Azoarcus sp. BH72

Azoarcus sp. BH72 was isolated from Kallar grass in Pakistan, a salt-tolerant plant growing in saline soils without fertilizer. The bacterium colonizes grass roots endophytically and fixes nitrogen, demonstrating that intimate nitrogen-fixing partnerships can form with grasses, not just legumes. This discovery energized research into extending rhizobial benefits to cereal crops, potentially revolutionizing global agriculture.

Azoarcus exhibits remarkable colonization behavior. In the rhizosphere, it swims toward roots using chemotaxis, then enters through cracks at lateral root emergence points. Once inside, it spreads through intercellular spaces, eventually reaching the xylem—the plant's water-conducting tissue. This systemic colonization places bacteria throughout the plant, creating distributed nitrogen-fixing capacity rather than the concentrated nodular strategy of rhizobia.

The bacterium's genome revealed capabilities for surviving inside plants: mechanisms to avoid triggering plant immune responses, to compete with other endophytes, and to adapt metabolism to the internal plant environment. Understanding these mechanisms guides efforts to engineer more effective plant-microbe partnerships. Azoarcus represents both a natural model for grass-endophyte symbiosis and a potential component of engineered systems designed to bring nitrogen fixation to crops that feed most of humanity.