Rhizobiales

Rhizobiales is an order of Gram-negative, predominantly aerobic bacteria within the class Alphaproteobacteria. Members occupy soils and plant-associated environments and exhibit diverse lifestyles, including nitrogen fixation, plant colonization, and, in some lineages, plant pathogenic interactions. The group is defined by phylogenetic relationships revealed by 16S rRNA gene sequences and comparative genomics.

A major feature of several Rhizobiales is symbiotic nitrogen fixation with leguminous plants. In these associations, bacterial partners infect root hairs and form specialized nodules in which they convert atmospheric nitrogen to ammonia, supplying fixed nitrogen to the plant in exchange for carbon compounds. The nodulation and nitrogen fixation processes are controlled by nodulation (nod) genes and nif genes, often carried on plasmids or genomic islands.

Prominent legume symbionts include Rhizobium, Sinorhizobium (Ensifer), Bradyrhizobium, Mesorhizobium, and Azorhizobium, each associated with different host ranges. Beyond symbiotic taxa, Rhizobiales also contains non-symbiotic and plant-pathogenic members. A well-known pathogen is Agrobacterium tumefaciens, the cause of crown gall disease; its Ti plasmid enables horizontal gene transfer to plant cells and has been widely exploited in plant genetic engineering.

Ecologically, Rhizobiales contribute to soil nitrogen inputs and broader biogeochemical cycles. Taxonomically, the order continues to be refined as genomic data integrate with traditional phenotypic characterizations, reflecting ongoing advances in bacterial systematics.