Soybean Tolerance to Soybean Aphid (\u3ci\u3eAphis glycines\u3c/i\u3e Matsumura) Herbivory

This research examined the soybean aphid tolerant soybean KS4202 and its possible role in soybean aphid management. Studies documented the stage at which the tolerance response was initially expressed, quantified the relationship between cumulative aphid-days (CADs) and yield loss, and determined economic injury levels (EILs) for the soybean aphid tolerant KS4202 soybean. At high aphid infestation levels, aphids negatively impacted yield for KS4202 and K03-4686 (susceptible) plants that were infested at V1 stage; however, KS4202 expressed tolerance to the soybean aphid at low aphid infestation levels. No significant differences in the yield parameters were detected for KS4202 when infested at any aphid infestation level; whereas K03-4686 was negatively impacted when CAD surpassed 10,000. The EILs calculated for KS4202 infested during reproductive stage soybean ranged from 826 to 3,415 aphids/plant, which were approximately 2.5-fold higher when compared to the EILs calculated for K03-4686. Studies were also conducted to document the expression of constitutive and induced defense-related genes in tolerant and susceptible soybeans in response to soybean aphid feeding. Differences in the expression of several JA-associated transcripts were observed between genotypes, suggesting that the constitutive expression of JA-associated transcripts may be important for KS4202 tolerance to soybean aphids, but not for K03-4686. The greater magnitude of PRX52, WRKY60 and PR1 induction in KS4202 relative to the susceptible genotype suggests that these transcripts may be contributing to the ability of KS4202 to tolerate high levels of oxidative stress. The use of peroxidase activity and relative expression of peroxidase transcripts as potential assays to phenotype aphid-tolerant recombinant inbred lines (RILs) were also investigated. Peroxidase basal levels were similar among the genotypes for plants at V1 and V3 stages. Overall, there was an indication that KS4202 had greater abundance of peroxidase (PRX52) than the high-yielding and susceptible genotypes in response to aphid feeding. Despite the evidence of PRX52 involvement in KS4202 tolerance, no direct relationship between PRX2 and aphid feeding was detected. The determination of EILs and identification of important mechanisms involved in plant tolerance is key for the development of successful breeding strategies and incorporation of this resistance into the IPM for soybean aphid. Advisors: Thomas E. Hunt and Tiffany M. Heng-Mos