Soutenance de thèse de M. Guilherme Silva Martins «  INVESTIGATION SUR LA FONCTION DU GÈNE AGO5 DANS LE DÉVELOPPEMENT DES PLANTES ET LA DÉFENSE ANTIVIRALE »

Description :

Président-rapporteur :

• Professeur Alexandre Maréchal, Ph.D., Faculté des sciences, Département de biologie, Université de Sherbrooke

Membres du jury :

• Professeur Peter Moffett, Ph.D., directeur de recherche, Faculté des sciences, Département de biologie, Université de Sherbrooke
• Pr Feng Qu, Ph.D., évaluateur externe, Department of Plant Pathology, The Ohio State University - College of Food, Agricultural and Environmental Sciences
• Professeur Kamal Bouarab, Ph.D., évaluateur interne, Département de biologie, Université de Sherbrooke

Sommaire :

RNA silencing is a conserved gene silencing mechanism mediated by small RNA (sRNA), and Argonaute (AGO) proteins are endoribonucleases central to the efficient functioning of this mechanism. AGO proteins recognize and bind to sRNA duplexes, originating from endogenous or viral double stranded RNA (dsRNA) processing, to target RNA that share sequence similarities with the loaded sRNA. Arabidopsis thaliana, the model plant used in this study, encodes 10 AGO proteins. The last decade was marked by a significant advancement in knowledge related to the involvement of those AGO proteins in distinct plant developmental pathways and their role in antiviral defense. In Arabidopsis-virus interactions, AGO1, AGO2 and AGO4 are among the most studied AGO proteins. This is mostly due to the order in which they were first identified, their involvement in many developmental processes or the roles in antiviral defense against a range of different viruses. However, other AGOs have also a function in plant growth and development and are also involved in plant immunity. AGO5, for instance, is involved in a few developmental processes mostly confined to reproductive tissues and showed to be induced in systemic leaves and restrict PVX systemic infection. Whether AGO5 has other roles in the plant life cycle and can restrict other plant viruses remais uncharted territory. Work done during my studies focused on the elucidation of the roles of AGO5 in flowering time and defense against viruses. Firstly, we have demonstrated that loss-of-function mutation in the AGO5 gene leads to early flowering time, and this is, at least, partially due to a precocious increase in SQUAMOSA-PROMOTER BINDING LIKE (SPL) transcription factors expression. Furthermore, we showed that AGO5 physically interacts with miR156, and the delay in flowering time in the miR156 overexpression line was abrogated in ago5-5 background, suggesting a role of AGO5 in age-dependent vegetative-to-reproductive phase transition. Previously, a study from our lab has shown that AGO5 works redundantly and cooperatively with AGO2 against potato virus X (PVX). In addition, AGO2 and AGO5 acts cooperatively in a spatiotemporally-regulated manner in which AGO2 is the first layer of defense and AGO5 is induced mostly in the distal tissue as part of a systemic antiviral defense (Brosseau and Moffett 2015). In an effort to investigate what induces AGO5 in systemic leaves, in our second study, we have discovered that a JA-related mechanism represses the AGO5 gene in Arabidopsis in a COI1 and MYC2-independent manner. Although AGO5 induction during virus infection seems to be unrelated to changes in JA biosynthesis or signaling, we used plants with increased AGO5 expression due to abolished JA biosynthesis to explore the effect of AGO5 against different plant viruses. In conclusion, this thesis enlightens not only a novel role of AGO5 in plant development but also an interaction between AGO5 and a JA-related mechanism in Arabidopsis.