Functional investigation of receptor-like kinase domains and motifs crucial for root nodule symbiosis

Seidler, Anna Isabel Mishal (2025): Functional investigation of receptor-like kinase domains and motifs crucial for root nodule symbiosis. Dissertation, LMU München: Faculty of Biology

Abstract

Arbuscular mycorrhiza (AM) symbiosis is a widespread, ancient symbiosis that facilitates nutrient uptake by land plants. In contrast, the intracellular uptake of nitrogen-fixing bacteria is rare and only occurs in a distinct phylogenetic clade, comprising the Fabales, Fagales, Rosales and Cucurbitales (FaFaCuRo clade). While a big variety of Fabales engage in root nodule symbiosis (RNS) with nitrogen-fixing bacteria, it is rare among the other members of the FaFaCuRo clade. This led to the assumption, that there was predisposition event at the root of this clade. The establishment of these two symbioses requires a set of shared genes, the so-called common symbiosis genes. Among them, is the Symbiosis Receptor-like Kinase (SymRK). SymRK homologues from the Eurosids I clade were described to complement RNS and AM symbiosis in the model species Lotus japonicus symrk mutant background, whereas SymRK from species outside that clade only complemented the AM symbiosis phenotype but not the RNS. This study focusses on the differences in complementation capacity of SymRK from Solanum lycopersicum (Tomato SymRK) and L. japonicus (Lotus SymRK), a Fabales species, to get a better understanding of SymRK evolution. We found that, in contrast to published data, the complementation of the L. japonicus symrk mutant with Tomato SymRK exhibited rare formation of infected nodules and very abundant primordia-like, but uninfected structures, that we named swellings. We wanted to clarify which domain of Lotus SymRK allows the protein to convey its full complementation capacity, i. e. abundant nodule without swelling formation. Therefore, we performed domain swap experiments by swapping the subdomains of the extracellular domain, the transmembrane domain and the intracellular domain. This experiment led to the conclusion that the intracellular domain plays an important role for the complementation capacity. Even though ubiquitination is known to play an important role for SymRK function, I did not observe differences in complementation capacity of Tomato SymRK and Lotus SymRK predicted ubiquitination site switch mutations. A further approach to come closer to the responsible amino acids was another domain swap experiment, swapping the intracellular domain in four different parts according to their amino acid conservation. This experiment though revealed that the role of the transmembrane domain or the extracellular domain is more important than suggested by the first domain swap experiment. Furthermore, the postulated evolutionary conservation of the intracellular domain of Nod-factor-receptors of Lotus japonicus (Lj) LjNFR1 and LjNFR5 with their homologues from Solanum lycopersicum (Sl) SlLYK1 and SlLYK10 could be observed. Taken together these results suggest distinct and complex evolutionary paths of Lotus japonicus receptor-like kinases enabling root symbiosis.

• Item Type: Theses (Dissertation, LMU Munich)
• Subjects: 500 Natural sciences and mathematics; 500 Natural sciences and mathematics > 570 Life sciences
• Faculties: Faculty of Biology
• Language: English
• Date of oral examination: 23. January 2025
• 1. Referee: Parniske, Martin
• Signature of the printed copy: 0001/UMC 31703