Towards an understanding on how RxLR-effector proteins are translocated from oomycetes into host cells

Suppression of Chitin-Triggered Immunity by Plant Fungal Pathogens: A Case Study of the Cucurbit Powdery Mildew Fungus Podosphaera xanthii

Fungal pathogens are significant plant-destroying microorganisms that present an increasing threat to the world's crop production. Chitin is a crucial component of fungal cell walls and a conserved MAMP (microbe-associated molecular pattern) that can be recognized by specific plant receptors, activating chitin-triggered immunity. The molecular mechanisms underlying the perception of chitin by specific receptors are well known in plants such as rice and Arabidopsis thaliana and are believed to function similarly in many other plants. To become a plant pathogen, fungi have to suppress the activation of chitin-triggered immunity. Therefore, fungal pathogens have evolved various strategies, such as prevention of chitin digestion or interference with plant chitin receptors or chitin signaling, which involve the secretion of fungal proteins in most cases. Since chitin immunity is a very effective defensive response, these fungal mechanisms are believed to work in close coordination. In this review, we first provide an overview of the current understanding of chitin-triggered immune signaling and the fungal proteins developed for its suppression. Second, as an example, we discuss the mechanisms operating in fungal biotrophs such as powdery mildew fungi, particularly in the model species Podosphaera xanthii, the main causal agent of powdery mildew in cucurbits. The key role of fungal effector proteins involved in the modification, degradation, or sequestration of immunogenic chitin oligomers is discussed in the context of fungal pathogenesis and the promotion of powdery mildew disease. Finally, the use of this fundamental knowledge for the development of intervention strategies against powdery mildew fungi is also discussed.

Multiple downy mildew effectors target the stress-related NAC transcription factor LsNAC069 in lettuce

To cause disease in lettuce, the biotrophic oomycete Bremia lactucae secretes potential RxLR effector proteins. Here we report the discovery of an effector-target hub consisting of four B. lactucae effectors and one lettuce protein target by a yeast-two-hybrid (Y2H) screening. Interaction of the lettuce tail-anchored NAC transcription factor, LsNAC069, with B. lactucae effectors does not require the N-terminal NAC domain but depends on the C-terminal region including the transmembrane domain. Furthermore, in Y2H experiments, B. lactucae effectors interact with Arabidopsis and potato tail-anchored NACs, suggesting that they are conserved effector targets. Transient expression of RxLR effector proteins BLR05 and BLR09 and their target LsNAC069 in planta revealed a predominant localization to the endoplasmic reticulum. Phytophthora capsici culture filtrate and polyethylene glycol treatment induced relocalization to the nucleus of a stabilized LsNAC069 protein, lacking the NAC-domain (LsNAC069ΔNAC ). Relocalization was significantly reduced in the presence of the Ser/Cys-protease inhibitor TPCK indicating proteolytic cleavage of LsNAC069 allows for relocalization. Co-expression of effectors with LsNAC069ΔNAC reduced its nuclear accumulation. Surprisingly, LsNAC069 silenced lettuce lines had decreased LsNAC069 transcript levels but did not show significantly altered susceptibility to B. lactucae. In contrast, LsNAC069 silencing increased resistance to Pseudomonas cichorii bacteria and reduced wilting effects under moderate drought stress, indicating a broad role of LsNAC069 in abiotic and biotic stress responses.

The Podosphaera xanthii haustorium, the fungal Trojan horse of cucurbit-powdery mildew interactions

The powdery mildew fungi are obligate biotrophic plant pathogens that develop a specialized structure for parasitism termed haustorium, which is responsible for nutrient uptake and factor exchange with the plant. In this work, we present a detailed microscopy analysis of the haustoria of the cucurbit powdery mildew fungus Podosphaera xanthii, a major limiting factor for cucurbit production worldwide. Despite being located inside plant epidermal cells, transmission electron microscopy (TEM) analysis showed the characteristic highly irregular outline of the extrahaustorial membrane that separates the extrahaustorial matrix of haustoria from the cytoplasm of the plant cell. TEM analysis also revealed the presence of some vesicles and electron-dense plaques of material surrounding the haustoria. In confocal microscopy analysis and aniline blue staining we found a positive correlation between haustorial development and deposition of callose, which is distributed as plaques around haustorial complex. In this study, a method for the isolation of P. xanthii haustoria was also adapted, which permitted the analysis of the formation of haustorial lobes and the visualization of vacuoles and the pool of vesicles inside the haustorial complex. Our findings suggested that the haustorial lobes were responsible for vesicular trafficking and most likely act as the main mediators of the fungus-plant dialogue. All of these findings were integrated into a model of the P. xanthii-host cellular interactions.

A novel structural effector from rust fungi is capable of fibril formation

It has been reported that filament-forming surface proteins such as hydrophobins are important virulence determinants in fungi and are secreted during pathogenesis. Such proteins have not yet been identified in obligate biotrophic pathogens such as rust fungi. Rust transferred protein 1 (RTP1p), a rust protein that is transferred into the host cytoplasm, accumulates around the haustorial complex. To investigate RTP1p structure and function, we used immunocytological, biochemical and computational approaches. We found that RTP1p accumulates in protuberances of the extra-haustorial matrix, a compartment that surrounds the haustorium and is separated from the plant cytoplasm by a modified host plasma membrane. Our analyses show that RTP1p is capable of forming filamentous structures in vitro and in vivo. We present evidence that filament formation is due to β-aggregation similar to what has been observed for amyloid-like proteins. Our findings reveal that RTP1p is a member of a new class of structural effectors. We hypothesize that RTP1p is transferred into the host to stabilize the host cell and protect the haustorium from degradation in later stages of the interaction. Thus, we provide evidence for transfer of an amyloid-like protein into the host cell, which has potential for the development of new resistance mechanisms against rust fungi.

The oomycete Pythium oligandrum expresses putative effectors during mycoparasitism of Phytophthora infestans and is amenable to transformation

The oomycete Pythium oligandrum is a mycoparasitic biocontrol agent that is able to antagonise several plant pathogens, and can promote plant growth. In order to test the potential usefulness of P. oligandrum as a biocontrol agent against late blight disease caused by the oomycete Phytophthora infestans, we investigated the interaction between P. oligandrum and Ph. infestans using the green fluorescent protein (GFP) as a reporter gene. A CaCl(2) and polyethylene-glycol-based DNA transformation protocol was developed for P. oligandrum and transformants constitutively expressing GFP were produced. Up to 56 % of P. oligandrum transformants showed both antibiotic resistance and fluorescence. Mycoparasitic interactions, including coiling of P. oligandrum hyphae around Ph. infestans hyphae, were observed with fluorescent microscopy. To gain further insights into the nature of P. oligandrum mycoparasitism, we sequenced 2376 clones from cDNA libraries of P. oligandrum mycelium grown in vitro, or on heat-killed Ph. infestans mycelium as the sole nutrient source. 1219 consensus sequences were obtained including transcripts encoding glucanases, proteases, protease inhibitors, putative effectors and elicitors, which may play a role in mycoparasitism. This represents the first published expressed sequence tag (EST) resource for P. oligandrum and provides a platform for further molecular studies and comparative analysis in the Pythiales.

A molecular insight into algal-oomycete warfare: cDNA analysis of Ectocarpus siliculosus infected with the basal oomycete Eurychasma dicksonii

Brown algae are the predominant primary producers in coastal habitats, and like land plants are subject to disease and parasitism. Eurychasma dicksonii is an abundant, and probably cosmopolitan, obligate biotrophic oomycete pathogen of marine brown algae. Oomycetes (or water moulds) are pathogenic or saprophytic non-photosynthetic Stramenopiles, mostly known for causing devastating agricultural and aquacultural diseases. Whilst molecular knowledge is restricted to crop pathogens, pathogenic oomycetes actually infect hosts from most eukaryotic lineages. Molecular evidence indicates that Eu. dicksonii belongs to the most early-branching oomycete clade known so far. Therefore Eu. dicksonii is of considerable interest due to its presumed environmental impact and phylogenetic position. Here we report the first large scale functional molecular data acquired on the most basal oomycete to date. 9873 unigenes, totalling over 3.5 Mb of sequence data, were produced from Sanger-sequenced and pyrosequenced EST libraries of infected Ectocarpus siliculosus. 6787 unigenes (70%) were of algal origin, and 3086 (30%) oomycete origin. 57% of Eu. dicksonii sequences had no similarity to published sequence data, indicating that this dataset is largely unique. We were unable to positively identify sequences belonging to the RXLR and CRN groups of oomycete effectors identified in higher oomycetes, however we uncovered other unique pathogenicity factors. These included putative algal cell wall degrading enzymes, cell surface proteins, and cyclophilin-like proteins. A first look at the host response to infection has also revealed movement of the host nucleus to the site of infection as well as expression of genes responsible for strengthening the cell wall, and secretion of proteins such as protease inhibitors. We also found evidence of transcriptional reprogramming of E. siliculosus transposable elements and of a viral gene inserted in the host genome.

Identification of appressorial and mycelial cell wall proteins and a survey of the membrane proteome of Phytophthora infestans

Proteins embedded in the cell wall and plasma membrane of filamentous oomycetes and fungi provide a means by which these organisms can interact with their local environment. However, cell wall and membrane proteins have often proved difficult to isolate using conventional proteomic techniques. Here we have used liquid chromatography tandem mass spectrometry (LC-MS/MS) to facilitate rapid and sensitive quantification of the cell wall proteome. We report the use of LC-MS/MS to identify differentially regulated proteins from the cell walls of three different lifecycle stages of the oomycete plant pathogen Phytophthora infestans: non-sporulating vegetative mycelium, sporulating mycelium, and germinating cysts with appressoria. We have also used quantitative real-time RT-PCR to confirm that the transcripts corresponding to some of these proteins, namely those identified in cell walls of germinating cysts with appressoria, accumulate differentially throughout the lifecycle. These proteins may, therefore, be important for pre-infective development and early pathogenicity. Up to 31 covalently and non-covalently bound cell wall-associated proteins were identified. All of the proteins identified in germinating cysts with appressoria, and several of those from mycelial fractions, were classified as putative effector or pathogen-associated molecular pattern (PAMP) molecules, including members of the CBEL family, the elicitin family, the crinkler (CRN) family and two transglutaminases. Thus, the cell wall of P. infestans may represent an important reservoir for surface-presented, apoplastic effectors or defence activation molecules. Proteins predicted to be cell surface proteins included IPI-B like proteins, mucins, cell wall-associated enzymes and annexin family members. Additionally we identified up to 27 membrane-associated proteins from Triton X-114 phase partitioned mycelial membrane preparations, producing the first inventory of oomycete membrane-associated proteins. Four of these proteins are small Rab-type G-proteins and several are associated with secretion.