Physiological traits associated with heat tolerance in bread wheat (Triticum aestivum L.)

Field experiments for evaluating heat tolerance-related physiological traits were conducted for two consecutive years using a mapping population of recombinant inbred lines (RILs) from the cross RAJ4014/WH730. Chlorophyll content (Chl) and chlorophyll fluorescence (CFL) were recorded under timely sown (TS) and late sown (LS) conditions. Late sowing exposes the terminal stage of plants to high temperature stress. Pooled analysis showed that CFL and Chl differed significantly under TS and LS conditions. The mean value of CFL (Fv/Fm) and Chl under both timely and late sown conditions were used as physiological traits for association with markers. Regression analysis revealed significant association of microsatellite markers viz., Xpsp3094 and Xgwm131 with coefficients of determination (R²) values for CFL (Fv/Fm) and Chl as 12 and 8 %, respectively. The correlation between thousand grain weight (TGW) with Chl and CFL were 14 and 7 % and correlation between grain wt./spike with Chl and CFL were 15 and 8 %, respectively. The genotypes showing tolerance to terminal heat stress as manifested by low heat susceptibility index (HSI = 0.43) for thousand grain weight, were also found having very low Chl, HSI (−0.52). These results suggest that these physiological traits may be used as a secondary character for screening heat-tolerant genotypes.     

Multi-locus genome-wide association studies reveal novel genomic regions associated with vegetative stage salt tolerance in bread wheat (Triticum aestivum L.)

Soil salinity is one of the typical abiotic stresses affecting sustainability of wheat production worldwide. In the present study, we performed a 35 K SNP genotyping assay on association panel of 135 diverse wheat genotypes evaluated for vegetative stage tolerance in hydroponics. Association analyses using five multi-locus GWAS models revealed 42 reliable QTNs for 10 salt tolerance associated traits. Among these 42 reliable QTNs, 9, 17 and 16 QTNs were associated with physiological, biomass and shoot ionic traits respectively. Novel major QTNs were identified for chlorophyll content, shoot fresh weight, seedling total biomass, Na⁺ and K⁺ concentration and Na⁺/K⁺ ratio in shoots. Further, 10 major QTNs showed significant effect on the corresponding salt tolerance traits. Gene ontology analysis of the associated genomic regions identified 58 candidate genes. The information generated in this study will be of potential value for improvement of salt tolerance of wheat cultivars using marker assisted selection.     

Identification of RAPD and ISSR markers for drought tolerance in wheat (Triticum aestivum L.)

Drought tolerance is the essential trait that needs to be incorporated in cereal crops, particularly those grown under the rainfed cultivation. Drought tolerance being contributed by several regions of the genome requires identification of these regions, using suitable molecular markers. Therefore, present investigation was aimed at analyzing the genetic diversity present among the cultivars of rainfed and the irrigated areas with respect to the drought tolerant trait. In all, 14 RAPD and 90 ISSR markers were used to identify these genomic regions. Out of 14 RAPD markers, one RAPD primer exhibited polymorphic banding pattern with 18.6 % polymorphism, clearly separating drought tolerant and drought susceptible genotypes. Out of 90 ISSR primers, only 3 ISSR primers revealed polymorphism in relation to the drought tolerance trait exhibiting 21.38 % polymorphism.     

Mapping loci associated with milling yield in wheat (Triticum aestivum L.)

A partial genetic linkage map constructed using 150 single seed descent (SSD) lines generated from a cross between the hexaploid wheat varieties Schomburgk and Yarralinka was used to identify loci controlling milling yield. Milling yield data were obtained using seed collected from field trials conducted at different sites over two seasons. The estimated broad-sense heritability of milling yield in this population was calculated as 0.48. In the preliminary analysis, two regions were identified on chromosomes 3A and 7D, which were significantly associated with milling yield and accounted for 22% and 19% of the genetic variation, respectively. Bulked segregant analysis in combination with AFLP identified other markers linked to these loci, as well as an additional region on chromosome 5A, which accounted for 19% of the genetic variation. The applicability of these markers as selection tools for breeding purposes is discussed.     

Mapping loci associated with flour colour in wheat (Triticum aestivum L.)

 An RFLP map constructed using 150 single seed descent (SSD) lines from a cross between two hexaploid wheat varieties (‘Schomburgk’בYarralinka’) was used to identify loci controlling flour colour. Flour colour data were obtained from field trials conducted over two seasons at different sites. The estimated heritability of this trait was calculated as 0.67. Two regions identified in the preliminary analysis on chromosomes 3A and 7A, accounted for 13% and 60% of the genetic variation respectively. A detailed analysis of the major locus on 7A was conducted through fine mapping of AFLP markers identified using bulked segregant analysis (BSA). Seven additional markers were identified by the BSA and mapped to the region of the 7A locus. The applicability of these markers to identify wheat lines with enhanced flour colour is discussed. Received: 30 September 1997 / Accepted: 4 February 1998     

Identification of three Wx proteins in wheat (Triticum aestivum L.)

Nullisomic analysis of waxy (Wx) protein of hexaploid wheat (Triticum aestivum L.) cv. “Chinese Spring” using two-dimensional polyacrylamide gel electrophoresis revealed that threeWx loci,Wx-A1, Wx-B1, andWx-D1, located on chromosome arms 7AS, 4AL, and 7DS, produce three distinct Wx subunit groups, subunit group-A (SGA), SGB, and SGD, respectively. SGA has a higher molecular weight and a more basic isoelectric point (pI) than the other two. SGB and SGD have the same molecular weight but a slightly different pI range. Owing to the detection of these three subunit groups, we were able to identify the expression of three waxy genes in wheat endosperm and to find two types of mutants among Japanese wheat cultivars, one lacking SGA and the others SGB. These results suggest the possibility of breeding a waxy wheat.     

Enzyme activities associated with developing wheat(Triticum aestivum L.) grain amyloplasts

Intact amyloplasts from endosperm of developing wheat grains have been isolated by first preparing the protoplasts and then fractionating the lysate of the protoplasts on percoll and ficoll gradients, respectively. Amyloplasts isolated as above were functional and not contaminated by cytosol or by organelles likely to be involved in carbohydrate metabolism. The enzyme distribution studies indicated that ADP-glucose pyrophosphorylase and starch synthase were confined to amyloplasts, whereas invertase, sucrose synthase, UDP-glucose pyrophosphorylase, hexokinase, phosphofructokinase-2 and fructose-2,6-P₂ase were absent fro the amyloplast and mainly confined to the cytosol. Triose-P isomerase, glyceraldehyde-3-P dehydrogenase, phosphohexose isomerase, phosphoglucomutase, phosphofructokinase, aldolase, PPi-fructose-6-P-1 phosphotransferase, and fructose-l,6-P₂ase, though predominantly cytosolic, were also present in the amyloplast. Based on distribution of enzymes, a probable pathway for starch biosynthesis in amyloplasts of developing wheat grains has been proposed.     

Genetics of tolerance to aluminium in wheat (Triticum aestivum L. Thell)

Preliminary studies indicated that aluminium-tolerance in wheat (Triticum aestivum L. Thell.) is a dominant character controlled by several genes. The present paper describes further work on localization and characterization of some of these genes in the genome of the medium Al tolerant wheat cultivar Chinese Spring (C.S.), using an aneuploid series (ditelosomics). Aluminium-tolerance of seedlings was assessed using the modified pulse method; the aluminium concentration in the nutrient solution causing irreversible damage to the root apical meristems on exposure for 24 h at 25°C was the measure of Al-tolerance. At least three different factors controlling Al-tolerance in the C.S. cultivar were located on chromosomes 5As, 2Dl and 4Dl. Significant differences were found in Al-uptake and accumulation in roots of the respective ditelosomic lines and euploid seedlings of C.S. Genes controlling Al-tolerance located in the D genome (2Dl and 4Dl) were not expressed in solution culture when genes located on 5As were missing, whereas some tolerance was observed in aneuploid lines in which genes from 5As were present while genes from 2Dl and 4Dl were missing. It is concluded that Al-tolerance genes located in A genome control the expression of other Al-tolerance genes located in the D genome. The implications of the obtained results for chromosome and gene manipulations in cereals are discussed.     

The wheat (Triticum aestivum L.) leaf proteome

The wheat leaf proteome was mapped and partially characterized to function as a comparative template for future wheat research. In total, 404 proteins were visualized, and 277 of these were selected for analysis based on reproducibility and relative quantity. Using a combination of protein and expressed sequence tag database searching, 142 proteins were putatively identified with an identification success rate of 51%. The identified proteins were grouped according to their functional annotations with the majority (40%) being involved in energy production, primary, or secondary metabolism. Only 8% of the protein identifications lacked ascertainable functional annotation. The 51% ratio of successful identification and the 8% unclear functional annotation rate are major improvements over most previous plant proteomic studies. This clearly indicates the advancement of the plant protein and nucleic acid sequence and annotation data available in the databases, and shows the enhanced feasibility of future wheat leaf proteome research.     

Silicon absorption by wheat (Triticum aestivum L.)

Although silicon (Si) is a quantitatively major inorganic constituent of higher plants the element is not considered generally essential for them. Therefore it is not included in the formulation of any of the solution cultures widely used in plant physiological research. One consequence of this state of affairs is that the absorption and transport of Si have not been investigated nearly as much as those of the elements accorded 'essential' status. In this paper we report experiments showing that Si is rapidly absorbed by wheat (Triticum aestivum L.) plants from solution cultures initially containing Si at 0.5 mM, a concentration realistic in terms of the concentrations of the element in soil solutions. Nearly mature plants (headed out) 'preloaded' with Si absorbed it at virtually the same rate as did plants grown previously in solutions to which Si had not been added. The rate of Si absorption increased by more than an order of magnitude between the 2-leaf and the 7-8 leaf stage, with little change thereafter. This revised version was published online in June 2006 with corrections to the Cover Date.     

Transcript-specific, single-nucleotide polymorphism discovery and linkage analysis in hexaploid bread wheat (Triticum aestivum L.)

Food security is a global concern and substantial yield increases in cereal crops are required to feed the growing world population. Wheat is one of the three most important crops for human and livestock feed. However, the complexity of the genome coupled with a decline in genetic diversity within modern elite cultivars has hindered the application of marker‐assisted selection (MAS) in breeding programmes. A crucial step in the successful application of MAS in breeding programmes is the development of cheap and easy to use molecular markers, such as single‐nucleotide polymorphisms. To mine selected elite wheat germplasm for intervarietal single‐nucleotide polymorphisms, we have used expressed sequence tags derived from public sequencing programmes and next‐generation sequencing of normalized wheat complementary DNA libraries, in combination with a novel sequence alignment and assembly approach. Here, we describe the development and validation of a panel of 1114 single‐nucleotide polymorphisms in hexaploid bread wheat using competitive allele‐specific polymerase chain reaction genotyping technology. We report the genotyping results of these markers on 23 wheat varieties, selected to represent a broad cross‐section of wheat germplasm including a number of elite UK varieties. Finally, we show that, using relatively simple technology, it is possible to rapidly generate a linkage map containing several hundred single‐nucleotide polymorphism markers in the doubled haploid mapping population of Avalon × Cadenza.     

Identification of catalytically active groups of wheat (Triticum aestivum L.) germ lipase

The active site of wheat germ lipase was studied by the Dixon method and chemical modification. The profile of curve logV = f(pH), pK and ionization heat values, lipase photoinactivation, and lipase inactivation with diethylpyrocarbonate and dicyclohexylcarbodiimide led us to assume that the active site of the enzyme comprises the carboxylic group of aspartic or glutamic acid and the imidazole group of histidine. Apparently, the OH-group of serine plays a key role in catalysis: as a result of incubation for 1 h in the presence of phenylmethylsulfonyl fluoride, the enzyme activity decreased by more than 70%. It is shown that ethylenediamine tetraacetate is a noncompetitive inhibitor of lipase. Wheat germs are very healthful because they are rich in vitamins, essential amino acids, and proteins. For this reason, wheat germs are widely used in food, medical, and feed mill industries [1-3]. However, their use is limited by instability during storage, which is largely determined by the effect of hydrolytic and redox enzymes. Representative enzymes of this group are lipase (glycerol ester hydrolase, EC 3.1.1.3), which hydrolyzes triglycerides of higher fatty acids, and lipoxygenase (EC 1.13.11.13), which oxidizes polyunsaturated higher fatty acids.     

Diversity of root-associated bacteria associated with field-grown canola (Brassica napus L.) and wheat (Triticum aestivum L.)

Little is known about the composition and diversity of the bacterial community associated with plant roots. The purpose of this study was to investigate the diversity of bacteria associated with the roots of canola plants grown at three field locations in Saskatchewan, Canada. Over 300 rhizoplane and 220 endophytic bacteria were randomly selected from agar-solidified trypticase soy broth, and identified using fatty acid methyl ester (FAME) profiles. Based on FAME profiles, 18 bacterial genera were identified with a similarity index >0.3, but 73% of the identified isolates belonged to four genera: Bacillus (29%), Flavobacterium (12%), Micrococcus (20%) and Rathayibacter (12%). The endophytic community had a lower Shannon-Weaver diversity index (1.35) compared to the rhizoplane (2.15), and a higher proportion of Bacillus, Flavobacterium, Micrococcus and Rathayibacter genera compared to rhizoplane populations. Genera identified in the endophytic isolates were also found in the rhizoplane isolates. Furthermore, principal component analysis indicated three clusters of bacteria regardless of their site of origin, i.e., rhizoplane or endophytic. In addition, the rhizoplane communities of canola and wheat grown at the same site differed significantly. These results indicate that diverse groups of bacteria are associated with field-grown plants and that endophytes are a subset of the rhizoplane community.     

Proteomic analysis on salicylic acid-induced salt tolerance in common wheat seedlings (Triticum aestivum L.)

The influence of salicylic acid (SA) on the salt tolerance mechanism in seedlings of common wheat (Triticum aestivum L.) was investigated using physiological measurements combined with global expression profiling (proteomics). In the present study, 0.5mM SA significantly reduced NaCl-induced growth inhibition in wheat seedlings, manifesting as increased fresh weights, dry weights, and photosynthetic pigments, but decreased lipid peroxidation. Two-week-old wheat seedlings treated with 0.5mM SA, 250mM NaCl and 250mM NaCl+0.5mM SA for 3days were used for the proteomic analyses. In total, 39 proteins differentially regulated by both salt and SA were revealed by 2D PAGE, and 38 proteins were identified by MALDI-TOF/TOF MS. The identified proteins were involved in various cellular responses and metabolic processes including signal transduction, stress defense, energy, metabolism, photosynthesis, and others of unknown function. All protein spots involved in signal transduction and the defense response were significantly upregulated by SA under salt stress, suggesting that these proteins could play a role in the SA-induced salt resistance in wheat seedlings.     

Chinese spring wheat (Triticum aestivum L.) chloroplast genome: Complete sequence and contig clones

Libraries of plasmid clones covering the entire chloroplast (cp) genome of the common wheat,Triticum aestivum cv. Chinese Spring were constructed and assembled into contig-clones. From these, we obtained the complete nucleotide sequence of wheat chloroplast DNA—a 134,540 bp circular DNA (DDBJ accession no. AB042240) containing four species of ribosomal RNA, 30 genes for 20 species of transfer RNA, and 71 protein coding genes. Additionally, we detected five unidentified open reading frames conserved among grasses. Plasmid clones are available on request.     

基于全基因组的河北省小麦品种遗传多样性分析

丰富的遗传变异对于提高作物的环境适应性和遗传改良进度至关重要。小麦是重要的粮食作物,河北省作为我国第三大小麦产区,在保障国家粮食安全中占有重要地位。建国以来,河北省审定了大量小麦品种,然而有关其遗传基础的研究却相对缺乏。本研究以1949年至2012年的169份河北省小麦品种为材料,利用覆盖小麦全基因组的SSR标记分析了供试小麦品种的遗传多样性。结果表明,供试河北省小麦品种的3个染色体组中,以B染色体组具有最高的遗传多样性,A组最低;7个同源群中,以第5、4群具有最高的多样性水平,而第7群最低;在21条染色体上的遗传多样性变化较大,以4A、2D具有较高的多样性水平,1A染色体最低。从品种的更新换代角度看,自1949年以来,尽管品种的等位基因频率在下降,河北省小麦品种的多样性水平基本呈现上升趋势。在此基础上,根据遗传相似系数,供试品种可聚为5大类,聚类结果既反映出河北省小麦品种多样性水平的复杂多样,也反映出品种的地域性分布特征。     

Identification of cytokinins in young wheat spikes (Triticum aestivum cv. Chinese spring)

Cytokinins in young wheat (Triticum aestivum cv. Chinese spring) spikes (2–15 mm) were isolated by immunoaffinity chromatography. High-performance liquid chromatography-radioimmunoassay and gas chromatography-mass spectrometry indicated that major cytokinins present weretrans-zeatin, ribosyl-trans-zeatin, ribosyldihydrozeatintrans-zeatin-9-glucoside, and the glucosides oftrans-zeatin, ribosyl-trans-zeatin, andtrans-zeatin-9-glucoside. Dihydrozeatin,iso-pentenyladenosine, andiso-pentenyladenine were also present but at lower concentrations.Mention of a trade name, proprietary product, or specific equipment does not constitute a guarantee or warranty by the U.S. Department of Agriculture and does not imply approval to the exclusion of other products that may also be suitable. R. C. Durley's present address is Biological Sciences, Monsanto Company, St. Louis, Missouri 63167.     

Low crop plant population densities promote pollen-mediated gene flow in spring wheat (Triticum aestivum L.)

Transgenic wheat is currently being field tested with the intent of eventual commercialization. The development of wheat genotypes with novel traits has raised concerns regarding the presence of volunteer wheat populations and the role they may play in facilitating transgene movement. Here, we report the results of a field experiment that investigated the potential of spring wheat plant population density and crop height to minimize gene flow from a herbicide-resistant (HR) volunteer population to a non-HR crop. Pollen-mediated gene flow (PMGF) between the HR volunteer wheat population and four conventional spring wheat genotypes varying in height was assessed over a range of plant population densities. Natural hybridization events between the two cultivars were detected by phenotypically scoring plants in F₁ populations followed by verification with Mendelian segregation ratios in the F_1:2 families. PMGF was strongly associated with crop yield components, but showed no association with flowering synchrony. Maximum observed PMGF was always less than 0.6%, regardless of crop height and density. The frequency of PMGF in spring wheat decreased exponentially with increasing plant population density, but showed no dependence on either crop genotype or height. However, increasing plant densities beyond the recommended planting rate of 300 cropped wheat plants m⁻² provided no obvious benefit to reducing PMGF. Nevertheless, our results demonstrate a critical plant density of 175–200 cropped wheat plants m⁻² below which PMGF frequencies rise exponentially with decreasing plant density. These results will be useful in the development of mechanistic models and best management practices that collectively facilitate the coexistence of transgenic and nontransgenic wheat crops.