Type 3 secretion system for nitrogen-fixing symbiosis – SymEffectors

Legumes have a tremendous ecological and agronomic impact due to their ability to interact symbiotically with nitrogen-fixing bacteria, called rhizobia.
A major advance in our understanding of this interaction was the discovery, in the early 1990s that it relies on the exchange of diffusible signal molecules between the two partners. Nod factors (NFs) were recognized as an essential bacterial signal to trigger the nodulation program of the host plant. For a long time, this key signal was assumed to be conserved in all nodulating rhizobia.
However, we challenged in 2007 this dogma by demonstrating that photosynthetic Bradyrhizobium strains nodulate some Aeschynomene species in the absence of NF synthesis using (a) so far unknown signal(s). It was proposed that this alternative symbiotic process represents the ground state or ancient state of the rhizobium-legume symbiosis. More recently, we demonstrated that non-photosynthetic bradyrhizobia, thanks to a type 3 secretion system (T3SS), could also activate nodulation in Aeschynomene independently of NFs. Since most photosynthetic Bradyrhizobium strains lack this secretory machinery, we conclude that at least two different NF-independent pathways exist in legumes, a T3SS-dependent one and a T3SS-independent one. The T3SS machinery, initially identified in animal and plant pathogens, permits the delivery of effector molecules inside the host cells where they suppress defense responses and favor the infection. Although it is known for more than a decade that rhizobia can use T3SS to modulate the plant immune system during nodulation, our work revealed that T3SS effector(s) can also directly activate the plant nodulation signaling pathway thereby bypassing NF recognition.

Our main goal in the SymEffectors project is to explore for the first time how some rhizobia use a pathogenic strategy to develop symbiosis with legumes. Our consortium, involving specialists in plant symbiosis and pathogenesis, will combine omics, bacterial and plant genetic approaches and cell biology techniques to dissect at a molecular level this new alternative route that permit the establishment of a nitrogen fixing symbiosis in plants.

We will focus on the following questions: What are the T3SS effector(s) that trigger nodule organogenesis and infection? What are the plant signaling pathways that are activated during the establishment of this symbiotic process? What is the degree of conservation between the two NF-independent symbiotic signaling pathways? What are the plant target(s) of the symbiotic effector(s)?

The knowledge on this ancestral nodulation program is expected to be very important to further exploit the rhizobium-legume symbiosis already existing in nature and should allow the development of new strategies to transfer nitrogen-fixing symbiosis to cereals.