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Since the introduction of the soybean aphid, Aphis glycines Matsumura, to the soybean growing regions of the United States, the soybean aphid has caused considerable economic damage and yield loss to soybean growers. The objectives of this research were to evaluate selected genotypes for resistance to the soybean aphid and characterize transcriptional changes in response to aphid feeding to better understand the underlying tolerant mechanism(s) in KS4202 and genes contributing to its tolerance response. A field study (2009) was conducted to evaluate selected soybean genotypes during their reproductive stages for resistance to A. glycines. The economic injury level (EIL) was reached in all genotypes during the 2009-growing season. Most of the genotypes showed no significant differences in yield or yield parameters with some minor exceptions for a few yield parameters. For KS4202, the average seed weight and the average number of seeds per pod for aphid infested treatments were significantly lower than their respective non-infested control plants. The mean number of aphids was significantly higher for KS4202 when compared to the other genotypes and the average peak number of aphids for this genotype was almost 5 times the economic threshold. The second component of this research was to characterize transcriptional changes in response to aphid feeding to better understand the underlying tolerant mechanism(s) in KS4202 and genes contributing to the tolerance response. Comparing gene expression levels between infested and control plants for KS4202, over 550 genes had a higher expression level in response to aphid feeding, while, over 650 genes had a lower expression level in response to aphid feeding. For K03-4686 (susceptible), over 150 genes had a higher expression level in response to aphid feeding, whereas, over 750 genes had a lower expression level when comparing infested to control plants. This research will significantly add to the understanding of the mechanisms of soybean aphid tolerance in soybeans and allow for the continual development of improved soybeans varieties with soybean aphid resistance.
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