Interested Members of the University Community are invited to attend the Final Oral Examination for the Degree of Doctor of Philosophy of JENNIFER WILKER of the Department of Plant Agriculture 

Date: TUESDAY, FEBRUARY 16
Time: 10 AM Eastern Standard Time 

Method:Via Zoom: Join Zoom Meeting 
Meeting ID: 997 2980 14091 
Passcode: 559663 

Thesis Title: Symbiotic Nitrogen Fixation in Heirloom, Landrace, and Modern Participatory-Bred and Conventionally-Bred Common Bean (Phaseolus vulgaris L.) 

Advisory Committee 
Dr. Peter Pauls, Advisor
Dr. Sally Humphries
Dr. Timothy Porch
Dr. Istvan Rajcan

Examination Committee
Dr. Manish Raizada, Chair
Dr. Roberto Papa, External Examiner
Dr. Peter Pauls
Dr. Timothy Porch
Dr. Jay Subramanian                                            

ABSTRACT 

Among legumes, common bean (Phaseolus vulgaris L.) is generally thought to have poor capacity for symbiotic nitrogen fixation (SNF). To reduce the need for expensive and environmentally non-sustainable fertilizer amendments to achieve high yields, bean growers worldwide would benefit from the development of bean varieties with stable, yield-sustaining nitrogen fixing capacity. In this thesis, 400 common bean genotypes were tested in inoculated, low-nitrogen field conditions to observe the diversity for SNF-related and agronomic traits. SNF was quantified using the natural abundance method in which isotope values were calculated from bean seeds, and genotypes that fixed large proportions of N were considered to have superior SNF capacity. In a panel of heirloom and conventional varieties it was confirmed that SNF capacity is greater in Middle American genotypes than in Andean genotypes, and that heirloom bean varieties have a wider range of SNF capacities than conventional varieties. In collaboration with an NGO in Honduras, a panel of Honduran bean varieties was curated, which included landraces, conventional varieties, and varieties developed through participatory plant breeding (PPB). It was found that genetic diversity was greater among Honduran landraces, and that these genotypes had higher SNF capacities than the other breeding categories. Candidate genes associated with SNF were identified in regions of the genome where nucleotide diversity was higher in the landraces and where population genetic differentiation was evident between the landraces and the PPB varieties. With the Middle American diversity panel (MDP) assembled by the Bean Coordinated Agricultural Project (BeanCAP), a genome wide association study (GWAS) was carried out to identify quantitative trait loci (QTL) associated with SNF traits in the entire panel and within each panel subpopulation. The largest number of QTL were found for seed nitrogen content (%N), followed by percent nitrogen derived from the atmosphere (%Ndfa) and leaf chlorophyll content. For most traits, QTL identified for the entire panel were not repeated in the panel subpopulations, nor were QTL shared between the subpopulations. Additionally, the QTL identified in the MDP are novel and do not co-localize to regions of the genome where SNF-related QTL were reported previously. Overall, it can be concluded that diversity for SNF in common bean is wide-ranging, and is found in germplasm as diverse as landraces, heirloom varieties, and varieties bred through conventional and participatory methods. This diversity can be employed in breeding efforts to generate varieties with improved nitrogen fixation for growers worldwide. 

PUBLICATIONS 

Wilker, Jennifer; Navabi, Alireza; Rajcan, Istvan; Marsolais, Frederic; Hill, Brett; Torkamaneh, Davoud; Pauls, K. Peter (2019). Agronomic Performance and Nitrogen Fixation of Heirloom and Conventional Dry Bean Varieties Under Low-Nitrogen Field Conditions. Frontiers in Plant Science, 2019-07-26, Vol.10. DOI: 10.3389/fpls.2019.00952 

Wilker, Jennifer; Humphries, Sally; Rosas-Sotomayor, Juan Carlos; Gómez Cerna, Marvin; Torkamaneh, Davoud; Edwards, Michelle; Navabi, Alireza; Pauls, K. Peter (2020). Genetic Diversity, Nitrogen Fixation, and Water Use Efficiency in a Panel of Honduran Common Bean (Phaseolus vulgaris L.) Landraces and Modern Genotypes. MDPI Plants, 2020-09-19, Vol.9 (9), DOI: 10.3390/plants9091238