Exploiting breakdown in nonhost effector-target interactions to boost host disease resistance

Pathogen perception and signaling in plant immunity

Plant diseases are a constant and serious threat to agriculture and ecological biodiversity. Plants possess a sophisticated innate immunity system capable of detecting and responding to pathogen infection to prevent disease. Our understanding of this system has grown enormously over the past century. Early genetic descriptions of plant disease resistance and pathogen virulence were embodied in the gene-for-gene hypothesis, while physiological studies identified pathogen-derived elicitors that could trigger defense responses in plant cells and tissues. Molecular studies of these phenomena have now coalesced into an integrated model of plant immunity involving cell surface and intracellular detection of specific pathogen-derived molecules and proteins culminating in the induction of various cellular responses. Extracellular and intracellular receptors engage distinct signaling processes but converge on many similar outputs with substantial evidence now for integration of these pathways into interdependent networks controlling disease outcomes. Many of the molecular details of pathogen recognition and signaling processes are now known, providing opportunities for bioengineering to enhance plant protection from disease. Here we provide an overview of the current understanding of the main principles of plant immunity, with an emphasis on the key scientific milestones leading to these insights.

The WY Domain of an RxLr Effector Drives Interactions with a Host Target Phosphatase to Mimic Host Regulatory Proteins and Promote Phytophthora infestans Infection

Plant pathogens manipulate the cellular environment of the host to facilitate infection and colonization that often lead to plant diseases. To accomplish this, many specialized pathogens secrete virulence proteins called effectors into the host cell, which subvert processes such as immune signaling, gene transcription, and host metabolism. Phytophthora infestans, the causative agent of potato late blight, employs an expanded repertoire of RxLR effectors with WY domains to manipulate the host through direct interaction with protein targets. However, our understanding of the molecular mechanisms underlying the interactions between WY effectors and their host targets remains limited. In this study, we performed a structural and biophysical characterization of the P. infestans WY effector Pi04314 in complex with the potato Protein Phosphatase 1-c (PP1c). We elucidate how Pi04314 uses a WY domain and a specialized C-terminal loop carrying a KVxF motif that interact with conserved surfaces on PP1c, known to be used by host regulatory proteins for guiding function. Through biophysical and in planta analyses, we demonstrate that Pi04314 WY or KVxF mutants lose their ability to bind PP1c. The loss of PP1c binding correlates with changes in PP1c nucleolar localization and a decrease in lesion size in plant infection assays. This study provides insights into the manipulation of plant hosts by pathogens, revealing how effectors exploit key regulatory interfaces in host proteins to modify their function and facilitate disease. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Novel stripe rust effector boosts the transcription of a host susceptibility factor through affecting histone modification to promote infection in wheat

Regulation of host gene expression to promote disease is a common strategy for plant pathogens. However, it remains unclear whether or not fungal pathogens manipulate host gene expression directly through secreted effectors with transcriptional activity. Here, we identified a fungal effector PstGTA1 from Puccinia striiformis f. sp. tritici (Pst), which has partial homology to the subunit of global transcriptional activator SNF2 from oyster. The transcriptional activating activity of PstGTA1 was validated in yeast, and the potential role of PstGTA1 in pathogenicity was assessed using gene silenced and overexpression transgenic wheat plants. Candidate targets regulated by PstGTA1 were screened by transcriptomic analysis, and the specific promoter region binding to PstGTA1 was further determined. PstGTA1 can be delivered to the wheat cell nucleus and contributes to the full virulence of Pst by targeting the promoter of TaSIG, a gene negatively regulating wheat immunity, and possibly activates its transcription by affecting the histone H3K4 acetylation level. Our study provides the first direct evidence for a fungal effector with transactivation activity modulating the transcription of a host specific susceptibility gene through promoter binding and histone acetylation.

RxLR Effectors: Master Modulators, Modifiers and Manipulators

Cytoplasmic effectors with an Arg-any amino acid-Arg-Leu (RxLR) motif are encoded by hundreds of genes within the genomes of oomycete Phytophthora spp. and downy mildew pathogens. There has been a dramatic increase in our understanding of the evolution, function, and recognition of these effectors. Host proteins with a wide range of subcellular localizations and functions are targeted by RxLR effectors. Many processes are manipulated, including transcription, post-translational modifications, such as phosphorylation and ubiquitination, secretion, and intracellular trafficking. This involves an array of RxLR effector modes-of-action, including stabilization or destabilization of protein targets, altering or disrupting protein complexes, inhibition or utility of target enzyme activities, and changing the location of protein targets. Interestingly, approximately 50% of identified host proteins targeted by RxLR effectors are negative regulators of immunity. Avirulence RxLR effectors may be directly or indirectly detected by nucleotide-binding leucine-rich repeat resistance (NLR) proteins. Direct recognition by a single NLR of RxLR effector orthologues conserved across multiple Phytophthora pathogens may provide wide protection of diverse crops. Failure of RxLR effectors to interact with or appropriately manipulate target proteins in nonhost plants has been shown to restrict host range. This knowledge can potentially be exploited to alter host targets to prevent effector interaction, providing a barrier to host infection. Finally, recent evidence suggests that RxLR effectors, like cytoplasmic effectors from fungal pathogen Magnaporthe oryzae, may enter host cells via clathrin-mediated endocytosis. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

"Order from disordered": Potential role of intrinsically disordered regions in phytopathogenic oomycete intracellular effector proteins

There is a continuous arms race between pathogens and their host plants. However, successful pathogens, such as phytopathogenic oomycetes, secrete effector proteins to manipulate host defense responses for disease development. Structural analyses of these effector proteins reveal the existence of regions that fail to fold into three-dimensional structures, intrinsically disordered regions (IDRs). Because of their flexibility, these regions are involved in important biological functions of effector proteins, such as effector-host protein interactions that perturb host immune responses. Despite their significance, the role of IDRs in phytopathogenic oomycete effector-host protein interactions is not clear. This review, therefore, searched the literature for functionally characterized oomycete intracellular effectors with known host interactors. We further classify regions that mediate effector-host protein interactions into globular or disordered binding sites in these proteins. To fully appreciate the potential role of IDRs, five effector proteins encoding potential disordered binding sites were used as case studies. We also propose a pipeline that can be used to identify, classify as well as characterize potential binding regions in effector proteins. Understanding the role of IDRs in these effector proteins can aid in the development of new disease-control strategies.

How to convert host plants into nonhosts

Recent research demonstrates that undermining interactions between pathogen effectors and their host target proteins can reduce infection. As more effector-target pairs are identified, their structures and interaction surfaces exposed, and there is the possibility of making multiple edits to diverse plant genomes, the desire to convert crops to nonhosts could become reality.

Symbiont-host interactome mapping reveals effector-targeted modulation of hormone networks and activation of growth promotion

Plants have benefited from interactions with symbionts for coping with challenging environments since the colonisation of land. The mechanisms of symbiont-mediated beneficial effects and similarities and differences to pathogen strategies are mostly unknown. Here, we use 106 (effector-) proteins, secreted by the symbiont Serendipita indica (Si) to modulate host physiology, to map interactions with Arabidopsis thaliana host proteins. Using integrative network analysis, we show significant convergence on target-proteins shared with pathogens and exclusive targeting of Arabidopsis proteins in the phytohormone signalling network. Functional in planta screening and phenotyping of Si effectors and interacting proteins reveals previously unknown hormone functions of Arabidopsis proteins and direct beneficial activities mediated by effectors in Arabidopsis. Thus, symbionts and pathogens target a shared molecular microbe-host interface. At the same time Si effectors specifically target the plant hormone network and constitute a powerful resource for elucidating the signalling network function and boosting plant productivity.

A tell tail sign: a conserved C-terminal tail-anchor domain targets a subset of pathogen effectors to the plant endoplasmic reticulum

The endoplasmic reticulum (ER) is the entry point to the secretory pathway and, as such, is critical for adaptive responses to biotic stress, when the demand for de novo synthesis of immunity-related proteins and signalling components increases significantly. Successful phytopathogens have evolved an arsenal of small effector proteins which collectively reconfigure multiple host components and signalling pathways to promote virulence; a small, but important, subset of which are targeted to the endomembrane system including the ER. We identified and validated a conserved C-terminal tail-anchor motif in a set of pathogen effectors known to localize to the ER from the oomycetes Hyaloperonospora arabidopsidis and Plasmopara halstedii (downy mildew of Arabidopsis and sunflower, respectively) and used this protein topology to develop a bioinformatic pipeline to identify putative ER-localized effectors within the effectorome of the related oomycete, Phytophthora infestans, the causal agent of potato late blight. Many of the identified P. infestans tail-anchor effectors converged on ER-localized NAC transcription factors, indicating that this family is a critical host target for multiple pathogens.

Cyclophilin effector Al106 of mirid bug Apolygus lucorum inhibits plant immunity and promotes insect feeding by targeting PUB33

Apolygus lucorum (Meyer-Dur; Heteroptera: Miridae) is a major agricultural pest infesting crops, vegetables, and fruit trees. During feeding, A. lucorum secretes a plethora of effectors into its hosts to promote infestation. However, the molecular mechanisms of these effectors manipulating plant immunity are largely unknown. Here, we investigated the molecular mechanism underlying the effector Al106 manipulation of plant-insect interaction by RNA interference, electrical penetration graph, insect and pathogen bioassays, protein-protein interaction studies, and protein ubiquitination experiment. Expression of Al106 in Nicotiana benthamiana inhibits pathogen-associated molecular pattern-induced cell death and reactive oxygen species burst, and promotes insect feeding and plant pathogen infection. In addition, peptidyl-prolyl cis-trans isomerase (PPIase) activity of Al106 is required for its function to inhibit PTI.Al106 interacts with a plant U-box (PUB) protein, PUB33, from N. benthamiana and Arabidopsis thaliana. We also demonstrated that PUB33 is a positive regulator of plant immunity. Furthermore, an in vivo assay revealed that Al106 inhibits ubiquitination of NbPUB33 depending on PPIase activity. Our findings revealed that a novel cyclophilin effector may interact with plant PUB33 to suppress plant immunity and facilitate insect feeding in a PPIase activity-dependent manner.

Tuning the Wavelength: Manipulation of Light Signaling to Control Plant Defense

The growth-defense trade-off in plants is a phenomenon whereby plants must balance the allocation of their resources between developmental growth and defense against attack by pests and pathogens. Consequently, there are a series of points where growth signaling can negatively regulate defenses and where defense signaling can inhibit growth. Light perception by various photoreceptors has a major role in the control of growth and thus many points where it can influence defense. Plant pathogens secrete effector proteins to manipulate defense signaling in their hosts. Evidence is emerging that some of these effectors target light signaling pathways. Several effectors from different kingdoms of life have converged on key chloroplast processes to take advantage of regulatory crosstalk. Moreover, plant pathogens also perceive and react to light in complex ways to regulate their own growth, development, and virulence. Recent work has shown that varying light wavelengths may provide a novel way of controlling or preventing disease outbreaks in plants.

Recent developments in plant-downy mildew interactions

Downy mildews are obligate oomycete pathogens that attack a wide range of plants and can cause significant economic impacts on commercial crops and ornamental plants. Traditionally, downy mildew disease control relied on an integrated strategies, that incorporate cultural practices, deployment of resistant cultivars, crop rotation, application of contact and systemic pesticides, and biopesticides. Recent advances in genomics provided data that significantly advanced understanding of downy mildew evolution, taxonomy and classification. In addition, downy mildew genomics also revealed that these obligate oomycetes have reduced numbers of virulence factor genes in comparison to hemibiotrophic and necrotrophic oomycetes. However, downy mildews do deploy significant arrays of virulence proteins, including so-called RXLR proteins that promote virulence or are recognized as avirulence factors. Pathogenomics are being applied to downy mildew population studies to determine the genetic diversity within the downy mildew populations and manage disease by selection of appropriate varieties and management strategies. Genome editing technologies have been used to manipulate host disease susceptibility genes in different plants including grapevine and sweet basil and thereby provide new soucres of resistance genes against downy mildews. Previously, it has proved difficult to transform and manipulate downy mildews because of their obligate lifestyle. However, recent exploitation of RNA interference machinery through Host-Induced Gene Silencing (HIGS) and Spray-Induced Gene Silencing (SIGS) indicate that functional genomics in downy mildews is now possible. Altogether, these breakthrough technologies and attendant fundamental understanding will advance our ability to mitigate downy mildew diseases.