Nodulation Adapted to Habitat Submergence

The semi-tropical legume Sesbania rostrata grows in the Sahel region of West Africa, thriving in temporarily flooded habitats, and exhibits versatility in nodulation. In both model and some crop legumes, nodulation occurs in root zone 1 with developing root hairs. Azorhizobia invade the epidermis via root hair curling (RHC) and membrane invagination. These responses are triggered by rhizobial Nod factors (NFs) perceived by plant receptors of the LysM-RLK type (Geurts et al., 2005). Intracellular root-hair invasion (zone-1 nodulation) also occurs in S. rostrata but only under well-aerated root growth (Goormachtig et al., 2004). Submergence interferes with RHC nodulation by accumulation of inhibiting ethylene concentrations. In natural habitats, nodules form by default at both lateral root bases (LRBs) of hydroponic roots and at the bases of stemlocated adventitious rootlets. Here, azorhizobial invasion occurs via direct intercellular cortical colonization at epidermal cracks. The cortical invasion skips epidermal responses, requires less stringent NF features, and depends on ethylene to induce local cell death and so create space for microbial infection (D’Haeze et al., 2003).