Genetic characterization of kernel polyphenol oxidases in wheat and related species

Polyphenol oxidase (PPO) activity causes undesirable darkening of raw Asian noodles and other wheat products. In this study we investigate the genetic origins and diversity of wheat kernel PPO. PPO was characterized via activity assays, antigenic staining, and Southern blots in Triticum aestivum, Triticum dicoccoides, Triticum durum, Triticum dicoccum, Triticum monococcum, Triticum urartu, Aegilops speltoides, and Aegilops tauschii. Among these species, PPO activity was well-correlated with antigenic staining intensity toward a wheat kernel-type PPO antibody. High PPO activity was observed in all three T. monococcum accessions (Am genome), one Ae. speltoides accession, one T. durum accession, and two hexaploid wheat cultivars. Southern blots suggested the presence of two or more kernel-type PPO genes in diploid progenitors of the hexaploid A, B, and D genomes. Whole-kernel PPO activity was evaluated in disomic substitution lines derived from three T. dicoccoides accessions in the background of T. durum ‘Langdon’. PPO activity was primarily associated with chromosome 2A and to a much lower degree with chromosome 2B. DNA sequence comparisons showed that the intron associated with the high PPO allele on chromosome 2AL of hexaploid wheat had 94% nucleotide identity with the homeologous intron found in T. monococcum, a species with high kernel PPO activity. This implies that the ancestral PPO allele on the A genome is one of the high activity, and the low PPO allele found in hexaploid wheat represents a relatively recent genetic alteration. Results confirm the presence of multiple kernel-type PPO genes in the diploid and tetraploid progenitors and relatives of hexaploid wheat. However, it is likely that relatively few of the many kernel-type PPO genes present in wheat contribute substantially to kernel PPO activity. A single genetic locus on homeologous group 2 chromosomes may be the primary cause of high PPO activity in wheat kernels.