Large-scale studies and biophysical analysis of systems involved in plant immunity.

Abstract

The field of plant immunity has progressed significantly in the last decade, driven primarily by both forward and reverse genetics and to a lesser extent by molecular biology techniques. However, many unknowns still remain before a more complete picture of this system can be achieved, which hinders our capacity to develop biotechnological solutions to ensure food safety for our growing population. Some of the problems that still need to be tackled relate to the multi-system involvement of some proteins, the interrelation of the different hormones, such as in trade-off systems, and the challenges of translating existing molecular knowledge into crop protection strategies. The goal of this thesis was to develop new methods and to adapt existing ones to address the challenges and push the boundaries of our knowledge of plant immunity as a system. We have adapted ClueGO analyses to visualize functionally grouped Gene Ontology (GO) terms specific to Arabidopsis. We developed a transcription factor- coregulator identification strategy based on double-transcriptome analyses. Finally, we have adapted a biophysical method, differential scanning fluorimetry (DSF). We tested the usefulness of these methods by interrogating different immune proteins/genes of the model plant Arabidopsis thaliana. Here is a summary of the major results obtained. In the realm of basal immunity, we discovered that clade I TGA transcription factors positively regulate this system by repressing WRKY transcription factors, which are negative regulators of the process. Furthermore, we have demonstrated that clade I TGA integrates into the growth- immunity trade-off system regulated by brassinosteroids by antagonizing the brassinosteroids-dependent suppression of basal immunity. In the realm of systemic acquired resistance (SAR), we have demonstrated that clade I TGA recruits a specific novel glutaredoxin as a corepressor to dampen the expression of a set of SAR-regulated genes controlled by salicylic acid (SA) and the SAR-orchestrator, NPR1. Finally, we demonstrated that NPR1 binds SA and that this interaction leads to the destabilization of NPR1. More importantly, the method used to show the latter is scalable and can be used to develop novel chemistries capable of deploying plant immunity in the field.