Wheat grain proteins: Past, present, and future

Background and Objectives

Research on wheat grain proteins is reviewed, including achievements over the past century and priorities for future research. The focus is on three groups of proteins that have major impacts on wheat quality and utilization: the gluten proteins which determine dough viscoelasticity but also trigger celiac disease in susceptible individuals, the puroindolines which are major determinants of grain texture and the amylase/trypsin inhibitors which are food and respiratory allergens and are implicated in triggering celiac disease and nonceliac wheat sensitivity.

Findings

Although earlier work focused on protein structure and properties, the development of genomics and high-sensitivity proteomics has resulted in the availability of a vast amount of information on the amino acid sequences of individual wheat proteins, including allelic variants of gluten proteins which are associated with good processing quality and of puroindolines, which are associated with a hard or soft grain texture, and on protein expression and polymorphism.

Conclusions

However, our ability to exploit this knowledge is limited by a lack of detailed understanding of the structure:function relationships of wheat proteins. In particular, we need to understand how the three-dimensional structures of the individual proteins determine their interactions with other grain components (to determine functional properties) and with the immune systems of susceptible consumers (to trigger adverse responses), how these interactions are affected by allelic variation, and how they can be manipulated.

Significance and Novelty

The article, therefore, identifies priorities for future research which should enable the adoption of a more rational approach to improving the quality of wheat grain proteins.

Exploring the End-Use Quality Potential of a Collection of Spanish Bread Wheat Landraces

Modern plant-breeding practices have narrowed the genetic base of wheat, such that there is a need to introduce new germplasms with underexploited diversity into breeding programs. Wheat landraces are a very valuable resource when searching for genetic variation, which not only possess increased adaptability, but also quality-related traits. Several studies have shown a wide genetic diversity in Spanish wheat landraces compared to other germplasm collections; therefore, the main objective of this study is to analyze the variability in a collection of 189 landraces from the Spanish National Plant Genetic Resources Centre (Centro de Recursos Fitogenéticos, CRF-INIA, Alcalá de Henares), in relation to end-use quality traits. We characterized the whole collection for high-molecular-weight glutenin and puroindoline allelic composition, and for gluten strength. In addition, grain protein content, grains per spike, and thousand kernel weight were evaluated in samples from four-year field trials. The relationship between glutenin composition and quality was evaluated, and some alleles strongly associated with high quality were identified in the collection, some of them specific for Iberian landraces. The results also show the presence of novel variability within high-molecular-weight glutenin and puroindolines, which needs to be characterized further in order to assess its influence on wheat quality. In addition, a set of landraces showing outstanding values for gluten quality and a good agronomic performance was selected for testing in field trials in order to evaluate the suitability of their direct use in cropping systems.

Insights into the Genetic Architecture of Bran Friability and Water Retention Capacity, Two Important Traits for Whole Grain End-Use Quality in Winter Wheat

Bran friability (particle size distribution after milling) and water retention capacity (WRC) impact wheat bran functionality in whole grain milling and baking applications. The goal of this study was to identify genomic regions and underlying genes that may be responsible for these traits. The Hard Winter Wheat Association Mapping Panel, which comprised 299 lines from breeding programs in the Great Plains region of the US, was used in a genome-wide association study. Bran friability ranged from 34.5% to 65.9% (median, 51.1%) and WRC ranged from 159% to 458% (median, 331%). Two single-nucleotide polymorphisms (SNPs) on chromosome 5D were significantly associated with bran friability, accounting for 11-12% of the phenotypic variation. One of these SNPs was located within the Puroindoline-b gene, which is known for influencing endosperm texture. Two SNPs on chromosome 4A were tentatively associated with WRC, accounting for 4.6% and 4.4% of phenotypic variation. The favorable alleles at the SNP sites were present in only 15% (friability) and 34% (WRC) of lines, indicating a need to develop new germplasm for these whole-grain end-use quality traits. Validation of these findings in independent populations will be useful for breeding winter wheat cultivars with improved functionality for whole grain food applications.

Anti-biofilm and sporicidal activity of peptides based on wheat puroindoline and barley hordoindoline proteins

The broad-spectrum activity of antimicrobial peptides (AMPs) and low probability of development of host resistance make them excellent candidates as novel bio-control agents. A number of AMPs are found to be cationic, and a small proportion of these are tryptophan-rich. The puroindolines (PIN) are small, basic proteins found in wheat grains with proposed roles in biotic defence of seeds and seedlings. Synthetic peptides based on their unique tryptophan-rich domain (TRD) display antimicrobial properties. Bacterial endospores and biofilms are highly resistant cells, with significant implications in both medical and food industries. In this study, the cationic PIN TRD-based peptides PuroA (FPVTWRWWKWWKG-NH2 ) and Pina-M (FSVTWRWWKWWKG-NH2 ) and the related barley hordoindoline (HIN) based Hina (FPVTWRWWTWWKG-NH2 ) were tested for effects on planktonic cells and biofilms of the common human pathogens including Pseudomonas aeruginosa, Listeria monocytogenes and the non-pathogenic Listeria innocua. All peptides showed significant bactericidal activity. Further, PuroA and Pina-M at 2 × MIC prevented initial biomass attachment by 85-90% and inhibited >90% of 6-h preformed biofilms of all three organisms. However Hina, with a substitution of Lys-9 with uncharged Thr, particularly inhibited Listeria biofilms. The PIN based peptides were also tested against vegetative cells and endospores of Bacillus subtilis. The results provided evidence that these tryptophan-rich peptides could kill B. subtilis even in sporulated state, reducing the number of viable spores by 4 log units. The treated spores appeared withered under scanning electron microscopy. The results establish the potential of these tryptophan-rich peptides in controlling persistent pathogens of relevance to food industries and human health. Copyright © 2016 European Peptide Society and John Wiley & Sons, Ltd.

Puroindoline allelic diversity in Indian wheat germplasm and identification of new allelic variants

Grain hardness is an important quality trait that influences product development in wheat. This trait is governed by variation in puroindoline proteins (PINA and PINB). Our study evaluated 551 Indian wheat germplasm lines for diversity in Pina and Pinb genes. Eighty-two lines were shortlisted for full length sequencing and grain hardness studies. Sequencing studies identified six unknown alleles: two for the Pina gene and four for the Pinb gene. Five of them were novel with non-synonymous changes in the corresponding amino acid sequences. Identified mutations in the deduced mature proteins and their pre- and pro-peptides influenced the hardness characteristics of the grain. We classified these 82 varieties into different hardness categories with reference to international and Indian systems of classification. The majority of Indian wheat varieties were categorized as hard. This study revealed that unexplored Indian wheat germplasm can be a good source of genetic variability for both Pina and Pinb genes, helping in marker-assisted breeding and in obtaining wheat with different textural properties.

Ultra-high performance liquid chromatography tandem mass spectrometry for the detection of durum wheat contamination or adulteration

In this work, an ultra-performance liquid chromatography electrospray ionization (UPLC-ESI)-MS/MS methodology based on multiple reaction monitoring (MRM) for the selective and sensitive detection and quantification of durum wheat adulteration has been developed and fully validated. The targeted analysis was performed by monitoring specific transitions at m/z 543.7 > 657.4 and m/z 543.7 > 299.2 of a species-specific marker derived from a tryptic peptide of puroindoline a (Pin-a), a cysteine-rich protein selectively present only in common wheat. In addition, two transitions at m/z 500.4 > 725.4 and m/z 500.4 > 561.9 of a reference peptide belonging to purothionin A-1, present in both species, were also monitored. The calibration curves obtained on binary mixtures with known percentages of common/durum wheat flours showed linearity (coefficient of regression, r ≥ 0.99) over concentrations that ranged between 80 and 1%. The limit of detection (LOD) and limit of quantification (LOQ) for the Pin-a marker in wheat flours were 0.01 and 0.03%, respectively. The identified Pin-a marker was also found to be highly diagnostic for the quantification of common wheat in raw materials (kernels) and processed products (pasta), thus offering new opportunities to assess food authenticity.

Did the house mouse (Mus musculus L.) shape the evolutionary trajectory of wheat (Triticum aestivum L.)?

Wheat (Triticum aestivum L.) is one of the most successful domesticated plant species in the world. The majority of wheat carries mutations in the Puroindoline genes that result in a hard kernel phenotype. An evolutionary explanation, or selective advantage, for the spread and persistence of these hard kernel mutations has yet to be established. Here, we demonstrate that the house mouse (Mus musculus L.) exerts a pronounced feeding preference for soft over hard kernels. When allele frequencies ranged from 0.5 to 0.009, mouse predation increased the hard allele frequency as much as 10-fold. Studies involving a single hard kernel mixed with ∼1000 soft kernels failed to recover the mutant kernel. Nevertheless, the study clearly demonstrates that the house mouse could have played a role in the evolution of wheat, and therefore the cultural trajectory of humankind.

Allelic diversity and molecular characterization of puroindoline genes in five diploid species of the Aegilops genus

Grain hardness is an important quality trait in wheat. This trait is related to the variation in, and the presence of, puroindolines (PINA and PINB). This variation can be increased by the allelic polymorphism present in the Aegilops species that are related to wheat. This study evaluated allelic Pina and Pinb gene variability in five diploid species of the Aegilops genus, along with the molecular characterization of the main allelic variants found in each species. This polymorphism resulted in 16 alleles for the Pina gene and 24 alleles for the Pinb gene, of which 10 and 17, respectively, were novel. Diverse mutations were detected in the deduced mature proteins of these alleles, which could influence the hardness characteristics of these proteins. This study shows that the diploid species of the Aegilops genus could be a good source of genetic variability for both Pina and Pinb genes, which could be used in breeding programmes to extend the range of different textures in wheat.