In the Rhizobium-legume symbiosis, compatible bacteria and host plants interact through an exchange of signals: Host compounds promote the expression of bacterial biosynthetic nod (nodulation) genes leading to the production of a lipochito-oligosaccharide signal, the Nod factor (NF). The particular array of nod genes carried by a given species of Rhizobium determines the NF structure synthesized and defines the range of legume hosts by which the bacterium is recognized. Purified NF can induce early host responses even in the absence of live Rhizobium One of the earliest known host responses to NF is an oscillatory behavior of cytoplasmic calcium, or calcium spiking, in root hair cells, initially observed in Medicago spp. and subsequently characterized in four other genera (D.W. Ehrhardt, R. Wais, S.R. Long [1996] Cell 85: 673-681; S.A. Walker, V. Viprey, J.A. Downie [2000] Proc Natl Acad Sci USA 97: 13413-13418; D.W. Ehrhardt, J.A. Downie, J. Harris, R.J. Wais, and S.R. Long, unpublished data). We sought to determine whether live Rhizobium trigger a rapid calcium spiking response and whether this response is NF dependent. We show that, in the Sinorhizobium melilotiMedicago truncatula interaction, bacteria elicit a calcium spiking response that is indistinguishable from the response to purified NF. We determine that calcium spiking is a nod gene-dependent host response. Studies of calcium spiking in M. truncatula and alfalfa (Medicago sativa) also uncovered the possibility of differences in early NF signal transduction. We further demonstrate the sufficiency of the nod genes for inducing calcium spiking by using Escherichia coli BL21 (DE3) engineered to express 11 S. meliloti nod genes.The Rhizobium-legume interaction initiates the development of a novel organ on the root of the host plant, the nodule, and its colonization by the bacteria, resulting in a nitrogen-fixing symbiosis. Within the first 12 to 24 h, bacteria trigger a series of microscopically visible morphological changes. In the epidermis, altered growth of root hair cells (root hair deformation) is followed by root hair curling. Bacteria concurrently induce renewed cortical cell division that will lead to the formation of a root nodule. Invasion structures, called infection threads, initiate within curled root hairs and grow into the developing nodule. Bacteria are eventually released from infection threads into the cells of the nodule, where they begin fixing nitrogen. Thus, in a compatible interaction, Rhizobium elicits root hair deformation and curling, infection thread development, and cell division in the root cortex leading to nodule formation. These morphological responses are considered to be the hallmarks of nodulation.Nodulation occurs only when compatible species of legumes and Rhizobium come into contact. Thus, Sinorhizobium meliloti interacts with Medicago spp. but not Vicia spp., which in turn form nodules in the presence of Rhizobium leguminosarum bv viciae. The specificity of the interaction is based on a reciprocal exchange of sig...