Members of the legume family of plants (e.g. peas, soybean) can form symbioses with nitrogen-fixing bacteria known as rhizobia. In return for receiving nitrogen-containing compounds (e.g. ammonia) from the rhizobia, the plant supplies the rhizobia with sugars and a home in special organs in the plant root called nodules. To set up a symbiosis communication between the legume and rhizobia is required. The plant releases flavonoids and the rhizobia responds by producing Nod factors (NFs). The NFs activate signalling pathways in the plant root that lead to nodule formation and infection of the rhizobia. The whole process is known as nodulation.
Interestingly, the flavonoids released by the plant can also induce the activation of the type III secretion system in rhizobia (1). Type III secretion systems are commonly found in pathogenic bacteria where they act like a needle to inject “effector” molecules into the host to subdue defences during infection. So, what role is it playing in nodulation?