CONTEXT, POSITIONING AND OBJECTIVES: Our aim is to increase our understanding of the role of pathogenicity factors of plant-pathogenic fungi, called effectors, in the infection process and in changes in virulence after host-shifts. Our central hypothesis is that the first step of host-shifts (the fixation of immune-escape mutations enabling to overcome non-host resistance) is followed by a second step of fine-tuning during which effector variants that efficiently interact with their target proteins in the novel host species are selected. Attempts to probe into the evolutionary, molecular and functional drivers of effector diversification have been hindered by the lack of large effector families identified in fungi, and thus the lack of good criteria to prioritize effectors for functional analysis. In this project, we overcome the methodological and conceptual barrier imposed by effector hyperdiversity by building on our recent discovery of an important, structurally conserved, but sequence-diverse family of effectors called MAX (for Magnaporthe Avrs and ToxB) in the model organism Magnaporthe oryzae, a fungal pathogen causing blast disease on rice and other cereals. In the highly multi-disciplinary MagMAX project, we will generate and integrate knowledge on protein structure, mode of action and genetic diversity to address four specific objectives: (1) Infer the diversification history of effector repertoires after host-shifts, (2) Understand the features determining the protein structure of effectors, (3) Decipher the virulence functions of fungal effectors by identifying the host proteins and processes they target, (4) Understand the molecular and eco-evolutionary factors driving fungal effector diversification. PROJECT ORGANISATION AND MEANS IMPLEMENTED: MagMAX will be coordinated by evolutionary microbiologist Dr Pierre Gladieux (INRA). MagMAX involves BGPI, which is among the best-qualified labs in functional and evolutionary studies of M. oryzae, and Centre de Biochimie Structurale, which is recognized for its expertise in studying protein biochemistry and structure biology. The project is organized in four work packages (WP) matching our four specific objectives. WP1 will infer the diversification history of the MAX effector repertoire, based on the production of high-quality re-sequencing data for a large sample set representative of the diversity of M. oryzae. WP2 will decipher the features governing the MAX effector fold, which is key to understand the effector’s mode of action, by modeling and experimental determination of their tertiary structure. WP3 will massively identify the host target proteins of MAX effectors using analyses of protein-protein interactions. WP4 will assess the hypothesis of molecular adaptation of MAX effectors to their targets combining information from other WPs about surface residues, surface of interactions and surface polymorphism. IMPACT: The world agricultural system needs to produce twice the amount of food before 2050. The emergence of new fungal plant pathogens poses a threat to global food security, and host-shifts are a major route for their emergence. Consequently, there is tremendous interest in identifying the factors driving the emergence of new fungal diseases. By combining diversity-, function- and structure-informed analysis of fungal effectors, our study will be pioneering, owing to its large-scale, multidisciplinarity and integrated nature, and as such should have a significant impact. Improved understanding of the biological features and molecular mechanisms underlying exploitation of new hosts will allow informed selection of which components of plant immunity to engineer to durably reduce the disease burden of cereals caused by fungi.