Effects of Nod Factors on Plants

Host-specific symbiotic interactions between rhizobia and leguminous plants result in nitrogen-fixing root nodule formation. This is achieved by a complex series of developmental changes in both partners involving spatially and temporally regulated expression of specific genes. The bacterial genes involved in nodulation (nod genes) code for the synthesis and excretion of specific signals, the Nod factors, that induce nodule organogenesis [1-4]. The core Nod factor structure consists of a tetra-or pentameric chitooligosaccharide that is N-acylated on the terminal non-reducing residue. The Nod factors, identified up to now from seven Rhizobium species, differ by substituents of the terminal sugar residues conferring the specific interaction with host plants. For example, the sulfate substitution on the reducing sugar residue of the R. meliloti Nod signals is indispensable for nodule induction on Medicago, while in the absence of the sulfate group R. meliloti gains the ability to trigger nodule formation on the non-host plant Vicia. The Nod factors act at pico-and nanomolar concentrations and all structural features of the molecules seem to contribute to their biological activity. The Nod signals induce various plant responses that result in root hair deformation, formation of preinfection threads, cortical cell division initiating the nodule primordium or nodule formation on certain host plants such as Medicago. The fact, that Medicago nodules can form spontaneously in the absence of bacteria, indicates that all steps of the nodule developmental pathway are controlled by the plant, uncoupled of bacterium infection, and are only triggered by the Nod factors.