Development and validation of a new co-dominant DNA marker for selecting the null allele of polyphenol oxidase gene <i>Ppo-D1</i> in common wheat (<i>Triticum aestivum</i> L.)

Polyphenol oxidase (PPO) is a key enzyme contributing to the time-dependent discoloration of wheat products. Developing cultivars with low PPO activity is one way to solve this problem. In this study, we focused on the <i>Ppo-D1</i> gene, which has the second highest effect on grain PPO activity after the <i>Ppo-A1</i> gene. Utilizing resequencing data, we found that the <i>Ppo-D1</i> gene in the common wheat line 'Fukuhonoka-NIL', which exhibits low PPO activity, has an approximately 3 kb deletion in the 3'UTR and a 73 bp deletion in the third exon. The deletion in the third exon indicated that this allele was the <i>ppo-D1d</i> allele, previously identified in the wheat D genome progenitor, <i>Aegilops tauschii</i> Coss. Additionally, the <i>ppo-D1d</i> allele in 'Fukuhonoka-NIL' had very low expression, suggesting that this allele is non-functional. We developed a new co-dominant DNA marker for distinguishing the <i>Ppo-D1a</i>, <i>Ppo-D1b</i> and <i>ppo-D1d</i> alleles and demonstrated that F₂ plants homozygous for the <i>ppo-D1d</i> allele exhibited significantly lower grain PPO activity. Additionally, we determined that the <i>ppo-D1d</i> allele likely originated from <i>Ae. tauschii</i> ssp. <i>tauschii</i> (lineage 1) accessions. The <i>ppo-D1d</i> allele has not previously been found in common wheat (<i>Triticum aestivum</i> L., AABBDD genome), and thus the DNA marker developed in this study will be helpful for introducing this allele in common wheat breeding programs.