Genetic characterization of kernel polyphenol oxidases in wheat and related species

An Overview of Factors Affecting the Functional Quality of Common Wheat (Triticum aestivum L.)

Wheat (Triticum aestivum L.) is one of the most important crops worldwide, and, as a resilient cereal, it grows in various climatic zones. Due to changing climatic conditions and naturally occurring environmental fluctuations, the priority problem in the cultivation of wheat is to improve the quality of the crop. Biotic and abiotic stressors are known factors leading to the deterioration of wheat grain quality and to crop yield reduction. The current state of knowledge on wheat genetics shows significant progress in the analysis of gluten, starch, and lipid genes responsible for the synthesis of the main nutrients in the endosperm of common wheat grain. By identifying these genes through transcriptomics, proteomics, and metabolomics studies, we influence the creation of high-quality wheat. In this review, previous works were assessed to investigate the significance of genes, puroindolines, starches, lipids, and the impact of environmental factors, as well as their effects on the wheat grain quality.

Polyphenol oxidase genes as integral part of the evolutionary history of domesticated tetraploid wheat

Studying and understanding the genetic basis of polyphenol oxidases (PPO)-related traits plays a crucial role in genetic improvement of crops. A tetraploid wheat collection (T. turgidum ssp., TWC) was analyzed using the 90K wheat SNP iSelect assay and phenotyped for PPO activity. A total of 21,347 polymorphic SNPs were used to perform genome-wide association analysis (GWA) in TWC and durum wheat sub-groups, detecting 23 and 85 marker-trait associations (MTA). In addition, candidate genes responsible for PPO activity were predicted. Based on the 23 MTAs detected in TWC, two haplotypes associated with low and high PPO activity were identified. Four SNPs were developed and validated providing one reliable marker (IWB75732) for marker assisted selection. The 23 MTAs were used to evaluate the genetic divergence (F_ST > 0.25) between the T. turgidum subspecies, providing new information important for understanding the domestication process of Triticum turgidum ssp. and in particular of ssp. carthlicum.

Determinants of wheat noodle color

Noodles are a leading food in the world, and color is a key determinant of consumer acceptance. In this review the two prominent forms of wheat noodles are considered: white salted and alkaline. Many of the preparation and evaluation strategies are the same for both, with prominence placed on 'brightness' (L*) or a lack of discoloration (ΔL*), and the absence of 'specks.' All raw noodles darken over time. Increasing the protein content of flours almost always translates into darker noodles. Greater discoloration is also associated with higher flour extraction rates, higher ash contents, and higher starch damage. Increasing storage time, dough water absorption, and temperature all often lead to greater discoloration. There is a large range in noodle color variation, and much of this variation is associated with genetics. Consequently, much research has been devoted to methods of screening germplasm, either as whole seeds, meals, flours, or noodle sheets. Polyphenol oxidase (PPO) is a primary culprit in noodle discoloration and has guided much of the research on noodle color. It is now possible to select germplasm with very low levels of PPO through the use of efficacious phenotype screens and the use of molecular markers. The success of this research has provided the opportunity to select wheat breeding lines with nil PPO activity, and to combine favorable alleles at multiple PPO loci. Yet, when noodles are prepared, we continue to observe discoloration. As our ability to minimize PPO activity increases, this 'non-PPO' discoloration has become more important. Perhaps the 'holy grail' is a noodle that never discolors, and has the 'perfect' level of a* (redness, zero?) and b* (yellowness/creaminess). Published 2018. This article is a U.S. Government work and is in the public domain in the USA.

Genetic characterization and expression analysis of wheat (Triticum aestivum) line 07OR1074 exhibiting very low polyphenol oxidase (PPO) activity

Characterized novel mutations present at Ppo loci account for the substantial reduction of the total kernel PPO activity present in a putative null Ppo - A1 genetic background. Wheat (Triticum aestivum) polyphenol oxidase (PPO) contributes to the time-dependent discoloration of Asian noodles. Wheat contains multiple paralogous and orthologous Ppo genes, Ppo-A1, Ppo-D1, Ppo-A2, Ppo-D2, and Ppo-B2, expressed in wheat kernels. To date, wheat noodle color improvement efforts have focused on breeding cultivars containing Ppo-D1 and Ppo-A1 alleles conferring reduced PPO activity. A major impediment to wheat quality improvement is a lack of additional Ppo alleles conferring reduced kernel PPO. In this study, a previously reported very low PPO line, 07OR1074, was found to contain a novel allele at Ppo-A2 and null alleles at the Ppo-A1 and Ppo-D1 loci. To examine the impact of each mutation upon kernel PPO, populations were generated from crosses between 07OR1074 and the hard white spring wheat cultivars Choteau and Vida. Expression analysis using RNA-seq demonstrated no detectable Ppo-A1 transcripts in 07OR1074 while Ppo-D1 transcripts were present at less than 10% of that seen in Choteau and Vida. Novel markers specific for the Ppo-D1 and Ppo-A2 mutations discovered in 07OR1074, along with the Ppo-A1 STS marker, were used to screen segregating populations. Evaluation of lines indicated a substantial genotypic effect on PPO with Ppo-A1 and Ppo-D1 alleles contributing significantly to total PPO in both populations. These results show that the novel mutations in Ppo-A1 and Ppo-D1 present in 07OR1074 are both important to lowering overall wheat seed PPO activity and may be useful to produce more desirable and marketable wheat-based products.

Genetic mapping of new seed-expressed polyphenol oxidase genes in wheat (Triticum aestivum L.)

Polyphenol oxidase (PPO) enzymatic activity is a major cause in time-dependent discoloration in wheat dough products. The PPO-A1 and PPO-D1 genes have been shown to contribute to wheat kernel PPO activity. Recently a novel PPO gene family consisting of the PPO-A2, PPO-B2, and PPO-D2 genes has been identified and shown to be expressed in wheat kernels. In this study, the sequences of these five kernel PPO genes were determined for the spring wheat cultivars Louise and Penawawa. The two cultivars were found to be polymorphic at each of the PPO loci. Three novel alleles were isolated from Louise. The Louise X Penawawa mapping population was used to genetically map all five PPO genes. All map to the long arm of homeologous group 2 chromosomes. PPO-A2 was found to be located 8.9 cM proximal to PPO-A1 on the long arm of chromosome 2A. Similarly, PPO-D1 and PPO-D2 were separated by 10.7 cM on the long arm of chromosome 2D. PPO-B2 mapped to the long arm of chromosome 2B and was the site of a novel QTL for polyphenol oxidase activity. Five other PPO QTL were identified in this study. One QTL corresponds to the previously described PPO-D1 locus, one QTL corresponds to the PPO-D2 locus, whereas the remaining three are located on chromosome 2B.

Alternative splicing in the coding region of Ppo-A1 directly influences the polyphenol oxidase activity in common wheat (Triticum aestivum L.)

Polyphenol oxidase (PPO) plays a crucial role in browning reactions in fresh and processed fruits and vegetables, as well as products made from cereal grains. Common wheat (Triticum aestivum L.) has a large genome, representing an interesting system to advance our understanding of plant PPO gene expression, regulation and function. In the present study, we characterized the expression of Ppo-A1, a major PPO gene located on wheat chromosome 2A, using DNA sequencing, semi-quantitative RT-PCR, PPO activity assays and whole-grain staining methods during grain development. The results indicated that the expression of the Ppo-A1b allele was regulated by alternative splicing of pre-mRNAs, resulting from a 191-bp insertion in intron 1 and one C/G SNP in exon 2. Eight mRNA isoforms were identified in developing grains based on alignments between cDNA and genomic DNA sequences. Only the constitutively spliced isoform b encodes a putative full-length PPO protein based on its coding sequence whereas the other seven spliced isoforms, a, c, d, e, f, g and h, have premature termination codons resulting in potential nonsense-mediated mRNA decay. The differences in expression of Ppo-A1a and Ppo-A1b were confirmed by PPO activity assays and whole grain staining, providing direct evidence for the influence of alternative splicing in the coding region of Ppo-A1 on polyphenol oxidase activity in common wheat grains.

Genetic diversity of Greek Aegilops species using different types of nuclear genome markers

Random Amplified Polymorphic DNA (RAPD) and Inter-Simple Sequence Repeat (ISSR) analyses were used to evaluate genetic variability and relationships of Greek Aegilops species. Thirty-eight accessions of seven Greek Aegilops species [Ae. triuncialis (genome UC), Ae. neglecta (UM), Ae. biuncialis (UM), Ae. caudata (C), Ae. comosa (M), Ae. geniculata (MU) and Ae. umbellulata (U)] as well as Triticum accessions were studied. Nineteen RAPD and ten ISSR primers yielded 344 and 170 polymorphic bands, respectively, that were used for the construction of dendrograms. Regardless of the similarity coefficient and marker type used, UPGMA placed 38 Aegilops accessions into one branch while the other branch consisted of wheat species. Within the Aegilops cluster, subgroups were identified that included species that shared the same genome or belonged to the same botanical section. Within the Triticum cluster, two robust subgroups were formed, one including diploid wheat and another including polyploid wheat. In conclusion, results showed that there is genetic diversity in the Greek Aegilops species studied, and clustering based on genetic similarities was in agreement with botanical classifications.