independent signaling. Pto and Prf associate in a coregulatory interaction that requires Pto kinase activity and N-myristoylation for signaling. Pto interacts with a unique Prf N-terminal domain outside of the NBARC-LRR domain and resides in a high molecular weight recognition complex dependent on the presence of Prf. In this complex, both Pto and Prf contribute to specific recognition of AvrPtoB. The data suggest that the role of Pto is confined to the regulation of Prf and that the bacterial effectors have evolved to target this coregulatory molecular switch.
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