Receptor proteins and redox control at the interface of symbiosis & immunity – DUALITY

The project exploits innovative new tools such as biosensors for in vivo spatio-temporal analysis of redox states; recently obtained genetic and genomic resources for Aeschynomene evenia; new Medicago truncatula plant mutants (in receptors and redox signalling), and new MtNFP interacting proteins.

WP1. Determine the role of the plant cellular redox state in different biotic conditions, as well as in NF-(in)dependent nodulation

Nitrate Reductase (NR), the first enzyme of the nitrogen reduction pathway in plants, is the best-characterised source of NO in plants. Interestingly, the double Mtnr1/2 mutant seems affected in nodulation capacity. We are now trying to determine whether this phenotype is correlated with a loss of NO production.

WP2. Define how Medicago truncatula LysM-RLKs control redox signalling for symbiosis and/or immunity

New Mtnfp mutants have been identified that have novel phenotypes in response to both Aphanomyces euteiches and rhizobia, providing excellent material to better understand how MtNFP jhas a dual role in controlling both symbiosis and immunity (P1 and P2).

WP3. Determine the connections between NF-independent nodulation, LysM-RLKs variations and redox balance in Aeschynomene evenia

LYR3 proteins of A. evenia and A. patula, which differ in their dependence or not on Nod factors, were shown to have contrasted LCO binding capacities, in accordance with sequence-based predictions (Bouchida et al., 2022) (P1 and P4).

WP4 Identification of MtNFP–dependent downstream signalling pathways and MtNFP interactors for symbiosis and/or immunity

A newly-evolved chimeric LysM-RLK in Medicago truncatula spp. tricycla R108, interacts with MtNFP and extends its Rhizobia symbiotic partnership (Luu et al. 2022) (P1 and P2).

How specific signal recognition through LysM-RLKs leads to the distinction of symbionts from pathogens, and what might be their functions in a NF-independent context is not well understood. To address this, DUALITY brings together 4 partners with complementary expertise and who are leaders in the fields of LysM receptors and symbiotic signalling in M. truncatula (P1), redox signalling (P2), plant immunity (P3) and Nod factor-independent symbiosis in A. evenia (P4).
This work is anticipated to provide breakthroughs to explain the mechanisms by which some members of an important family of plant receptors largely involved in plant-microbe interactions, transmit microbial signalling via changes in redox balance that control defence to pathogens and symbiosis establishment. In turn, such knowledge may provide leads to simultaneously maintain efficient symbiosis and increase disease resistance in order to promote sustainable agriculture.

1. Bouchiba Y, Esque J, Cottret L, Maréchaux M, Gaston M, Gasciolli V, Keller K, Nouwen N, Gully D, Arrighi J-F, Gough C, Lefebvre B, Barbe S, Bono J-J. (2022) (P1 and P4)
An integrated approach reveals how lipo-chitooligosaccharides interact with the lysin motif receptor-like kinase MtLYR3. Protein Science; 31:e4327. doi.org/10.1002/pro.4327
2. Luu T-H, Ourth A, Pouzet C, Pauly N, Cullimore J (2022) (P1 and P2)
A newly-evolved chimeric lysin-motif receptor-like kinase in Medicago truncatula spp. tricycla extends its Rhizobia symbiotic partnership. New Phytologist doi: 10.1111/nph. 18270

Plants benefit from two major symbiotic interactions with soil microbes, the Arbuscular Mycorrhizal (AM) and the Rhizobium-Legume (RL) symbioses, to improve their mineral nutrition. They generally establish such associations following recognition of symbiotic chitin-derived molecules called Myc-LCOs and Nod factors (NFs) when they are produced by AM fungi and rhizobia, respectively. These symbiotic signal molecules are perceived by members of the plant LysM-domain Receptor-Like Kinase (LysM-RLK) family, which also controls the perception of pathogenic chitinic molecules. In the model legume Medicago truncatula, we have shown that four LysM-RLK receptors are particularly important for symbiosis and/or immunity; MtNFP, MtLYK3, MtLYR3 and MtLYK9. We have also shown the presence of these four LysM-RLKs in Aeschynomene evenia, which is an exception in legumes in not requiring NFs for the RL symbiosis. How specific signal recognition through LysM-RLKs leads to the distinction of symbionts from pathogens, and what might be their functions in a NF-independent context is not well understood.
Upon perception of symbiotic signals or pathogenic elicitors the regulation of Reactive Oxygen Species (ROS) and nitrogen species homeostasis, including hydrogen peroxide and nitric oxide (NO), constitutes one of the earliest known plant responses. In M. truncatula, symbiotic signals induce a rapid change in ROS and can block a pathogen-induced ROS burst, both effects being dependent on MtNFP. This supports close connections between MtNFP and ROS regulation for symbiosis and immunity. We have also shown that ROS and NO can have positive roles in both the AM and the RL symbioses, and that in A. evenia, ROS production and cell collapse are early steps of NF-independent symbiosis. We hypothesise that (i) the fine-tuned spatio-temporal dynamics of ROS and NO production via LysM receptors is a critical component of the mechanisms that enable M. truncatula to distinguish between symbiotic and pathogenic signals, (ii) LysM-RLK and ROS/NO have different roles in the establishment of the NF-independent RL symbiosis in A. evenia.
The originality of the DUALITY project is to study LysM-RLK receptors and redox state that both have crucial, and closely linked, roles in symbiosis and immunity. Our objectives are to determine (1) the precise “signatures” of ROS and NO production in different biotic conditions, as well as in both NF-dependent and NF-independent nodulation; (2) the roles of key LysM-RLKs in the regulation of redox signalling; (3) the variations to LysM-RLKs and their roles in influencing redox control to enable NF-independent nodulation; and (4) how MtNFP controls interactions with contrasting outcomes. To tackle these aims, DUALITY brings together 4 partners who are leaders in the fields of LysM receptors and symbiotic signalling in M. truncatula (P1), redox signalling (P2), plant immunity (P3) and Nod factor-independent symbiosis in A. evenia (P4). The project will exploit innovative new tools such as biosensors for in vivo spatio-temporal analysis of redox states; recently obtained genetic and genomic resources for A. evenia; new M. truncatula plant mutants (in receptors and redox signalling), and new MtNFP interacting proteins.
This work is anticipated to provide breakthroughs to explain the mechanisms by which some members of an important family of plant receptors largely involved in plant-microbe interactions, transmit microbial signalling via changes in redox balance that control defence to pathogens and symbiosis establishment. In turn, such knowledge may provide leads to simultaneously maintain efficient symbiosis and increase disease resistance in order to promote sustainable agriculture.