A Phytophthora infestans RXLR effector targets a potato ubiquitin-like domain-containing protein to inhibit the proteasome activity and hamper plant immunity

An unconventional effector MoRpa12 targeting host nuclei is essential for the development and pathogenicity of Magnaporthe oryzae

RNA polymerase I (Pol I) is a multi-subunit protein complex associated with the transcription of most ribosomal RNA molecules in all eukaryotes. Rpa12 is a small subunit of the Pol I catalytic core and plays a critical role in RNA cleavage, transcription initiation and elongation during proliferation in yeast and mammals. However, the function of Rpa12 in phytopathogenic fungi has not yet been characterized. Here, we present the functional characterization of MoRpa12, a homologue of the yeast Rpa12, in Magnaporthe oryzae. MoRpa12 shows upregulation during the infection phase, and MoRpa12-GFP exhibits nuclear localization at different developmental stages of M. oryzae and translocates into the nuclei of plant cells after fungal penetration. The MoRpa12 mutants also exhibit significant defects on mitosis, autophagy, oxidative stress tolerance, cell wall integrity, septin ring assembly, lipid and glycogen metabolism, and pathogenicity. The four cysteine residues at the amino terminus of this protein are critical for the nuclear localization of MoRpa12, and their site-directed mutagenesis affects the localization, fungal invasion, and full virulence of M. oryzae. In conclusion, our findings indicate that MoRpa12 functions as an unconventional secreted effector targeting host nuclei and is essential for the fungal growth and plant infection of M. oryzae.

Transcriptomic analysis of Phytophthora infestans races and evaluation of their pathogenicity on potato

Phytophthora infestans causes potato late blight and significantly impacts potato production. The pathogen's remarkable adaptability and ability to generate new physiological races through virulence variation undermine varietal resistance, posing challenges for disease prevention and control. To explore the genetic mechanisms behind these, different physiological races of P. infestans were inoculated on potato leaves and assayed through transcriptomics combined with metabolic profiling methods. It was found that the DL04 strain, which carries virulence factor 3, exhibited a high level of pathogenicity. Biologically, DL04 showed more rapid growth and denser mycelial structures compared to most other strains, indicating enhanced pathogenicity. DL04 induced a greater enrichment of carbon metabolism, biosynthesis of amino acids, and glycolysis/gluconeogenesis pathways during the infection process. This led to the upregulation of genes related to cell hydrolysis, accelerating leaf infection and contributing to its higher level of pathogenicity. The reasons for the differences in pathogenicity among different physiological races of P. infestans were clarified at the transcriptional level. This finding provides valuable insights into the genetic basis of P. infestans pathogenicity and offer critical information for developing effective control strategies, breeding for disease resistance, and improving potato production practice.

High-quality assembly of the chromosomal genome for Flemingia macrophylla reveals genomic structural characteristics

Flemingia macrophylla, a prominent shrub species within the Fabaceae family, is widely distributed across China and Southeast Asia. In addition to its ecological importance, it possesses notable medicinal value, with its roots traditionally used for treating rheumatism, enhancing blood circulation, and alleviating joint pain. We employed Nanopore sequencing platforms to generate a high-quality reference genome for F. macrophylla, with an assembled genome size of 1.01 Gb and a contig N50 of 59.43 Mb. A total of 33,077 protein-coding genes were predicted, and BUSCO analysis indicated a genome completeness of 99%. Phylogenomic analyses showed that F. macrophylla is most closely related to Cajanus cajan among the sampled taxa, with an estimated divergence time of 13.2-20.0 MYA. Evidence of whole-genome duplication (WGD) events was detected in F. macrophylla, C. cajan, and P. vulgaris, with these species sharing two WGD events. The unique gene families in F. macrophylla are associated with strong resistance to both abiotic and biotic stress, supporting its remarkable ecological adaptability. Furthermore, gene family expansion analysis revealed a significant enrichment of genes related to secondary metabolites biosynthesis, providing a molecular basis for its high medicinal value. In summary, this study provides a foundational genomic resource for F. macrophylla, offering valuable insights into its genetic architecture, evolutionary history, and potential applications in medecine and agriculture. The comprehensive analyses lay the groundwork for future research into the species's medicinal properties and evolutionary biology.

Ubiquitin Receptor RPN13-Mediated "Candidatus Liberibacter asiaticus" Virulence Effector Degradation to Positively Regulate Immunity

Ubiquitin-proteasome is a conserved mechanism that regulates cellular responses and disease resistance in plants. However, the regulation role of ubiquitin-proteasome in the pathogenicity of "Candidatus Liberibacter asiaticus" (CLas), the causal agent of citrus Huanglongbing, one of the most serious citrus diseases, remains poorly defined. In this study, we identified a CLas effector, SDE5640 (CLIBASIA_05640), which downregulates salicylic acid signaling pathway genes and partial 26S proteasome genes in SDE5640-transgenic citrus shoots. SDE5640 suppresses proteasome activity to promote bacterial infection. Intriguingly, RPN13, a known component of the proteasome, was first identified as a new ubiquitin receptor. Overexpression of NbRPN13 leads to the degradation of SDE5640 via the 26S proteasome system. We further revealed that SDE5640 can be ubiquitinated in planta and that NbRPN13 promotes the degradation of SDE5640 by binding to ubiquitinated SDE5640. Furthermore, transient coexpression of SDE5640 and CsRPN13 enhances citrus resistance against Xanthomonas. Together, these results demonstrate that RPN13 recognizes the ubiquitination site of SDE5640 and specifically degrades it using the ubiquitin-proteasome system, which provides evidence for the role of ubiquitin-proteasome in CLas-citrus interactions.

Genome-wide screening for virulent candidate secreted effector protein macromolecules in Magnaporthe oryzae

Rice blast, caused by Magnaporthe oryzae (M. oryzae), is a severe threat to rice production globally. The pathogen counters rice immunity by secreting effectors that disrupt host defenses. In this study, we conducted a comprehensive genome-wide screening to identify candidate secreted effector proteins (CSEPs) in M. oryzae. Using a new bioinformatics pipeline, we predicted 577 CSEPs and analyzed their sequence features and functional annotations. We found that these effectors have distinct sequence signatures, such as high cysteine content, and are involved in infection and immune suppression. Phylogenetic analysis revealed M. oryzae's close relationship with other pathogenic fungi and the conservation of certain CSEPs across species. Expression analysis during infection indicated a role of CSEPs in the pathogenic process and the ability to inhibit plant necrosis. Finally, we validated the function of three candidate effector proteins through gene disruption mutant analysis including pathogenesis testing in rice. This study provides a foundation for understanding M. oryzae pathogenicity and may aid in developing resistance strategies against rice blast.

Post-translational modifications control the signal at the crossroads of plant-pathogen interactions

The co-evolution of plants and pathogens has enabled them to 'outsmart' each other by promoting their own defence responses and suppressing those of the other. While plants are reliant on their sophisticated immune signalling pathways, pathogens make use of effector proteins to achieve the objective. This entails rapid regulation of underlying molecular mechanisms for prompt induction of associated signalling events in both plants as well as pathogens. The past decade has witnessed the emergence of post-translational modification (PTM) of proteins as a key a factor in modulating cellular responses. The ability of PTMs to expand the functional diversity of the proteome and induce rapid changes at the appropriate time enables them to play crucial roles in the regulation of plant-pathogen interactions. Therefore, this review will delve into the intricate interplay of five major PTMs involved in plant defence and pathogen countermeasures. We discuss how plants employ PTMs to fortify their immune networks, and how pathogen effectors utilize/target host modification systems to gain entry into plants and cause disease. We also emphasize the need for identification of novel PTMs and propose the use of PTM pathways as potential targets for genome editing approaches.

Roadmap to Success: How Oomycete Plant Pathogens Invade Tissues and Deliver Effectors

Filamentous plant pathogens threaten global food security and ecosystem resilience. In recent decades, significant strides have been made in deciphering the molecular basis of plant-pathogen interactions, especially the interplay between pathogens' molecular weaponry and hosts' defense machinery. Stemming from interdisciplinary investigations into the infection cell biology of filamentous plant pathogens, recent breakthrough discoveries have provided a new impetus to the field. These advances include the biophysical characterization of a novel invasion mechanism (i.e., naifu invasion) and the unraveling of novel effector secretion routes. On the plant side, progress includes the identification of components of cellular networks involved in the uptake of intracellular effectors. This exciting body of research underscores the pivotal role of logistics management by the pathogen throughout the infection cycle, encompassing the precolonization stages up to tissue invasion. More insight into these logistics opens new avenues for developing environmentally friendly crop protection strategies in an era marked by an imperative to reduce the use of agrochemicals.

The Phytophthora infestans effector Pi05910 suppresses and destabilizes host glycolate oxidase StGOX4 to promote plant susceptibility

Phytophthora infestans is a notorious oomycete pathogen that causes potato late blight. It secretes numerous effector proteins to manipulate host immunity. Understanding mechanisms underlying their host cell manipulation is crucial for developing disease resistance strategies. Here, we report that the conserved RXLR effector Pi05910 of P. infestans is a genotype-specific avirulence elicitor on potato variety Longshu 12 and contributes virulence by suppressing and destabilizing host glycolate oxidase StGOX4. By performing co-immunoprecipitation, yeast-two-hybrid assays, luciferase complementation imaging, bimolecular fluorescence complementation and isothermal titration calorimetry assays, we identified and confirmed potato StGOX4 as a target of Pi05910. Further analysis revealed that StGOX4 and its homologue NbGOX4 are positive immune regulators against P. infestans, as indicated by infection assays on potato and Nicotiana benthamiana overexpressing StGOX4 and TRV-NbGOX4 plants. StGOX4-mediated disease resistance involves enhanced reactive oxygen species accumulation and activated the salicylic acid signalling pathway. Pi05910 binding inhibited enzymatic activity and destabilized StGOX4. Furthermore, mutagenesis analyses indicated that the 25th residue (tyrosine, Y25) of StGOX4 mediates Pi05910 binding and is required for its immune function. Our results revealed that the core RXLR effector of P. infestans Pi05910 suppresses plant immunity by targeting StGOX4, which results in decreased enzymatic activity and protein accumulation, leading to enhanced plant susceptibility.

PTI-ETI synergistic signal mechanisms in plant immunity

Plants face a relentless onslaught from a diverse array of pathogens in their natural environment, to which they have evolved a myriad of strategies that unfold across various temporal scales. Cell surface pattern recognition receptors (PRRs) detect conserved elicitors from pathogens or endogenous molecules released during pathogen invasion, initiating the first line of defence in plants, known as pattern-triggered immunity (PTI), which imparts a baseline level of disease resistance. Inside host cells, pathogen effectors are sensed by the nucleotide-binding/leucine-rich repeat (NLR) receptors, which then activate the second line of defence: effector-triggered immunity (ETI), offering a more potent and enduring defence mechanism. Moreover, PTI and ETI collaborate synergistically to bolster disease resistance and collectively trigger a cascade of downstream defence responses. This article provides a comprehensive review of plant defence responses, offering an overview of the stepwise activation of plant immunity and the interactions between PTI-ETI synergistic signal transduction.

Chloroplast protein StFC-II was manipulated by a Phytophthora effector to enhance host susceptibility

Oomycete secretes a range of RxLR effectors into host cells to manipulate plant immunity by targeting proteins from several organelles. In this study, we report that chloroplast protein StFC-II is hijacked by a pathogen effector to enhance susceptibility. Phytophthora infestans RxLR effector Pi22922 is activated during the early stages of P. infestans colonization. Stable overexpression of Pi22922 in plants suppresses flg22-triggered reactive oxygen species (ROS) burst and enhances leaf colonization by P. infestans. A potato ferrochelatase 2 (FC-II, a nuclear-encoded chloroplast-targeted protein), a key enzyme for heme biosynthesis in chloroplast, was identified as a target of Pi22922 in the cytoplasm. The pathogenicity of Pi22922 in plants is partially dependent on FC-II. Overexpression of StFC-II decreases resistance of potato and Nicotiana benthamiana against P. infestans, and silencing of NbFC-II in N. benthamiana reduces P. infestans colonization. Overexpression of StFC-II increases heme content and reduces chlorophyll content and photosynthetic efficiency in potato leaves. Moreover, ROS accumulation both in chloroplast and cytoplasm is attenuated and defense-related genes are down-regulated in StFC-II overexpression transgenic potato and N. benthamiana leaves. Pi22922 inhibits E3 ubiquitin ligase StCHIP-mediated StFC-II degradation in the cytoplasm and promotes its accumulation in chloroplasts. In summary, this study characterizes a new mechanism that an oomycete RxLR effector suppresses host defenses by promoting StFC-II accumulation in chloroplasts, thereby compromising the host immunity and promoting susceptibility.

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.

Ubiquitination from the perspective of plant pathogens

The constant battle of survival between pathogens and host plants has played a crucial role in shaping the course of their co-evolution. However, the major determinants of the outcome of this ongoing arms race are the effectors secreted by pathogens into host cells. These effectors perturb the defense responses of plants to promote successful infection. In recent years, extensive research in the area of effector biology has reported an increase in the repertoire of pathogenic effectors that mimic or target the conserved ubiquitin-proteasome pathway. The role of the ubiquitin-mediated degradation pathway is well known to be indispensable for various aspects of a plant's life, and thus targeting or mimicking it seems to be a smart strategy adopted by pathogens. Therefore, this review summarizes recent findings on how some pathogenic effectors mimic or act as one of the components of the ubiquitin-proteasome machinery while others directly target the plant's ubiquitin-proteasome system.

The effectors of Phytophthora infestans impact host immunity upon regulation of antagonistic hormonal activities

MAIN CONCLUSION

Phytophthora infestans effectors manipulate the antagonism of host hormones to interfere with the immune response of plants at different infection stages. Phytophthora infestans (P. infestans) poses a serious threat to global crop production, and its effectors play an indispensable role in its pathogenicity. However, the function of these effectors during the switch from biotrophy to necrotrophy of P. infestans remains unclear. Further research on the effectors that manipulate the antagonistic response of host hormones is also lacking. In this study, a coexpression analysis and infection assays were performed to identify distinct gene expression changes in both P. infestans and tomato. During the switch from biotrophy to necrotrophy, P. infestans secretes three types of effectors to interfere with host salicylic acid (SA), jasmonic acid (JA), ethylene (ET), and abscisic acid (ABA) levels. The three aforementioned effectors also regulate the host gene expression including NPR1, TGA2.1, PDF1.2, NDR1, ERF3, NCED6, GAI4, which are involved in hormone crosstalk. The changes in plant hormones are mediated by the three types of effectors, which may accelerate infection and drive completion of the P. infestans lifecycle. Our findings provide new insight into plant‒pathogen interactions that may contribute to the prevention growth of hemibiotrophic pathogens.