Molecular cloning and expression analysis of multiple polyphenol oxidase genes in developing wheat (Triticum aestivum) kernels

Polyphenol oxidase (PPO) is a major cause of time-dependent discoloration in raw wheat (Triticum aestivum) flour dough. The PPO-A1 and PPO-D1 genes have previously been implicated in dough discoloration. However, wheat contains multiple PPO genes. The goal of this study was to identify and quantify expression levels for PPO genes relevant to wheat quality. Three novel sequences were identified and found to be orthologous to one another and paralogous to the previously described PPO-A1/PPO-D1 group. The new genes localized to homeologous group 2 chromosomes. We propose naming these new genes PPO-A2, PPO-B2, and PPO-D2. Real-time PCR analysis determined that in the wheat cultivar ‘Alpowa’, PPO-A1a, PPO-A2b, PPO-D1b and PPO-D2b were all expressed to substantial levels in developing wheat kernels, while PPO-B2b was not. Transcript levels varied over the course of grain development, with peak levels observed at 9–16 days post-anthesis. These results show that wheat kernel PPO activity is the result of at least two orthologous families of two paralogous genes and that some of these genes are expressed to several-fold greater levels than others. The novel PPO-2 genes described here together account for 72% of PPO transcripts in developing kernels of the wheat cultivar Alpowa.