Recent advances in the genetics underlying wheat grain protein content and grain protein deviation in hexaploid wheat

Wheat is one of the most important global crops and selection for better performance has been ongoing since ancient times. As a quantitative trait controlled by the interplay of several genomic loci and under the strong influence of the environment, grain protein content (GPC) is of major interest in breeding programs. Here, we review the most recent contributions to the genetics underlying wheat GPC and grain protein deviation (GPD, representing the relationship between grain protein content and yield), together with the performance of genomic prediction models characterizing these traits. A total of 364 significant loci related to GPC and GPD are positioned on the hexaploid wheat genome, highlighting genomic regions where significant independent QTL overlap, with special focus on two regions located on chromosomes 3A and 5A. Some of the corresponding homoeologous sequences co-locate with significant independent QTL reported on the B and D subgenomes. Overlapping independent QTL from different studies are indicative of genomic regions exhibiting stability across environments and genotypes, with promising candidates for improving grain quality.     

水氮互作对小麦籽粒蛋白质、淀粉含量及其组分的影响

以两个不同品质类型的小麦品种（强筋品种豫麦34、弱筋品种豫麦50）为材料,在大田条件下,研究了3个灌水处理（W₁：拔节水;W₂：拔节水+花后15 d灌浆水;W₃：拔节水+灌浆水+花后28 d麦黄水）和3个氮肥水平（0、150、270 kg·hm^-2）对籽粒蛋白质、淀粉含量及其组分的影响.结果表明：270 kg·hm^-2的施氮量有利于提高强筋小麦（豫麦34）籽粒蛋白质含量,籽粒清蛋白、醇溶蛋白和谷蛋白含量明显提高,谷/醇增大;支链淀粉和总淀粉含量提高,直/支下降;籽粒产量增加.弱筋小麦（豫麦50）在150 kg·hm^-2 的施氮量下,清蛋白和醇溶蛋白含量增加,球蛋白和谷蛋白含量下降,谷/醇降低;支链淀粉和总淀粉含量提高;不施氮肥或氮肥施用过多（270 kg·hm^-2）均影响籽粒蛋白质和淀粉的积累,使产量下降.W₂处理促进了籽粒蛋白质和淀粉积累,W₁或W₃处理均不利于籽粒蛋白质和淀粉积累,且导致籽粒产量下降.水、氮互作效应中,强筋和弱筋小麦分别以全生育期270 kg·hm^-2和150 kg·hm^-2施氮量配合拔节水+灌浆水（W₂）为比较理想的水氮运筹方式.     

Grain protein and grain yield of tritordeum in comparison to wheat and triticale

The new species of cereal × Tritordeum Ascherson et Graebner (Hordeum chilense Roem. et Shultz × Triticum ssp.) has a grain protein concentration (GPC) of up to 25%. The relationship between GPC and yield, and the factors responsible for the high GPC of tritordeum were examined and compared in field experiments. Three experimental tritordeum lines, two early and a later released (recombined and secondary tritordeums) were compared to wheat (cv. Cajeme) and triticale cultivars (cv. Trujillo). GPC's were 19%–22% for recombined tritordeums, 16% for the secondary tritordeum, 12–15% for wheats and 11% for triticale. Grain yields of the recombined and secondary tridordeum were 17–33% and 45–57% that of the wheats and triticale, respectively. Reducing grain sink size by spikelet removal resulted in an increased GPC of remaining grains. Considering all species together there were a strong inverse relationship between GPC and grain yield (GY) per main ear (GPC=26–4.76 ln GY; r²=0.82). In another experiment, frost damage to an early sown treatment of wheat reduced sink size. Harvest index (HI) of early sown wheat was reduced from 0.45 to 0.19, values comparable to that of tritordeum. Having similar HI, the GPC of the early sown wheat was the same as an early sown tritordeum (around 18%). Data for total N uptake and the N concentration of plant tissue during the growing season indicated that enhanced N uptake and remobilisation were not responsible for tritordeum's high GPC. These results suggest that the high GPC of the early lines of tritordeum is a consequence of the small grain yield concentrating the grain protein.     

Precise mapping of a locus affecting grain protein content in durum wheat

Grain protein content (GPC) is an important factor in pasta and breadmaking quality, and in human nutrition. It is also an important trait for wheat growers because premium prices are frequently paid for wheat with high GPC. A promising source for alleles to increase GPC was detected on chromosome 6B of Triticum turgidum var. dicoccoides accession FA-15-3 (DIC). Two previous quantitative trait locus (QTL) studies found that the positive effect of DIC-6B was associated to a single locus located between the centromere and the Nor-B2 locus on the short arm of chromosome 6B. Microsatellite markers Xgwm508 and Xgwm193 flanking the QTL region were used in this study to develop 20 new homozygous recombinant substitution lines (RSLs) with crossovers between these markers. These 20 RSLs, plus nine RSLs developed in previous studies were characterized with four new RFLP markers located within this chromosome segment. Grain protein content was determined in three field experiments organized as randomized complete block designs with ten replications each. The QTL peaks for protein content were located in the central region of a 2.7-cM interval between RFLP markers Xcdo365 and Xucw67 in the three experiments. Statistical analyses showed that almost all lines could be classified unequivocally within low- and high- protein groups, facilitating the mapping of this trait as a single Mendelian locus designated Gpc-6B1. The Gpc-6B1 locus was mapped 1.5-cM proximal to Xcdo365 and 1.2-cM distal to Xucw67. These new markers can be used to reduce the size of the DIC chromosome segment selected in marker-assisted selection programs. Markers Nor-B2 and Xucw66 flanking the previous two markers can be used to select against the DIC segment and reduce the linkage drag during the transfer of Gpc-6B1 into commercial bread and pasta wheat varieties. The precise mapping of the high GPC gene, the high frequency of recombinants recovered in the targeted region, and the recent development of a tetraploid BAC library including the Gpc-6B1 DIC allele are the first steps towards the map-based cloning of this gene.Communicated by J. Dvorak     

Detection of QTLs for grain protein content in durum wheat

Grain protein content (GPC) of durum wheat (Triticum turgidum L. var. durum) is an important trait for the nutritional value of grain and for influencing the technological property of flour. Protein content is a quantitative trait negatively correlated with grain yield, thus increase in protein quantity usually results in yield reduction. This study was initiated to introgress alleles for high GPC from var. dicoccoides into durum wheat germplasm by the backcross inbred line (BIL) method and to identify molecular markers linked to high GPC alleles not associated with depressing effects on yield. The backcross line 3BIL-85 with high GPC and similar grain yield to the recurrent parent was backcrossed to Latino, and the generations F₂, F₃ and F₄ were evaluated for GPC and yield per spike (GYS) in three field trials. Three QTLs with major effects on GPC were detected on chromosome arms 2AS, 6AS and 7BL, identified by the markers Xcfa2164, XP39M37 ₍₂₅₀₎ and Xgwm577 _, respectively. Multiple regression analysis indicated that the three QTLs explained all the genetic variances of the trait. The high GPC parental line 3BIL-85 was not significantly different from the recurrent parent Latino for GYS, but the phenotypic correlation coefficient between GPC and GYS had negative values (from −0.02 to −0.28) in each trial, although it was statistically significant only in the F3 progeny trial. No co-located QTL for GYS was detected, excluding the hypothesis that the putative QTLs for GPC were indirect QTLs for low grain yield. The negative protein-yield response could be due to: (a) co-location of grain yield per spike QTLs with reduced phenotypic effects not detectable by the experimental design or statistical procedures, or to (b) opposite pleiotropic gene effects due to the major bio-energetic requirements for synthesis of protein then carbohydrates. Mapping loci by BILs should enable the production of near-isogenic lines in which the individual effects of each QTL can be examined in detail without confounding variations due to other putative QTLs. An erratum to this article can be found at     

小麦品种蛋白质品质性状稳定性研究

用陕西省关中小麦品种区域试验所选用的12个小麦品种（品系）在12个试点的数据资料。分析了品种,环境及品种与环境互作（CEI）对籽粒硬度,蛋白质含量,沉淀值及湿面筋含量的影响。结果表明：基因型效应对所有品质参数均有显著影响,基因型与环境互作对沉淀值影响较大,而对籽粒硬度,蛋白质含量与湿面筋含量影响较小,环境效应对湿面筋含量和好粒硬度影响较大,而对蛋白质含量与沉淀值影响较小,蛋白质品质参数的回归系数（b值）表明,基因型对不同环境的反应存在着显著差异。对于籽粒硬度,蛋白质含量与湿面筋含量回归偏差显著的品种很少,这表明线性回归模式占了基因型变异的绝大部分,一些品种的沉淀值显著偏离了回归,上述结果表明,要改善小麦品种的蛋白质品质,也应重视环境对小麦蛋白质品质的影响。     

Analysis of the functions of TaGW2 homoeologs in wheat grain weight and protein content traits

GW2 is emerging as a key genetic determinant of grain weight in cereal crops; it has three homoeologs (TaGW2‐A1, ‐B1 and ‐D1) in hexaploid common wheat (Triticum aestivum L.). Here, by analyzing the gene editing mutants that lack one (B1 or D1), two (B1 and D1) or all three (A1, B1 and D1) homoeologs of TaGW2, several insights are gained into the functions of TaGW2‐B1 and ‐D1 in common wheat grain traits. First, both TaGW2‐B1 and ‐D1 affect thousand‐grain weight (TGW) by influencing grain width and length, but the effect conferred by TaGW2‐B1 is stronger than that of TaGW2‐D1. Second, there exists functional interaction between TaGW2 homoeologs because the TGW increase shown by a double mutant (lacking B1 and D1) was substantially larger than that of their single mutants. Third, both TaGW2‐B1 and ‐D1 modulate cell number and length in the outer pericarp of developing grains, with TaGW2‐B1 being more potent. Finally, TaGW2 homoeologs also affect grain protein content as this parameter was generally increased in the mutants, especially in the lines lacking two or three homoeologs. Consistent with this finding, two wheat end‐use quality‐related parameters, flour protein content and gluten strength, were considerably elevated in the mutants. Collectively, our data shed light on functional difference between and additive interaction of TaGW2 homoeologs in the genetic control of grain weight and protein content traits in common wheat, which may accelerate further research on this important gene and its application in wheat improvement.     

Inheritance of deeper root length and grain yield in half-diallel durum wheat (Triticum durum) crosses

Inheritance, heterosis and combining ability of deeper root length (DRL) and grain yield (GY) were investigated in durum wheat populations obtained from half‐diallel crossings among five parental lines differing in their DRL and GY. The study was conducted with the final objective of identifying parent lines to be used in a breeding programme to develop drought‐tolerant wheat varieties. General combining ability and specific combining ability effects were significant for both traits; however, additive gene effects were predominant over non‐additive effects. Partial dominance was ambidirectional for DRL and unidirectional for GY. Lines INRAT69 and Omrabia conferred DRL whereas Omrabia and Khiar transmitted high GY to their respective progenies. In the studied material, both characters were controlled mainly by dominant alleles, but they could also be attributed to recessive alleles although less frequently. Both broad‐sense and strict‐sense heritabilities were high for DRL, confirming the importance of additive gene effects, whereas strict‐sense heritability for GY was average, indicating the importance of interaction effects as compared with the additive effects; this could mean reduced selection efficiency for the latter trait. Thus, the expected genetic progress per cycle of selection will be lower for GY compared with DRL. Omrabia should be included in the breeding programme as a parent so that while maintaining high GY, resulting progeny should be better able to resist drought through DRL.     

氮磷水平与气象条件对春小麦籽粒蛋白质含量形成动态的影响

以3个品质类型春小麦品种的施肥和播期试验为基础,通过建立籽粒蛋白质含量形成动态的拟合方程,定量揭示籽粒蛋白质含量形成动态及氮磷肥与气象条件的影响效应。结果表明,灌浆期籽粒蛋白质含量随时间动态变化的普遍规律符合一元三次多项式曲线,即呈自开花始先降低后升高的单谷曲线变化。氮磷肥与气象条件的影响及基因型差异通过方程特征量而体现。氮水平增加,高蛋白品种蛋白质含量增加,动态曲线谷值和峰值均明显提高且出现时间分别提前和推后。磷水平增加,高蛋白强筋品种蛋白质含量降低,高蛋白中筋品种则增加;高蛋白品种谷值和峰值均提高且出现时间推迟。低蛋白品种蛋白质含量随氮磷肥变化不明显且幅度很小。在没有水分胁迫的情况下,光温互作是影响籽粒蛋白质含量动态形成的首要因子,其次为降水;而气温日较差则为最敏感因子。较高光温条件互作前提下,增加灌浆期温度日较差使高蛋白品种蛋白质含量提高,低蛋白品种则降低。高蛋白相比于低蛋白品种更易受氮磷水平和气象条件影响。     

Variation in phosphorus and sulfur content shapes the genetic architecture and phenotypic associations within the wheat grain ionome

Dissection of the genetic basis of wheat ionome is crucial for understanding the physiological and biochemical processes underlying mineral accumulation in seeds, as well as for efficient crop breeding. Most of the elements essential for plants are metals stored in seeds as chelate complexes with phytic acid or sulfur‐containing compounds. We assume that the involvement of phosphorus and sulfur in metal chelation is the reason for strong phenotypic correlations within ionome. Adjustment of element concentrations for the effect of variation in phosphorus and sulfur seed content resulted in drastic change of phenotypic correlations between the elements. The genetic architecture of wheat grain ionome was characterized by quantitative trait loci (QTL) analysis using a cross between durum and wild emmer wheat. QTL analysis of the adjusted traits and two‐trait analysis of the initial traits paired with either P or S considerably improved QTL detection power and accuracy, resulting in the identification of 105 QTLs and 617 QTL effects for 11 elements. Candidate gene search revealed some potential functional associations between QTLs and corresponding genes within their intervals. Thus, we have shown that accounting for variation in P and S is crucial for understanding of the physiological and genetic regulation of mineral composition of wheat grain ionome and can be implemented for other plants.     

Gene networks in hexaploid wheat: interacting quantitative trait loci for grain protein content

In hexaploid wheat, single-locus and two-locus quantitative trait loci (QTL) analyses for grain protein content (GPC) were conducted using two different mapping populations (PI and PII). Main effect QTLs (M-QTLs), epistatic QTLs (E-QTLs) and QTL x environment interactions (QE, QQE) were detected using two-locus analyses in both the populations. Only a few QTLs were common in both the analyses, and the QTLs and the interactions detected in the two populations differed, suggesting the superiority of two-locus analysis and the need for using several mapping populations for QTL analysis. A sizable proportion of genetic variation for GPC was due to interactions (28.59% and 54.03%), rather than to M-QTL effects (7.24% and 7.22%), which are the only genetic effects often detected in the majority of QTL studies. Even E-QTLs made a marginal contribution to genetic variation (2.68% and 6.04%), thus suggesting that the major part of genetic variation is due to changes in gene networks rather than the presence or absence of specific genes. This is in sharp contrast to the genetic dissection of pre-harvest sprouting tolerance conducted by us earlier, where interaction effects were not substantial, suggesting that the nature of genetic variation also depends on the nature of the trait.     

我国旱地春小麦产量及主要农艺指标的变异分析

采用4年、13个品种(系)、18个试点组成的全国旱地春小麦区域试验产量资料,通过联合方差分析和基因型及其与环境互作（GGE）双标图分析,研究了基因型、环境、基因型与环境互作效应（GEI）对产量变异的影响及品种的产量稳定性.结果表明：环境对产量变异的影响远大于基因型和GEI,环境引起的产量变异占87.5%～92.0%．互作因素中以地点×基因型的互作效应最大,基因型×年份的互作效应最小.我国旱地春小麦基因型多年多点的平均产量水平为2550 kg·hm^-2.产量三要素中,千粒重受环境的影响最小.影响产量变异的主要环境因子有：≥10 ℃年积温、生育期降雨量、平均气温、海拔、年降雨量和无霜期.产量与单位面积穗数（0.675^**）、穗粒数（0.581^**）、千粒重（0.456^**）呈极显著正相关,产量三要素间也呈正相关（0.244～0.480^**）,处于可同步提高范围.     

Leaf-applied sodium chloride promotes cadmium accumulation in durum wheat grain

Cadmium (Cd) accumulation in durum wheat grain is a growing concern. Among the factors affecting Cd accumulation in plants, soil chloride (Cl) concentration plays a critical role. The effect of leaf NaCl application on grain Cd was studied in greenhouse-grown durum wheat (Triticum turgidum L. durum, cv. Balcali-2000) by immersing (10 s) intact flag leaves into Cd and/or NaCl-containing solutions for 14 times during heading and dough stages. Immersing flag leaves in solutions containing increasing amount of Cd resulted in substantial increases in grain Cd concentration. Adding NaCl alone or in combination with the Cd-containing immersion solution promoted accumulation of Cd in the grains, by up to 41%. In contrast, Zn concentrations of grains were not affected or even decreased by the NaCl treatments. This is likely due to the effect of Cl complexing Cd and reducing positive charge on the metal ion, an effect that is much smaller for Zn. Charge reduction or removal (CdCl₂⁰ species) would increase the diffusivity/lipophilicity of Cd and enhance its capability to penetrate the leaf epidermis and across membranes. Of even more significance to human health was the ability of Cl alone to penetrate leaf tissue and mobilize and enhance shoot Cd transfer to grains, yet reducing or not affecting Zn transfer.     

The dynamics of protein body formation in developing wheat grain

Wheat is a major source of protein in the diets of humans and livestock but we know little about the mechanisms that determine the patterns of protein synthesis in the developing endosperm. We have used a combination of enrichment with ¹⁵N glutamine and NanoSIMS imaging to establish that the substrate required for protein synthesis is transported radially from its point of entrance in the endosperm cavity across the starchy endosperm tissues, before becoming concentrated in the cells immediately below the aleurone layer. This transport occurs continuously during grain development but may be slower in the later stages. Although older starchy endosperm cells tend to contain larger protein deposits formed by the fusion of small protein bodies, small highly enriched protein bodies may also be present in the same cells. This shows a continuous process of protein body initiation, in both older and younger starchy endosperm cells and in all regions of the tissue. Immunolabeling with specific antibodies shows that the patterns of enrichment are not related to the contents of gluten proteins in the protein bodies. In addition to providing new information on the dynamics of protein deposition, the study demonstrates the wider utility of NanoSIMS and isotope labelling for studying complex developmental processes in plant tissues.     

A selection strategy to accommodate genotype-by-environment interaction for grain yield of wheat: managed-environments for selection among genotypes

Selection for grain yield among wheat lines is complicated by large line-by-environment (L × E) interactions in Queensland, Australia. Early generation selection is based on an evaluation of many lines in a few environments. The small sample of environments, together with the large L × E interaction, reduces the realised response to selection. Definition of a series of managed-environments which provides discrimination among lines, which is relevant to the target production-environments, and can be repeated over years, would facilitate early generation selection. Two series of managed-environments were conducted. Eighteen managed-environments were generated in Series-1 by manipulating nitrogen and water availability, together with the sowing date, at three locations. Nine managed-environments based on those from Series-1 were generated in Series-2. Line discrimination for grain yield in the managed-environments was compared to that in a series of 16 random production-environments. The genetic correlation between line discrimination in the managed-environments and that in the production-environments was influenced by the number and combination of managed-environments. Two managed-environment selection regimes, which gave a high genetic correlation in both Series-1 and 2, were identified. The first used three managed-environments, a high input (low water and nitrogen stress) environment with early sowing at three locations. The second used six managed-environments, a combination of a high input (low water and nitrogen stress) and medium input (water and nitrogen stress) with early sowing at three locations. The opportunities for using managed-environments to provide more reliable selection among lines in the Queensland wheat breeding programme and its potential limitations are discussed.     

基因型和地域分布对小麦籽粒氨基酸含量影响的研究

根据河南省7个试验基点的10个小麦基因型籽粒中17种氨基酸含量的测定结果,对不同基因型氨基酸含量差异及其地域分布进行了研究,结果表明,小麦籽粒氨基酸含量不仅存在基因型的差异,而且在很大程度上受生态环境条件的影响,环境间氨基酸含量的变异明显大于基因型,约为基因型间变异的1．5倍。不同试点所对应的气候分布、土壤类型与氨基酸含量变异有较大的吻合程度,表现出随湿润条件增加,氨基酸含量和必需氨基酸、非必需氨基酸总量逐渐下降,而非必需氨基酸占氨基酸总量的比例呈逐渐上升趋势,与人体代谢关系密切的赖氨酸化学记分较 低,且变异较小。     

Detection of grain protein content QTLs across environments in tetraploid wheats

Grain protein content (GPC) is an important quality factor in both durum and bread wheats. GPC is considered to be a polygenic trait influenced by environmental factors and management practice. The objectives of this study were both to compare the quantitative trait loci (QTL) for GPC in a population of 65 recombinant inbred lines of tetraploid wheats evaluated in three locations for several years (eight data sets), and to investigate the genetic relationship among GPC and grain yield. QTLs were determined based on the Messapia × dicoccoides linkage map which covers 217 linked loci on the 14 chromosomes with 42 additional loci as yet unassigned to linkage groups. The map extends to 1352 cM; the average distance between adjacent markers was 6.3 cM. Seven QTLs for GPC, located on the chromosome arms 4BS, 5AL, 6AS (two loci), 6BS, 7AS and 7BS, were detected that were significant in at least one environment at P<0.001 or in at least two environments at P<0.01. One QTL was significant in all but one environment, two were significant in four or five environments, and four were significant in two out of eight environments. Six out of seven protein content QTLs had pleiotropic effects or were associated to QTLs for grain yield and explained the negative correlation among GPC and yield components. The present results support the concept that studies conducted in a single environment are likely to underestimate the number of QTLs that can influence a trait and that the phenotypic data for a quantitative trait should be collected over a range of locations to identify putative QTLs and determine their phenotypic effects.     

Epistasis and genotype-environment interaction for quantitative trait loci affecting flowering time in Arabidopsis thaliana

A major goal of evolutionary biology is to understand the genetic architecture of the complex quantitative traits that may lead to adaptations in natural populations. Of particular relevance is the evaluation of the frequency and magnitude of epistasis (gene–gene and gene–environment interaction) as it plays a controversial role in models of adaptation within and among populations. Here, we explore the genetic basis of flowering time in Arabidopsis thaliana using a series of quantitative trait loci (QTL) mapping experiments with two recombinant inbred line (RIL) mapping populations [Columbia (Col) x Landsberg erecta (Ler), Ler x Cape Verde Islands (Cvi)]. We focus on the response of RILs to a series of environmental conditions including drought stress, leaf damage, and apical damage. These data were explicitly evaluated for the presence of epistasis using Bayesian based multiple-QTL genome scans. Overall, we mapped fourteen QTL affecting flowering time. We detected two significant QTL–QTL interactions and several QTL–environment interactions for flowering time in the Ler x Cvi population. QTL–environment interactions were due to environmentally induced changes in the magnitude of QTL effects and their interactions across environments – we did not detect antagonistic pleiotropy. We found no evidence for QTL interactions in the Ler x Col population. We evaluate these results in the context of several other studies of flowering time in Arabidopsis thaliana and adaptive evolution in natural populations.