The genetics of gliadin and glutenin,the major storage proteins of the wheat endosperm

This paper reviews our present knowledge of the chromosomal location of the genes that control the synthesis of gliadins and glutenins, the two major groups of storage protein in the endosperm of wheat (Triticum aestivum). Allelic relationships and genetic linkage between genes are also discussed. The areas that require futher investigation are identified.     

Physicochemical control of durum wheat grain filling and glutenin polymer assembly under different temperature regimes

Durum wheat is grown in the Mediterranean area where drought and high temperature frequently prevail and impact grain texture, composition and yield. The purpose of this work was to examine the effect of high temperature on grain development and final composition according to the timing of exposure. High temperature (up to 27.5 °C) was applied either during the linear grain filling or drying phases or during whole grain development. The dynamics of grain dry mass, water, glutenin polymers, and protein bodies during grain development were determined. Irrespective of high temperature timing, the arrest of grain filling was observed at 45.9% grain moisture content. At that point, starch granules included in endosperm cells reached their physical packing limit, limiting further deposits. HT applied before physiological maturity shortened the duration of grain filling and resulted in a significant increase in grain protein concentration and in the proportion of vitreous grain. Late formation of sodium dodecyl sulfate (SDS)-insoluble glutenin polymers below 32% grain moisture content was also favored. The ability of wheat storage protein to form a viscoelastic matrix embedding starch granules at the beginning of grain desiccation is proposed to be mandatory for gaining vitreous grains and a high proportion of SDS-insoluble glutenin polymers.     

Polymerisation of gluten proteins in developing wheat grain as affected by desiccation

The breadmaking quality of wheat is affected by the composition of gluten proteins and the polymerisation of subunits that are synthesised and accumulated in developing wheat grain. The biological mechanisms and time course of these events during grain development are documented, but not widely confirmed. Therefore, the aim of this study was to monitor the accumulation of gluten protein subunits and the size distribution of protein aggregates during grain development. The effect of desiccation on the polymerisation of gluten proteins and the functional properties of gluten were also studied. The results showed that the size of glutenin polymers remained consistently low until yellow ripeness (YR), while it increased during grain desiccation after YR. Hence, this polymerisation process was presumed to be initiated by desiccation. A similar polymerisation event was also observed when premature grains were dried artificially. The composition of gluten proteins, the ratios of glutenin to gliadin and high molecular weight-glutenin subunits to low molecular weight-glutenin subunits, in premature grain after artificial desiccation showed close association with the size of glutenin polymers in artificially dried grain. Functional properties of gluten in these samples were also associated with polymer size after artificial desiccation.     

Developmental and environmental effects on the assembly of glutenin polymers and the impact on grain quality of wheat

Wheat kernel development can be divided into three phases i.e. cell division, cell enlargement and dehydration. Accumulation of gluten proteins continues till the end of the cell enlargement phase. During the dehydration phase, post-translational polymerization of the glutenin subunits occurs to form very large glutenin polymers. Assembly of the glutenin polymers has been monitored by increase in the unextractable polymeric protein. Lines possessing HMW-GS related to dough strength (e.g. 5 + 10) started accumulating large polymers several days earlier than lines with HMW-GS related to dough weakness (e.g. 2 + 12) and maintained their higher amounts till maturity. This may be explained by faster polymerization resulting from a higher concentration of cysteine residues in the x-type HMW-GS.     

Multiple heat and drought events affect grain yield and accumulations of high molecular weight glutenin subunits and glutenin macropolymers in wheat

Spring wheat plants were subjected to water deficit and/or high temperature episodes at spikelet initiation, anthesis or both stages. The stresses modified the early dough stage and maturity, shortened the kernel desiccation period and caused grain yield loss. Plants subjected to stress at the early growth stages had higher grain yields than the non-early-stressed plants when stress reoccurred at anthesis. Concentrations of high molecular weight glutenin subunits in grain were up-regulated by the single early drought, the early drought combined with late heat and the double drought stress treatments, but was down-regulated by the early heat and double heat stress events. Concentration of glutenin macropolymers was increased by the single early drought episode, the single late drought and heat events, as well as the early drought combined with the late heat stress, but was reduced by the early heat stress and double heat events.     

Effects of ozone treatment on the molecular properties of wheat grain proteins

Ozone is a powerful and highly reactive oxidizing agent, which has found increasing applications in the field of grain processing. However, in some cases, O₃ can potentially promote oxidation and/or degradation of the chemical constituents of grains. Experiments were carried out to evaluate the specific effects of gaseous ozone on the molecular properties of wheat grain proteins and their consequences on the bread-making quality of the resulting flours.Ozonation causes a significant reduction in the SDS solubility of the wheat prolamins, which can reasonably be attributed to conjugate effects of an increase in molecular dimensions and an increase in the compactness of the protein polymers initially present. In fact, our results demonstrate that this general reinforcement of the aggregative status of prolamins due to ozonation of wheat grains results from (i) the formation of new intermolecular S-S bonds, (ii) to a lesser extent, the formation of other types of intermolecular covalent cross-links (dityrosine cross-links) and finally, (iii) significant changes in secondary structure. By significantly affecting the molecular properties of wheat grain prolamins, ozone leads to profound changes in the rheological properties (i.e. increase in the tenacity and a great limitation of the extensibility) of the flours and/or doughs obtained.     

Chemical imaging: the distribution of ions and molecules in developing and mature wheat grain

The ability to combine topographical and in situ chemical analysis of individual cereal grains, without recourse to fractionation, offers an opportunity to determine the distribution of functionally- and nutritionally-important components. Three such technologies are reviewed, including immunolocation using monoclonal antibodies specific for different types of wheat prolamins, secondary ion mass spectroscopy (SIMS) to detect the presence of inorganic elements such as sodium and sulphur, and infrared (including Raman and Fourier-transform infrared [FT-IR]) microspectroscopy to determine the distribution of biopolymers across the grain. Immunolabelling has shown that the distribution of prolamin proteins changes across the endosperm, with the outer endosperm containing a much greater proportion of prolamins than the inner endosperm. SIMS has shown, for the first time, the presence of Na⁺ in the phytin granules and that sulphur is enriched at the boundary between the starch granules and the protein matrix. Raman microspectroscopy has been used to investigate the distribution of proteins and the phenolic compound, ferulic acid, across the grain, whilst FT-IR has been used to define the microheterogeneity of arabinoxylans in endosperm cell walls. These methods highlight how in situ analysis can yield new insights into grain composition and how this may be altered by environmental conditions during grain development.     

Biochemical changes in wheat during storage at three temperatures

Biochemical changes in wheat grains stored at 10, 25 and 45 °C for six months were studied. A significant decrease in pH and an increase in titratable acidity was observed during storage of wheat grains at 25 °C and 45 °C. Moisture contents of wheat grains decreased by 15% at 25 °C and 26% at 45 °C during six months of storage. A significant decrease in water soluble amylose (20–28%) along with an increase in insoluble amylose contents (7.6–17%) were observed during storage at 25 and 45 °C. Amylase activity of the samples showed a decrease as the storage progressed. Total soluble sugars increased by 9% at 10 °C and 12% at 25 °C; a 37% decrease was observed after six months storage at 45 °C. Total available lysine decreased by 18.0% and 22.6% at 25 and 45 °C, respectively, after six months storage. In vitro protein digestibility of wheat grains decreased by 5.00% at 25 °C and 10.28% at 45 °C during six months of storage. However, no significant biochemical changes occurred during storage at 10 °C.     

Effect of high temperature on albumin and globulin accumulation in the endosperm proteome of the developing wheat grain

The accumulation of KCl-soluble/methanol-insoluble albumins and globulins was investigated in the endosperm of developing wheat (Triticum aestivum, L. cv. Butte 86) grain produced under a moderate (24 °C/17 °C, day/night) or a high temperature regimen (37 °C/28 °C) imposed from 10 or 20 days post-anthesis (dpa) until maturity. Proteins were separated by 2-DE and developmental profiles for nearly 200 proteins were analyzed by hierarchical clustering. Comparison of protein profiles across physiologically equivalent stages of grain fill revealed that high temperature shortened, but did not substantially alter, the developmental program. Accumulation of proteins shifted from those active in biosynthesis and metabolism to those with roles in storage and protection against biotic and abiotic stresses. Few proteins responded transiently when plants were transferred to the high temperature regimens, but levels of a number of proteins were altered during late stages of grain development. Specific protein responses depended on whether the high temperature regimens were initiated early or mid development. Some of the heat responsive proteins have been implicated in gas bubble stabilization in bread dough and others are suspected food allergens.     

Atmospheric CO2 enrichment changes the wheat grain proteome

Spring wheat (Triticum aestivum L. cv. Triso) was grown in a free-air CO₂ enrichment (FACE) field experiment in order to gain information on CO₂-induced effects on grain composition and quality at maturity. A proteome analysis was performed using two-dimensional gel electrophoresis (2-DE) and protein identification was done with mass spectrometry (MALDI-TOF MS). In elevated CO₂ (526 μl l⁻¹), an increase of 13.5% in grain yield was observed relative to 375 μl l⁻¹ at a low level of significance (P = 0.528). Total grain protein concentration was decreased by 3.5% at a high level of statistical significance. Most importantly, a number of statistically significant changes within the grain proteome were observed, as the levels of 32 proteins were affected by elevated CO₂: 16 proteins were up-regulated and 16 were down-regulated. Our experiment demonstrates that high-CO₂ can markedly affect the proteome of mature wheat grain. The potential role of the proteins, changed in response to CO₂ enrichment, is discussed as some may affect grain quality. For the task of selecting cultivars resistant to CO₂-induced quality loss, we propose to consider the proteins affected by elevated CO₂ identified in this work here.     

Polymerization of glutenin during grain development in near-isogenic wheat lines differing at Glu-D1 and Glu-B1 in greenhouse and field

The polymerization of glutenin polymers was monitored by measuring the Unextractable Polymeric Protein (UPP) at 3-day intervals after anthesis for four pairs of near-isogenic wheat lines. Two pairs, the variety Lance, differing at the Glu-D1 locus (HMW-GS 5+10 or 2+12) and the variety Halberd, differing at the Glu-B1 locus (HMW-GS 7+9 or 20x+20y) were grown in the field (2000) and twice in the greenhouse (2000 and 2001). Two other pairs, the varieties Warigal and Avocet, differing at the Glu-D1 locus (HMW-GS 5+10 or 2+12) were grown in the greenhouse in 2001. The behavior of all lines was consistent from greenhouse and field plantings in that the lines possessing strength-associated HMW-GS (5+10 at Glu-D1 and 7+9 at Glu-B1) showed an increase in accumulation of larger glutenin polymers (measured by UPP) at an earlier stage during grain filling than the lines with allelic counterparts (HMW-GS 2+12 at Glu-D1 and 20x+20y at Glu-B1). In all cases, the increases were maintained until maturity, paralleling the greater dough strength of flour from these lines, measured by mixograph dough development time.     

Width of clover strips and wheat rows influence grain yield in winter wheat/white clover intercropping

Cereal–legume intercropping offers potential benefits in low-input cropping systems, where nutrient inputs, in particular nitrogen (N), are limited. In the present study, winter wheat (Triticum aestivum L.) and white clover (Trifolium repens L.) were intercropped by sowing the wheat into rototilled strips in an established stand of white clover.A field experiment was performed in two fields starting in two different years to explore the effects of width of the wheat rows and clover strips on the competition between the species and on wheat yields. The factors were intercropping (clover sole crop, wheat sole crop and wheat/clover intercropping), rototilled band width, sowing width and wheat density in a factorial experimental design that enabled some of the interactions between the factors to be estimated. The measurements included grain yield, ear density, grain weight, grain N concentration, dry matter and N in above-ground biomass of wheat, clover and weeds and profiles of photosynthetic active radiation (PAR) within the crop canopy.Intercropping of winter wheat and clover resulted in wheat grain yield decreases of 10–25% compared with a wheat sole crop. The yield reductions were likely caused by interspecific competition for light and N during vegetative growth, and for soil water during grain filling. N uptake in the wheat intercrop increased during late season growth, resulting in only small differences in total N uptake between wheat intercrops and sole crops, but increased grain N concentrations in the intercrop. Interspecific competition during vegetative wheat growth was reduced by increasing width of the rototilled strips from 7 to 14 cm, resulting in higher grain yields and increased grain N uptake. Increasing the sowing width of the wheat crop from 3 to 6 cm increased interspecific interactions and reduced wheat intraspecific competition during the entire growing season, leading to improved grain yields and higher grain N uptake.     

Temperature variations during grain filling obtained in growth tunnel experiments and its influence on protein content,polymer build-up and gluten viscoelastic properties in wheat

The aim of this study was to investigate effects of temperature during grain filling on gluten quality characteristics at a lower to moderate temperature range. Experiments with two wheat varieties grown in field covered by polypropylene tunnels during grain filling were performed in two seasons. Mean day temperature differences achieved within the tunnel were approximately 2–2.5 °C from the open to the closed end. There were significant effects of temperature on grain maturity time, thousand grain weight and protein content. The resistance to stretching of the gluten doughs increased with the increasing day temperatures. This was reflected in the proportion of unextractable polymeric proteins (UPP). The results suggest that increases in temperature within this temperature range affect the polymerization of polymeric proteins, giving higher molecular weights, and hence increased Rmax and stronger gluten. The two varieties differed in their response to temperature. In addition, there were seasonal variations in gluten functionality that may be associated with fluctuations in day temperatures between the seasons.