Rhizobium-legume symbioses: Molecular signals elaborated by rhizobia that are important for nodulati

Abstract

Symbiotic nitrogen fixation has traditionally been considered the domain of rhizobia belonging to α-proteobacteria and leguminous plants. Recent discovery of members of β-proteobacteria capable of nitrogen reduction indicates a greater diversity among nitrogen fixing symbionts than previously realized. Intense research on nitrogen-fixing bacteria conducted during the past few decades has shed light on the molecular signals and thye intricate biochemical events that occur when the rhizobia interact with legumes. In this paper, we have carefully documented these advances on the present understanding of nodule formation which is an orchestrated sequence of events evidenced by changes in root morphology at the site of infection. Attachment of the bacteria to the legume roots results in root hair curling and deformation forming a pocket, which serves to entrap the rhizohia thus facilitating subsequent events of nodulation. Initiation and continuation of the nodulation process is dependent upon a constant exchange of chemical signals between the host legume and the bacteria. Several bacterial and plant-derived components play a crucial role in establishing symbiotic association. Flavonoids released by the legumes and lipochiooligosaccharides (LCO) or nodulation factors (Nod factors) secreted by the rhizobia are the two principal signal molecules involved in establishing symbioses. Nodulation genes set apart the rhizobia from other nitrogen fixing and endophytic bacteria. They encode proteins essential for the production of a unique set of molecules, known as Nod factors and other complex organic molecules integral to the symbiotic relationship between these bacteria and the legumes. Divided into three groups according to their respective functions, nod genes are designated as regulatory, common, or host specific. We observe that our increased knowledge and understanding of the symbiotic process, however, has resulted in minimal transfer of technology for use in commercial agricultural production and suggest that further research on the manufacture and use of flavonoids and Nod-factors to enhance nodulation in cultivated legumes such as soybean can be a promising area.