Pathogenicity of Pseudomonas syringae is dependent on a type III secretion system, which secretes a suite of virulence effector proteins into the host cytoplasm, and the production of a number of ...toxins such as coronatine (COR), which is a mimic of the plant hormone jasmonate-isoleuce (JA-Ile). Inside the plant cell, effectors target host molecules to subvert the host cell physiology and disrupt defenses. However, despite the fact that elucidating effector action is essential to understanding bacterial pathogenesis, the molecular function and host targets of the vast majority of effectors remain largely unknown. Here, we found that effector HopX1 from Pseudomonas syringae pv. tabaci (Pta) 11528, a strain that does not produce COR, interacts with and promotes the degradation of JAZ proteins, a key family of JA-repressors. We show that hopX1 encodes a cysteine protease, activity that is required for degradation of JAZs by HopX1. HopX1 associates with JAZ proteins through its central ZIM domain and degradation occurs in a COI1-independent manner. Moreover, ectopic expression of HopX1 in Arabidopsis induces the expression of JA-dependent genes, represses salicylic acid (SA)-induced markers, and complements the growth of a COR-deficient P. syringae pv. tomato (Pto) DC3000 strain during natural bacterial infections. Furthermore, HopX1 promoted susceptibility when delivered by the natural type III secretion system, to a similar extent as the addition of COR, and this effect was dependent on its catalytic activity. Altogether, our results indicate that JAZ proteins are direct targets of bacterial effectors to promote activation of JA-induced defenses and susceptibility in Arabidopsis. HopX1 illustrates a paradigm of an alternative evolutionary solution to COR with similar physiological outcome.
Plants are engaged in a continuous co-evolutionary struggle for dominance with their pathogens. The outcomes of these interactions are of particular importance to human activities, as they can have ...dramatic effects on agricultural systems. The recent convergence of molecular studies of plant immunity and pathogen infection strategies is revealing an integrated picture of the plant-pathogen interaction from the perspective of both organisms. Plants have an amazing capacity to recognize pathogens through strategies involving both conserved and variable pathogen elicitors, and pathogens manipulate the defence response through secretion of virulence effector molecules. These insights suggest novel biotechnological approaches to crop protection.
Plant perception of pathogen-associated molecular patterns (PAMPs) triggers a phosphorylation relay leading to PAMP-triggered immunity (PTI). Despite increasing knowledge of PTI signaling, how immune ...homeostasis is maintained remains largely unknown. Here we describe a forward-genetic screen to identify loci involved in PTI and characterize the Arabidopsis calcium-dependent protein kinase CPK28 as a negative regulator of immune signaling. Genetic analyses demonstrate that CPK28 attenuates PAMP-triggered immune responses and antibacterial immunity. CPK28 interacts with and phosphorylates the plasma-membrane-associated cytoplasmic kinase BIK1, an important convergent substrate of multiple pattern recognition receptor (PRR) complexes. We find that BIK1 is rate limiting in PTI signaling and that it is continuously turned over to maintain cellular homeostasis. We further show that CPK28 contributes to BIK1 turnover. Our results suggest a negative regulatory mechanism that continually buffers immune signaling by controlling the turnover of this key signaling kinase.
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•A forward-genetic screen identifies loci regulating Arabidopsis immune signaling•MOB1/CPK28 negatively regulates immune signaling triggered by infection and PAMPs•CPK28 interacts with and phosphorylates the central immune regulator BIK1•CPK28 buffers immune responses and contributes to BIK1 turnover
The amplitude of immune responses must be tightly controlled. Using a forward-genetic screen, Monaghan et al. show that the Arabidopsis calcium-regulated protein kinase CPK28 buffers immunity and regulates turnover of the kinase BIK1, which is a key rate-limiting component of the plant immune system.
SARM1 (sterile alpha and TIR motif containing 1) is responsible for depletion of nicotinamide adenine dinucleotide in its oxidized form (NAD
) during Wallerian degeneration associated with ...neuropathies. Plant nucleotide-binding leucine-rich repeat (NLR) immune receptors recognize pathogen effector proteins and trigger localized cell death to restrict pathogen infection. Both processes depend on closely related Toll/interleukin-1 receptor (TIR) domains in these proteins, which, as we show, feature self-association-dependent NAD
cleavage activity associated with cell death signaling. We further show that SARM1 SAM (sterile alpha motif) domains form an octamer essential for axon degeneration that contributes to TIR domain enzymatic activity. The crystal structures of ribose and NADP
(the oxidized form of nicotinamide adenine dinucleotide phosphate) complexes of SARM1 and plant NLR RUN1 TIR domains, respectively, reveal a conserved substrate binding site. NAD
cleavage by TIR domains is therefore a conserved feature of animal and plant cell death signaling pathways.
The filamentous oomycete plant pathogen Phytophthora infestans causes late blight, an economically important disease, on members of the nightshade family (Solanaceae), such as the crop plants potato ...and tomato. The related plant Nicotiana benthamiana is a model system to study plant-pathogen interactions, and the susceptibility of N. benthamiana to Phytophthora species varies from susceptible to resistant. Little is known about the extent to which plant basal immunity, mediated by membrane receptors that recognise conserved pathogen-associated molecular patterns (PAMPs), contributes to P. infestans resistance.
We found that different species of Phytophthora have varying degrees of virulence on N. benthamiana ranging from avirulence (incompatible interaction) to moderate virulence through to full aggressiveness. The leucine-rich repeat receptor-like kinase (LRR-RLK) BAK1/SERK3 is a major modulator of PAMP-triggered immunity (PTI) in Arabidopsis thaliana and N. benthamiana. We cloned two NbSerk3 homologs, NbSerk3A and NbSerk3B, from N. benthamiana based on sequence similarity to the A. thaliana gene. N. benthamiana plants silenced for NbSerk3 showed markedly enhanced susceptibility to P. infestans infection but were not altered in resistance to Phytophthora mirabilis, a sister species of P. infestans that specializes on a different host plant. Furthermore, silencing of NbSerk3 reduced the cell death response triggered by the INF1, a secreted P. infestans protein with features of PAMPs.
We demonstrated that N. benthamiana NbSERK3 significantly contributes to resistance to P. infestans and regulates the immune responses triggered by the P. infestans PAMP protein INF1. In the future, the identification of novel surface receptors that associate with NbSERK3A and/or NbSERK3B should lead to the identification of new receptors that mediate recognition of oomycete PAMPs, such as INF1.
Plants and animals use innate immunity as a first defense against pathogens, a costly yet necessary tradeoff between growth and immunity. In Arabidopsis, the regulatory leucine-rich repeat ...receptor-like kinase (LRR-RLK) BAK1 combines with the LRR-RLKs FLS2 and EFR in pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) and the LRR-RLK BRI1 in brassinosteroid (BR)-mediated growth. Therefore, a potential tradeoff between these pathways mediated by BAK1 is often postulated. Here, we show a unidirectional inhibition of FLS2-mediated immune signaling by BR perception. Unexpectedly, this effect occurred downstream or independently of complex formation with BAK1 and associated downstream phosphorylation. Thus, BAK1 is not rate-limiting in these pathways. BRs also inhibited signaling triggered by the BAK1-independent recognition of the fungal PAMP chitin. Our results suggest a general mechanism operative in plants in which BR-mediated growth directly antagonizes innate immune signaling.
Perception of pathogen-associated molecular patterns (PAMPs), such as bacterial flagellin (or the peptide flg22), by surface-localized receptors activates defense responses and subsequent immunity. ...In a previous forward-genetic screen aimed at the identification of Arabidopsis (Arabidopsis thaliana) flagellin-insensitive (fin) mutants, we isolated fin4, which is severely affected in flg22-triggered reactive oxygen species (ROS) bursts. Here, we report that FIN4 encodes the chloroplastic enzyme ASPARTATE OXIDASE (AO), which catalyzes the first irreversible step in the de novo biosynthesis of NAD. Genetic studies on the role of NAD have been hindered so far by the lethality of null mutants in NAD biosynthetic enzymes. Using newly identified knockdown fin alleles, we found that AO is required for the ROS burst mediated by the NADPH oxidase RBOHD triggered by the perception of several unrelated PAMPs. AO is also required for RBOHD-dependent stomatal closure. However, full AO activity is not required for flg22-induced responses that are RBOHD independent. Interestingly, although the fin4 mutation dramatically affects RBOHD function, it does not affect functions carried out by other members of the RBOH family, such as RBOHC and RBOHF. Finally, we determined that AO is required for stomatal immunity against the bacterium Pseudomonas syringae. Altogether, our work reveals a novel specific requirement for AO activity in PAMP-triggered RBOHD-dependent ROS burst and stomatal immunity. In addition, the availability of viable mutants for the chloroplastic enzyme AO will enable future detailed studies on the role of NAD metabolism in different cellular processes, including immunity, in Arabidopsis.
Stripe rust caused by the fungus Puccinia striiformis f.sp. tritici (Pst) is a major constraint to wheat production worldwide. The molecular events that underlie Pst pathogenicity are largely ...unknown. Like all rusts, Pst creates a specialized cellular structure within host cells called the haustorium to obtain nutrients from wheat, and to secrete pathogenicity factors called effector proteins. We purified Pst haustoria and used next-generation sequencing platforms to assemble the haustorial transcriptome as well as the transcriptome of germinated spores. 12,282 transcripts were assembled from 454-pyrosequencing data and used as reference for digital gene expression analysis to compare the germinated uredinospores and haustoria transcriptomes based on Illumina RNAseq data. More than 400 genes encoding secreted proteins which constitute candidate effectors were identified from the haustorial transcriptome, with two thirds of these up-regulated in this tissue compared to germinated spores. RT-PCR analysis confirmed the expression patterns of 94 effector candidates. The analysis also revealed that spores rely mainly on stored energy reserves for growth and development, while haustoria take up host nutrients for massive energy production for biosynthetic pathways and the ultimate production of spores. Together, these studies substantially increase our knowledge of potential Pst effectors and provide new insights into the pathogenic strategies of this important organism.
Plants deploy extracellular and intracellular immune receptors to sense and restrict pathogen attacks. Rapidly evolving pathogen effectors play crucial roles in suppressing plant immunity but are ...also monitored by intracellular nucleotide-binding, leucine-rich repeat immune receptors (NLRs), leading to effector-triggered immunity (ETI). Here, we review how NLRs recognize effectors with a focus on direct interactions and summarize recent research findings on the signalling functions of NLRs. Coiled-coil (CC)-type NLR proteins execute immune responses by oligomerizing to form membrane-penetrating ion channels after effector recognition. Some CC-NLRs function in sensor-helper networks with the sensor NLR triggering oligomerization of the helper NLR. Toll/interleukin-1 receptor (TIR)-type NLR proteins possess catalytic activities that are activated upon effector recognition-induced oligomerization. Small molecules produced by TIR activity are detected by additional signalling partners of the EDS1 lipase-like family (enhanced disease susceptibility 1), leading to activation of helper NLRs that trigger the defense response.
The ins and outs of rust haustoria Garnica, Diana P; Nemri, Adnane; Upadhyaya, Narayana M ...
PLoS pathogens,
09/2014, Letnik:
10, Številka:
9
Journal Article
Recenzirano
Odprti dostop
The HMC septum undergoes complex changes during host wall penetration and haustorial maturation, including occlusion of the central pore, thereby preventing continuity of the cytoplasmic contents ...throughout the hyphae 7. ...the HMC and haustorium are separated from the hyphae, which may aid the development of independent transcriptional and metabolic programs in these cells. ...the development of systems to screen candidate effectors for their role in disease 33 will expand our understanding of these important proteins and increase the options to control rust pathogenic fungi.
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