Peter R. Shewry

Seed storage proteins: structures and biosynthesis.

in them. The presence of these groups may allow the plant to maintain high levels of storage protein synthesis despite variations in sulfur availability. The strict tissue specificity of seed storage protein synthesis contrasts with that of tuber storage proteins, which may be synthesized in vegetative tissues under unusual conditions (for example, in vitro or after removal of tubers) (Shewry, 1995). A second common property of seed storage proteins is their presence in the mature seed in discrete deposits called protein bodies, whose origin has been the subject of some dispute and may in fact vary both between and within species. Finally, all storage protein fractions are mixtures of components that exhibit polymorphism both within single genotypes and among genotypes of the same species. This polymorphism arises from the presence of multigene families and, in some cases, proteolytic processing and glycosylation.

High molecular weight subunits of wheat glutenin

The high molecular weight (HMW) subunits of wheat glutenin are of considerable interest because of their relationship to breadmaking quality. We review recent studies of their genetics, amino acid sequences and conformations, and discuss how they may be assembled to form disulphide-bonded polymers that confer elasticity on wheat dough. We also speculate on how their structure and functionality may be explored using protein engineering and expression in microorganisms or in developing seeds of transgenic plants.

Location of β-amylase sequences in wheat and its relatives

SummaryA β-amylase cDNA clone isolated from barley has been used to locate β-amylase encoding sequences on wheat, rye, and Aegilops umbellulata chromosomes by hybridisation to restriction endonuclease digested DNA obtained from wheat aneuploid and wheat-alien addition lines. Structural genes were identified on homoeologous group 4 and 5 chromosomes, confirming the results of isozyme studies. In addition, a further set of structural genes was found on homoeologous group 2 chromosomes. It is proposed that there are two homoeoallelic series, β-Amy-1 on group 4 or 5 chromosomes, and β-Amy-2 on group 2 chromosomes. Evidence is presented that each locus contains one or two β-amylase structural genes, and it is suggested that the large number of isozymes seen upon IEF are due to post-translational modifications.

The classification and nomenclature of wheat gluten proteins: A reassessment

The nomenclature, relationships and classification of wheat gluten proteins are discussed in relation to recently-reported amino acid sequences of individual components. Although all gliadin and glutenin components show some degree of sequence relationship, three groups of closely related proteins can be recognised. These are high molecular weight (HMW) prolamins (HMW subunits of glutenin), the S-poor prolamins (ω-gliadins) and the S-rich prolamins. The latter group includes gtiadins (α- β-, γ-) and glutenins (LMW subunits). It is concluded that the classical division into gliadins and glutenins is based on a secondary characteristic, the formation of interor intra- molecular disulphide bonds, rather than on homology of the primary amino acid sequences.

Aphid alarm pheromone produced by transgenic plants affects aphid and parasitoid behavior

The alarm pheromone for many species of aphids, which causes dispersion in response to attack by predators or parasitoids, consists of the sesquiterpene (E)-β-farnesene (Eβf). We used high levels of expression in Arabidopsis thaliana plants of an Eβf synthase gene cloned from Mentha × piperita to cause emission of pure Eβf. These plants elicited potent effects on behavior of the aphid Myzus persicae (alarm and repellent responses) and its parasitoid Diaeretiella rapae (an arrestant response). Here, we report the transformation of a plant to produce an insect pheromone and demonstrate that the resulting emission affects behavioral responses at two trophic levels.

Biotechnology of breadmaking: unraveling and manipulating the multi-protein gluten complex.

Breadmaking is one of humankinds oldest technologies, being established some 4,000 years ago. The ability to make leavened bread depends largely on the visco-elastic properties conferred to wheat doughs by the gluten proteins. These allow the entrapment of carbon dioxide released by the yeast, giving rise to a light porous structure. One group of gluten proteins, the high molecular weight (HMW) subunits, are largely responsible for gluten elasticity, and variation in their amount and composition is associated with differences in elasticity (and hence quality) between various types of wheat. These proteins form elastomeric polymers stabilized by inter-chain disulphide bonds, and detailed studies of their structures have led to models for die mechanism of elasticity. This work has also provided a basis for direct improvement of wheat quality by transformation with additional HMW subunit genes.

Analysis of HMW glutenin subunits encoded by chromosome 1A of bread wheat (Triticum aestivum L.) indicates quantitative effects on grain quality.

SummaryA gene encoding the high-molecular-weight (HMW) subunit of glutenin 1Ax1 was isolated from bread wheat cv Hope. Comparison of the deduced amino acid sequence with that previously reported for an allelic subunit, 1Ax2*, showed only minor differences, which were consistent with both subunits being associated with good bread-making quality. Quantitative analyses of total protein extracts from 22 cultivars of bread wheat showed that the presence of either subunit 1Ax1 or 1Ax2*, when compared with a null allele, resulted in an increase in the proportion of HMW subunit protein from ca. 8 to 10% of the total. It is suggested that this quantitative increase in HMW subunit protein may account for the association of 1Ax subunits with good quality.

Allergens to wheat and related cereals

Wheat is one of the major crops grown, processed and consumed by humankind and is associated with both intolerances (notably coeliac disease) and allergies.

Molecular evolution of the seed storage proteins of barley, rye and wheat

The major storage proteins (prolamins) of barley, rye and wheat are characterized by the presence of two or more unrelated structural domains, one of which contains repeated sequences. Because of this repetitive structure and their restricted distribution (only in grasses), it has been suggested that the prolamins are of recent origin. Contrary to this hypothesis, we show that parts of the non-repetitive domain of one group of prolamins are homologous with sequences present in a large group of seed proteins from monocotyledonous and dicotyledonous plants; including Bowman-Birk protease inhibitors, cereal inhibitors of alpha-amylase and trypsin, and 2 S globulin storage proteins of castor bean and oil seed rape. This implies an ancient origin for these non-repetitive domains. The origins of the repetitive domains are not known but may lie within the grasses.

Phenolic Acids in Wheat Varieties in the HEALTHGRAIN Diversity Screen

The amounts and compositions of free, conjugated, bound, and total phenolic acids were determined in 175 samples of wheat flour grown on a single site in 2005. The highest contents of total phenolic acids were found in flours of winter wheat (1171 microg/g) with average levels of 658 microg/g total phenolics across all of the wheat genotypes. Winter wheats showed a range of >3.5-fold across the concentration range for total phenolic acids. Spelt genotypes displayed the narrowest (1.9-fold) range of total phenolic acid concentration. The concentrations of phenolic acids in the different phenolic acid fractions were in the order bound > conjugated > free, with bound phenolic acids making up around 77% of the total phenolic acid concentration and free phenolic acids constituting between 0.5 and 1%. The results indicate that there is genetic diversity in phenolic acid content and that it should be possible to selectively breed for lines with high contents of phenolic components.