Water-tolerant nodulation is an adaptation of legumes that grow in wet or temporarily flooded habitats. This nodulation mode takes place at lateral root bases via intercellular bacterial invasion in cortical infection pockets. The tropical legume Sesbania rostrata has become a model for the study of the molecular basis of crack entry nodulation compared with root hair curl nodulation. For intercellular invasion, Nodulation Factor (NF) signalling recruits an ethylene-dependent, common Sym gene-independent pathway, leading to local cell death. The NF structure requirements are less stringent than for intracellular invasion in root hairs, which is correlated with a very specific NF-induced calcium spiking signature, presumably necessary for correct gene expression to assemble a functional entry complex in the epidermis.