PRG-Wiki is an open and daily update space about plant resistance gene, in which all information about this family is stored, curated and discussed. The purpose of our work is creating a worldwide community working on plant resistance genes with a constant update on all aspects of this research field and to encourage scientists to be actors of the discussion and of the data exchange. PRG-Wiki actually stores more than 112 reference resistance gene and 104335 putative disease resistance gene. Through the wiki pages any contributor can suggest changes to the PRG database and directly update it with new data, new information and with corrections of wrong information.
Plants are constantly exposed to a large and diverse array of microbes; however, most plants are immune to the majority of potential invaders and susceptible to only a small subset of pathogens. The cytoskeleton comprises a dynamic intracellular framework that responds rapidly to biotic stresses and supports numerous fundamental cellular processes including vesicle trafficking, endocytosis and the spatial distribution of organelles and protein complexes. For years, the actin cytoskeleton has been assumed to play a role in plant innate immunity against fungi and oomycetes, based largely on static images and pharmacological studies. To date, however, there is little evidence that the host-cell actin cytoskeleton participates in responses to phytopathogenic bacteria. Here, we quantified the spatiotemporal changes in host-cell cytoskeletal architecture during the immune response to pathogenic and non-pathogenic strains of Pseudomonas syringae pv. tomato DC3000. Two distinct changes to host cytoskeletal arrays were observed that correspond to distinct phases of plant-bacterial interactions i.e. the perception of microbe-associated molecular patterns (MAMPs) during pattern-triggered immunity (PTI) and perturbations by effector proteins during effector-triggered susceptibility (ETS). We demonstrate that an immediate increase in actin filament abundance is a conserved and novel component of PTI. Notably, treatment of leaves with a MAMP peptide mimic was sufficient to elicit a rapid change in actin organization in epidermal cells, and this actin response required the host-cell MAMP receptor kinase complex, including FLS2, BAK1 and BIK1. Finally, we found that actin polymerization is necessary for the increase in actin filament density and that blocking this increase with the actin-disrupting drug latrunculin B leads to enhanced susceptibility of host plants to pathogenic and non-pathogenic bacteria.
Jessica L. Henty-Ridilla, Masaki Shimono, Jiejie Li, Jeff H. Chang, Brad Day, Christopher J. Staiger
Transcriptional regulation in host cells plays a crucial role in the establishment of plant defense and associated cell death in response to pathogen attack. Here, we review our current knowledge of the transcriptional control of plant defenses with a focus on the MYB family of transcription factors (TFs). Within this family, the Arabidopsis MYB protein AtMYB30 is a key regulator of plant defenses and one of the best characterized MYB regulators directing defense-related transcriptional responses. The crucial role played by AtMYB30 in the regulation of plant disease resistance is underlined by the finding that AtMYB30 is targeted by the Xanthomonas type III effector XopD resulting in suppression of AtMYB30-mediated plant defenses. Moreover, the function of AtMYB30 is also tightly controlled by plant cells through protein-protein interactions and post-translational modifications (PTMs). AtMYB30 studies highlight the importance of cellular dynamics for defense-associated gene regulation in plants. Finally, we discuss how AtMYB30 and other MYB TFs mediate the interplay between disease resistance and other stress responses.
Bacterial papain-like protease AvrPphB cleaves kinases in host plant cell
Activity-based probe for AvrPphB inspired on its unique substrate specificity
Secretion of ProAvrPphB through narrow type III pilus requires prodomain
Prodomain removal is essential to trigger cell death in resistant plants
The AvrPphB effector of Pseudomonas syringae is a papain-like protease that is injected into the host plant cell and cleaves specific kinases to disrupt immune signaling. Here, we used the unique substrate specificity of AvrPphB to generate a specific activity-based probe. This probe displays various AvrPphB isoforms in bacterial extracts, upon secretion and inside the host plant. We show that AvrPphB is secreted as a proprotease and that secretion requires the prodomain, but probably does not involve a pH-dependent unfolding mechanism. The prodomain removal is required for the ability of AvrPphB to trigger a hypersensitive cell death in resistant host plants, presumably since processing exposes a hidden acylation site required for subcellular targeting in the host cell. We detected two active isoforms of AvrPphB in planta, of which the major one localizes exclusively to membranes.
Genome sequences of plant fungal pathogens have enabled the identification of effectors that cooperatively modulate the cellular environment for successful fungal growth and suppress host defense. Identification and characterization of novel effector proteins are crucial to understand pathogen virulence and host plant defense mechanisms. Previous reports indicate that the Pseudomonas syringae pv. tomato DC3000 type III secretion system (T3SS) can be used to study how non-bacterial effectors manipulate dicot plant cell function using the Effector Detector Vector (pEDV) system. Here we report a pEDV-based effector delivery system in which the T3SS of Burkholderia glumae, an emerging rice pathogen, is used to translocate the AVR-Pik and AVR-Pii effectors of fungal pathogen Magnaporthe oryzae to rice cytoplasm. The translocated AVR-Pik and AVR-Pii showed avirulence activity when tested in rice cultivars containing the cognate R genes. AVR-Pik reduced and delayed the hypersensitive response triggered by B. glumae in the non-host plant Nicotiana benthamiana indicative of an immunosuppressive virulence activity. AVR proteins fused with fluorescent protein and nuclear localization signal were delivered by B. glumae T3SS and observed in the nuclei of infected cells in rice, wheat, barley and N. benthamiana. Our bacterial T3SS-enabled eukaryotic effector delivery and subcellular localization assays provide a useful method to identify and study effector functions in monocot plants.
Shailendra Sharma, Shiveta Sharma, Akiko Hirabuchi, Kentaro Yoshida, Koki Fujisaki, Akiko Ito, Aiko Uemura, Ryohei Terauchi, Sophien Kamoun, Kee Hoon Sohn, Jonathan D.G. Jones and Hiromasa Saitoh
One of the most efficient plant resistance reactions to pathogen attack is the hypersensitive response, a form of programmed cell death at infection sites. The Arabidopsis transcription factor MYB30 is a positive regulator of hypersensitive cell death responses. Here we show that MIEL1 (MYB30-Interacting E3 Ligase1), an Arabidopsis RING-type E3 ubiquitin ligase that interacts with and ubiquitinates MYB30, leads to MYB30 proteasomal degradation and downregulation of its transcriptional activity. In non-infected plants, MIEL1 attenuates cell death and defence through degradation of MYB30. Following bacterial inoculation, repression of MIEL1 expression removes this negative regulation allowing sufficient MYB30 accumulation in the inoculated zone to trigger the hypersensitive response and restrict pathogen growth. Our work underlines the important role played by ubiquitination to control the hypersensitive response and highlights the sophisticated fine-tuning of plant responses to pathogen attack. Overall, this work emphasizes the importance of protein modification by ubiquitination during the regulation of transcriptional responses to stress in eukaryotic cells.
Daniel Marino, Solène Froidure, Joanne Canonne, Sara Ben Khaled, Mehdi Khafif, Cécile Pouzet, Alain Jauneau, Dominique Roby and Susana Rivas
Plant LysM proteins control the perception of microbial-derived N-acetylglucosamine compounds for the establishment of symbiosis or activation of plant immunity. This raises questions about how plants, and notably legumes, can differentiate friends and foes using similar molecular actors and whether any receptors can intervene in both symbiosis and resistance.
To study this question, nfp and lyk3 LysM-receptor like kinase mutants of Medicago truncatula that are affected in the early steps of nodulation, were analysed following inoculation with Aphanomyces euteiches, a root oomycete. The role of NFP in this interaction was further analysed by overexpression of NFP and by transcriptome analyses.
nfp, but not lyk3, mutants were significantly more susceptible than wildtype plants to A. euteiches, whereas NFP overexpression increased resistance. Transcriptome analyses on A. euteiches inoculation showed that mutation in the NFP gene led to significant changes in the expression of c. 500 genes, notably involved in cell dynamic processes previously associated with resistance to pathogen penetration. nfp mutants also showed an increased susceptibility to the fungus Colletotrichum trifolii.
These results demonstrate that NFP intervenes in M. truncatula immunity, suggesting an unsuspected role for NFP in the perception of pathogenic signals.
Amy Huei-Yi Lee; Maggie A. Middleton; David S. Guttman; Darrell Desveaux
Bacterial phytopathogens utilize a myriad of virulence factors to modulate their plant hosts in order to promote successful pathogenesis. One potent virulence strategy is to inject these virulence proteins into plant cells via the type III secretion system. Characterizing the host targets and the molecular mechanisms of type III secreted proteins, known as effectors, has illuminated our understanding of eukaryotic cell biology. As a result, these effectors can serve as molecular probes to aid in our understanding of plant cellular processes, such as immune signalling, vesicle trafficking, cytoskeleton stability and transcriptional regulation. Furthermore, given that effectors directly and specifically interact with their targets within plant cells, these virulence proteins have enormous biotechnological potential for manipulating eukaryotic systems.
A common response by plants to fungal attack is deposition of callose, a (1,3)-β-glucan polymer, in the form of cell wall thickenings called papillae, at site of wall penetration. While it has been generally believed that the papillae provide a structural barrier to slow fungal penetration, this idea has been challenged in recent studies of Arabidopsis (Arabidopsis thaliana), where fungal resistance was found to be independent of callose deposition. To the contrary, we show that callose can strongly support penetration resistance when deposited in elevated amounts at early time points of infection. We generated transgenic Arabidopsis lines that express POWDERY MILDEW RESISTANT4 (PMR4), which encodes a stress-induced callose synthase, under the control of the constitutive 35S promoter. In these lines, we detected callose synthase activity that was four times higher than that in wild-type plants 6 h post inoculation with the virulent powdery mildew Golovinomyces cichoracearum. The callose synthase activity was correlated with enlarged callose deposits and the focal accumulation of green fluorescent protein-tagged PMR4 at sites of attempted fungal penetration. We observed similar results from infection studies with the nonadapted powdery mildew Blumeria graminis f. sp. hordei. Haustoria formation was prevented in resistant transgenic lines during both types of powdery mildew infection, and neither the salicylic acid-dependent nor jasmonate-dependent pathways were induced. We present a schematic model that highlights the differences in callose deposition between the resistant transgenic lines and the susceptible wild-type plants during compatible and incompatible interactions between Arabidopsis and powdery mildew.
Dorothea Ellinger, Marcel Naumann, Christian Falter, Claudia Zwikowics, Torsten Jamrow, Chithra Manisseri, Shauna C. Somerville, and Christian A. Voigt
A fundamental question regarding any immune system is how it can discriminate between pathogens and non-pathogens. Here, we discuss how this discrimination can be mediated by a surveillance system distinct from pattern-recognition receptors that recognize conserved microbial patterns. It can be based instead on the ability of the host to sense perturbations in host cells induced by bacterial toxins or 'effectors' that are encoded by pathogenic microorganisms. Such 'effector-triggered immunity' was previously demonstrated mainly in plants, but recent data confirm that animals can also use this strategy.
Lynda M. Stuart, Nicholas Paquette & Laurent Boyer
An annotated high-quality draft genome sequence for Xanthomonas campestris pv. campestris race 1 strain Xca5 (originally described as X. campestris pv. armoraciae), the causal agent of black rot on Brassicaceae plants, has been determined. This genome sequence is a valuable resource for comparative genomics within the campestris pathovar.
Pyrenophora tritici-repentis is a necrotrophic fungus causal to the disease tan spot of wheat, whose contribution to crop loss has increased significantly during the last few decades. Pathogenicity by this fungus is attributed to the production of host-selective toxins (HST), which are recognized by their host in a genotype-specific manner. To better understand the mechanisms that have led to the increase in disease incidence related to this pathogen, we sequenced the genomes of three P. tritici-repentis isolates. A pathogenic isolate that produces two known HSTs was used to assemble a reference nuclear genome of approximately 40 Mb composed of 11 chromosomes that encode 12,141 predicted genes. Comparison of the reference genome with those of a pathogenic isolate that produces a third HST, and a nonpathogenic isolate, showed the nonpathogen genome to be more diverged than those of the two pathogens. Examination of gene-coding regions has provided candidate pathogen-specific proteins and revealed gene families that may play a role in a necrotrophic lifestyle. Analysis of transposable elements suggests that their presence in the genome of pathogenic isolates contributes to the creation of novel genes, effector diversification, possible horizontal gene transfer events, identified copy number variation, and the first example of transduplication by DNA transposable elements in fungi. Overall, comparative analysis of these genomes provides evidence that pathogenicity in this species arose through an influx of transposable elements, which created a genetically flexible landscape that can easily respond to environmental changes.
Viola A. Manning, Iovanna Pandelova, Braham Dhillon, Larry J. Wilhelm, Stephen B. Goodwin, Aaron M. Berlin, Melania Figueroa, Michael Freitag, James K. Hane, Bernard Henrissa, Wade H. Holman, Chinnappa D. Kodira, Joel Martin, Richard P. Oliver, Barbara Robbertse, Wendy Schackwitz, David C. Schwartz, Joseph W. Spatafora, B. Gillian Turgeon, Chandri Yandava, Sarah Young, Shiguo Zhou, Qiandong Zeng, Igor V. Grigoriev, Li-Jun Ma, and Lynda M. Ciuffetti
Zymoseptoria tritici is an important fungal pathogen on wheat that originated in the Fertile Crescent. Its closely related sister species Z. pseudotritici and Z. ardabiliae infect wild grasses in the same region. This recently emerged host–pathogen system provides a rare opportunity to investigate the evolutionary processes shaping the genome of an emerging pathogen. Here, we investigate genetic signatures in plant cell wall degrading enzymes (PCWDEs) that are likely affected by or driving coevolution in plant-pathogen systems. We hypothesize four main evolutionary scenarios and combine comparative genomics, transcriptomics, and selection analyses to assign the majority of PCWDEs in Z. tritici to one of these scenarios. We found widespread differential transcription among different members of the same gene family, challenging the idea of functional redundancy and suggesting instead that specialized enzymatic activity occurs during different stages of the pathogen life cycle. We also find that natural selection has significantly affected at least 19 of the 48 identified PCWDEs. The majority of genes showed signatures of purifying selection, typical for the scenario of conserved substrate optimization. However, six genes showed diversifying selection that could be attributed to either host adaptation or host evasion. This study provides a powerful framework to better understand the roles played by different members of multigene families and to determine which genes are the most appropriate targets for wet laboratory experimentation, for example, to elucidate enzymatic function during relevant phases of a pathogen’s life cycle.
Patrick C. Brunner, Stefano F.F. Torriani, Daniel Croll, Eva H. Stukenbrock, and Bruce A. McDonald
Transcription activator-like effectors (TALEs) have revolutionized the field of genome engineering. We present here a systematic assessment of TALE DNA recognition, using quantitative electrophoretic mobility shift assays and reporter gene activation assays. Within TALE proteins, tandem 34-amino acid repeats recognize one base pair each and direct sequence-specific DNA binding through repeat variable di-residues (RVDs). We found that RVD choice can affect affinity by four orders of magnitude, with the relative RVD contribution in the order NG > HD ∼ NN ≫ NI > NK. The NN repeat preferred the base G over A, whereas the NK repeat bound G with 103-fold lower affinity. We compared AvrBs3, a naturally occurring TALE that recognizes its target using some atypical RVD-base combinations, with a designed TALE that precisely matches ‘standard’ RVDs with the target bases. This comparison revealed unexpected differences in sensitivity to substitutions of the invariant 5′-T. Another surprising observation was that base mismatches at the 5′ end of the target site had more disruptive effects on affinity than those at the 3′ end, particularly in designed TALEs. These results provide evidence that TALE–DNA recognition exhibits a hitherto un-described polarity effect, in which the N-terminal repeats contribute more to affinity than C-terminal ones.
Joshua F. Meckler, Mital S. Bhakta, Moon-Soo Kim, Robert Ovadia, Chris H. Habrian, Artem Zykovich, Abigail Yu, Sarah H. Lockwood, Robert Morbitzer, Janett Elsäesser, Thomas Lahaye, David J. Segal, and Enoch P. Baldwin
The type III secretion apparatus (T3SA) is a multi-protein complex central to the virulence of many Gram-negative pathogens. Currently, the mechanisms controlling the hierarchical addressing of needle subunits, translocators and effectors to the T3SA are still poorly understood. In Shigella, MxiC is known to sequester effectors within the cytoplasm prior to receiving the activation signal from the needle. However, molecules involved in linking the needle and MxiC are unknown. Here, we demonstrate a molecular interaction between MxiC and the predicted inner-rod component MxiI suggesting that this complex plugs the T3SA entry gate. Our results suggest that MxiI–MxiC complex dissociation facilitates the switch in secretion from translocators to effectors. We identified MxiCF206S variant, unable to interact with MxiI, which exhibits a constitutive secretion phenotype although it remains responsive to induction. Moreover, we identified the mxiIQ67A mutant that only secretes translocators, a phenotype that was suppressed by coexpression of the MxiCF206S variant. We demonstrated the interaction between MxiI and MxiC homologues in Yersinia and Salmonella. Lastly, we identified an interaction between MxiC and chaperone IpgC which contributes to understanding how translocators secretion is regulated. In summary, this study suggests the existence of a widely conserved T3S mechanism that regulates effectors secretion.
Youness Cherradi, Lionel Schiavolin, Simon Moussa, Alaeddine Meghraoui, Ahmed Meksem, Latéfa Biskri, Mohamed Azarkan, Abdelmounaaïm Allaoui and Anne Botteaux
Pathogens target important components of host immunity to cause disease. The Pseudomonas syringae type III-secreted effector HopU1 is a mono-ADP-ribosyltransferase required for full virulence on Arabidopsis thaliana. HopU1 targets several RNA-binding proteins including GRP7, whose role in immunity is still unclear. Here, we show that GRP7 associates with translational components, as well as with the pattern recognition receptors FLS2 and EFR. Moreover, GRP7 binds specifically FLS2 and EFR transcripts in vivo through its RNA recognition motif. HopU1 does not affect the protein–protein associations between GRP7, FLS2 and translational components. Instead, HopU1 blocks the interaction between GRP7 and FLS2 and EFR transcripts in vivo. This inhibition correlates with reduced FLS2 protein levels upon Pseudomonas infection in a HopU1-dependent manner. Our results reveal a novel virulence strategy used by a microbial effector to interfere with host immunity.
Valerie Nicaise, Anna Joe, Byeong-ryool Jeong, Christin Korneli, Freddy Boutrot, Isa Westedt, Dorothee Staiger, James R Alfano and Cyril Zipfel
Rhizoctonia solani is a major fungal pathogen of rice (Oryza sativa L.) that causes great yield losses in all rice-growing regions of the world. Here we report the draft genome sequence of the rice sheath blight disease pathogen, R. solani AG1 IA, assembled using next-generation Illumina Genome Analyser sequencing technologies. The genome encodes a large and diverse set of secreted proteins, enzymes of primary and secondary metabolism, carbohydrate-active enzymes, and transporters, which probably reflect an exclusive necrotrophic lifestyle. We find few repetitive elements, a closer relationship to Agaricomycotina among Basidiomycetes, and expand protein domains and families. Among the 25 candidate pathogen effectors identified according to their functionality and evolution, we validate 3 that trigger crop defence responses; hence we reveal the exclusive expression patterns of the pathogenic determinants during host infection.
Aiping Zheng, Runmao Lin, Danhua Zhang, Peigang Qin, Lizhi Xu, Peng Ai, Lei Ding, Yanran Wang, Yao Chen, Yao Liu, Zhigang Sun, Haitao Feng, Xiaoxing Liang, Rongtao Fu, Changqing Tang, Qiao Li, Jing Zhang, Zelin Xie, Qiming Deng, Shuangcheng Li, Shiquan Wang, Jun Zhu, Lingxia Wang, Huainian Liu and Ping Li
Xylan is a major structural component of plant cell wall and the second most abundant plant polysaccharide in nature. Here, by combining genomic and functional analyses, we provide a comprehensive picture of xylan utilization by Xanthomonas campestris pv campestris (Xcc) and highlight its role in the adaptation of this epiphytic phytopathogen to the phyllosphere. The xylanolytic activity of Xcc depends on xylan-deconstruction enzymes but also on transporters, including two TonB-dependent outer membrane transporters (TBDTs) which belong to operons necessary for efficient growth in the presence of xylo-oligosaccharides and for optimal survival on plant leaves. Genes of this xylan utilization system are specifically induced by xylo-oligosaccharides and repressed by a LacI-family regulator named XylR. Part of the xylanolytic machinery of Xcc, including TBDT genes, displays a high degree of conservation with the xylose-regulon of the oligotrophic aquatic bacterium Caulobacter crescentus. Moreover, it shares common features, including the presence of TBDTs, with the xylan utilization systems of Bacteroides ovatus and Prevotella bryantii, two gut symbionts. These similarities and our results support an important role for TBDTs and xylan utilization systems for bacterial adaptation in the phyllosphere, oligotrophic environments and animal guts. Guillaume Déjean, Servane Blanvillain-Baufumé, Alice Boulanger, Armelle Darrasse, Thomas Dugé de Bernonville, Anne-Laure Girard, Sébastien Carrére, Stevie Jamet, Claudine Zischek, Martine Lautier, Magali Solé, Daniela Büttner, Marie-Agnès Jacques, Emmanuelle Lauber, Matthieu Arlat
Nicolas Denancé's insight:
The second story from Matthieu Arlat and Emmanuelle Lauber group reporting the role of Xanthomonas TBDTs.
Beneficial associations between plants and arbuscular mycorrhizal fungi play a major role in terrestrial environments and in the sustainability of agroecosystems. Proteins, microRNAs, and small molecules have been identified in model angiosperms as required for the establishment of arbuscular mycorrhizal associations and define a symbiotic ‘toolkit’ used for other interactions such as the rhizobia–legume symbiosis. Based on recent studies, we propose an evolutionary framework for this toolkit. Some components appeared recently in angiosperms, whereas others are highly conserved even in land plants unable to form arbuscular mycorrhizal associations. The exciting finding that some components pre-date the appearance of arbuscular mycorrhizal fungi suggests the existence of unknown roles for this toolkit and even the possibility of symbiotic associations in charophyte green algae.
Under the endosymbiont hypothesis, over a billion years ago a heterotrophic eukaryote entered into a symbiotic relationship with a cyanobacterium (the cyanobiont). This partnership culminated in the plastid that has spread to forms as diverse as plants and diatoms. However, why primary plastid acquisition has not been repeated multiple times remains unclear. Here, we report a possible answer to this question by showing that primary plastid endosymbiosis was likely to have been primed by the secretion in the host cytosol of effector proteins from intracellular Chlamydiales pathogens. We provide evidence suggesting that the cyanobiont might have rescued its afflicted host by feeding photosynthetic carbon into a chlamydia-controlled assimilation pathway.
Steven G. Ball, Agathe Subtil, Debashish Bhattacharya, Ahmed Moustafa, Andreas P.M. Weber, Lena Gehre, Christophe Colleoni, Maria-Cecilia Arias, Ugo Cenci, and David Dauvillée
Cell-cell signaling in Xylella fastidiosa has been implicated in the coordination of traits enabling colonization in plant hosts as well as insect vectors. This cell density-dependent signaling has been attributed to a diffusible signaling factor (DSF) produced by the DSF synthase RpfF. DSF produced by related bacterial species are unsaturated fatty acids, but that of X. fastidiosa was thought to be different from those of other taxa. We describe here the isolation and characterization of an X. fastidiosa DSF (XfDSF) as 2(Z)-tetradecenoic acid. This compound was isolated both from recombinant Erwinia herbicola expressing X. fastidiosa rpfF and from an X. fastidiosa rpfC deletion mutant that overproduces DSF. Since an rpfF mutant is impaired in biofilm formation and underexpresses the hemagglutinin-like protein-encoding genes hxfA and hxfB, we demonstrate that these traits can be restored by ca. 0.5 µM XfDSF but not by myristic acid, the fully saturated tetradecenoic acid. A phoA-based X. fastidiosa biosensor that assesses DSF-dependent expression of hxfA or hxfB revealed a high level of molecular specificity of DSF signaling.
IMPORTANCE X. fastidiosa causes diseases in many important plants, including grape, where it incites Pierce’s disease. Virulence of X. fastidiosa for grape is coordinated by cell-cell signaling molecules, designated DSF (Diffusible Signaling Factor). Mutants blocked in DSF production are hypervirulent for grape, suggesting that virulence is suppressed upon DSF accumulation and that disease could be controlled by artificial elevation of the DSF level in plants. In this work, we describe the isolation of the DSF produced by X. fastidiosa and the verification of its biological activity as an antivirulence factor. We also have developed X. fastidiosa DSF biosensors to evaluate the specificity of cell-cell signaling to be investigated.
Ellen D. Beaulieu, Michael Ionescu, Subhadeep Chatterjee, Kenji Yokota, Dirk Trauner, and Steven Lindow
Phytopathogenic fungi frequently contain dispensable chromosomes, some of which contribute to host range or pathogenicity. In Leptosphaeria maculans, the stem canker agent of oilseed rape (Brassica napus), the minichromosome was previously suggested to be dispensable, without evidence for any role in pathogenicity.
Using genetic and genomic approaches, we investigated the inheritance and molecular determinant of an L. maculans–Brassica rapa incompatible interaction.
Single gene control of the resistance was found, while all markers located on the L. maculans minichromosome, absent in the virulent parental isolate, co-segregated with the avirulent phenotype. Only one candidate avirulence gene was identified on the minichromosome, validated by complementation experiments and termed AvrLm11. The minichromosome was frequently lost following meiosis, but the frequency of isolates lacking it remained stable in field populations sampled at a 10-yr time interval, despite a yearly sexual stage in the L. maculans life cycle.
This work led to the cloning of a new ‘lost in the middle of nowhere’ avirulence gene of L. maculans, interacting with a B. rapa resistance gene termed Rlm11 and introgressed into B. napus. It demonstrated the dispensability of the L. maculans minichromosome and suggested that its loss generates a fitness deficit.