Molecular mechanisms of plant defense against pathogens / Mécanismes moléculaires de défense des plantes contre les pathogènes

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We study two molecular mechanisms by which plants defend themselves against viruses. RNA silencing encompasses a number of related processes that regulate gene expression of endogenous genes, as well as foreign nucleic acids, through the recognition and processing of double-stranded RNA. Most plant viruses produce double-stranded RNA replication intermediates that can be targeted by the host RNA silencing machinery. Dicer proteins process double-stranded RNAs into defined small RNAs and these in turn are bound by Argonaute (AGO) endoribonucleases that target and cleave complementary single-stranded RNA. At the same time, plant viruses encode suppressors or RNA silencing (VSRs), which interfere with the RNA silencing machinery. We have developed a platform of functional and genetic assays to analyse the antiviral activities of AGO proteins from Arabidopsis thaliana and other species. In doing so, we have identified three key proteins, AGO2, AGO4 and AGO5 that target viral RNAs. In particular, we find that these proteins not only overcome VSR activity, but, unlike other members of the AGO family, they possess properties that allow them to access viral RNA, which is normally protected inside viral replication complexes. We will undertake structure-function, cell biological and genetic analyses to understand how certain AGO proteins are able to target viral RNAs and how viruses counteract this defense. In addition, we will study the role of natural variation in AGO proteins in determining virus host range, its role in plant fitness and its utility in developing virus-resistant plants. ***We also study a type of innate immunity known as effector-mediated immunity (ETI) that is mediated by immune receptor-like proteins (NB-LRR proteins) that recognize all types of pathogens and induce a dramatic suite of defence responses. We have shown that ETI signaling leads to an inhibition of viral mRNA translation. In addition, although ETI does not cause a global inhibition of protein translation, using a translatomic approach, we find that large numbers of transcripts have altered translational status during ETI signaling. We will characterize the mechanisms and factors that inhibit viral translation as well as characterizing the involvement of translationally regulated transcripts during ETI defense responses. In addition, we will characterize the role of the global translational regulator TOR in the growth-to-defense transition that is necessary for effective pathogen defense. These approaches will employ a variety of plant model systems, genetics, biochemistry, cell and molecular biology.