Question

how does rhizobium bacteria know that they are residing in the root nodules of leguminous plants

Answer 1

Rhizobium bacteria are free living in the soil but they need a plant host to fix nitrogen. These bacteria are able to sense flavonoids released by roots of leguminous plants. This leads to the bacteria releasing nod factors that are picked up by the leguminous roots. This brings about deformations in root hair called nodules where bacteria enter by endocytosis and reside.