The soybean symbiosis with rhizobia results in new root organs, nodules, in which the bacteria fix nitrogen for the plant. To balance the benefits and costs of this interaction, nodule numbers are tightly regulated by plants via autoregulation of nodulation (AON), which involves systemic root-shoot signalling. The NARK kinase plays a central role in the shoot where it perceives root-derived signals (peptides) and controls the generation of the shoot-to-root signals through currently unknown mechanisms. Importantly, a similar mechanism, but now involving root-localized NARK, is induced by nitrate to negatively control nodulation, but detailed insight into that signalling cascade is also lacking. I will identify new molecular players in the NARK pathway to better understand local and systemic control of nodulation, contributing to the optimization of nitrogen fixation for sustainable agriculture. The study of the NARK-dependent root and leaf phosphoproteomes in response to respectively nitrate and AON, will reveal new regulated phosphorylated proteins, which will be investigated through biochemical and genetic approaches to demonstrate their importance. The analysis of transgenic lines with changed levels of protein expression will further reveal whether these new components play an important role during AON or nitrate-controlled nodulation and will establish whether these pathways influence the bean protein content, an important agricultural yield parameter.