A null allele of the polyphenol oxidase gene Ppo-A1 in hexaploid wheat originates from tetraploid wheat

Polyphenol oxidase (PPO) has a major effect on the time-dependent darkening of noodle products. Development of varieties with low PPO activity is one method to eliminate this problem and respond to the demands of consumers. Among the common wheat (Triticum aestivum L.) Ppo genes, Ppo-A1 has the highest effect on grain PPO activity, and several Ppo-A1 alleles with low or no activity have been reported, with the recently identified Ppo-A1i allele being the most desirable allele. In this study, reverse-transcription polymerase chain reaction analysis confirmed that this allele was not expressed and was therefore functionally null. We also determined that Ppo-A1i has an approximately 3 kb insertion in the second intron. Taking advantage of the insertion sequence, we developed a new co-dominant marker, PPO18Plus, capable of distinguishing Ppo-A1a, Ppo-A1b, and Ppo-A1i. In addition, we determined that the Ppo-A1i allele is identical to and originates from the Ppo-A1g allele of tetraploid wheat. The durum wheat (Triticum turgidum ssp. durum (Desf.) Husn.) Ppo-A1g allele has been used to improve pasta color in durum wheat breeding programs. Thus, the PPO18Plus marker developed here will be very useful in both hexaploid and durum wheat breeding programs.