A new symbiotic pathway for rhizobial infection and nodulation in legumes – SymWay

A new symbiotic pathway for rhizobial infection and nodulation in legumes

Legumes are of prime agronomic and ecological importance thanks to their ability to develop a nitrogen-fixing symbiosis with rhizobia that are hosted in a specialized root organ, the nodule. Research performed on two model legumes has identified many genes important for this symbiosis and led to a general scheme where rhizobia produce Nod factor signal molecules that are perceived by plant LysM-RLK receptors, which, in turn, activate a Nod signalling pathway controlling an intracellular infection process through infection thread formation, concomitantly with nodule organogenesis. However, this symbiotic process is not universal. Some legumes (25% of genera) such as lupin, peanut and Aeschynomene spp., use another symbiotic mechanism not involving infection threads. It is considered simpler but remains understudied. Among Aeschynomene spp., several are remarkable because they are efficiently nodulated by photosynthetic Bradyrhizobium strains not producing Nod factors. Therefore, alternative symbiotic pathways exist to trigger nodulation without Nod factor recognition and infection thread formation.

To decipher the mechanisms of this so-called Nod-independent symbiosis, we established Aeschynomene evenia as a working model during the ANR project “AeschyNod”, and recently generated a high-quality genome for this species. By combining a forward genetic screen for nodulation mutants in A. evenia with a genomic survey of genes known to have symbiotic roles in model legumes, we demonstrated the lack of involvement in A. evenia of key genes required for rhizobial perception and intracellular infection in model legumes, while downstream components of Nod signaling are conserved. The major breakthrough of this work was the identification of two novel actors essential for establishment of this Nod-independent symbiosis, AeRLCK (Receptor-Like Cytoplasmic Kinase) and AeCRK (Cysteine-rich Receptor-like Kinase). RLCKs and CRKs are central players in plant signalling pathways, through interaction with ligand-binding receptor kinases (RLKs). We hypothesize that AeRLCK and AeCRK play key roles by interacting with receptors and other actors to mediate perception of photosynthetic Bradyrhizobium, signal transduction and infection, but their mode of action remains to be discovered.

Characterization of AeRLCK and AeCRK is crucial to uncover the molecular basis of the Nod independent symbiosis. Our main goals in the SymWay project are i) to characterize the biological functions of AeRLCK and AeCRK, ii) to identify AeRLCK and AeCRK interacting proteins and their roles in nodulation, and iii) to understand how some legume species have evolved a Nod-independent symbiotic process through the recruitment of these actors. For these purposes, our consortium involves three partners, including two leaders in the study of the rhizobium-legume symbiosis; an expert on the Nod-independent process in Aeschynomene and a specialist in the characterization of symbiotic receptors. The third partner has strong expertise in bioinformatics. Together, we will make use of a series of genomic, genetic, transcriptomic, molecular biology and protein biochemistry approaches and exploit comparative symbiotic systems.

Deciphering the molecular mechanisms of the Nod-independent symbiosis, through characterization of AeRLCK and AeCRK, should lead to a new paradigm for how the rhizobium-legume symbiosis can be established and, thus, significantly broaden our views on the evolution and diversity of the mechanisms underpinning this highly beneficial plant-microbe interaction. In turn, acquired knowledge in Aeschynomene legumes could be transferred to crop legumes using a similar infection process (no infection threads) via breeding programs. This project is also expected to provide important novel leads to design adapted strategies aimed at transferring nitrogen-fixing symbiosis to cereals.