Plant Innate Immune System Research Articles

Recognition of bacterial plant pathogens: local, systemic and transgenerational immunity

Bacterial pathogens can cause multiple plant diseases and plants rely on their innate immune system to recognize and actively respond to these microbes. The plant innate immune system comprises extracellular pattern recognition receptors that recognize conserved microbial patterns and intracellular nucleotide binding leucine-rich repeat (NLR) proteins that recognize specific bacterial effectors delivered into host cells. Plants lack the adaptive immune branch present in animals, but still afford flexibility to pathogen attack through systemic and transgenerational resistance. Here, we focus on current research in plant immune responses against bacterial pathogens. Recent studies shed light onto the activation and inactivation of pattern recognition receptors and systemic acquired resistance. New research has also uncovered additional layers of complexity surrounding NLR immune receptor activation, cooperation and sub-cellular localizations. Taken together, these recent advances bring us closer to understanding the web of molecular interactions responsible for coordinating defense responses and ultimately resistance.

Recent Advances in Plant NLR Structure, Function, Localization, and Signaling

Nucleotide-binding domain leucine-rich repeat (NLR) proteins play a central role in the innate immune systems of plants and vertebrates. In plants, NLR proteins function as intracellular receptors that detect pathogen effector proteins directly, or indirectly by recognizing effector-induced modifications to other host proteins. NLR activation triggers a suite of defense responses associated with programed cell death (PCD). The molecular mechanisms underlying NLR activation, and how activation is translated into defense responses, have been particularly challenging to elucidate in plants. Recent reports, however, are beginning to shed some light. It is becoming clear that plant NLR proteins are targeted to diverse sub-cellular locations, likely depending on the locations where the effectors are detected. These reports also indicate that some NLRs re-localize following effector detection, while others do not, and such relocalization may reflect differences in signaling pathways. There have also been recent advances in understanding the structure of plant NLR proteins, with crystal structures now available for the N-terminal domains of two well-studied NLRs, a coiled-coil (CC) domain and a Toll-interleukin Receptor (TIR). Significant improvements in molecular modeling have enabled more informed structure-function studies, illuminating roles of intra- and inter-molecular interactions in NLR activation regulation. Several independent studies also suggest that intracellular trafficking is involved in NLR-mediated resistance. Lastly, progress is being made on identifying transcriptional regulatory complexes activated by NLRs. Current models for how plant NLR proteins are activated and how they induce defenses are discussed, with an emphasis on what remains to be determined.

MAMP (microbe-associated molecular pattern) triggered immunity in plants

Plants are sessile organisms that are under constant attack from microbes. They rely on both preformed defenses, and their innate immune system to ward of the microbial pathogens. Preformed defences include for example the cell wall and cuticle, which act as physical barriers to microbial colonization. The plant immune system is composed of surveillance systems that perceive several general microbe elicitors, which allow plants to switch from growth and development into a defense mode, rejecting most potentially harmful microbes. The elicitors are essential structures for pathogen survival and are conserved among pathogens. The conserved microbe-specific molecules, referred to as microbe- or pathogen-associated molecular patterns (MAMPs or PAMPs), are recognized by the plant innate immune systems pattern recognition receptors (PRRs). General elicitors like flagellin (Flg), elongation factor Tu (EF-Tu), peptidoglycan (PGN), lipopolysaccharides (LPS), Ax21 (Activator of XA21-mediated immunity in rice), fungal chitin, and β-glucans from oomycetes are recognized by plant surface localized PRRs. Several of the MAMPs and their corresponding PRRs have, in recent years, been identified. This review focuses on the current knowledge regarding important MAMPs from bacteria, fungi, and oomycetes, their structure, the plant PRRs that recognizes them, and how they induce MAMP-triggered immunity (MTI) in plants.

Rhizobacteria Bacillus subtilis restricts foliar pathogen entry through stomata

Plants exist in a complex multitrophic environment, where they interact with and compete for resources with other plants, microbes and animals. Plants have a complex array of defense mechanisms, such as the cell wall being covered with a waxy cuticle serving as a potent physical barrier. Although some pathogenic fungi infect plants by penetrating through the cell wall, many bacterial pathogens invade plants primarily through stomata on the leaf surface. Entry of the foliar pathogen, Pseudomonas syringae pathovar tomato DC3000 (hereafter PstDC3000), into the plant corpus occurs through stomatal openings, and consequently a key plant innate immune response is the transient closure of stomata, which delays disease progression. Here, we present evidence that the root colonization of the rhizobacteria Bacillus subtilis FB17 (hereafter FB17) restricts the stomata-mediated pathogen entry of PstDC3000 in Arabidopsis thaliana. Root binding of FB17 invokes abscisic acid (ABA) and salicylic acid (SA) signaling pathways to close light-adapted stomata. These results emphasize the importance of rhizospheric processes and environmental conditions as an integral part of the plant innate immune system against foliar bacterial infections.

Advances on molecular mechanisms of plant-pathogen interactions

Plants have established a complicated immune defense system during co-evolution with pathogens. The innate immune system of plants can be generally divided into two levels. One, named PAMP-triggered immunity (PTI), is based on the recognition of pathogen-associated molecular patterns by pattern-recognition receptors, which confers resistance to most pathogenic microbes. The other begins in cytoplasm and mainly relies on recognition of microbial effectors by plant resistance proteins in direct or indirect ways, which then initiates potent defense responses. This process, termed effector-triggered immunity (ETI), is necessary for defense against pathogens that can secret effectors to suppress the first level of immunity. Activation of these two layers of immunity in plant is based on distinguishing and recognition of "self" and "non-self" signals. Recognition of "non-self" signals can activate signal cascades, such as MAPK cascades, which will then induce defense gene expression and corresponding defense responses. In this review, we focused on underlying molecular mechanisms of plant-pathogen interactions and the latest advances of the PTI and ETI signaling network.

How do plants achieve immunity? Defence without specialized immune cells

Vertebrates have evolved a sophisticated adaptive immune system that relies on an almost infinite diversity of antigen receptors that are clonally expressed by specialized immune cells that roam the circulatory system. These immune cells provide vertebrates with extraordinary antigen-specific immune capacity and memory, while minimizing self-reactivity. Plants, however, lack specialized mobile immune cells. Instead, every plant cell is thought to be capable of launching an effective immune response. So how do plants achieve specific, self-tolerant immunity and establish immune memory? Recent developments point towards a multilayered plant innate immune system comprised of self-surveillance, systemic signalling and chromosomal changes that together establish effective immunity.

A Rolling Stone Gathers No Moss, but Resistant Plants Must Gather Their MOSes

The detection of pathogenic microbes by plant resistance (R) proteins and the subsequent activation of R protein-mediated immunity constitute an important layer in the plant innate immune system. Most R genes encode proteins with nucleotide-binding (NB) and leucine-rich repeat (LRR) domains. The autoimmune mutant suppressor of npr1, constitutive 1 (snc1), that constitutively activates resistance signaling, is a unique model used in our laboratory to dissect the details of TIR (Toll/Interleukin1 receptor)-NB-LRR, protein-mediated defense responses. Suppressor screens of snc1 yielded 15 modifier of snc1 (mos) complementation groups containing second-site mutations, and resulted in the identification of 13 novel MOS genes via either positional cloning or T-DNA tagging. Characterizations of the mos mutants have revealed important roles for transcriptional regulation, RNA processing, protein modifications, and nucleocytoplasmic trafficking in R protein-mediated immunity. The MOS genes have taught us a great deal about the complex mechanisms surrounding R protein activation. Future in-depth genetic and biochemical analyses will further enhance our knowledge of how R proteins are deliberately activated and how specific, targeted immunity is achieved in plants.

Tracing the origin and evolutionary history of plant nucleotide‐binding site–leucine‐rich repeat (NBS‐LRR) genes

Plant disease resistance genes (R genes) encode proteins that function to monitor signals indicating pathogenic infection, thus playing a critical role in the plant's defense system. Although many studies have been performed to explore the functional details of these important genes, their origin and evolutionary history remain unclear. In this study, focusing on the largest group of R genes, the nucleotide-binding site-leucine-rich repeat (NBS-LRR) genes, we conducted an extensive genome-wide survey of 38 representative model organisms and obtained insights into the evolutionary stage and timing of NBS-LRR genes. Our data show that the two major domains, NBS and LRR, existed before the split of prokaryotes and eukaryotes but their fusion was observed only in land plant lineages. The Toll/interleukin-1 receptor (TIR) class of NBS-LRR genes probably had an earlier origin than its nonTIR counterpart. The similarities of the innate immune systems of plants and animals are likely to have been shaped by convergent evolution after their independent origins. Our findings start to unravel the evolutionary history of these important genes from the perspective of comparative genomics and also highlight the important role of reorganizing pre-existing building blocks in generating evolutionary novelties.

Chemical diversity on display in the plant innate immune systems of closely-related species

Chemical diversity on display in the plant innate immune systems of closely-related species

The Strawberry Plant Defense Mechanism: A Molecular Review

Strawberry, a small fruit crop of great importance throughout the world, has been considered a model plant system for Rosaceae, and is susceptible to a large variety of phytopathogenic organisms. Most components and mechanisms of the strawberry defense network remain poorly understood. However, from current knowledge, it seems clear that the ability of a strawberry plant to respond efficiently to pathogens relies first on the physiological status of injured tissue (pre-formed mechanisms of defense) and secondly on the general ability to recognize and identify the invaders by surface plant receptors, followed by a broad range of induced mechanisms, which include cell wall reinforcement, production of reactive oxygen species, phytoalexin generation and pathogenesis-related protein accumulation. Dissection of these physiological responses at a molecular level will provide valuable information to improve future breeding strategies for new strawberry varieties and to engineer strawberry plants for durable and broad-spectrum disease resistance. In turn, this will lead to a reduction in use of chemicals and in environmental risks. Advances in the understanding of the molecular interplay between plant (mainly those considered model systems) and various classes of microbial pathogens have been made in the last two decades. However, major progress in the genetics and molecular biology of strawberry is still needed to uncover fully the way in which this elaborate plant innate immune system works. These fundamental insights will provide a conceptual framework for rational human intervention through new strawberry research approaches. In this review, we will provide a comprehensive overview and discuss recent advances in molecular research on strawberry defense mechanisms against pathogens.

An Autoactive Mutant of the M Flax Rust Resistance Protein Has a Preference for Binding ATP, Whereas Wild-Type M Protein Binds ADP

Resistance (R) proteins are key regulators of the plant innate immune system and are capable of pathogen detection and activation of the hypersensitive cell death immune response. To understand the molecular mechanism of R protein activation, we undertook a phenotypic and biochemical study of the flax nucleotide binding (NB)-ARC leucine-rich repeat protein, M. Using Agrobacterium-mediated transient expression in flax cotyledons, site-directed mutations of key residues within the P-loop, kinase 2, and MHD motifs within the NB-ARC domain of M were shown to affect R protein function. When purified using a yeast expression system and assayed for ATP and ADP, these mutated proteins exhibited marked differences in the quantity and identity of the bound nucleotide. ADP was bound to recombinant wild-type M protein, while the nonfunctional P-loop mutant did not have any nucleotides bound. In contrast, ATP was bound to an autoactive M protein mutated in the highly conserved MHD motif. These data provide direct evidence supporting a model of R protein function in which the "off" R protein binds ADP and activation of R protein defense signaling involves the exchange of ADP for ATP.

Arabidopsis MPK3 and MPK6 Play Different Roles in Basal and Oligogalacturonide- or Flagellin-Induced Resistance against Botrytis cinerea

Mitogen-activated protein kinases (MAPKs) are fundamental components of the plant innate immune system. MPK3 and MPK6 are Arabidopsis (Arabidopsis thaliana) MAPKs activated by pathogens and elicitors such as oligogalacturonides (OGs), which function as damage-associated molecular patterns, and flg22, a well-known microbe-associated molecular pattern. However, the specific contribution of MPK3 and MPK6 to the regulation of elicitor-induced defense responses is not completely defined. In this work we have investigated the roles played by these MAPKs in elicitor-induced resistance against the fungal pathogen Botrytis cinerea. Analysis of single mapk mutants revealed that lack of MPK3 increases basal susceptibility to the fungus, as previously reported, but does not significantly affect elicitor-induced resistance. Instead, lack of MPK6 has no effect on basal resistance but suppresses OG- and flg22-induced resistance to B. cinerea. Overexpression of the AP2C1 phosphatase leads to impaired OG- and flg22-induced phosphorylation of both MPK3 and MPK6, and to phenotypes that recapitulate those of the single mapk mutants. These data indicate that OG- and flg22-induced defense responses effective against B. cinerea are mainly dependent on MAPKs, with a greater contribution of MPK6.

Conservation and clade-specific diversification of pathogen-inducible tryptophan and indole glucosinolate metabolism in Arabidopsis thaliana relatives

• A hallmark of the innate immune system of plants is the biosynthesis of low-molecular-weight compounds referred to as secondary metabolites. Tryptophan-derived branch pathways contribute to the capacity for chemical defense against microbes in Arabidopsis thaliana. • Here, we investigated phylogenetic patterns of this metabolic pathway in relatives of A. thaliana following inoculation with filamentous fungal pathogens that employ contrasting infection strategies. • The study revealed unexpected phylogenetic conservation of the pathogen-induced indole glucosinolate (IG) metabolic pathway, including a metabolic shift of IG biosynthesis to 4-methoxyindol-3-ylmethylglucosinolate and IG metabolization. By contrast, indole-3-carboxylic acid and camalexin biosyntheses are clade-specific innovations within this metabolic framework. A Capsella rubella accession was found to be devoid of any IG metabolites and to lack orthologs of two A. thaliana genes needed for 4-methoxyindol-3-ylmethylglucosinolate biosynthesis or hydrolysis. However, C. rubella was found to retain the capacity to deposit callose after treatment with the bacterial flagellin-derived epitope flg22 and pre-invasive resistance against a nonadapted powdery mildew fungus. • We conclude that pathogen-inducible IG metabolism in the Brassicaceae is evolutionarily ancient, while other tryptophan-derived branch pathways represent relatively recent manifestations of a plant-pathogen arms race. Moreover, at least one Brassicaceae lineage appears to have evolved IG-independent defense signaling and/or output pathway(s).

The role of lipopolysaccharide and peptidoglycan, two glycosylated bacterial microbe-associated molecular patterns (MAMPs), in plant innate immunity.

In an environment that is rich in potentially pathogenic microorganisms, the survival of higher eukaryotic organisms depends on efficient pathogen sensing and rapidly mounted defence responses. Such protective mechanisms are found in all multicellular organisms, and are collectively referred to as 'innate immunity'. Innate immunity is the first line of defence against invading microorganisms in vertebrates and the only line of defence in invertebrates and plants. Bacterial glycoconjugates, such as lipopolysaccharides (LPSs) from the outer membrane of Gram-negative bacteria and peptidoglycan (PGN) from the cell walls of both Gram-positive and Gram-negative bacteria, have been found to act as elicitors of plant innate immunity. These conserved, indispensable, microbe-specific molecules are also referred to as 'microbe-associated molecular patterns' (MAMPs). MAMPs are recognized by the plant innate immune system through the action of pattern recognition receptors (PRRs). A greater insight into the mechanisms of MAMP recognition and the description of PRRs for different microbial glycoconjugates will have considerable impact on the improvement of plant health and disease resistance. Here, the current knowledge about LPS and PGN as MAMPs is reviewed.

System of innate immunity in plants

The review deals with the mechanisms of innate immunity in plants with a focus on families of pattern-recognition receptors and regard for recent data on complete sequencing of the genomes of several plant species. Plants utilize several families of such receptors, both membrane-bound and cytoplasmic ones, which contain conservative leucine-rich repeats. The lack of adaptive immunity and there to related rearrangements in genes encoding immune receptors in plants are partly compensated by mechanisms of "specific" immunity to counter particular pathogens; such mechanisms being fully encoded within a plant genome. At the level of intracellular signal transduction, as well as in respect of effector mechanisms, similarities between plant and animal innate immune systems can be found, although the latter has many additional aspects.

Antimicrobial peptides and proteins of the horse - insights into a well-armed organism

Antimicrobial peptides play a pivotal role as key effectors of the innate immune system in plants and animals and act as endogenous antibiotics. The molecules exhibit an antimicrobial activity against bacteria, viruses, and eukaryotic pathogens with different specificities and potencies depending on the structure and amino-acid composition of the peptides. Several antimicrobial peptides were comprehensively investigated in the last three decades and some molecules with remarkable antimicrobial properties have reached the third phase of clinical studies. Next to the peptides themselves, numerous organisms were examined and analyzed regarding their repertoire of antimicrobial peptides revealing a huge number of candidates with potencies and properties for future medical applications. One of these organisms is the horse, which possesses numerous peptides that are interesting candidates for therapeutical applications in veterinary medicine. Here we summarize investigations and knowledge on equine antimicrobial peptides, point to interesting candidates, and discuss prospects for therapeutical applications.

Differential expression of cytochrome oxidase and ALY-family genes in resistant and susceptible tomato cultivars (Solanum lycopersicum) inoculated with Tomato bushy stunt virus

Tomato cultivars (TY20, TY70/84 and TY70/70) were mechanically inoculated with purified virus preparations of Tomato bushy stunt virus (TBSV). Inoculated plant tissues were collected after one day, one week and two weeks post-inoculation, respectively. The P19 gene was amplified using specific RT-PCR from the three examined tomato cultivars. Our main result is that the expression of the P19 gene was high in both TY70/84 and TY70/70, but it was very low in case of TY20 tomato cultivars. Messenger RNA of the inoculated and healthy plants was scanned using the differential display technique to discover the up- and down-regulated genes induced or suppressed in the infected plants. Three down-regulated and four up-regulated genes were observed. Sequence analysis revealed that the 400 bp up-regulated genes amplified from cultivars TY70/84 and TY70/70 were cytochrome oxidase genes. Expression of these genes was highest in cultivar TY70/84, followed by cultivar TY70/70; it was considerably low in cultivar TY20 and the control plants. The other four up-regulated genes belonged to the ALY gene family. We conclude that the expression of P19, cytochrome oxidase, and ALY family genes were highly expressed in inoculated plants with TBSV compared to the control plants, which suggest that these genes are members of the plant innate immune system. Keywords Differential expression; Tomato (Solanum lycopersicum); Tomato bushy stunt virus (TBSV); P19 gene; Cytochrome oxidase; Aly-family genes

Molecular battles between plant and pathogenic bacteria in the phyllosphere

The phyllosphere, i.e., the aerial parts of the plant, provides one of the most important niches for microbial colonization. This niche supports the survival and, often, proliferation of microbes such as fungi and bacteria with diverse lifestyles including epiphytes, saprophytes, and pathogens. Although most microbes may complete the life cycle on the leaf surface, pathogens must enter the leaf and multiply aggressively in the leaf interior. Natural surface openings, such as stomata, are important entry sites for bacteria. Stomata are known for their vital role in water transpiration and gas exchange between the plant and the environment that is essential for plant growth. Recent studies have shown that stomata can also play an active role in limiting bacterial invasion of both human and plant pathogenic bacteria as part of the plant innate immune system. As counter-defense, plant pathogens such as Pseudomonas syringae pv tomato (Pst) DC3000 use the virulence factor coronatine to suppress stomate-based defense. A novel and crucial early battleground in host-pathogen interaction in the phyllosphere has been discovered with broad implications in the study of bacterial pathogenesis, host immunity, and molecular ecology of bacterial diseases.

99th Dahlem Conference on Infection, Inflammation and Chronic Inflammatory Disorders: Innate immune responses in plants

Plants rely exclusively upon mechanisms of innate immunity. Current concepts of the plant innate immune system are based largely on two forms of immunity that engage distinct classes of immune receptors. These receptors enable the recognition of non-self structures that are either conserved between members of a microbial class or specific to individual strains of a microbe. One type of receptor comprises membrane-resident pattern recognition receptors (PRRs) that detect widely conserved microbe-associated molecular patterns (MAMPs) on the cell surface. A second type of mainly intracellular immune sensors, designated resistance (R) proteins, recognizes either the structure or function of strain-specific pathogen effectors that are delivered inside host cells. Phytopathogenic microorganisms have evolved a repertoire of effectors, some of which are delivered into plant cells to sabotage MAMP-triggered immune responses. Plants appear to have also evolved receptors that sense cellular injury by the release and perception of endogenous damage-associated molecular patterns (DAMPs). It is possible that the integration of MAMP and DAMP responses is critical to mount robust MAMP-triggered immunity. This signal integration might help to explain why plants are colonized in nature by remarkably diverse and seemingly asymptomatic microbial communities.

Glyco-conjugates as elicitors or suppressors of plant innate immunity

Innate immunity is the first line of defense against invading microorganisms in vertebrates and the only line of defense in invertebrates and plants. Bacterial glyco-conjugates, such as lipopolysaccharides (LPS) from the outer membrane of Gram-negative bacteria and peptidoglycan (PGN) from the cell walls of both Gram-positive and Gram-negative bacteria, and fungal and oomycete glycoconjugates such as oligosaccharides derived from the cell wall components beta-glucan, chitin and chitosan, have been found to act as elicitors of plant innate immunity. These conserved indispensable microbe-specific molecules are also referred to as microbe-associated molecular patterns (MAMPs). Other glyco-conjugates such as bacterial extracellular polysaccharides (EPS) and cyclic glucan have been shown to suppress innate immune responses, thus conversely promoting pathogenesis. MAMPs are recognized by the plant innate immune system though the action of pattern recognition receptors (PRRs). A greater insight into the mechanisms of MAMP recognition and the description of PRRs for different microbial glyco-conjugates will have considerable impact on the improvement of plant health and disease resistance. Here we review the current knowledge about the bacterial MAMPs LPS and PGN, the fungal MAMPs beta-glucan, chitin and chitosan oligosaccharides and the bacterial suppressors EPS and cyclic glucan, with particular reference to the chemical structures of these molecules, the PRRs involved in their recognition (where these have been defined), and possible mechanisms underlying suppression.