Thorsten Nürnberger

A flagellin-induced complex of the receptor FLS2 and BAK1 initiates plant defence

Plants sense potential microbial invaders by using pattern-recognition receptors to recognize pathogen-associated molecular patterns (PAMPs). In Arabidopsis thaliana, the leucine-rich repeat receptor kinases flagellin-sensitive 2 (FLS2) (ref. 2) and elongation factor Tu receptor (EFR) (ref. 3) act as pattern-recognition receptors for the bacterial PAMPs flagellin and elongation factor Tu (EF-Tu) (ref. 5) and contribute to resistance against bacterial pathogens. Little is known about the molecular mechanisms that link receptor activation to intracellular signal transduction. Here we show that BAK1 (BRI1-associated receptor kinase 1), a leucine-rich repeat receptor-like kinase that has been reported to regulate the brassinosteroid receptor BRI1 (refs 6,7), is involved in signalling by FLS2 and EFR. Plants carrying bak1 mutations show normal flagellin binding but abnormal early and late flagellin-triggered responses, indicating that BAK1 acts as a positive regulator in signalling. The bak1-mutant plants also show a reduction in early, but not late, EF-Tu-triggered responses. The decrease in responses to PAMPs is not due to reduced sensitivity to brassinosteroids. We provide evidence that FLS2 and BAK1 form a complex in vivo, in a specific ligand-dependent manner, within the first minutes of stimulation with flagellin. Thus, BAK1 is not only associated with developmental regulation through the plant hormone receptor BRI1 (refs 6,7), but also has a functional role in PRR-dependent signalling, which initiates innate immunity.

Innate immunity in plants and animals: striking similarities and obvious differences.

Summary:u2002 Innate immunity constitutes the first line of defense against attempted microbial invasion, and it is a well‐described phenomenon in vertebrates and insects. Recent pioneering work has revealed striking similarities between the molecular organization of animal and plant systems for nonself recognition and anti‐microbial defense. Like animals, plants have acquired the ability to recognize invariant pathogen‐associated molecular patterns (PAMPs) that are characteristic of microbial organisms but which are not found in potential host plants. Such structures, also termed general elicitors of plant defense, are often indispensable for the microbial lifestyle and, upon receptor‐mediated perception, inevitably betray the invader to the plants surveillance system. Remarkable similarities have been uncovered in the molecular mode of PAMP perception in animals and plants, including the discovery of plant receptors resembling mammalian Toll‐like receptors or cytoplasmic nucleotide‐binding oligomerization domain leucine‐rich repeat proteins. Moreover, molecular building blocks of PAMP‐induced signaling cascades leading to the transcriptional activation of immune response genes are shared among the two kingdoms. In particular, nitric oxide as well as mitogen‐activated protein kinase cascades have been implicated in triggering innate immune responses, part of which is the production of anti‐microbial compounds. In addition to PAMP‐mediated pathogen defense, disease resistance programs are often initiated upon plant‐cultivar‐specific recognition of microbial race‐specific virulence factors, a recognition specificity that is not known from animals.

Of PAMPs and Effectors: The Blurred PTI-ETI Dichotomy

Typically, pathogen-associated molecular patterns (PAMPs) are considered to be conserved throughout classes of microbes and to contribute to general microbial fitness, whereas effectors are species, race, or strain specific and contribute to pathogen virulence. Both types of molecule can trigger plant immunity, designated PAMP-triggered and effector-triggered immunity (PTI and ETI, respectively). However, not all microbial defense activators conform to the common distinction between PAMPs and effectors. For example, some effectors display wide distribution, while some PAMPs are rather narrowly conserved or contribute to pathogen virulence. As effectors may elicit defense responses and PAMPs may be required for virulence, single components cannot exclusively be referred to by one of the two terms. Therefore, we put forward that the distinction between PAMPs and effectors, between PAMP receptors and resistance proteins, and, therefore, also between PTI and ETI, cannot strictly be maintained. Rather, as illustrated by examples provided here, there is a continuum between PTI and ETI. We argue that plant resistance is determined by immune receptors that recognize appropriate ligands to activate defense, the amplitude of which is likely determined by the level required for effective immunity.

Bacterial Effectors Target the Common Signaling Partner BAK1 to Disrupt Multiple MAMP Receptor-Signaling Complexes and Impede Plant Immunity

Successful pathogens have evolved strategies to interfere with host immune systems. For example, the ubiquitous plant pathogen Pseudomonas syringae injects two sequence-distinct effectors, AvrPto and AvrPtoB, to intercept convergent innate immune responses stimulated by multiple microbe-associated molecular patterns (MAMPs). However, the direct host targets and precise molecular mechanisms of bacterial effectors remain largely obscure. We show that AvrPto and AvrPtoB bind the Arabidopsis receptor-like kinase BAK1, a shared signaling partner of both the flagellin receptor FLS2 and the brassinosteroid receptor BRI1. This targeting interferes with ligand-dependent association of FLS2 with BAK1 during infection. It also impedes BAK1-dependent host immune responses to diverse other MAMPs and brassinosteroid signaling. Significantly, the structural basis of AvrPto-BAK1 interaction appears to be distinct from AvrPto-Pto association required for effector-triggered immunity. These findings uncover a unique strategy of bacterial pathogenesis where virulence effectors block signal transmission through a key common component of multiple MAMP-receptor complexes.

Receptor-Mediated Increase in Cytoplasmic Free Calcium Required for Activation of Pathogen Defense in Parsley

Transient influx of Ca2+ constitutes an early element of signaling cascades triggering pathogen defense responses in plant cells. Treatment with the Phytophthora sojae–derived oligopeptide elicitor, Pep-13, of parsley cells stably expressing apoaequorin revealed a rapid increase in cytoplasmic free calcium ([Ca2+]cyt), which peaked at ∼1 μM and subsequently declined to sustained values of 300 nM. Activation of this biphasic [Ca2+]cyt signature was achieved by elicitor concentrations sufficient to stimulate Ca2+ influx across the plasma membrane, oxidative burst, and phytoalexin production. Sustained concentrations of [Ca2+]cyt but not the rapidly induced [Ca2+]cyt transient peak are required for activation of defense-associated responses. Modulation by pharmacological effectors of Ca2+ influx across the plasma membrane or of Ca2+ release from internal stores suggests that the elicitor-induced sustained increase of [Ca2+]cyt predominantly results from the influx of extracellular Ca2+. Identical structural features of Pep-13 were found to be essential for receptor binding, increases in [Ca2+]cyt, and activation of defense-associated responses. Thus, a receptor-mediated increase in [Ca2+]cyt is causally involved in signaling the activation of pathogen defense in parsley.

Specific Bacterial Suppressors of MAMP Signaling Upstream of MAPKKK in Arabidopsis Innate Immunity

Plants and animals possess innate immune systems to prevent infections and are effectively nonhosts for most potential pathogens. The molecular mechanisms underlying nonhost immunity in plants remain obscure. In Arabidopsis, nonhost/nonpathogenic Pseudomonas syringae sustains but pathogenic P. syringae suppresses early MAMP (microbe-associated molecular pattern) marker-gene activation. We performed a cell-based genetic screen of virulence factors and identified AvrPto and AvrPtoB as potent and unique suppressors of early-defense gene transcription and MAP kinase (MAPK) signaling. Unlike effectors of mammalian pathogens, AvrPto and AvrPtoB intercept multiple MAMP-mediated signaling upstream of MAPKKK at the plasma membrane linked to the receptor. In transgenic Arabidopsis, AvrPto blocks early MAMP signaling and enables nonhost P. syringae growth. Deletions of avrPto and avrPtoB from pathogenic P. syringae reduce its virulence. The studies reveal a fundamental role of MAMP signaling in nonhost immunity, and a novel action of type III effectors from pathogenic bacteria.

Innate immunity in plants and animals: emerging parallels between the recognition of general elicitors and pathogen-associated molecular patterns

Recent findings have highlighted remarkable similarities in the innate pathogen defense systems of plants, animals and insects. Pathogen-associated molecular patterns (PAMP) that are similar to those activating innate immune responses in animals have been shown to mediate the activation of plant defense. Moreover, recognition complexes that are structurally related to animal PAMP receptors are now being discovered in plants, suggesting a common evolutionary origin of pathogen defense systems in higher eukaryotes.

Signal transmission in the plant immune response

Genetic and biochemical dissection of signaling pathways regulating plant pathogen defense has revealed remarkable similarities with the innate immune system of mammals and Drosophila. Numerous plant proteins resembling eukaryotic receptors have been implicated in the perception of pathogen-derived signal molecules. Receptor-mediated changes in levels of free calcium in the cytoplasm and production of reactive oxygen species and nitric oxide constitute early events generally observed in plant-pathogen interactions. Positive and negative regulation of plant pathogen defense responses has been attributed to mitogen-activated protein kinase cascades. In addition, salicylic acid, jasmonic acid and ethylene are components of signaling networks that provide the molecular basis for specificity of plant defense responses. This article reviews recent advances in our understanding of early signaling events involved in the establishment of plant disease resistance.

The BRI1-Associated Kinase 1, BAK1, Has a Brassinolide-Independent Role in Plant Cell-Death Control

Programmed cell death (PCD) is a common host response to microbial infection [1-3]. In plants, PCD is associated with immunity to biotrophic pathogens, but it can also promote disease upon infection by necrotrophic pathogens [4]. Therefore, plant cell-suicide programs must be strictly controlled. Here we demonstrate that the Arabidopsis thaliana Brassinosteroid Insensitive 1 (BRI1)-associated receptor Kinase 1 (BAK1), which operates as a coreceptor of BRI1 in brassinolide (BL)-dependent plant development, also regulates the containment of microbial infection-induced cell death. BAK1-deficient plants develop spreading necrosis upon infection. This is accompanied by production of reactive oxygen intermediates and results in enhanced susceptibility to necrotrophic fungal pathogens. The exogenous application of BL rescues growth defects of bak1 mutants but fails to restore immunity to fungal infection. Moreover, BL-insensitive and -deficient mutants do not exhibit spreading necrosis or enhanced susceptibility to fungal infections. Together, these findings suggest that plant steroid-hormone signaling is dispensable for the containment of infection-induced PCD. We propose a novel, BL-independent function of BAK1 in plant cell-death control that is distinct from its BL-dependent role in plant development.

Non‐host resistance in plants: new insights into an old phenomenon

SUMMARY Resistance of an entire plant species to all isolates of a microbial species is referred to as non-host or species resistance. An interplay of both constitutive barriers and inducible reactions comprises the basis for this most durable form of plant disease resistance. Activation of inducible plant defence responses is probably brought about by the recognition of invariant pathogen-associated molecular patterns (PAMP) that are characteristic of whole classes of microbial organisms. PAMP perception systems and PAMP-induced signalling cascades partially resemble those known to mediate activation of innate immune responses in animals, suggesting an evolutionarily ancient molecular concept of non-self recognition and immunity in eukaryotes. Genetic dissection has recently provided clues for SNARE-complex-mediated exocytosis and directed vesicle trafficking in executing plant non-host resistance. Recent functional analysis of bacterial effector proteins indicates that establishment of infection in susceptible plants is associated with suppression of plant species resistance.