Nod factors are the main bacterial signal molecules that trigger nodule formation on legume plants. They control bacterial invasion as well as de novo cell division for primordium formation and are perceived by receptor complexes. In most legumes, the rhizobia enter via curled root hairs and are guided to the primordium cells by intracellular infection threads. Bacteria that enter the infection threads produce Nod factors and keep on generating these signal molecules till they are taken up by the plant cells and differentiate into nitrogen fixing bacteroids. We are studying several aspects of Nod factor signaling in the model legume Medicago truncatula but also in the water-tolerant tropical legume Sesbania rostrata that has evolved an alternative epidermis-free invasion way as an adaptation to periodically submerged habitats.