Journey to Nodule Formation: From Molecular Dialogue to Nitrogen Fixation

Abstract

AbstractSoil bacteria collectively called rhizobia establish a unique beneficial interaction with most legumes and a few nonleguminous plants in the family of Ulmaceae (Parasponia sp.). This endosymbiosis between rhizobia and the host plant leads to the formation of N2-fixing nodules, in which bacteria are provided with a carefully regulated oxygen and carbon supply allowing the bacteria to reduce nitrogen efficiently for the plant. The early steps of this symbiosis begin with exchange of signals, a molecular dialogue between the symbiotic partners. The chemical signals involved determine a high degree of host specificity. The release of the plant-derived chemicals to the rhizosphere activates the expression of the bacterial nodulation genes, resulting in the production of lipochitooligosaccharide signaling molecules called Nod factors. Nod factors induce a cascade of events in the host plant that leads to the formation of nodule primordia. Since nodulation and the subsequent nitrogen fixation are energy-intensive processes, the host plant maintains a balance between cost and benefit by limiting the number of nodules that form through autoregulation of nodulation. Functional nodules are not maintained throughout the life cycle of plants and nodules senesce when they get old. This review examines the current state of knowledge of signals and mechanisms involved in the early establishment of rhizobium–legume symbioses, focusing on how the host plant induces Nod factor production, how these signals are perceived and transduced in the host plant, and the physiological and morphological changes associated with root nodule development.KeywordsArbuscular MycorrhizalRoot HairInfection ThreadNodule SenescenceCalcium SpikeThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.