Variation in two PPO genes associated with polyphenol oxidase activity in seeds of common wheat

Polyphenol oxidase (PPO) is often regarded as a major factor resulting in time-dependent darkening and discoloration of Asian noodles and other wheat end products. To understand the relationship between variation of PPO genes and PPO activity of seed, the three PPO genes, which express in immature wheat grain, were investigated in 216 common wheat cultivars. The results indicated that only TaPPO-A1 and TaPPO-D1 showed high polymorphisms related to PPO activity. Two alleles in both TaPPO-A1 (TaPPO-A1a and TaPPO-A1b) and TaPPO-D1 (TaPPO-D1a and TaPPO-D1b) were detected using denaturing polyacrylamide gel electrophoresis. Wheat cultivars with TaPPO-A1b usually showed higher PPO activity than those with TaPPO-A1a. The TaPPO-D1a allele was often found in lower-PPO-activity cultivars, compared with TaPPO-D1b. The sequencing results of DNA fragments confirmed that two introns existed in TaPPO-D1 like TaPPO-A1. Some variation of introns was detected in the two alleles of TaPPO-D1. During seed development, the high-PPO-activity cultivar, Yangmai 158 (TaPPO-A1b/TaPPO-D1b) showed higher transcription of the two PPO genes, in comparison to low-activity cultivar, Yongchuanbaimai (TaPPO-A1a/TaPPO-D1a). These results suggested that variation in introns may influence the transcription of TaPPO-A1 and TaPPO-D1 in immature seeds of wheat.