UC Riverside

Legume-rhizobium symbiosis has become a model system for studying beneficial symbiosis between eukaryotic hosts and bacterial symbionts. Hosts are predicted to exert control over beneficial bacterial symbionts to prevent the evolution and spread of exploitative genotypes. Since nitrogen is exchanged in legume-rhizobium symbiosis and legumes can acquire nitrogen from soils, there can context dependent effects from nitrogen-enriched soils that can dramatically alter legume-rhizobium symbiosis. Anthropogenic nitrogen deposition has been enriching terrestrial ecosystems for more than a century and is accelerating in some regions. One prediction is that nitrogen enrichment can relax selection to maintain host control traits, leading to evolution and spread of exploitative rhizobia and/or an breakdown in symbiosis if legumes cease to gain benefit from rhizobial infection.

Lotus strigosus is an annual legume that experiences variable nitrogen deposition and mineral nitrogen contexts across California. To examine the potential for adaptation to mineral nitrogen enrichment in L. strigosus, inbred lines of L. strigosus from populations that have experienced either little nitrogen deposition or more than seven decades of intense nitrogen deposition were exposed to a simulated nitrogen deposition gradient and Bradyrhizobium that vary in growth benefit provided to L. strigosus. To examine the effects of mineral nitrogen saturation on a host control in a legume that has historically experienced nitrogen-poor soils, L. strigosus from a pristine site were exposed to growth saturation mineral nitrogen and infected with either combinations of Bradyrhizobium strains that vary in benefit to L. strigosus and individual strains that vary in benefit.

Lotus exhibited little evidence of adaptation to use mineral nitrogen more efficiently in response to simulated nitrogen deposition. Symbisois with Bradyrhizobium was reduced at very high fertilizer levels, but this was most likely because of nitrogen toxicity rather than a loss of benefit from symbiosis.

Host control traits in L. strigosus from a pristine site over pairs of Bradyrhizobium strains were resilient to growth saturating nitrogen, contrary to expectations if such traits are costly. When inoculated with single strains of Bradyrhizobium in different mineral nitrogen contexts, L. strigosus exhibited fine tuned investment in Bradyrhizobium to prevent exploitation.