LAYING THE GROUNDWORK TO BREED FOR MUTUALISMS:TRACKING LONG-TERM ADAPTATION OF NITROGEN-FIXING RHIZOBIA TO ALFALFA IN AGRICULTURAL FIELDS

The development of extensive genomic and bioinformatics resources in legume-rhizobium systems allows us to create novel sequence-based methods to answer fundamental questions about rhizobial fitness and the genomic basis of adaptation to abiotic and biotic contexts. We propose to apply these cutting-edge, lab-based methods to ongoing Pennsylvania State University variety trials of the perennial forage and cover crop alfalfa (Medicago sativa). This proposal will:Objective 1: Translate sequencing methodologies to the field to measure seasonal and alfalfa-mediated evolutionary processes in established varietal trialsObjective 2: Test a novel evolution-informed phenotyping pipeline to measure the ability of alfalfa varieties to reward helpful strainsObjective 3: Identify rhizobia candidate genes underlying: 1) adaptation to alfalfa nodules in agricultural fields, 2) competitive success across alfalfa varieties and environmental variation, and 3) N-fixation benefits to plantsIn particular, this proposal aims to assess the potential to select legume varieties that can maintain large populations of mutualistic rhizobia via rewards mechanisms. To accomplish this goal, we will employ interconnected field and greenhouse-based experiments. In Aim 1, we will measure seasonal- and host-mediated selection in alfalfa nodules in the variety trials over multiple years. In the greenhouse, we will assess the competitive strain fitness of field isolates (Aim 2) and quantify single-strain benefits (Aim 3).Aim 1:Measure seasonal- and variety-mediated selection on rhizobia in alfalfa variety trialsAim 2: Screen rhizobia isolates for competitive fitness across alfalfa varieties and environmental variabilityAim 3: Use single-strain inoculations to test N-fixation benefits to each alfalfa variety