Plant innate immunity: An updated insight into defense mechanism

Plants are invaded by an array of pathogens of which only a few succeed in causing disease. The attack by others is countered by a sophisticated immune system possessed by the plants. The plant immune system is broadly divided into two, viz. microbial-associated molecular-patterns-triggered immunity (MTI) and effector-triggered immunity (ETI). MTI confers basal resistance, while ETI confers durable resistance, often resulting in hypersensitive response. Plants also possess systemic acquired resistance (SAR), which provides long-term defense against a broad-spectrum of pathogens. Salicylic-acid-mediated systemic acquired immunity provokes the defense response throughout the plant system during pathogen infection at a particular site. Trans-generational immune priming allows the plant to heritably shield their progeny towards pathogens previously encountered. Plants circumvent the viral infection through RNA interference phenomena by utilizing small RNAs. This review summarizes the molecular mechanisms of plant immune system, and the latest breakthroughs reported in plant defense. We discuss the plant–pathogen interactions and integrated defense responses in the context of presenting an integral understanding in plant molecular immunity.     

The Ankyrin-Repeat Transmembrane Protein BDA1 Functions Downstream of the Receptor-Like Protein SNC2 to Regulate Plant Immunity

Plants utilize a large number of immune receptors to recognize pathogens and activate defense responses. A small number of these receptors belong to the receptor-like protein family. Previously, we showed that a gain-of-function mutation in the receptor-like protein SNC2 (for Suppressor of NPR1, Constitutive2) leads to constitutive activation of defense responses in snc2-1D mutant plants. To identify defense signaling components downstream of SNC2, we carried out a suppressor screen in the snc2-1D mutant background of Arabidopsis (Arabidopsis thaliana). Map-based cloning of one of the suppressor genes, BDA1 (for bian da; "becoming big" in Chinese), showed that it encodes a protein with amino-terminal ankyrin repeats and carboxyl-terminal transmembrane domains. Loss-of-function mutations in BDA1 suppress the dwarf morphology and constitutive defense responses in snc2-1D npr1-1 (for nonexpressor of pathogenesis-related genes1,1) and also result in enhanced susceptibility to bacterial pathogens. In contrast, a gain-of-function allele of bda1 isolated from a separate genetic screen to search for mutants with enhanced pathogen resistance was found to constitutively activate cell death and defense responses. These data suggest that BDA1 is a critical signaling component that functions downstream of SNC2 to regulate plant immunity.     

植物与病原微生物互作分子基础的研究进展

植物在与病原微生物共同进化过程中形成了复杂的免疫防卫体系。植物的先天免疫系统可大致分为两个层面。第一个层面的免疫基于细胞表面的模式识别受体对病原物相关分子模式的识别, 该免疫过程被称为病原物相关分子模式触发的免疫(PAMP-triggered immunity, PTI), 能帮助植物抵抗大部分病原微生物; 第二个层面的免疫起始于细胞内部, 主要依靠抗病基因编码的蛋白产物直接或间接识别病原微生物分泌的效应子并且激发防卫反应, 来抵抗那些能够利用效应子抑制第一层面免疫的病原微生物, 这一过程被称为效应子触发的免疫(Effector-triggered immunity, ETI)。这两个层面的免疫都是基于植物对“自我”及“非我”的识别, 依靠MAPK级联等信号网络, 将识别结果传递到细胞核内, 调控相应基因的表达, 做出适当的免疫应答。本文着重阐述了植物与病原微生物互作过程中不同层面的免疫反应所发生主要事件的分子基础及研究进展。     

植物与病原微生物互作分子基础的研究进展

植物在与病原微生物共同进化过程中形成了复杂的免疫防卫体系。植物的先天免疫系统可大致分为两个层面。第一个层面的免疫基于细胞表面的模式识别受体对病原物相关分子模式的识别,该免疫过程被称为病原物相关分子模式触发的免疫(PAMP-triggered immunity,PTI),能帮助植物抵抗大部分病原微生物;第二个层面的免疫起始于细胞内部,主要依靠抗病基因编码的蛋白产物直接或间接识别病原微生物分泌的效应子并且激发防卫反应,来抵抗那些能够利用效应子抑制第一层面免疫的病原微生物,这一过程被称为效应子触发的免疫(Effector-triggered immunity,ETI)。这两个层面的免疫都是基于植物对"自我"及"非我"的识别,依靠MAPK级联等信号网络,将识别结果传递到细胞核内,调控相应基因的表达,做出适当的免疫应答。本文着重阐述了植物与病原微生物互作过程中不同层面的免疫反应所发生主要事件的分子基础及研究进展。     

EHD2 inhibits ligand-induced endocytosis and signaling of the leucine-rich repeat receptor-like protein LeEix2

Plants are constantly being challenged by aspiring pathogens. In order to protect themselves, plants have developed numerous defense mechanisms that are either specific or non-specific to the pathogen. Pattern recognition receptors can trigger plant defense responses in response to specific ligands or patterns. EIX (ethylene-inducing xylanase) triggers a defense response via the LeEix2 receptor, while bacterial flagellin triggers plant innate immunity via the FLS2 receptor. Endocytosis has been suggested to be crucial for the process in both cases. Here we show that the EIX elicitor triggers internalization of the LeEix2 receptor. Treatment with endocytosis, actin or microtubule inhibitors greatly reduced the internalization of LeEix2. Additionally, we demonstrate that plant EHD2 binds to LeEix2 and is an important factor in its internalization and in regulation of the induction of defense responses such as the hypersensitive response, ethylene biosynthesis and induction of pathogenesis-related protein expression in the case of EIX/LeEix2 (an LRR receptor lacking a kinase domain), but does not appear to be involved in the FLS2 system (an LRR receptor possessing a kinase domain). Our results suggest that various endocytosis pathways are involved in the induction of plant defense responses.     

A receptor-like cytoplasmic kinase phosphorylates the host target RIN4, leading to the activation of a plant innate immune receptor

Plants have evolved sophisticated surveillance systems to recognize pathogen effectors delivered into host cells. RPM1 is an NB-LRR immune receptor that recognizes the Pseudomonas syringae effectors AvrB and AvrRpm1. Both effectors associate with and affect the phosphorylation of RIN4, an immune regulator. Although the kinase and the specific mechanisms involved are unclear, it has been hypothesized that RPM1 recognizes phosphorylated RIN4. Here, we identify RIPK as a RIN4-interacting receptor-like protein kinase that phosphorylates RIN4. In response to bacterial effectors, RIPK phosphorylates RIN4 at amino acid residues T21, S160, and T166. RIN4 phosphomimetic mutants display constitutive activation of RPM1-mediated defense responses and RIN4 phosphorylation is induced by AvrB and AvrRpm1 during P. syringae infection. RIPK knockout lines exhibit reduced RIN4 phosphorylation and blunted RPM1-mediated defense responses. Taken together, our results demonstrate that the RIPK kinase associates with and modifies an effector-targeted protein complex to initiate host immunity.     

Receptor-like kinases and receptor-like proteins: keys to pathogen recognition and defense signaling in plant innate immunity

Plants have evolved multiple layers of defense against various pathogens in the environment. Receptor-like kinases/proteins (RLKs/RLPs) are on the front lines of the battle between plants and pathogens since they are present at the plasma membrane and perceive signature molecules from either the invading pathogen or damaged plant tissue. With a few notable exceptions, most RLKs/RLPs are positive regulators of plant innate immunity. In this review, we summarize recently discovered RLKs/RLPs that are involved in plant defense responses against various classes of pathogens.We also describe what is currently known about the mechanisms of RLK-mediated initiation of signaling via protein-protein interactions and phosphorylation.     

Two-component elements mediate interactions between cytokinin and salicylic acid in plant immunity

Recent studies have revealed an important role for hormones in plant immunity. We are now beginning to understand the contribution of crosstalk among different hormone signaling networks to the outcome of plant-pathogen interactions. Cytokinins are plant hormones that regulate development and responses to the environment. Cytokinin signaling involves a phosphorelay circuitry similar to two-component systems used by bacteria and fungi to perceive and react to various environmental stimuli. In this study, we asked whether cytokinin and components of cytokinin signaling contribute to plant immunity. We demonstrate that cytokinin levels in Arabidopsis are important in determining the amplitude of immune responses, ultimately influencing the outcome of plant-pathogen interactions. We show that high concentrations of cytokinin lead to increased defense responses to a virulent oomycete pathogen, through a process that is dependent on salicylic acid (SA) accumulation and activation of defense gene expression. Surprisingly, treatment with lower concentrations of cytokinin results in increased susceptibility. These functions for cytokinin in plant immunity require a host phosphorelay system and are mediated in part by type-A response regulators, which act as negative regulators of basal and pathogen-induced SA-dependent gene expression. Our results support a model in which cytokinin up-regulates plant immunity via an elevation of SA-dependent defense responses and in which SA in turn feedback-inhibits cytokinin signaling. The crosstalk between cytokinin and SA signaling networks may help plants fine-tune defense responses against pathogens.     

Phosphorylation of receptor-like cytoplasmic kinases by bacterial Flagellin

Molecular mechanisms that distinguish self and non-self are fundamental in innate immunity to prevent infections in plants and animals. Recognition of the conserved microbial components triggers immune responses against a broad spectrum of potential pathogens. In Arabidopsis, bacterial flagellin was perceived by a leucine-rich repeat-receptor-like kinase (LRR-RLK) FLS2. Upon flagellin perception, FLS2 forms a complex with another LRR-RLK BAK1. The intracellular signaling events downstream of FLS2/BAK1 receptor complex are still poorly understood. We recently identified a receptor-like cytoplasmic kinase (RLCK) BIK1 that associates with flagellin receptor complex to initiate plant innate immunity. BIK1 is rapidly phosphorylated upon flagellin perception in an FLS2- and BAK1-dependent manner. BAK1 directly phosphorylates BIK1 with an in vitro kinase assay. Plants have evolved a large number of RLCK genes involved in a wide range of biological processes. We provided evidence here that additional RLCKs could also be phosphorylated by flagellin and may play redundant role with BIK1 in plant innate immunity.     

ER Quality Control Components UGGT and STT3a Are Required for Activation of Defense Responses in Bir1-1

The receptor-like kinase SUPPRESSOR OF BIR1, 1 (SOBIR1) functions as a critical regulator in plant immunity. It is required for activation of cell death and defense responses in Arabidopsis bak1-interacting receptor-like kinase 1,1 (bir1-1) mutant plants. Here we report that the ER quality control component UDP-glucose:glycoprotein glucosyltransferase (UGGT) is required for the biogenesis of SOBIR1 and mutations in UGGT suppress the spontaneous cell death and constitutive defense responses in bir1-1. Loss of function of STT3a, which encodes a subunit of the oligosaccharyltransferase complex, also suppresses the autoimmune phenotype in bir1-1. However, it has no effect on the accumulation of SOBIR1, suggesting that additional signaling components other than SOBIR1 may be regulated by ER quality control. Our study provides clear evidence that ER quality control play critical roles in regulating defense activation in bir1-1.     

Plant Immunity Induced by Oligogalacturonides Alters Root Growth in a Process Involving Flavonoid Accumulation in Arabidopsis thaliana

Plants adapt to challenging environmental factors by modulating morphogenetic processes. Although it has been speculated that activation of defense responses against pathogens leads to plant growth adjustment, little is known about developmental and architectural responses to defense stimulators. In this report we evaluated the activity of oligogalacturonides (OGs), a class of molecules directly involved in plant immunity, to modulate root system architecture in Arabidopsis thaliana. We show that OGs induce PAD3 expression and camalexin synthesis, two well-known markers of defense responses. These effects were related to primary root growth inhibition and increased lateral root and root hair formation, which are reminiscent of altered auxin responses. Cellular analysis showed that the effect of these compounds on primary root growth was due to changes in cell elongation and increased flavonoid accumulation at the root elongation region. Moreover, the observations that similar changes in primary root growth were induced by naphthylphthalamic acid supply and that auxin- or flavonoid-related mutants tir1, doc1, pgp1, pgp4, pgp19, and tt4-1 show differential responses to primary root growth inhibition by OGs suggest that auxin homeostasis plays a role in the oligogalacturonide-induced alteration of root cell patterning. Our results suggest that OGs might play a dual function in adaptation of plants to pathogen challenge by inducing defense responses and plant architecture adjustment.     

Plant autoimmunity—fresh insights into an old phenomenon

The plant immune system is well equipped to ward off the attacks of different types of phytopathogens. It primarily relies on two types of immune sensors—plasma membrane-resident receptor-like kinases and intracellular nucleotide-binding domain leucine-rich repeat (NLRs) receptors that engage preferentially in pattern- and effector-triggered immunity, respectively. Delicate fine-tuning, in particular of the NLR-governed branch of immunity, is key to prevent inappropriate and deleterious activation of plant immune responses. Inadequate NLR allele constellations, such as in the case of hybrid incompatibility, and the mis-activation of NLRs or the absence or modification of proteins guarded by these NLRs can result in the spontaneous initiation of plant defense responses and cell death—a phenomenon referred to as plant autoimmunity. Here, we review recent insights augmenting our mechanistic comprehension of plant autoimmunity. The recent findings broaden our understanding regarding hybrid incompatibility, unravel candidates for proteins likely guarded by NLRs and underline the necessity for the fine-tuning of NLR expression at various levels to avoid autoimmunity. We further present recently emerged tools to study plant autoimmunity and draw a cross-kingdom comparison to the role of NLRs in animal autoimmune conditions.

Increasing evidence indicates that plant autoimmunity is in many cases the consequence of the inappropriate activation of cytoplasmic immune sensors.     

Plant systems for recognition of pathogen-associated molecular patterns

Research of the last decade has revealed that plant immunity consists of different layers of defense that have evolved by the co-evolutional battle of plants with its pathogens. Particular light has been shed on PAMP- (pathogen-associated molecular pattern) triggered immunity (PTI) mediated by pattern recognition receptors. Striking similarities exist between the plant and animal innate immune system that point for a common optimized mechanism that has evolved independently in both kingdoms. Pattern recognition receptors (PRRs) from both kingdoms consist of leucine-rich repeat receptor complexes that allow recognition of invading pathogens at the cell surface. In plants, PRRs like FLS2 and EFR are controlled by a co-receptor SERK3/BAK1, also a leucine-rich repeat receptor that dimerizes with the PRRs to support their function. Pathogens can inject effector proteins into the plant cells to suppress the immune responses initiated after perception of PAMPs by PRRs via inhibition or degradation of the receptors. Plants have acquired the ability to recognize the presence of some of these effector proteins which leads to a quick and hypersensitive response to arrest and terminate pathogen growth.     

Fungal Pathogens: The Battle for Plant Infection

The attempted infection of a plant by a pathogen, such as a fungus or an Oomycete, may be regarded as a battle whose major weapons are proteins and smaller chemical compounds produced by both organisms. Indeed, plants produce an astonishing plethora of defense compounds that are still being discovered at a rapid pace. This pattern arose from a multi-million year, ping-pong?type co-evolution, in which plant and pathogen successively added new chemical weapons in this perpetual battle. As each defensive innovation was established in the host, new ways to circumvent it evolved in the pathogen. This complex co-evolution process probably explains not only the exquisite specificity observed between many pathogens and their hosts, but also the ineffectiveness or redundancy of some defensive genes which often encode enzymes with overlapping activities. Plants evolved a complex, multi-level series of structural and chemical barriers that are both constitutive or preformed and inducible. These defenses may involve strengthening of the cell wall, hypersensitive response (HR), oxidative burst, phytoalexins and pathogenesis-related (PR) proteins. The pathogen must successfully overcome these obstacles before it succeeds in causing disease. In some cases, it needs to modulate or modify plant cell metabolism to its own benefit and/or to abolish defense reactions. Central to the activation of plant responses is timely perception of the pathogen by the plant. A crucial role is played by elicitors which, depending on their mode of action, are broadly classified into nonspecific elicitors and highly specific elicitors or virulence effector/avirulence factors. A protein battle for penetration is then initiated, marking the pathogen attempted transition from extracellular to invasive growth before parasitism and disease can be established. Three major types of defense responses may be observed in plants: non-host resistance, host resistance, and host pathogenesis. Plant innate immunity may comprise a continuum from non-host resistance involving the detection of general elicitors to host-specific resistance involving detection of specific elicitors by R proteins. It was generally assumed that non-host resistance was based on passive mechanisms and that nonspecific rejection usually arose as a consequence of the non-host pathogen failure to breach the first lines of plant defense. However, recent evidence has blurred the clear-cut distinction among non-host resistance, host-specific resistance and disease. The same obstacles are also serious challenges for host pathogens, reducing their success rate significantly in causing disease. Indeed, even susceptible plants mount a (insufficient) defense response upon recognition of pathogen elicited molecular signals. Recent evidence suggests the occurrence of significant overlaps between the protein components and signalling pathways of these types of resistance, suggesting the existence of both shared and unique features for the three branches of plant innate immunity.     

BON1 interacts with the protein kinases BIR1 and BAK1 in modulation of temperature-dependent plant growth and cell death in Arabidopsis

The Arabidopsis copine gene BON1 encodes a calcium-dependent phospholipid-binding protein involved in plant growth homeostasis and disease resistance. However, the biochemical and molecular mechanisms by which BON1 modulates plant growth and defense responses are not well understood. Here, we show that BON1 interacts physically with the leucine-rich-repeat receptor-like kinases BIR1 (BAK1-interacting receptor-like kinase 1) and pathogen-associated molecular pattern (PAMP) receptor regulator BAK1 in vitro and in vivo. Additionally, bon1 and bir1 mutants exhibit synergistic interaction. While a bir1 null mutant has similar growth and cell-death defects compared with bon1, a bir1 bon1 double mutant displays more severe phenotypes than does the single mutants. The bon1-1 and bir1-1 phenotypes are partially suppressed by overexpression of BIR1 and BON1, respectively. Furthermore, the bir1 phenotype is attenuated by a loss-of-function mutation in the resistance (R) gene SNC1 (Suppressor of npr1-1, constitutive 1), which mediates defense responses in bon1. Intriguingly, BON1 and BIR1 can be phosphorylated by BAK1 in vitro. Our findings suggest that BIR1 functions as a negative regulator of plant resistance and that BON1 and BIR1 might modulate both PAMP- and R protein-triggered immune responses.     

Self/nonself perception and recognition mechanisms in plants: a comparison of self-incompatibility and innate immunity

Analyses of emerging concepts indicate that parallels exist between self-incompatibility and pathogen recognition. In the case of surveillance of 'nonself', plant immune responses are triggered either by pattern recognition receptors (PRRs) that detect conserved pathogen-associated molecular patterns (PAMPs) or by resistance (R) proteins recognizing isolate-specific pathogen effectors. PAMP detection is an important component of innate immunity in plants and serves as an early warning system for the presence of potential pathogens and activation of plant defense mechanisms. In the Brassicaceae, the recognition of 'self' and self-incompatibility are components of a receptor-ligand based mechanism that utilizes an S receptor kinase (SRK) to perceive and reject 'self'-pollen. SRK is an S-domain receptor-like kinase (RLK), which in turn is part of the RLK family, some members of which represent PRRs involved in the detection of PAMPs. S-domain RLKs also occur in species that do not exhibit self-incompatibility and are up-regulated in response to wounding, PAMPs and pathogen recognition. Although evolution may have driven expansion of certain RLK families to serve roles in particular physiological processes, this may not exclude these receptor types from functioning in different programs. Recent findings on self/nonself recognition are reviewed and conceptual and mechanistic links between microbial recognition and self-incompatibility are discussed.     

Plant receptor-like serine threonine kinases: roles in signaling and plant defense

Plants are hosts to a wide array of pathogens from all kingdoms of life. In the absence of an active immune system or combinatorial diversifications that lead to recombination-driven somatic gene flexibility, plants have evolved different strategies to combat both individual pathogen strains and changing pathogen populations. The receptor-like kinase (RLK) gene-family expansion in plants was hypothesized to have allowed accelerated evolution among domains implicated in signal reception, typically a leucine-rich repeat (LRR). Under that model, the gene-family expansion represents a plant-specific adaptation that leads to the production of numerous and variable cell surface and cytoplasmic receptors. More recently, it has emerged that the LRR domains of RLK interact with a diverse group of proteins leading to combinatorial variations in signal response specificity. Therefore, the RLK appear to play a central role in signaling during pathogen recognition, the subsequent activation of plant defense mechanisms, and developmental control. The future challenges will include determinations of RLK modes of action, the basis of recognition and specificity, which cellular responses each receptor mediates, and how both receptor and kinase domain interactions fit into the defense signaling cascades. These challenges will be complicated by the limited information that may be derived from the primary sequence of the LRR domain. The review focuses upon implications derived from recent studies of the secondary and tertiary structures of several plant RLK that change understanding of plant receptor function and signaling. In addition, the biological functions of plant and animal RLK-containing receptors were reviewed and commonalities among their signaling mechanisms identified. Further elucidated were the genomic and structural organizations of RLK gene families, with special emphasis on RLK implicated in resistance to disease and development.     

Challenges in understanding RLK function

Plants use receptor-like kinases (RLKs) to transduce extracellular signals into the cell. Recent advancements in RLK research include the cloning of the BRASSINOSTEROID INSENSITIVE 1 and CLAVATA 1 genes, revealing RLK roles in development. Our understanding of RLK function has also been broadened by transgenic approaches in the study of the RLKs pollen receptor kinase 1, and wall associated kinase 1. These results extend the observations that RLKs function in developmental processes and plant defense responses. Additionally, expression based studies suggest roles for other newly reported RLKs in development and light responses. Taken together, the studies confirm the importance of RLKs in diverse plant processes, yet major challenges remain. These include identifying ligands that activate RLKs and characterizing downstream pathways. These challenges can be conquered by coordinated efforts from investigators using molecular, genetic, and biochemical approaches.