Sequence adaptation of Symbiosis Receptor-like Kinase (SymRK) enabling nitrogen-fixing root nodule development

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Molecular innovations underlying the evolution of novel traits are the drivers of biological diversification. The identification of these drivers provides the milestones within a roadmap to synthetically install or optimize this trait. Fossils of the earliest land plants carry structures of arbuscular mycorrhiza (AM), a symbiosis with nutrient-delivering fungi that most present day land plants still engage in. In contrast, the root nodule symbiosis (RNS) with nitrogen-fixing bacteria is much younger, and restricted to a single phylum comprising four orders of plants, the Fabales, Fagales, Cucurbitales and Rosales. The genetic toolkit underlying the intracellular accommodation of AM fungi is largely conserved across land plants. Parts of it have been co-opted during the evolution of RNS, including a set of signal transduction components such as the Symbiosis Receptor-like Kinase (SymRK). We obtained evidence that during evolution, SymRK underwent super-functionalization; it acquired novel molecular features that facilitated the development of RNS, while maintaining its conserved function for AM. In this project, we will explore the amino acid sequence diversity among SymRK orthologs with the goal to identify critical sequence adaptations that underlie the rhizobial infection of plant cells. We will study the mechanistic consequences of these sequence variations at the cellular and molecular level with a special focus on the SymRK protein interactome. Unravelling protein-protein interactions modulated by these adaptations will pinpoint the specific mechanistic innovations facilitating the symbiotic infection process of plant cells by nitrogen-fixing bacteria.