Sinorhizobium meliloti

Sinorhizobium meliloti has become the model organism for understanding how nitrogen-fixing symbioses work at the molecular level. Its partnership with alfalfa (Medicago sativa) has been dissected gene by gene, revealing the intricate signaling dialogue between plant and bacterium. This deep understanding enables engineering efforts to extend nitrogen fixation to crops that currently require fertilizers—potentially revolutionizing agriculture while reducing its environmental footprint.

The symbiosis begins with chemical conversation. Alfalfa roots release flavonoids that S. meliloti detects, triggering production of Nod factors—lipochitooligosaccharide signals that the plant recognizes. The plant responds by curling root hairs around bacteria and initiating nodule development. Inside developing nodules, bacteria differentiate into bacteroids—specialized nitrogen-fixing forms that can no longer reproduce. This differentiation represents commitment: bacteria sacrifice reproductive potential for partnership benefits.

S. meliloti research revealed that symbiotic quality varies within bacterial populations. Some strains fix more nitrogen; some fix less but reproduce more within nodules. Plants have evolved sanctions against cheating: nodules containing less-effective bacteria receive fewer resources and support smaller bacterial populations. This policing mechanism maintains cooperation despite evolutionary pressure toward selfishness. The S. meliloti system demonstrates that stable partnerships require enforcement mechanisms, not just mutual benefit—a lesson applicable to any collaborative relationship.