A review of the occurrence of Grain softness protein-1 genes in wheat (Triticum aestivum L.).

Grain softness protein-1 (Gsp-1) is a small, 495-bp intronless gene found throughout the Triticeae tribe at the distal end of group 5 chromosomes. With the Puroindolines, it constitutes a key component of the Hardness locus. Gsp-1 likely plays little role in grain hardness, but has direct interest due to its utility in phylogeny and its role in arabinogalactan peptides. Further role(s) remain to be identified. In the polyploid wheats, Triticum aestivum and T. turgidum, the gene is present in a homoeologous series. Since its discovery, there have been conflicting reports and data as to the number of Gsp-1 genes and the level of sequence polymorphism. Little is known about allelic variation within a species. In the simplest model, a single Gsp-1 gene is present in each wheat and Aegilops tauschii genome. The present review critically re-examines the published and some unpublished data (sequence available in the NCBI nucleotide and MIPS Wheat Genome Databases). A number of testable hypotheses are identified, and include the level of polymorphism that may represent (and define) different Gsp-1 alleles, the existence of a fourth Gsp-1 gene, and the apparent, at times, high level of naturally-occurring or artifactual gene chimeras. In summary, the present data provide firm evidence for at most, three Gsp-1 genes in wheat, although there are numerous data that suggest a more complex model.

The distal portion of the short arm of wheat (Triticum aestivum L.) chromosome 5D controls endosperm vitreosity and grain hardness.

Kernel vitreosity is an important trait of wheat grain, but its developmental control is not completely known. We developed back-cross seven (BC(7)) near-isogenic lines in the soft white spring wheat cultivar Alpowa that lack the distal portion of chromosome 5D short arm. From the final back-cross, 46 BC(7)F(2) plants were isolated. These plants exhibited a complete and perfect association between kernel vitreosity (i.e. vitreous, non-vitreous or mixed) and Single Kernel Characterization System (SKCS) hardness. Observed segregation of 10:28:7 fit a 1:2:1 Chi-square. BC(7)F(2) plants classified as heterozygous for both SKCS hardness and kernel vitreosity (n = 29) were selected and a single vitreous and non-vitreous kernel were selected, and grown to maturity and subjected to SKCS analysis. The resultant phenotypic ratios were, from non-vitreous kernels, 23:6:0, and from vitreous kernels, 0:1:28, soft:heterozygous:hard, respectively. Three of these BC(7)F(2) heterozygous plants were selected and 40 kernels each drawn at random, grown to maturity and subjected to SKCS analysis. Phenotypic segregation ratios were 7:27:6, 11:20:9, and 3:28:9, soft:heterozygous:hard. Chi-square analysis supported a 1:2:1 segregation for one plant but not the other two, in which cases the two homozygous classes were under-represented. Twenty-two paired BC(7)F(2):F(3) full sibs were compared for kernel hardness, weight, size, density and protein content. SKCS hardness index differed markedly, 29.4 for the lines with a complete 5DS, and 88.6 for the lines possessing the deletion. The soft non-vitreous kernels were on average significantly heavier, by nearly 20%, and were slightly larger. Density and protein contents were similar, however. The results provide strong genetic evidence that gene(s) on distal 5DS control not only kernel hardness but also the manner in which the endosperm develops, viz. whether it is vitreous or non-vitreous.

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.

Prevalence of Puroindoline D1 and Puroindoline b-2 variants in U.S. Pacific Northwest wheat breeding germplasm pools, and their association with kernel texture.

Kernel texture is a major factor influencing the classification and end use properties of wheat (Triticum aestivum L.), and is mainly controlled by the Puroindoline a (Pina) and Puroindoline b (Pinb) genes. Recently, a new puroindoline gene, Puroindoline b-2 (Pin b-2), was identified. In this study, 388 wheat cultivars and advanced breeding lines from the U.S. Pacific Northwest were investigated for frequencies of Puroindoline D1 alleles and Pinb-2 variants 2 and 3. Results indicated that Pinb-D1b (74.0%) was the predominant genotype among hard wheats (N = 196), the only other hard allele encountered was Pina-D1b (26.0%). Across all varieties, Pinb-2v3 was the predominant genotype (84.5%) compared with Pinb-2v2 (15.5%). However, among 240 winter wheat varieties (124 soft white, 15 club, 68 hard red and 33 hard white varieties), all carried Pinb-2v3. Among spring wheats, Pinb-2v2 and Pinb-2v3 frequencies were more variable (soft white 25.0:75.0, hard red 58.2:41.8 and hard white 40.0:60.0, respectively). Kernel texture variation was analyzed using 247 of the 388 wheat varieties grown in multi-location factorial trials in up to 7 crop years. The range of variety means among the four groups, soft winter, soft spring, hard winter and hard spring, was on the order of 15-25 single kernel characterization system (SKCS) Hardness Index. The least significant difference for each of these trials ranged from 2.8 to 5.6 SKCS Hardness Index. Observations lead to the conclusion that Pinb-2 variants do not exert a prominent effect on kernel texture, however, Pinb-2 variants do identify features of wheat germ plasm structure in the U.S. Pacific Northwest.

Physical mapping and a new variant of Puroindoline b-2 genes in wheat.

Puroindoline a and b proteins soften the endosperm of wheat kernels. When the underlying puroindoline genes are altered by mutation or are deleted, kernels become harder. Thus, puroindoline a and b (Pina and Pinb) play an important role in wheat quality and utilization. Recently, additional Pinb genes have been reported. In the present report, we provide corroborating coding and additional 5' and 3' flanking sequence for three Pinb variants: Pinb-2v1, Pinb-2v2, and Pinb-2v3. Additionally, a new Pinb variant, Pinb-2v4, is reported. All four variants were physically mapped using Chinese Spring (CS) diteolosomics, nullisomic-tetrasomics, and CS-Cheyenne disomic substitution lines. Results place Pinb-2v1 on 7DL, Pinb-2v2 on 7BL, Pinb-2v3 on 7B, and Pinb-2v4 on 7AL. Pinb-2v1 and Pinb-2v4 were present in all cvs. examined: CS, Cheyenne, Recital, Wichita and Winsome. Pinb-2v2 was present in CS and Recital; Pinb-2v3 was present in Cheyenne, Wichita, and Winsome. These results are not wholly consistent with prior research and additional studies will be required to reconcile discrepancies. The discovery of Pinb-2v4 and the mapping of all four variants will contribute to a better understanding of gene duplication events in wheat and their bearing on wheat kernel texture and grain utilization.