Salicylic acid (SA) is considered a key hormone driving plant immunity. Yet, our view on the complexity in microbe-induced SA signalling among plant species is limited. This is because most knowledge on SA perception, signalling and its role in plant immunity comes from research in Arabidopsis thaliana. Excitingly, lead findings suggest the presence of a novel signalling cascade in the crop tomato (Solanum lycopersicum), dubbed the NPR1-independent SA (NIS) signalling pathway, that acts separately and in parallel with the known, canonical NPR1-dependent cascade. The main goal of this PhD project is to complete the current plant immunity models with this NIS pathway and accurately reflect the situation for dicots. This will comprise protein-protein interactomics analyses with previously determined transcriptional repressor (NIS1), E3 Ub ligase (RING/NIS2) and receptor (NLR/NIS3) proteins. Furthermore, functional analyses will unravel the molecular machinery of the NIS pathway. These analyses include the evaluation of NIS1 stability in different conditions to determine why and how SA triggers NIS1 degradation, and if RING/NIS2 is involved. Additionally, the function of NLR/NIS3 as a potential novel SA receptor will be analysed. Finally, the role of the NIS and NPR pathways in SA and pathogen responses in tomato will be evaluated. This way, this project will generate paradigm-shifting fundamental findings in plant immunity.