Domestication, Breeding for Disease Resistance and Nitrogen Availability Effects in the Legume-Rhizobia Symbiosis

Legume crops are unique in their capacity to obtain nitrogen by associating with beneficial nitrogen fixing bacteria called rhizobia. Biological nitrogen fixation by rhizobia offers an attractive alternative to chemical-nitrogen fertilization as it comes without polluting byproducts. Evolutionary tradeoffs between above and belowground traits can lead to the disruption of host control over microbiota. My first chapter investigated how plant control over symbiosis has evolved under artificial selection. We found that although cowpeas experienced a modest decrease in genetic diversity during early domestication, they responded efficiently to variation in symbiotic effectiveness by forming numerous root associations with rhizobia. Thus, the early stages of domestication have not led to a substantial degradation of host control over symbiosis. Most importantly, this work uncovered segregating variation of host control traits in cowpea, revealing that these traits can be selected upon. My second chapter studied how legumes form root nodules to gain fixed nitrogen from rhizobia and how access to nitrogen in soil appears to adaptively allow plants to discriminate among these sources. Plants were inoculated with their symbiont as well as fertilized with different concentrations of organic nitrogen molecules that simulate symbiotically fixed nitrogen and inorganic sources that can be accessed in the soil. Results showed that net benefits of nodulation were reduced or eliminated under all forms of extrinsic fertilization. Additionally, even when symbiosis imposed significant costs, hosts did not reduce investment into nodulation or nitrogen fixation when exposed to aspartic acid, unlike with other nitrogen sources. Finally, I studied the effects of breeding for disease resistance in symbiosis by using near isogenic cowpea cultivars both susceptible and resistant to root knot nematodes. We used effective and ineffective (non-beneficial) rhizobial inoculants to test the responses of four different cowpea cultivars that vary in the degree of resistance. Our results show that the resistant cultivars respond similarly to wildtype progenitor hosts when inoculated with both effective and ineffective rhizobial strains, which suggest that breeding for disease resistance against nematodes has not affected the host plant’s capacity to differentiate beneficial rhizobia from ineffective strains