Submitted to: Theoretical and Applied Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/22/2007
Publication Date: 2/14/2007
Citation: Massa, A.N., Beecher, B.S., Morris, C.F. 2007. Polyphenol oxidase (PPO) in wheat and wild relatives: Molecular evidence for a multigene family. Theoretical and Applied Genetics, 1239-1247. Interpretive Summary: In order to better understand the organization of wheat polyphenol oxidase (PPO) and its gene family structure, genomic sequences from diploid, tetraploid, and hexaploid wheat species (wild and domestic) were examined in this study. PPO has been determined to be the cause of darkening and discoloration in noodles and other wheat-based products. This darkening has a huge impact on consumer acceptance of various wheat products. It seems that wheat PPO genes may have evolved through duplication into a multigene family. This study is particularly interested in how this evolution took place, and how the genes are characterized and expressed in the developing kernel.
Technical Abstract: Wheat polyphenol oxidase (PPO) is the major cause of browning reactions that discolor Asian noodles and other wheat products. It has been hypothesized that genes encoding wheat PPOs may have evolved by gene duplication into a multigene family. Here we characterized PPO genomic sequences from diploid (Triticum monococcum, T. urartu, Aegilops tauschii and Ae. speltoides), tetraploid (T. turgidum, subspecies dicoccoides and durum) and hexaploid (T. aestivum cultivars Klasic and ID377s) wheat species to gain a better understanding of the structure and organization of PPO genes. DNA fragments were amplified from a highly polymorphic and phylogenetic informative region of the gene. As a result, we obtained highly discriminative sequences. Three distinct PPOs, obtained from the A genome of T. monococcum, provided evidence for gene duplication events (paralogous loci). Furthermore, the number of sequences obtained for bread and durum wheat was higher than the expected number of orthologous loci. Sequence comparison revealed nucleotide and structural diversity, and detected five sequence intron types, all with a common insertion position. This was hypothesized to be homologous to that of intron 2 of previously reported wheat PPOs. A MITE of the Stowaway family accounted for the major difference between the five intervening sequences, and was unique to T. aestivum cv. Klasic. Nucleotide and structural diversity, together with well-resolved phylogenetic trees, provided molecular evidence to support the hypothesis of a PPO multigene family structure and organization.