Rhizobium
Introduction
Rhizobium is a genus of Gram-negative bacteria associated with the formation of root nodules in legumes. These bacteria are known for their ability to fix atmospheric nitrogen (N2) into a form that is usable by plants, a process known as biological nitrogen fixation.
Classification and Taxonomy
Rhizobium belongs to the family Rhizobiaceae, within the order Rhizobiales, in the class Alphaproteobacteria. The genus was first described by the German botanist Heinrich Anton de Bary in 1888. The taxonomy of Rhizobium has undergone significant changes over the years, with several species being reclassified into new genera such as Sinorhizobium and Mesorhizobium.
Morphology and Physiology
Rhizobium bacteria are rod-shaped and measure approximately 1-3 micrometers in length. They are motile, possessing one to several flagella at one or both ends of the cell. Rhizobium species are facultative aerobes, meaning they can grow in both the presence and absence of oxygen. They are also capable of metabolizing a wide range of organic compounds as sources of carbon and energy.
Symbiosis with Legumes
The symbiotic relationship between Rhizobium and legumes is a mutualistic one, where both the plant and the bacteria benefit. The plant provides the bacteria with a protected environment and nutrients in the form of organic acids, while the bacteria supply the plant with fixed nitrogen. This symbiosis is initiated by a complex chemical dialogue between the two partners, involving the exchange of signal molecules such as flavonoids and nodulation (Nod) factors.
Nitrogen Fixation
The ability of Rhizobium to fix atmospheric nitrogen is facilitated by the enzyme nitrogenase, which is encoded by the nif genes. Nitrogen fixation occurs within the root nodules, where the bacteria are enclosed in plant cells in a specialized structure known as a symbiosome. The nitrogenase enzyme is sensitive to oxygen, and the plant host provides a low-oxygen environment within the nodules for the enzyme to function effectively.
Genomics
The genomes of several Rhizobium species have been sequenced, providing insights into their metabolic capabilities and symbiotic interactions with legumes. The genome of Rhizobium typically consists of a single circular chromosome and one or more plasmids. The symbiotic genes involved in nodulation and nitrogen fixation are often located on a symbiotic plasmid or integrated into the chromosome in a symbiotic island.
Industrial and Agricultural Importance
Rhizobium plays a crucial role in sustainable agriculture due to its ability to enrich soil with nitrogen through symbiosis with legumes. This reduces the need for synthetic nitrogen fertilizers, which are costly and have negative environmental impacts. Rhizobium inoculants are commercially available and are commonly used in agriculture to enhance the productivity of legume crops.