Upon submergence, Azorhizobium caulinodans infects the semiaquatic legume Sesbania rostrata via the intercellular crack entry process, resulting in lateral root-based nodules. A gene encoding a gibberellin (GA) 20-oxidase, SrGA20ox1, involved in GA biosynthesis, was transiently up-regulated during lateral root base nodulation. Two SrGA20ox1 expression patterns were identified, one related to intercellular infection and a second observed in nodule meristem descendants. The infection-related expression pattern depended on bacterially produced nodulation (Nod) factors. Pharmacological studies demonstrated that GAs were involved in infection pocket and infection thread formation, two Nod factor-dependent events that initiate lateral root base nodulation, and that they were also needed for nodule primordium development. Moreover, GAs inhibited the root hair curling process. These results show that GAs are Nod factor downstream signals for nodulation in hydroponic growth.Legume plants develop a symbiotic interaction with rhizobia by forming root nodules in which the bacteria fix atmospheric nitrogen. Nodule formation integrates several developmental processes, such as induction of cortical and pericycle cell division and rhizobial invasion, which are coordinated in time and space. The onset of the symbiosis is marked by a complex exchange of signals, involving plant flavonoids and bacterial nodulation (Nod) factors. Recognition of specific Nod factors will switch on the nodulation program in the legume host.The best known mode of invasion is the root hair curling (RHC) mechanism that is used by most crop legumes and the model legumes barrel medic (Medicago truncatula) and Lotus japonicus. Rhizobia induce growing root hairs to curl in the root zone I, just above the root meristem, whereby a rhizobial microcolony is entrapped. Local cell wall degradation and subsequent inward growth of the root hair plasma membrane result in the formation of an infection thread (IT) that guides the bacteria to the cortical cells. RHC is Nod factor dependent, and purified compatible Nod factors trigger several nodulation-related effects within the root hair, such as deformation, gene expression, Ca 21 spiking, membrane depolarization, and ion effluxes (Oldroyd and Downie, 2004). Several compo-

Background: Biological nitrogen fixation is a prokaryotic process that plays an essential role in the global nitrogen cycle. Azorhizobium caulinodans ORS571 has the dual capacity to fix nitrogen both as free-living organism and in a symbiotic interaction with Sesbania rostrata. The host is a fastgrowing, submergence-tolerant tropical legume on which A. caulinodans can efficiently induce nodule formation on the root system and on adventitious rootlets located on the stem.