Receptor-like cytoplasmic kinases are pivotal components in pattern recognition receptor-mediated signaling in plant immunity

Innate immunity is generally initiated with recognition of conserved pathogen-associated molecular patterns (PAMPs). PAMPs are perceived by pattern recognition receptors (PRRs), leading to activation of a series of immune responses, including the expression of defense genes, ROS production and activation of MAP kinase. Recent progress has indicated that receptor-like cytoplasmic kinases (RLCKs) are directly activated by ligand- activated PRRs and initiate pattern -triggered immunity (PTI) in both Arabidopsis and rice. To suppress PTI, pathogens inhibit the RLCKs by many types of effectors, including AvrAC, AvrPphB and Xoo1488. In this review, we summarize recent advances in RLCK-mediated PTI in plants.     

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.     

Receptor-like protein kinases: the keys to response

Plants are constantly challenged by changes in temperature, light, nutrient conditions, and exposure to pathogens and by other fluctuations in their environment. The molecular basis of how plants respond to these external factors is an active area of investigation. Plant cells often use receptors at the cell surface to sense environmental changes, and then transduce this information via activated signaling pathways to trigger adaptive responses. In Arabidopsis, the receptor-like protein kinase (RLK) gene family contains more than 600 members, many of which are likely to respond to the external challenges presented by an ever-changing environment. RLKs are involved in hormonal response pathways, cell differentiation, plant growth and development, self-incompatibility, and symbiont and pathogen recognition.     

New faces in plant innate immunity: heterotrimeric G proteins

Co-existence of species seems to inevitably result in origin of parasitism and hence development of molecular mechanisms of attack and defense. Certain similarities between plant and animal defense systems point to an ancient inheritance of the innate immunity. Heterotrimeric G proteins are structurally conserved signaling molecules connecting plasma membrane bound receptors to cytoplasmic effectors. They were found in most eukaryotic organisms. Their role in human pathophysiology and animal diseases was well established. In plants these proteins were also recently implicated in innate immunity. However, molecular mechanisms governed by G proteins and providing resistance against plant pathogens seem to be different from those in animal systems and largely remain elusive. In this review we attempted to sketch current ideas of plant defense system and to present a contemporary status of heterotrimeric G proteins in plant innate immunity.     

Ubiquitination of pattern recognition receptors in plant innate immunity

Lacking an adaptive immune system, plants largely rely on plasma membrane‐resident pattern recognition receptors (PRRs) to sense pathogen invasion. The activation of PRRs leads to the profound immune responses that coordinately contribute to the restriction of pathogen multiplication. Protein post‐translational modifications dynamically shape the intensity and duration of the signalling pathways. In this review, we discuss the specific regulation of PRR activation and signalling by protein ubiquitination, endocytosis and degradation, with a particular focus on the bacterial flagellin receptor FLS2 (flagellin sensing 2) in Arabidopsis.     

R proteins as fundamentals of plant innate immunity

Plants are attacked by a wide spectrum of pathogens, being the targets of viruses, bacteria, fungi, protozoa, nematodes and insects. Over the course of their evolution, plants have developed numerous defense mechanisms including the chemical and physical barriers that are constitutive elements of plant cell responses locally and/or systemically. However, the modern approach in plant sciences focuses on the evolution and role of plant protein receptors corresponding to specific pathogen effectors. The recognition of an invader’s molecules could be in most cases a prerequisite sine qua non for plant survival. Although the predicted three-dimensional structure of plant resistance proteins (R) is based on research on their animal homologs, advanced technologies in molecular biology and bioinformatics tools enable the investigation or prediction of interaction mechanisms for specific receptors with pathogen effectors. Most of the identified R proteins belong to the NBS-LRR family. The presence of other domains (including the TIR domain) apart from NBS and LRR is fundamental for the classification of R proteins into subclasses. Recently discovered additional domains (e.g. WRKY) of R proteins allowed the examination of their localization in plant cells and the role they play in signal transduction during the plant resistance response to biotic stress factors. This review focuses on the current state of knowledge about the NBS-LRR family of plant R proteins: their structure, function and evolution, and the role they play in plant innate immunity.     

Receptor-like kinases take center stage in plant biology

正Like the animals,higher plants must perceive and interpret numerous external and endogenous cues to properly program growth and development and physiological responses.Animal cells use diverse receptors to perceive external signals to regulate cellular processes.Thus,receptor-tyrosine kinases(RTKs)are responsible for perceiving growth regu-     

Pathogen-induced defense and innate immunity in macroalgae

Animals and vascular plants are known to defend themselves facultatively against pathogens, with innate receptors mediating their resistance. Macroalgal defense against microorganisms, in contrast, has until recently been regarded mainly as constitutive. Indeed, many macroalgae appear to be chemically defended at constantly high levels, and this is possibly one of the reasons why the first evidence of pathogen-aroused resistance in a macroalga was detected only a decade ago. Here, I summarize the results of studies that indicate the existence of pathogen-activated or pathogen-induced macroalgal defense. Most indications so far come from molecular investigations, which revealed major functional similarities among the defense systems of distant macroalgal clades and the innate immune systems of vascular plants and metazoans. Homologies exist in the primary and secondary defense-activating signals, as well as in the enzymes that are involved and the cellular responses that are activated. This strongly suggests that innate immunity also exists in relatively distinct macroalgal clades. However, a macroalgal receptor still needs to be isolated and characterized, and the molecular concept of macroalgal receptor-mediated immunity needs to be complemented with an ecological perspective on pathogen-induced defense, to develop a joint neuroecological perspective on seaweed-microbe interactions.     

The role of NDR1 in pathogen perception and plant defense signaling

The biochemical and cellular function of NDR1 in plant immunity and defense signaling has long remained elusive. Herein, we describe a novel role for NDR1 in both pathogen perception and plant defense signaling, elucidated by exploring a broader, physiological role for NDR1 in general stress responses and cell wall adhesion. Based on our predictive homology modeling, coupled with a structure-function approach, we found that NDR1 shares a striking similarity to mammalian integrins, well-characterized for their role in mediating the interaction between the extracellular matrix and stress signaling. ndr1-1 mutant plants exhibit higher electrolyte leakage following pathogen infection, compared to wild type Col-0. In addition, we observed an altered plasmolysis phenotype, supporting a role for NDR1 in maintaining cell wall-plasma membrane adhesions through mediating fluid loss under stress.Key words: NDR1, integrin, RGD, plant defenseNON-RACE SPECIFIC DISEASE RESISTANCE-1(NDR1) was first identified as playing an essential role in plant defense activation following the perception of the bacterial phytopathogen Pseudomonas syringae pv tomato DC3000.¹ Over the past decade, the role of NDR1 in plant defense signaling has emerged through the elucidation of the genetic interactions in which NDR1 participates. This includes the activation of the coiled-coil nucleotide binding site leucine rich repeat (CC-NB-LRR) family of resistance (R) proteins. In parallel, the function of ENHANCED DISEASE SUCEPTIBILITY-1 (EDS1 ²) has been extensively described through its genetic and biochemical relationship with the activation of Toll-Interleukin Receptor (TIR)-NB-LRR R-proteins.³ As central activators required for defense signaling, NDR1 and EDS1 represent critical nodes required for the activation of host resistance.     

Pathogen recognition and innate immunity

Microorganisms that invade a vertebrate host are initially recognized by the innate immune system through germline-encoded pattern-recognition receptors (PRRs). Several classes of PRRs, including Toll-like receptors and cytoplasmic receptors, recognize distinct microbial components and directly activate immune cells. Exposure of immune cells to the ligands of these receptors activates intracellular signaling cascades that rapidly induce the expression of a variety of overlapping and unique genes involved in the inflammatory and immune responses. New insights into innate immunity are changing the way we think about pathogenesis and the treatment of infectious diseases, allergy, and autoimmunity.     

Pattern recognition molecules and innate immunity to parasites

Recent pioneering advances in understanding how plants, insects and worms eliminate pathogens has led to the realization that innate immunity plays a vital role in protecting humans from infection. This comprehensive review examines the molecules involved in innate immune responses, how they act to control parasites and if their engagement can explain many immune features characteristic of parasitic infections.     

Autophagy and plant innate immunity

Plant innate immunity is often associated with specialized programmed cell death at or near the site of pathogen infection. Despite the isolation of several lesion mimic mutants, the molecular mechanisms that regulate cell death during an immune response remain obscure. Recently, autophagy, an evolutionarily conserved process of bulk protein and organelle turnover, was shown to play an important role in limiting cell death initiated during plant innate immune responses. Consistent with its role in plants, several studies in animals also demonstrate that the autophagic machinery is involved in innate as well as adaptive immunities. Here, we review the role of autophagy in plant innate immunity. Because autophagy is observed in healthy and dying plant cells, we will also examine whether autophagy plays a protective or a destructive role during an immune response.     

Sugars and plant innate immunity

Sugars are involved in many metabolic and signalling pathways in plants. Sugar signals may also contribute to immune responses against pathogens and probably function as priming molecules leading to pathogen-associated molecular patterns (PAMP)-triggered immunity and effector-triggered immunity in plants. These putative roles also depend greatly on coordinated relationships with hormones and the light status in an intricate network. Although evidence in favour of sugar-mediated plant immunity is accumulating, more in-depth fundamental research is required to unravel the sugar signalling pathways involved. This might pave the way for the use of biodegradable sugar-(like) compounds to counteract plant diseases as cheaper and safer alternatives for toxic agrochemicals.     

AGC kinases in plant development and defense

More than 100,000 publications demonstrate that AGC kinases are important regulators of growth, metabolism, proliferation, cell divison, survival and apoptosis in mammalian systems.¹ Mutation and/or dysregulation of these kinases contribute to the pathogenesis of many human diseases, including cancer and diabetes. Although AGC kinases are also present in plants, little is known about their functions. We demonstrated that the AGC kinase OXIDATIVE SIGNAL-INDUCIBLE1 (OXI1/AGC2-1) regulate important developmental processes and defense responses in plants. The summary of recent progress also demonstrates that we are only beginning to understand the role of this kinase pathway in plants.Key words: AGC kinases, reactive oxygen species, plant stress, plant microbe interaction, plant pathogen     

Peptidoglycan recognition proteins: a novel family of four human innate immunity pattern recognition molecules.

The innate immune system recognizes microorganisms through a series of pattern recognition receptors that are highly conserved in evolution. Insects have a family of 12 peptidoglycan recognition proteins (PGRPs) that recognize peptidoglycan, a ubiquitous component of bacterial cell walls. We report cloning of three novel human PGRPs (PGRP-L, PGRP-Ialpha, and PGRP-Ibeta) that together with the previously cloned PGRP-S, define a new family of human pattern recognition molecules. PGRP-L, PGRP-Ialpha, and PGRP-Ibeta have 576, 341, and 373 amino acids coded by five, seven, and eight exons on chromosomes 19 and 1, and they all have two predicted transmembrane domains. All mammalian and insect PGRPs have at least three highly conserved C-terminal PGRP domains located either in the extracellular or in the cytoplasmic (or in both) portions of the molecules. PGRP-L is expressed in liver, PGRP-Ialpha and PGRP-Ibeta in esophagus (and to a lesser extent in tonsils and thymus), and PGRP-S in bone marrow (and to a lesser extent in neutrophils and fetal liver). All four human PGRPs bind peptidoglycan and Gram-positive bacteria. Thus, these PGRPs may play a role in recognition of bacteria in these organs.     

Trained immunity: a memory for innate host defense

Immune responses in vertebrates are classically divided into innate and adaptive, with only the latter being able to build up immunological memory. However, although lacking adaptive immune responses, plants and invertebrates are protected against reinfection with pathogens, and invertebrates even display transplant rejection. In mammals, past "forgotten" studies demonstrate cross-protection between infections independently of T and B cells, and more recently memory properties for NK cells and macrophages, prototypical cells of innate immunity, have been described. We now posit that mammalian innate immunity also exhibits an immunological memory of past insults, for which we propose the term "trained immunity." Understanding trained immunity will revolutionize our view of host defense and immunological memory, and could lead to defining a new class of vaccines and immunotherapies.