The diverse roles of NB-LRR proteins in plants

Plant innate immunity relies on specialised immune receptors that can detect and defend against a wide variety of microbes. The first group of receptors comprises the transmembrane pathogen- or pattern-recognition receptors (PRRs), which respond to slowly evolving pathogen- or microbe-associated molecular patterns (PAMPs/MAMPs). The second group of immune receptors is formed by the polymorphic disease resistance (R) proteins that detect microbe-derived effector proteins. Most R proteins are members of the nucleotide binding leucine-rich repeat (NB-LRR) class. Although this class comprises one of the biggest protein families in plants, relatively few have been functionally characterised to date. The question rises whether all NB-LRRs function as immune receptors, or that they might have alternative functions. The answer is: yes, they do have alternative functions that are different from the immune receptor function. This review summarises the current knowledge about non-immune receptor signal transduction functions of NB-LRRs in plants.     

Post-translational modifications in effectors and plant proteins involved in host–pathogen conflicts

Molecular interplay between two species is largely driven by protein–protein interactions and protein modifications that set the pace of co-evolution in these species. During host–pathogen interactions, proteins involved in virulence and defence impart tempospatial dynamic post-translational modifications (PTMs) to gain advantage for the causative species. Pathogens mainly cause disease in plant hosts by secreting elicitors (peptides and small molecules) or proteins in the inter- and intracellular space of host cells. These pathogen proteins have evolved a wide array of sophisticated mechanisms to manipulate host responses, including resistance. Through a set of diverse events ranging from PTMs to post-translational oligomerization, these proteins are able to enhance virulence and suppress the otherwise elaborate plant immune system. Similarly, PTMs adapted by host proteins often lead to the activation of a robust defence response. Insights into the PTMs of pathogen and host proteins are therefore germane to the understanding of the co-evolutionary arms race. This review summarizes the characterization of PTMs in pathogen effectors and their target host proteins. Based on this, a metaphorical view of host–pathogen conflicts is proposed, where PTMs act as molecular pivots in a 3D combinatorial game model – a novel abstraction of the arms race, where these molecular pivots restore the balance of competition between the two organisms.     

Emerging roles of molecular chaperones in plant innate immunity

Molecular chaperones and co-chaperones are proteins that aid in the folding and assembly of other macromolecular structures or complexes. Although growing evidence suggests that molecular chaperones and co-chaperones play critical roles in plant innate immunity, the molecular mechanism of how these proteins contribute to defense signaling remains largely unknown. In this review, we highlight the cytoplasmic chaperones, endoplasmic reticulum (ER) resident lectin chaperones, and enzymes that have been identified from genetic and “omic” approaches and shown to be involved in plant–microbe interactions. We also discuss the roles of molecular chaperones in plant innate immunity and emerging mechanisms underlying the biogenesis of plant pathogen-associated molecular pattern receptors in the ER.     

Molecular basis and regulatory mechanisms underlying fungal insecticides' resistance to solar ultraviolet irradiation

Resistance to solar ultraviolet (UV) irradiation is crucial for field-persistent control efficacies of fungal formulations against arthropod pests, because their active ingredients are formulated conidia very sensitive to solar UV wavelengths. This review seeks to summarize advances in studies aiming to quantify, understand and improve conidial UV resistance. One focus of studies has been on the many sets of genes that have been revealed in the postgenomic era to contribute to or mediate UV resistance in the insect pathogens serving as main sources of fungal insecticides. Such genetic studies have unveiled the broad basis of UV-resistant molecules including cytosolic solutes, cell wall components, various antioxidant enzymes, and numerous effectors and signaling proteins, that function in developmental, biosynthetic and stress-responsive pathways. Another focus has been on the molecular basis and regulatory mechanisms underlying photorepair of UV-induced DNA lesions and photoreactivation of UV-impaired conidia. Studies have shed light upon a photoprotective mechanism depending on not only one or two photorepair-required photolyases, but also two white collar proteins and other partners that play similar or more important roles in photorepair via interactions with photolyases. Research hotspots are suggested to explore a regulatory network of fungal photoprotection and to improve the development and application strategies of UV-resistant fungal insecticides. © 2021 Society of Chemical Industry.     

Expression-based genotyping of the rice blast resistance genes in the elite maintainer line Yixiang1B

Plants employ extracellular immune receptors to perceive conserved pathogen-associated molecular patterns (PAMPs), triggering the first layer of defense known as pattern-triggered immunity (PTI). The understanding of PTI is mainly based on studies focusing on leaves. Plants are vulnerable to attack by various root pathogens including plant-parasitic nematodes. Evidence is accumulating that phytonematodes utilize their secreted effectors to suppress PTI to enable infection. PTI assays used for characterizing nematode effectors are often conducted in a non-host plant or tissue, such as leaves, because of lacking of root assays. Thus, there is a need for PTI assays in roots of host plants. Here, we tested two bacterial PAMPs (flg22 and flgII-28) and two nonpathogenic bacteria (Pseudomonas fluorescens and P. syringae strain DC3000 ΔhrcQ-U) for their ability to induce PTI responses, including the induction of defense gene expression and callose deposition, in roots of tomato and potato. We found that flg22 and the two nonpathogenic bacteria are potent in inducing defense gene expression and callose deposition in tested roots, demonstrating for the first time induction of PTI in roots of solanaceous plants. Effectors GrCEP12 and Hs10A06 were previously indicated to be involved in PTI suppression. Consistently, upon elicitor treatment, roots of transgenic plants overexpressing GrCEP12 and Hs10A06, respectively, showed a reduced level of defense gene expression or no induction of callose deposition compared to control roots. Taken together, our established root PTI assays represent a valuable tool that will facilitate the study of phytonematodes and potentially other root pathogens in their manipulation of plant immunity.     

Classical and molecular genetics of <Emphasis Type="Italic">Bremia lactucae</Emphasis>, cause of lettuce downy mildew

Lettuce downy mildew caused by Bremia lactucae has long been a model for understanding biotrophic oomycete–plant interactions. Initial research involved physiological and cytological studies that have been reviewed earlier. This review provides an overview of the genetic and molecular analyses that have occurred in the past 25 years as well as perspectives on future directions. The interaction between B. lactucae and lettuce (Lactuca sativa) is determined by an extensively characterized gene-for-gene relationship. Resistance genes have been cloned from L. sativa that encode proteins similar to resistance proteins isolated from other plant species. Avirulence genes have yet to be cloned from B. lactucae, although candidate sequences have been identified on the basis of motifs present in secreted avirulence proteins characterized from other oomycetes. Bremia lactucae has a minimum of 7 or 8 chromosome pairs ranging in size from 3 to at least 8 Mb and a set of linear polymorphic molecules that range in size between 0.3 and 1.6 Mb and are inherited in a non-Mendelian manner. Several methods indicated the genome size of B. lactucae to be ca. 50 Mb, although this is probably an underestimate, comprising approximately equal fractions of highly repeated sequences, intermediate repeats, and low-copy sequences. The genome of B. lactucae still awaits sequencing. To date, several EST libraries have been sequenced to provide an incomplete view of the gene space. Bremia lactucae has yet to be transformed, but regulatory sequences from it form components of transformation vectors used for other oomycetes. Molecular technology has now advanced to the point where rapid progress is likely in determining the molecular basis of specificity, mating type, and fungicide insensitivity.     

Accumulation of pathogenesis-related proteins in the epidermis of tomato leaves infected by Cladosporium fulvum

Upon infection byCladosporium fulvum, tomato plants start to produce pathogenesis-related (PR) proteins. The PR proteins 1,3-β-glucanase, chitinase, and PR-1b accumulated near the stomata in the lower epidermis ofC. fulvum-inoculated tomato leaves as could be determined by immunolocalization with polyclonal antibodies. However, no differences in accumulation of PR proteins between a compatible and an incompatible interaction were found. Results obtained from enzyme activity measurements of 1,3-β-glucanase and chitinase on similar leaf material as used for the immunolocalization did not fully reflect the immunolocalization data. The antibodies possibly detect only the extracellular but not the intracellular enzymes. The accumulation of PR proteins near the stomata might be part of a general defence response of plants against pathogens and potential pathogens.     

稻曲病菌SIX效应蛋白参与调控植物免疫

为明确稻曲病菌Ustilaginoidea virens的SIX(secreted in xylem)效应蛋白功能,利用生物信息学软件预测6个SIX1蛋白和1个SIX2蛋白的分泌特性,并通过酵母分泌系统对其分泌特性进行验证,在烟草上通过根癌农杆菌Agrobacterium tumefaciens介导的瞬时表达系统检测SIX蛋白对小鼠促凋亡蛋白BAX、致病疫霉Phytophthora infestans激发子INF1、大豆疫霉Phytophthora sojae病原相关分子模式糖基水解酶XEG1引起烟草细胞坏死的作用,在烟草上表达SIX蛋白后接种辣椒疫霉Phytophthora capsici并观察其对辣椒疫霉侵染的作用。结果显示:所检测的SIX蛋白中有5个含有预测的信号肽,2个被预测为非经典分泌蛋白,但经酵母分泌系统验证均具有分泌功能;大部分SIX蛋白能够抑制BAX、INF1、XEG1引起的烟草细胞坏死,但抑制坏死的情况并不完全一致;大部分含信号肽的SIX蛋白能够促进辣椒疫霉侵染,UV8b＿3638去信号肽突变体也能够促进辣椒疫霉侵染。表明大多数SIX蛋白参与调控植物免疫,且能促进病原...     

害虫对Bt杀虫蛋白抗性相关受体蛋白互作网络分析

为对Bt杀虫蛋白与化学杀虫剂的协同使用提供理论指导，通过蛋白互作分析方法，以黑腹果蝇Drosophila melanogaster为模型，对化学杀虫剂与Bt杀虫剂作用受体以及抗性相关蛋白的互作网络进行研究。结果表明，Bt杀虫蛋白的27个受体在黑腹果蝇中共存在50个互作蛋白，其中有39个蛋白只与Bt杀虫蛋白抗性相关，11个蛋白还与化学杀虫剂抗性相关，这2类杀虫剂抗性互作网络交集较小，并且重叠蛋白的变异引起交互抗性的概率不大。表明Bt杀虫蛋白与化学杀虫剂产生交互抗性的概率较小，可以将Bt杀虫剂与化学杀虫剂协同使用，提高害虫防治效果，并有效克服或延缓害虫抗性产生。     

Components of different signalling pathways regulated by a new orthologue of AtPROPEP1 in tomato following infection by pathogens

Plants express different defence mechanisms in response to pathogens. Understanding the recognition of pathogen‐associated molecular patterns (PAMPs) by specific receptors, and the role of endogenous signals such as AtPep1 that regulate expression of genes in Arabidopsis thaliana, has aided the understanding of the defence mechanisms in different species. The aim of this study was to identify possible orthologous sequences of AtPROPEPs in tomato (Solanum lycopersicum) and characterize its role in resistance to necrotrophic pathogens. The presence of an orthologue of the A. thaliana AtPROPEP1 gene in S. lycopersicum, SlPROPEP, by in silico analysis, is reported here. This has 96% identity with the C‐terminal region of a previously described potato peptide, another possible orthologue of AtPep1. A virus‐induced gene silencing (VIGS) system was employed to investigate the role of the SlPROPEP. Silencing of SlPROPEP in tomato made plants more susceptible to Pythium dissotocum; approximately 30% of SlPROPEP‐silenced plants showed stem constriction compared with 4% in control plants. Furthermore, quantification of P. dissotocum by qPCR revealed that the increase in symptom severity in SlPROPEP‐silenced plants was associated with a 15 times increase in growth of the pathogen compared to control plants. Silencing of SlPROPEP also resulted in decreased expression of genes involved in plant defence against pathogens, such as PR‐1, PR‐5, ERF1, LOX‐D and DEF2. These results suggest that SlPROPEP is involved in tomato resistance to P. dissotocum and probably acts as a pathogen‐associated molecular pattern through signalling pathways mediated by jasmonic acid/ethylene (JA/ET).     

Toxigenicity and pathogenicity of <Emphasis Type="Italic">Fusarium poae</Emphasis> and <Emphasis Type="Italic">Fusarium avenaceum</Emphasis> on wheat

In a field experiment between 2004 and 2006, 14 winter wheat varieties were inoculated with either a mixture of three isolates of F. poae or a mixture of three isolates of F. avenaceum. In a subsequent climate chamber experiment, the wheat variety Apogee was inoculated with individual single conidium isolates derived from the original poly conidium isolates used in the field. Disease symptoms on wheat heads were visually assessed, and the yield as well as the fungal incidence on harvested grains (field only) was determined. Furthermore, grains were analysed using LC-MS/MS to determine the content of Fusarium mycotoxins. In samples from field and climate chamber experiments, 60 to 4,860 μg kg⁻¹ nivalenol and 2,400 to 17,000 μg kg⁻¹ moniliformin were detected in grains infected with F. poae and F. avenaceum, respectively. Overall, isolate mixtures and individual isolates of F. avenaceum proved to be more pathogenic than those of F. poae, leading to a higher disease level, yield reductions up to 25%, and greater toxin contamination. For F. poae, all variables except for yield were strongly influenced by variety (field) and by isolate (climate chamber). For F. avenaceum, variety had a strong effect on all variables, but isolate effects on visual disease were not reflected in toxin production. Correlations between visual symptoms, fungal incidence, and toxin accumulation in grains are discussed.     

Tolerance to a non-host isolate of Verticillium dahliae in tomato

Tolerance to Verticillium spp. is a condition in which a host plant develops few symptoms despite substantial colonization by the pathogen. In the present paper we have shown that Craigella tomatoes are tolerant to a non-host isolate of V. dahliae, Dvd E6. Symptom expression was used to quantify disease and quantitative PCR to assess the amount of fungus in the stems. The classical incompatible and compatible interactions between Craigella resistant or Craigella susceptible near isolines and V. dahliae, race 1 were used for comparative purposes. Additional experiments using cytological assessment and quantitative PCR showed that in the tolerant interactions one plant defence response, vascular coating, was deployed as effectively as in resistant plants, limiting pathogen distribution. However, a second defence response, which causes the cyclical elimination of fungus from the stem in the classical interactions either does not occur or is substantially delayed in tolerant plants. Thus, the Verticillium population remains stable and substantial throughout the studied time course.     

Hrp-dependent biotrophic mechanism of virulence: How has it evolved in tumorigenic bacteria?

A mechanism of virulence mediated byhrp-genes is present in many Gram-negative bacterial pathogens. It involves delivery of effector proteins into host cellsvia the type III secretion system (TTSS) and the interaction of TTSS effectors with plant proteins. These interactions may either promote responses beneficial to the pathogen or trigger the hypersensitive response if an effector is recognized by corresponding resistance protein.Pantoea agglomerans, which is widespread in nature mainly as an epiphyte, has evolved into ahrp-dependent and host-specific tumorigenic pathogen by acquiring a plasmid containing a pathogenicity island (PAI). This PAI harbors ahrp-gene cluster, and genes encoding for TTSS effector proteins and biosynthesis of IAA and cytokinins. The results reviewed describe how the interplay between negative-acting and positive-acting TTSS effectors determines the transformation ofP. agglomerans into two related pathovars. Furthermore, the PAI’s structure supports the premise that these pathovars are recently evolved pathogens. Finally, the possible interaction between TTSS effectors and phytohormones for gall formation is proposed.     

Identification of wheat blue dwarf phytoplasma effectors targeting plant proliferation and defence responses

The identification of effectors from pathogenic microbes is one of the most important subjects for elucidating infection mechanisms. Wheat blue dwarf (WBD) phytoplasma causes dwarfism, witches' broom, and yellow leaf tips in wheat plants, resulting in severe yield loss in northwestern China. In this study, 37 candidate effector proteins were transiently expressed in Nicotiana benthamiana. Plants expressing the SAP11‐like protein SWP1 exhibited typical witches' broom. Interestingly, another protein, SWP11, induced both cell death and defence responses, including H₂O₂ accumulation and callose deposition. Analysis by qRT‐PCR was used to show that a marker gene of the hypersensitive response, HIN1, and three pathogenesis‐related genes, PR1, PR2 and PR3, were significantly up‐regulated in leaves of N. benthamiana expressing SWP11. In addition, SWP12 and SWP21 (TENGU‐like) were shown to suppress SWP11‐, BAX‐, and/or INF1‐induced cell death. These results indicated that SWP21 has a distinct role in virulence compared with TENGU and that WBD phytoplasma possesses effectors that target plant proliferation and defence responses. The ability of these effectors to trigger or suppress plant immunity provides new insights into the phytoplasma–plant interaction.     

Target the core: durable plant resistance against filamentous plant pathogens through effector recognition

Plant pathogens colonize their host through the secretion of effector proteins that modulate plant metabolism and immune responses to their benefit. Plants evolve towards effector recognition, leading to host immunity. Typically, pathogen effectors are targets for recognition through plant receptors that are encoded by resistance genes. Resistance gene mediated crop immunity puts a tremendous pressure on pathogens to adapt and alter their effector repertoire to overcome recognition. We argue that the type of effector that is recognized by the host may have considerable implications on the durability of resistance against filamentous plant pathogens. Effector genes that are conserved among pathogens and reside in core genome regions are most likely to hold indispensable virulence functions. Consequently, the cost for the pathogen to overcome recognition by the host is higher than for diversified, host‐specific effectors with a quantitative impact on virulence. Consequently, resistance genes that directly target conserved effector proteins without the interception of other effector proteins are potentially excellent resistance resources. © 2019 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.