The rhizobial type III effector ErnA confers the ability to form nodules in legumes

Significance Legumes have a tremendous ecological and agronomic importance due to their ability to interact symbiotically with nitrogen-fixing rhizobia. In most of the rhizobial–legume symbioses, the establishment of the interaction requires the plant perception of the bacterial lipochitooligosaccharide Nod factor signal. However, some bradyrhizobia can activate the symbiosis differently, thanks to their type III secretion system, which delivers effector proteins into the host cell. Here, we demonstrate that this symbiotic process relies on a small set of effectors playing distinct and complementary roles. Most remarkably, a nuclear-targeted effector named ErnA conferred the ability to form nodules. The understanding of this alternative pathway toward nitrogen-fixing symbiosis could pave the way for designing new strategies to transfer nodulation into cereals.