IDENTIFYING RHIZOBIA PARTNERS FOR COLD-HARDY HAIRY VETCH

The overarching goal of this project is to elucidate how interactions amongtemperature, plant genotype, and microbial symbionts drive nodulation and BNF of the important cover crop, hairyvetch (Vicia villosa Roth). The legume/rhizobia symbiosis is perhaps the earliestexample of human use of beneficial microbes to boost crop yields. Thismutualism depends on rhizobia soil bacteria, which form organs callednodules on legume roots and fix atmospheric nitrogen (N) into a plant-available form, in exchange for carbon resources from the plant.Prior to widespread use of synthetic N fertilizers in the 20th century, thissymbiotic exchange, called biological nitrogen fixation (BNF), accountedfor almost all usable N entering agroecosystems. While synthetic Nfertilizers led to breakthroughs in agricultural yields, they also havenegative environmental impacts, especially the fossil fuels required for theirproduction and their contribution to water pollution and nitrous oxideemissions.One way to mitigate these negative effects is to increase BNFefficiency (N2 fixed per CO2 respired) to reduce reliance on syntheticfertilizers and maintain crop yields. There is increasing interest amongfarmers and industry in leveraging plant-microbe interactions to reducereliance on synthetic N inputs without sacrificing crop productivity.Improving BNF efficiency could prevent plants from investing too much carbon innodules, which could decrease yields. N-per-C efficiency is known to vary among legume hosts and rhizobia strains, yet neither this ratio nor itseffects on the rate or timing of plant and nodule growth are widely measured.Because U.S. crop production is dominated by summer field crops, it is critical toimplement efficient BNF in a manner that fits into these systems. Winter annual cover crops area promising option because they are planted after cash crop harvest in the fall, grown through thewinter, and terminated and incorporated into soil in the spring. Leguminous cover cropscontribute biologically fixed nitrogen for use by following cash crops. However, the coldclimates of northern states such as Minnesota and Wisconsin limit cover crop options, as fewspecies can successfully establish during fall. Hairy vetch (Vicia villosa Roth) is a legume thatcan be fall-seeded and reliably over-winter inthe upper Midwest. In severalcases, hairy vetch has been found to produce more N than other legume cover crops and toproduce enough N to support a subsequent corn crop.Recenthairy vetch breeding efforts led to thedevelopment of varieties cold-hardy enough to survive winter as far north as USDA planthardiness zone 3a. However, cold temperatures are well known to negatively impact the legume-rhizobia symbiosis, and lead to variable or insufficient N accumulationacross fields. There are two main approaches to improve biological nitrogen fixation in legumes: applying rhizobia inoculants which contain superior strains to seeds at planting,or planting legume cultivars which select more efficient rhizobia strains from the soilenvironment. I will conduct research in both areas to overcome low-temperature barriers to hairyvetch BNF: 1) develop a pipeline for inoculant development based on rhizobia traits that lead toimproved BNF at low temperatures and 2) identify hairy vetch varieties best able to select forefficient rhizobia strains from upper Midwest soils.