Submitted to: Theoretical and Applied Genetics
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/17/2007
Publication Date: 4/11/2007
Citation: He, X.Y., He, Z.H., Zhang, L.P., Sun, D.J., Morris, C.F., Fuerst, E.P., Xia, X.C. 2007. Allelic variation of polyphenol oxidase (PPO) genes located on chromosomes 2A and 2D and development of functional markers for the PPO genes in common wheat. Theoretical and Applied Genetics 115:47-58. Interpretive Summary: Polyphenol oxidase (PPO) is associated with browning and discoloration of pasta, pan bread, steam bread, and especially Asian noodles. It is essential for scientists in wheat breeding programs to develop wheat cultivars with low PPO activity for good end-use quality. Many studies to date implied that PPO activity is mainly conditioned by the genes located on the homoeologous group 2 chromosomes in wheat. It is desirable also to develop functional STS markers for PPO genes with high levels of precision in distinguishing different alleles associated with high vs. low PPO activity. Since no functional markers for the PPO gene on chromosome 2D have been developed so far, the main purpose of this study was to characterize the PPO genes on chromosomes 2A and 2D, and to develop functional STS markers for the PPO gene on chromosome 2D.
Technical Abstract: Polyphenol oxidase (PPO) activity is highly related to the undesirable browning of wheat-based end products, especially Asian noodles. Characterization of PPO genes and the development of their functional markers are of great importance for marker-assisted selection in wheat breeding. In the present study, complete genomic DNA sequences of two PPO genes, one each located on chromosomes 2A and 2D and their allelic variants were characterized by means of in silico cloning and experimental validation. Sequences were aligned at both DNA and protein levels. Two haplotypes on chromosome 2D showed 95.2% sequence identity at the DNA level, indicating much more sequence diversity than those on chromosome 2A with 99.6% sequence identity. Both of the PPO genes on chromosome 2A and 2D contain an open reading frame (ORF) of 1,731 bp, encoding a PPO precursor peptide of 577 amino acids with a predicted molecular mass of ~64 kD. Two complementary dominant STS markers, PPO16 and PPO29, were developed based on the PPO gene haplotypes located on chromosome 2D; they amplify a 713-bp fragment in cultivars with low PPO activity and a 490-bp fragment in those with high PPO activity, respectively. The two markers were mapped on chromosome 2DL using a doubled haploid population derived from the cross Zhongyou 9507/CA9632, and a set of nullisomic-tetrasomic lines and ditelosomic line 2DS of Chinese Spring. QTL analysis indicated that the PPO gene co-segregated with the two STS markers and was closely linked to SSR marker Xwmc41 on chromosome 2DL, explaining from 9.6 to 24.4% of the phenotypic variance for PPO activity across three environments. In order to simultaneously detect PPO loci on chromosomes 2A and 2D, a multiplexed marker combination PPO33/PPO16 was developed and yielded distinguishable DNA patterns in a number of cultivars. The STS marker PPO33 for the PPO gene on chromosome 2A was developed from the same gene sequences as PPO18 that we reported previously, and can amplify a 481-bp and a 290-bp fragment from cultivars with low and high PPO activity, respectively. A total of 217 Chinese wheat cultivars and advanced lines were used to validate the association between the polymorphic fragments and grain PPO activity. The results showed that the marker combination PPO33/PPO16 is efficient and reliable for evaluation PPO activity and can be used in wheat breeding programs aimed for noodle and other end product quality improvement.