Plants and Microbes

Effectors secreted by the bacterial type III system play a central role in the interaction between Gram-negative bacterial pathogens and their host plants. Recent advances in the effector studies have helped cementing several key concepts concerning bacterial pathogenesis, plant immunity, and plant–pathogen co-evolution. Type III effectors use a variety of biochemical mechanisms to target specific host proteins or DNA for pathogenesis. The identifications of their host targets led to the identification of novel components of plant innate immune system. Key modules of plant immune signaling pathways such as immune receptor complexes and MAPK cascades have emerged as a major battle ground for host–pathogen adaptation. These modules are attacked by multiple type III effectors, and some components of these modules have evolved to actively sense the effectors and trigger immunity.

The rice XA21 receptor confers immunity to the Gram-negative bacterial pathogen, Xanthomonas oryzae pv. oryzae (Xoo) upon recognition of the conserved microbial signature AxYS22. Here, we demonstrate that the intracellular kinase domain of XA21 translocates to the nucleus upon AxYS22-mediated perception and that this translocation event is required for XA21-mediated immunity.

Ustilago maydis is a biotrophic pathogen causing maize (Zea mays) smut disease. Transcriptome profiling of infected maize plants indicated that a gene encoding a putative cystatin (CC9) is induced upon penetration by U. maydis wild type. By contrast, cc9 is not induced after infection with the U. maydis effector mutant Δpep1, which elicits massive plant defenses. Silencing of cc9 resulted in a strongly induced maize defense gene expression and a hypersensitive response to U. maydis wild-type infection. Consequently, fungal colonization was strongly reduced in cc9-silenced plants, while recombinant CC9 prevented salicylic acid (SA)–induced defenses. Protease activity profiling revealed a strong induction of maize Cys proteases in SA-treated leaves, which could be inhibited by addition of CC9. Transgenic maize plants overexpressing cc9-mCherry showed an apoplastic localization of CC9. The transgenic plants showed a block in Cys protease activity and SA-dependent gene expression. Moreover, activated apoplastic Cys proteases induced SA-associated defense gene expression in naïve plants, which could be suppressed by CC9. We show that apoplastic Cys proteases play a pivotal role in maize defense signaling. Moreover, we identified cystatin CC9 as a novel compatibility factor that suppresses Cys protease activity to allow biotrophic interaction of maize with the fungal pathogen U. maydis.

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Nucleotide binding-leucine rich repeat (NB-LRR) proteins function as intracellular receptors for the detection of pathogens in both plants and animals. Despite their central role in innate immunity, the molecular mechanisms that govern NB-LRR activation are poorly understood. The Arabidopsis NB-LRR protein RPS5 detects the presence of the Pseudomonas syringae effector protein AvrPphB by monitoring the status of the Arabidopsis protein kinase PBS1. AvrPphB is a cysteine protease that targets PBS1 for cleavage at a single site within the activation loop of PBS1. Using a transient expression system in the plant Nicotiana benthamiana and stable transgenic Arabidopsis plants we found that both PBS1 cleavage products are required to activate RPS5 and can do so in the absence of AvrPphB. We also found, however, that the requirement for cleavage of PBS1 could be bypassed simply by inserting five amino acids at the PBS1 cleavage site, which is located at the apex of the activation loop of PBS1. Activation of RPS5 did not require PBS1kinase function, thus RPS5 appears to sense a subtle conformational change in PBS1, rather than cleavage. This finding suggests that NB-LRR proteins may function as fine-tuned sensors of alterations in the structures of effector targets.

The genetically tractable endophytic fungus Piriformospora indica is able to colonize the root cortex of a great variety of different plant species with beneficial effects to its hosts and it represents a suitable model system to study symbiotic interactions. Recent cytological studies in barley and Arabidopsis showed that upon penetration of the root, P. indica establishes a biotrophic interaction during which fungal cells are encased by the host plasma membrane. Large-scale transcriptional analyses of fungal and plant responses have shown that perturbance of plant hormone homeostasis and secretion of fungal lectins and other small proteins (effectors) may be involved in the evasion and suppression of host defenses at these early colonization steps. At later stages P. indica is found more often in moribund host cells where it secretes a large variety of hydrolytic enzymes that degrade proteins. This strategy of colonizing plants is reminiscent of that of hemibiotrophic fungi, although a defined shift to necrotrophy with massive host cell death is missing. Instead, the association with the plant root leads to beneficial effects for the host such as growth promotion, increased resistance to root as well as leaf pathogens, and increased tolerance to abiotic stresses. This review describes current advances in understanding the components of P. indica endophytic lifestyle from molecular and genomic analyses.

The model plant Arabidopsis thaliana exhibits extensive natural variation in resistance to parasites. Immunity is often conferred by resistance (R) genes that permit recognition of specific races of a disease. The number of such R genes and their distribution are poorly understood. In this study, we investigated the basis for resistance to the downy mildew agent Hyaloperonospora arabidopsidis ex parasitica (Hpa) in a global sample of A. thaliana. We implemented a combined genome-wide mapping of resistance using populations of recombinant inbred lines and a collection of wild A. thaliana accessions. We tested the interaction between 96 host genotypes collected worldwide and five strains of Hpa. Then, a fraction of the species-wide resistance was genetically dissected using six recently constructed populations of recombinant inbred lines. We found that resistance is usually governed by single dominant R genes that are concentrated in four genomic regions only. We show that association genetics of resistance to diseases such as downy mildew enables increased mapping resolution from quantitative trait loci interval to candidate gene level. Association patterns in quantitative trait loci intervals indicate that the pool of A. thaliana resistance sources against the tested Hpa isolates may be predominantly confined to six RPP (Resistance to Hpa) loci isolated in previous studies. Our results suggest that combining association and linkage mapping could accelerate resistance gene discovery in plants.

LA nature toute prodigue qu'elle est nous fournit peu de Plantes d'un aussi grand usàge que le Safran. Ses fleurs font agréables à la vue & à l'odorat. Son pistile est consideré comme une chose précieuse. Il entre dans les apprefts de cuisine; il fournit aux Teinturiers une très belle couleur; les Médecins l'employent très utilements dans plusieurs maladies: sa Fanne même & ses pétales servent dans le pays où on le cultive, à faire du Fourrage pour les bestiaux.

   Mais semblable en cela aux Plantes les plus précieuses, celle-ci est tendre & délicate, & ne peut être conservée que par des soins proportionnés à ses usages.

   C'est pourquoy quelque précaution que les habitans du Gaslinois qui la cultivent prennent pour sa conservation , elle ne laisse pas d'être attaquée de plusieurs maladies, qui toutes tendent à la détruire.

   De toutes celles ausquelles cette Plante est sujette, il n'y en a point de plus dangereuse, ni qui lui soit plus nuisible que celle que les habitans du pays appellent la Mort. Et j'ai été surpris des desordres que cause cette maladie dans les endroits qui ont le malheur d'en être affligés.

   Et qui ne le seroit pas en effet, de voir qu'une Plante attaquée d'une maladie devient meurtrière des autres de son espece? En avoit-on jusqu'ici remarqué de contagieuses Epidemiques dans les Plantes? Celle qui attaque l'Oignon du Safran est cependant de cette nature,. puisque semblable à la peste des animaux , elle gâte les Oignons voisins , & bientost l'extrémité du champ le sentiroit de la contagion si l'on n'empêchoit la communication par une profonde tranchée qu'il est essentiel de faire dès le commencement du Printemps; parce que la Mort qui fait beaucoup de progrès dans cette saison, n'en fait presque point dans les autres; circonstance digne de remarque, dans la saison où les Plantes paroissent le plus en état de résister à la contagion, elles y succombent, & périssent en plus grand nombre.

Plant diseases caused by Phytophthora species will remain an ever increasing threat to agriculture and natural ecosystems. Phytophthora literally means plant destroyer, a name coined in the 19th century by Anton de Bary when he investigated the potato disease that set the stage for the Great Irish Famine. Phytophthora infestans, the causal agent of potato late blight, was the first species in a genus that at present has over 100 recognized members. In the last decade, the number of recognized Phytophthora species has nearly doubled and new species are added almost on a monthly basis. Here we present an overview of the 10 clades that are currently distinguished within the genus Phytophthora with special emphasis on new species that have been described since 1996 when Erwin and Ribeiro published the valuable monograph ‘Phytophthora diseases worldwide’ (35).

Necrosis and ethylene inducing-like proteins (NLP) are widely distributed in eukaryotic and prokaryotic plant pathogens and are considered to be important virulence factors. We identified a total of 70 potential Phytophthora sojae NLP genes but 37 were designated as pseudogenes. Sequence alignment of the remaining 33 NLPs delineated six groups. Three of these groups include proteins with an intact hepta-peptide (GHRHDWE) motif, which is important for necrosis inducing acitivity, whereas the motif is not conserved in the other groups. A total of 19 representative NLP genes were assessed for necrosis-inducing activity by heterologous expression in Nicotiana benthamiana. Surprisingly, only 8 genes trigger cell death. The expression of the NLP genes in P. sojae was examined, distinguishing 20 expressed and 13 non-expressed NLP genes. Real-time RT-PCR results indicate that most NLPs are highly expressed during cyst germination and infection stages. Amino acid substitution ratios (Ka/Ks) of 33 NLP sequences from four distinct P. sojae strains resulted in identification of positive selection sites in a distinct NLP group. Overall our study indicates that expansion and pseudogenization of the P. sojae NLP family results from an ongoing birth-and-death process, and that varying patterns of expression, necrosis-inducing activity, and positive selection suggest that NLPs have diversified in function.

Plant disease-resistance (R) proteins are thought to function as receptors for ligands produced directly or indirectly by pathogen avirulence (Avr) proteins. The biochemical functions of most Avr proteins are unknown, and the mechanisms by which they activate R proteins have not been determined. In Arabidopsis, resistance to Pseudomonas syringae strains expressing AvrPphB requires RPS5, a member of the class of R proteins that have a predicted nucleotide-binding site and leucine-rich repeats, and PBS1, a protein kinase. AvrPphB was found to proteolytically cleave PBS1, and this cleavage was required for RPS5-mediated resistance, which indicates that AvrPphB is detected indirectly via its enzymatic activity.

Plants are engaged in a continuous co-evolutionary struggle for dominance with their pathogens. The outcomes of these interactions are of particular importance to human activities, as they can have dramatic effects on agricultural systems. The recent convergence of molecular studies of plant immunity and pathogen infection strategies is revealing an integrated picture of the plant–pathogen interaction from the perspective of both organisms. Plants have an amazing capacity to recognize pathogens through strategies involving both conserved and variable pathogen elicitors, and pathogens manipulate the defence response through secretion of virulence effector molecules. These insights suggest novel biotechnological approaches to crop protection.