Rhizobacteria are root-colonizing bacteria that form symbiotic relationships with many plants. The name comes from the Greek rhiza, meaning root. Though parasitic varieties of rhizobacteria exist, the term usually refers to bacteria that form a relationship beneficial for both parties (mutualism). Such bacteria are often referred to as plant growth-promoting rhizobacteria, or PGPRs.
The mechanism by which PGPRs promote plant growth is not clearly understood, but it is believed to be by the production of siderophores, assimilation of minerals, production of phytohormones, and symbiotic nitrogen fixation. PGPR bacteria include Pseudomonas putida, Azospirillum fluorescens, and Azospirillum lipoferum.
Though microbial inoculants can be beneficial for crops, they are not widely used in industrial agriculture, as large-scale application techniques have yet to become economically viable. A notable exception is the use of rhizobial inoculants for legumes such as peas. Inoculation with PGPRs ensure efficient nitrogen fixation, and they have been employed in North American agriculture for over 100 years.