|PhD project on effector proteins of plant pathogens and on the molecular mechanisms by which they aid the infection process (e.g. suppression of plant defense). This project is part of the Centre of Biosystems Genomics (CBSG) and and will be carried out in the Dept. Biology, Utrecht.
Information: Guido Van den Ackerveken, Plant-Microbe Interactions, Utrecht University
PG7, Oomycete effectors stimulating plant disease susceptibility
In recent years it was discovered that oomycete pathogens, such as Phytophthora infestans and downy mildews, produce effector proteins that have their activity inside the host cell. These proteins are not “injected” into the host cell by a type III-like machinery as used by pathogenic bacteria such as Pseudomonas syringae, but are secreted by the pathogen and are then able to reach the host cell cytoplasm through an as yet unknown mechanism. The effector proteins that are translocated have a consensus RXLR sequence C-terminal of the signal peptide that is required for the transport into the host cell. The proposed role of the RXLR effectors is to suppress plant defense or to modulate other host cell processes that increase susceptibility and enhance pathogen virulence. In addition oomycetes secrete many other proteins during the infection process that have an extracellular activity, e.g. hydrolases, proteases, elicitin-like proteins, necrosis-inducing like proteins, etc., and that may contribute to establish a successful infection.
The genomes of P. infestans, Hyaloperonospora parasitica (downy mildew of Arabidopsis) and other oomycetes encode hundreds of potential effector proteins. Several of these proteins have been identified in our lab by proteomics and EST analysis. The question is: “what is the function of these secreted pathogen proteins in the infection process?”. The overall aim of this project is to understand the function of oomycete effectors, from H. parasitica and P. infestans, which stimulate plant disease susceptibility. The focus will be on those effectors that potentially affect host gene expression or metabolism (in collaboration with PG8).
The approach taken in this project starts with the selection of effectors that are conserved between H. parasitica and P. infestans (by bioinformatics and comparative genomics, with PG5, and BB16) as well as on species-specific effectors that stimulate disease susceptibility. To analyse the potential disease stimulating activity of oomycete effectors the corresponding genes will be cloned in plant expression vectors and transformed to Arabidopsis. Lines will be generated that express (35S and inducible) oomycete effectors in planta. By testing various pathogens lines can be selected that show enhanced disease susceptibility. The selections will be classified in lines that show suppression of plant defenses and lines that are more susceptible due to positive effects on the infection process. For this latter class of lines we will study changes in gene expression and metabolite content in order to understand the mechanism by which the selected effectors stimulate disease susceptibility.
The innovative aspect is in the combination of tools and expertise from different groups within the CBSG to mine and functionally study the information that is available in the five oomycete genomes. Detailed comparative genomics will allow the identification of genes and gene families that are evolutionary dynamic and that constitute candidate key virulence genes. By combining the power of bioinformatics / comparative genomics (BB16) with experiments in the Arabidopsis-downy mildew system we intend to discover important functions for selected effector proteins. The Arabidopsis system is ideally suited for this as it allows the rapid generation of large numbers of transgenic lines, the highly efficient analysis of gene expression changes (using available DNA microarrays), and the possibility to identify a large number of compounds through metabolic analysis. This will generate novel data on the activities of oomycete effectors and the mechanisms that make plants susceptible to oomycete diseases. This knowledge on effector function will be exploited to generate new approaches for resistance breeding against oomycete pathogen, i.e. in potato against P. infestans.