Since PPO is known to be the major cause of browning reactions that discolour Asian noodles and other wheat products, a better understanding of PPO gene structure should contribute to minimizing the deleterious effects of PPO via wheat breeding and improvement. A PPO gene model has emerged that identifies two clusters of three genes each, that is, the genes in wheat exist in two evolutionary/expression groups: kernel and 'green tissue' (paralogous loci). This paper describes our recent progress on the kernel PPO gene structure and copy number, including evidence for gene duplication events (Massa et al, 2007). Furthermore, the number of sequences obtained for bread and durum wheat was higher than the number expected for orthologous loci. Nucleotideand structural diversity, together with well-resolved phylogenetic trees, provided molecular evidence to support the hypothesis of a PPO multigene family structure and organization in wheat.
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