Azospirillum brasilense

Azospirillum brasilense represents a different model of plant-microbe mutualism than nodule-forming rhizobia. Rather than forming specialized structures, Azospirillum lives in the rhizosphere—the soil zone surrounding roots—and on root surfaces. It fixes nitrogen, but more importantly, it produces plant hormones that enhance root growth, improving the plant's ability to access water and nutrients. This associative rather than intimate relationship provides benefits to both partners without the commitment of nodule formation.

The looser partnership offers flexibility that nodule symbiosis cannot. Azospirillum associates with grasses, cereals, and other non-legumes that cannot form nodules. It provides benefits to plants regardless of their genetic capacity for symbiosis. Commercial Azospirillum inoculants are used on wheat, corn, and sugarcane, often increasing yields by 10-30% while reducing fertilizer requirements. The bacterium demonstrates that mutualism exists on a spectrum—from casual associations to intimate symbiosis—with different arrangements suiting different contexts.

Azospirillum research revealed that plant growth promotion involves more than nitrogen fixation. The bacterium produces auxins, cytokinins, and gibberellins that directly stimulate plant growth. It also outcompetes plant pathogens in the rhizosphere, providing protection. This multi-service provision makes Azospirillum valuable even when soil nitrogen is adequate. The lesson: successful partnerships often deliver multiple benefits beyond the obvious primary value, with secondary services sometimes mattering more than the headline capability.