The genetic characterization of a nil polyphenol oxidase (PPO) trait for the improvement of end product quality in wheat (Triticum aestivum)
Hystad, Steven Michael.
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Wheat (Triticum aestivum) polyphenol oxidase (PPO) contributes to the time dependent discoloration of Asian noodles. Wheat contains multiple paralogous and orthologous PPO genes expressed in wheat kernels, Ppo-A1, Ppo-D1, Ppo-A2, Ppo-D2, and Ppo-B2. To date, wheat improvement efforts have focused on breeding cultivars containing Ppo-D1 and Ppo-A1 alleles conferring reduced PPO activity. A major impediment to wheat quality improvement is a lack of additional PPO alleles conferring reduced kernel PPO. Thus, the discovery of novel low or null PPO alleles is critical to further reduce PPO and improve noodle color stability. The objective of this study was to 1) Characterize mutations present in a low PPO line 07OR1074 and create molecular markers associated with newly characterized alleles; 2) assess the allelic impact of the newly characterized alleles on kernel PPO activity and upon Chinese white salted noodle quality. To characterize additional mutations present and develop molecular markers, genomic clones were created for each known PPO gene and sequenced. Previously reported very low PPO line, 07OR1074, was found to contain a novel allele at Ppo-A2 and null alleles at Ppo-A1 and Ppo-D1 loci and molecular markers were created that encompassed each mutation. To determine the allelic impact of the new alleles on kernel PPO activity, F3:4 lines were genotyped with the novel molecular markers, grown in the field in Bozeman, Montana, and assayed for kernel PPO activity. Evaluation of lines indicated a substantial genotypic effect on PPO with Ppo-A1 and Ppo-D1 loci contributing significantly (P <.001) to total PPO. To determine the effect of the null PPO alleles upon noodle quality Chinese white salted noodles were produced from both refined white flour and whole wheat flour. The results clearly demonstrate that the null PPO alleles at Ppo-A1 and Ppo-D1 loci produce noodles that are brighter (greater L*), more red (greater a*), and more yellow (greater b*) at 24 and 48 hours. These results show that mutations in Ppo-A1 and Ppo-D1 genes are important to lowering overall wheat kernel PPO activity producing a more desirable and marketable product.