Expression of high zinc efficiency of Aegilops tauschii and Triticum monococcum in synthetic hexaploid wheats

The effect of varied zinc (Zn) supply on shoot and root dry matter production, severity of Zn deficiency symptoms and Zn tissue concentrations was studied in two Triticum turgidum (BBAA) genotypes and three synthetic hexaploid wheat genotypes by growing plants in a Zn-deficient calcareous soil under greenhouse conditions with (+Zn=5 mg kg^-1 soil) and without (−Zn) Zn supply. Two synthetic wheats (BBAADD) were derived from two different Aegilops tauschii (DD) accessions using same Triticum turgidum (BBAA), while one synthetic wheat (BBAAAA) was derived from Triticum turgidum (BBAA) and Triticum monococcum (AA). Visible symptoms of Zn deficiency, such as occurrence of necrotic patches on leaves and reduction in shoot elongation developed more rapidly and severely in tetraploid wheats than in synthetic hexaploid wheats. Correspondingly, decreases in shoot and root dry matter production due to Zn deficiency were higher in tetraploid wheats than in synthetic hexaploid wheats. Transfer of the DD genome from Aegilops tauschii or the AA genome from Triticum monococcum to tetraploid wheat greatly improved root and particularly shoot growth under Zn-deficient, but not under Zn-sufficient conditions. Better growth and lesser Zn deficiency symptoms in synthetic hexaploid wheats than in tetraploid wheats were not accompanied by increases in Zn concentration per unit dry weight, but related more to the total amount of Zn per shoot, especially in the case of synthetic wheats derived from Aegilops tauschii. This result indicates higher Zn uptake capacity of synthetic wheats. The results demonstrated that the genes for high Zn efficiency from Aegilops tauschii (DD) and Triticum monococcum (AA) are expressed in the synthetic hexaploid wheats. These wheat relatives can be used as valuable sources of genes for improvement of Zn efficiency in wheat. This revised version was published online in June 2006 with corrections to the Cover Date.     

Expression and suppression of resistance to greenbug (Homoptera: Aphididae) in synthetic hexaploid wheats derived from Triticum dicoccum x Aegilops tauschii crosses

Fifty-eight synthetic hexaploid wheats, developed by crossing Triticum dicoccum Schrank. and Aegilops tauschii (Coss.) Schmal., were evaluated at the seedling stage, together with their parents, for resistance to greenbug (Schizaphis graminum Rondani) under greenhouse conditions. Seedlings of different synthetic hexaploids showed large phenotypic differences for resistance. All the T. dicoccum parents were susceptible, while high levels of resistance were observed in some of the Ae. tauschii parents. Of the synthetic hexaploids derived from resistant Ae. tauschii parents, a high proportion (76%) showed levels of resistance to the greenbug biotype used that were comparable to those of the resistant parent. While there were clear indications of the presence of suppressor genes for greenbug resistance in the A and/or B genomes of T. dicoccum in some synthetics, positive epistatic interaction was also found in synthetic hexaploids with higher levels of resistance than that of either parent. Resistance from different Ae. tauschii accessions was expressed differently when crossed with the same T. dicoccum, indicating diversity among the resistance genes present in the test synthetic hexaploid wheats. Based on resistance reactions, the genes conferring greenbug resistance in these synthetic hexaploids are probably different from resistance genes previously transferred to wheat from Ae. tauschii.     

Seed protein and isozyme variations in Triticum tauschii (Aegilops squarrosa)

Sixty Triticum tauschii (Aegilops squarrosa, 2n=2x=14, DD) accessions were evaluated for the variability of high-molecular-weight (HMW) glutenins, gliadins and isozymes of seed esterase, -amylase and glucose-phosphate isomerase. Wide variability was observed for HMW-glutenins and gliadins. The implications of unique HMW-glutenin alleles for quality parameters are discussed. Isozyme evaluations indicated more variability for the Est-D ^t 5 locus as compared to the Est-D5 of bread-wheat. The polymorphism for -Amy-D ^t 1 was less than that of -Amy-D1. Similar to the bread-wheat situation, Gpi-D ^t 1 showed no polymorphism. The variability observed with the traits evaluated can be readily observed in T. turgidum x T. tauschii synthetic hexaploids (2n=6x=42, AABBDD) suggesting that T. tauschii accessions may be a rich source for enhancing the genetic variability of T. aestivum cultivars.     

Nitrate reductases in hexaploid and tetraploid wheats and Aegilops

Summary Nitrate reductase activity (NR activity), protein content (NR protein) and polypeptides were compared in shoots of Triticum aestivum ssp. vulgare (L.) cv Fidel (bread wheat, AABBDD genome), Triticum dicoccum cv Vernal (AABB genome), Aegilops squarrosa var. strangulata (DD genome) and the amphiploid 365 (AABBDD genome), produced by crossing T. dicoccum cv Vernal and Ae. squarrosa var. strangulata. Constitutive NR protein and activity were found in shoots of all seedlings grown without nitrate, with the highest activity in the bread wheat. The inducible NR protein and activity developed upon the addition of nitrate. A 116-K polypeptide was identified as the main component of the NR from the bread wheat, while a faint, sometimes discernable 94-K band appeared on Western blots. Only one NR polypeptide could be identified in Ae. squarrosa —the 94 K. An intermediary situation was observed with the tetraploid T. dicoccum and the amphiploid: The 94-K polypeptide was the only one separated from NR of seedlings grown in the absence of nitrate. The 116-K polypeptide appeared after the addition of nitrate. The intensity of its band on the gel increased with the duration of the nitrate treatment. When comparing Ae. squarrosa and T. dicoccum, the constitutive isozyme (94-K polypeptide) was found in the D as well as in the AB genomes, while the inducible NR (116-K polypeptide) was absent from the D genome. Addition of the D genome into the AB genome slightly reinforced the expression of the inducible form (AB genome expression) in the amphiploid wheat. We postulate that the inducible form of NR in the bread wheat resulted from an evolutionary selection pressure favoured by cultivation.     

The structure of the Aegilops tauschii genepool and the evolution of hexaploid wheat

 Polymorphism in the lengths of restriction fragments at 53 single-copy loci, the rRNA locus Nor3, and the high-molecular-weight glutenin locus Glu1 was investigated in the D genome of hexaploid Triticum aestivum and that of Aegilops tauschii, the source of the T. aestivum D genome. The distribution of genetic variation in Ae. tauschii suggests gene flow between Ae. tauschii ssp. strangulata and ssp. tauschii in Iran but less in Transcaucasia. The “strangulata” genepool is wider than it appears on the basis of morphology and includes ssp. strangulata in Transcaucasia and southeastern (SE) Caspian Iran and ssp. tauschii in north-central Iran and southwestern (SW) Caspian Iran. In the latter region, Ae. tauschii morphological varieties ‘meyeri’ and ‘typica’ are equidistant to ssp. strangulata in Transcaucasia, and both belong to the “strangulata” genepool. A model of the evolution of Ae. tauschii is presented. On the geographic region basis, the D genomes of all investigated forms of T. aestivum are most closely related to the “strangulata” genepool in Transcaucasia, Armenia in particular, and SW Caspian Iran. It is suggested that the principal area of the origin of T. aestivum is Armenia, but the SW coastal area of the Caspian Sea and a corridor between the two areas may have played a role as well. Little genetic differentiation was found among the D genomes of all investigated free-threshing and hulled forms of T. aestivum, and all appear to share a single D-genome genepool, in spite of the fact that several Ae. tauschii parents were involved in the evolution of T. aestivum. Received: 17 November 1997 / Accepted: 17 March 1998     

Characterisation of high- and low-molecular weight glutenin subunits associated to the D genome of Aegilops tauschii in a collection of synthetic hexaploid wheats

Synthetic hexaploid wheats (2n=6x=42, AABBDD) involving genomes from Triticum turgidum (2n= 4x=28, AABB) and Aegilops tauschii (2n=2x=14, DD) have been produced as a means for introducing desirable characteristics into bread wheat. In the present work we describe the genetic variability present at the Glu-D ^t 1 and Glu-D ^t 3 loci, encoding high- (HMW) and low-molecular-weight (LMW) glutenin subunits respectively, derived from Ae. tauschii, using electrophoretic and chromatographic methods, in a collection of synthetic hexaploid wheats. A wide variation both in mobility and surface hydrophobicity of HMW glutenin subunits was observed between different accessions of Ae. tauschii used in the production of the synthetic hexaploids. A combination of electrophoretic and chromatographic methods improves the identification of HMW glutenin subunits; in fact subunits with identical apparent mobility were revealed to have a different surface hydrophobicity by reversed-phase high performance liquid chromatography. None of the Dx5^t subunits present in Ae. tauschii showed the presence of the extra cysteine residue found in the HMW glutenin subunit Dx5 of Triticum aestivum, as revealed by selective amplification with polymerase chain reaction (PCR). The wide variability and the high number of subunits encoded by the Glu-D ^t 3 locus suggests that Ae. tauschii may be a rich source for enhancing the genetic variability of glutenin subunits in bread wheat and improving bread-making properties. Received: 3 March 2001 / Accepted: 23 March 2001     

The expression of salt tolerance from Triticum tauschii in hexaploid wheat

Summary Accessions of Triticum tauschii (Coss.) Schmal. (D genome donor to hexaploid wheat) vary in salt tolerance and in the rate that Na⁺ accumulates in leaves. The aim of this study was to determine whether these differences in salt tolerance and leaf Na⁺ concentration would be expressed in hexaploid wheat. Synthetic hexaploids were produced from five T. tauschii accessions varying in salt tolerance and two salt-sensitive T. turgidum cultivars. The degree of salt tolerance of the hexaploids was evaluated as the grain yield per plant in 150 mol m^-3 NaCl relative to grain yield in 1 mol m^-3 NaCl (control). Sodium concentration in leaf 5 was measured after the leaf was fully expanded. The salt tolerance of the genotypes correlated negatively with the concentration of Na⁺ in leaf 5. The salt tolerance of the synthetic hexaploids was greater than the tetraploid parents primarily due to the maintenance of kernel weight under saline conditions. Synthetic hexaploids varied in salt tolerance with the source of their D genome which demonstrates that genes for salt tolerance from the diploid are expressed at the hexaploid level.     

Sequencing chromosome 5D of Aegilops tauschii and comparison with its allopolyploid descendant bread wheat (Triticum aestivum)

Flow cytometric sorting of individual chromosomes and chromosome‐based sequencing reduces the complexity of large, repetitive Triticeae genomes. We flow‐sorted chromosome 5D of Aegilops tauschii, the D genome donor of bread wheat and sequenced it by Roche 454 GS FLX platform to approximately 2.2x coverage. Repetitive sequences represent 81.09% of the survey sequences of this chromosome, and Class I retroelements are the prominent type, with a particular abundance of LTR/Gypsy superfamily. Nonrepetitive sequences were assembled to cover 17.76% of the total chromosome regions. Up to 6188 nonrepetitive gene loci were predicted to be encoded by the 5D chromosome. The numbers and chromosomal distribution patterns of tRNA genes suggest abundance in tRNA^Lys and tRNA^Met species, while the nonrepetitive assembly reveals tRNA^Ala species as the most abundant type. A comparative analysis of the genomic sequences of bread wheat and Aegilops chromosome 5D indicates conservation of gene content. Orthologous unique genes, matching Aegilops 5D sequences, numbered 3730 in barley, 5063 in Brachypodium, 4872 in sorghum and 4209 in rice. In this study, we provide a chromosome‐specific view into the structure and organization of the 5D chromosome of Ae. tauschii, the D genome ancestor of bread wheat. This study contributes to our understanding of the chromosome‐level evolution of the wheat genome and presents a valuable resource in wheat genomics due to the recent hybridization of Ae. tauschii genome with its tetraploid ancestor.     

Inheritance and expression of stripe rust resistance in common wheat (Triticum aestivum) transferred from Aegilops tauschii and its utilization

Stripe rust is one of the most destructive diseases for wheat crops in China. Two stripe rust physiological strains, i.e. CYR30 (intern. name: 175E191) and CYR31 (intern. name: 293E175) have been the dominant and epidemic physiological strains since 1994. One Aegilops tauschii accession (SQ-214) from CIMMYT was found immune from or highly resistant to Chinese new stripe rust races CYR30 and CYR31 at adult stage. SQ-214 was crossed with a highly susceptible Ae. tauschii accession As-80. Analysis of data from F1-F2 populations of SQ-214/As-80 revealed that the resistance was controlled by a single dominant gene. To exploit the resistance for wheat breeding, SQ-214 was crossed with Chinese Spring (CS) and backcrossed by two Chinese commercial wheat varieties MY26 and SW3243. The resistance from SQ-214 was suppressed in the F1 hybrids (CS/SQ-214) and the F2 population of CS/SQ-214//MY26. However, the resistance of SQ-214 was expressed in several F2 individuals of CS/SQ-214//SW3243. Eleven advanced lines with high level of resistance to the Chinese stripe rust CYR30 and CYR31 have been developed. This result suggested that SW3243 does not suppress the expression of the Chinese stripe rust and should be used as wheat germplasm for exploiting resistance of Ae. tauschii in wheat breeding. The gliadin electrophoretic pattern of the eleven advanced lines with high stripe rust resistances was compared with their parents SQ-214, CS and SW3243 by acid polyacrylamide gel electrophoresis. The omega-gliadin bands of Gli-Dt1 in Ae. tauschii SQ-214 were transferred to some advanced lines and freely expressed in common wheat genetic background. One of advanced lines possesses a null Gli-D1 allele, where the omega-gliadin bands encoding by the Gli-D1 allele were absent. The potential utilization of this advanced line for wheat quality and stripe rust resistance breeding is also discussed in this paper.     

Resistance to greenbug (Heteroptera: Aphididae) biotype I in Aegilops tauschii synthetic wheats

The greenbug, Schizaphis graminum (Rondani), is a major pest of wheat in North America, reducing U.S. wheat production by 60 to 100 million dollars each year. In this research, 149 wheat lines containing genes from Aegilops tauschii (Coss.) Schmal. were evaluated for resistance to greenbug biotype I. More than 50% of the lines sustained moderate foliar chlorosis from greenbug feeding, and approximately one third of all the lines were highly resistant. All lines with chlorosis scores similar to the resistant control 'Largo' expressed high levels of antibiosis, producing greenbug populations with mean weights ranging from 0.05 to 11.8 mg. There was no significant difference between greenbug weights on these lines and those reared on 'Largo', but the mean weight of individuals reared on the susceptible control 'Thunderbird' was significantly greater than those reared on 'Largo' or any of the test lines. The mean population size of greenbugs produced on plants of each line was significantly correlated with mean greenbug weight. Tolerance was not evident in any of the lines examined, but was unexpectedly apparent in 'Thunderbird' at a level similar to that in the tolerant control cultivar 'Largo'.     

Phylogenetic relationships of Triticum tauschii the D genome donor to hexaploid wheat

Summary In crosses between T. tauschii (D ^t) accesions, their polymorphic gliadin forms were inherited as blocks of gliadin components -Gli-D ^t1, Gli-D ^t2 — as single Mendelian characters. From the progeny of four tri-parental crosses (test-crosses), HMW glutenin subunits derived from T. tauschii (Glu-D ^t1) segregated as alleles of the Glu-D1 locus in bread wheat. In three of the tri-parental crosses, a small proportion (2.5%) of the progeny with atypical segregation patterns, were identified through somatic chromosome counts, to be aneuploids (1.9% hypoploids and 0.6% hyperploids). Chromosomal mapping studies revealed that the synteny of genes for HMW glutenin subunits and gliadins in T. tauschii are conserved in the D genome homologue (chromosome 1D) of T. aestivum. The map distance between the Glu-D1/-D ^t1 and Gli-D1/-D ^t1 loci was calculated to be 63.5 cM, while a linkage to the centromere of 7.7–9.7 cM was estimated for the Glu-D1/-D ^t1 locus.     

Phylogenetic relationships of Triticum tauschii,the D genome donor to hexaploid wheat

Summary Isoelectric focusing of seed esterase (Est-5) isozymes in 79 T. tauschii accessions from diverse sources revealed the presence of six different seed esterase phenotypes. In one of these phenotypes, exclusive to a var. meyeri accession (AUS 18989), no detectable enzymatic activity was observed. Segregation in crosses between T. tauschii (D^t) accessions confirmed three of the seed esterase phenotypes to be alleles of the designated Est-D ^t5 gene locus; the inheritance pattern of these isozymes was not affected by the subspecies differences between the parents. On the bases of variation in Est-5 and their Glu-1 and Gli-1 gene loci (in a previous study in this series), only three strangulata accessions showed consistent homology with their prevalent gene expression in the D genome of hexaploid wheat. The implications of these observations for further interpreting the phyletic nature of the D genome donor in natural hexaploid wheat synthesis are also reported.     

Characterization of low-molecular-weight-glutenin subunit genes from the D-genome of Triticum aestivum,Aegilops crassa,Ae. cylindrica and Ae. tauschii

Twenty low-molecular-weight-glutenin subunit (LMW-GS) gene sequences from the D-genome from Aegilops crassa (2n = 4x = 28), Ae. cylindrica (2n = 4x = 28), Ae. tauschii (2n = 2x = 14) and Triticum aestivum (2n = 6x = 42) were obtained using five sets of specific allele primer pairs. Only the sequences of the first primer pair were complete coding sequences (cds) of LMW-GS, and had 305, 304, 306 and 305 LMW-m amino acid residues in Ae. crassa, Ae. cylindrica, Ae. tauschii and T. aestivum, respectively. The repetitive domain and repeat motif numbers of all LMW glutenin subunits showed eight conserved cysteine residues that lead to the same functional activity in different genome. Based on DNA and predicted protein sequences, phylogenetic trees for all sets of sequences were drawn. At the DNA level, the species closest to T. aestivum for the second, third, fourth and fifth set of sequences were Ae. cylindrica, Ae. tauschii and Ae. crassa, respectively. At the protein level, the species closest to T. aestivum based on the first, second and fifth set of sequences were Ae. cylindrica, Ae. crassa and Ae. crassa, respectively. For other sets of sequences, bread wheat proved to be a distinct species. The LMW-GS gene sequences have been recorded in the GenBank with accession numbers JQ726549–JQ726568JQ726549JQ726550JQ726551JQ726552JQ726553JQ726554JQ726555JQ726556JQ726557JQ726558JQ726559JQ726560JQ726561JQ726562JQ726563JQ726564JQ726565JQ726566JQ726567JQ726568.     

Genetic analysis of Aegilops tauschii -derived seedling resistance to leaf rust in synthetic hexaploid wheat

Seedling resistance to leaf rust available in the synthetic hexaploid wheat line Syn137 was characterised by means of cytogenetic and linkage mapping. Monosomic analysis located a single dominant gene for leaf rust resistance on chromosome 5D. Molecular mapping not only confirmed this location but also positioned the gene to the distal part of the long arm of chromosome 5D. A test of allelism showed that the gene, tentatively named LrSyn137, is independent but closely linked to Lr1. It appears that Syn137 is occasionally heterogeneous for Lr1 since the analysis of the Lr1-specific marker RGA567-5 in the genetic mapping population indicated the presence of Lr1. Syn137 represents another source of genetic variation that can be useful for the diversification of leaf rust resistance in wheat cultivars.     

节节麦和硬粒小麦-簇毛麦双二倍体间杂种的产生及其细胞遗传学研究

通过胚培养产生了节节麦和硬粒小麦-簇毛麦双二倍体间的杂种。结果表明节节麦和硬粒小麦-簇毛麦双二倍体杂交以节节麦作母本较易结实,3个组合的结实率分别为59.18%、67.72%和60.22%,胚培成苗率分别为39.13%、38.10%和50%。杂种F_1生活力旺盛,形态像父本硬粒小麦-簇毛麦双二倍体。杂种自交可孕,3个组合自交结实率平均为7.63%。杂种F_1(ABVD)的减数分裂平均构型为25.36个单价体,1.21个二价体和0.06个三价体,平均每个细胞交叉结频率为1.38,高于“中国春”单倍体的配对频率,推测V组和A、B、D组染色体间有部分同源关系。节节麦和硬粒小麦-簇毛麦双二倍体杂交可能是产生八倍体(AABBDDVV)的又一途径。     

Hybridization of Aegilops caudata with Triticum durum,Triticum aestivum and the Detection of Alien Chromatins

Aegilops caudata L. carries resistance genes against wheat diseases as well as genes of high crude protein and lysine contents, which can be useful for wheat improvement. An amphiploid of Triticum durum - Ac. caudata was synthesized and the hybridization of T. aestivum with the amphiploid Am 8 was carried out. Chromosome in situ hybridization was carried out for the PMC (pollen mother cell) of the synthesized amphiploid (AABBCC) and ( T. aestivum Beijing 837 × Am 8) F2 by using the pAeca 212 as a probe. The results showed that the 7 bivalents from C genome had hybridization signals in the amphiploid. The detection for F2(Beijing 837× Am 8) indicated that translocation, even (pure) home translocation, occurred in F2 generations spontaneously. The study showed the bright prospect in transferring alien resistance genes from C genome to wheat.     

Pm34: a new powdery mildew resistance gene transferred from Aegilops tauschii Coss. to common wheat (Triticum aestivum L.)

Powdery mildew is a major fungal disease in wheat growing areas worldwide. A novel source of resistance to wheat powdery mildew present in the germplasm line NC97BGTD7 was genetically characterized as a monogenic trait in greenhouse and field trials using F₂ derived lines from a NC97BGTD7 X Saluda cross. Microsatellite markers were used to map and tag this resistance gene, now designated Pm34. Three co-dominant microsatellite markers linked to Pm34 were identified and their most likely order was established as: Xbarc177-5D, 5.4cM, Pm34, 2.6cM, Xbarc144-5D, 14cM, Xgwm272-5D. These microsatellite markers were previously mapped to the long arm of the 5D chromosome and their positions were confirmed using Chinese Spring nullitetrasomic Nulli5D-tetra5A and ditelosomic Dt5DL lines. Pm2, the only other known Pm gene on chromosome 5D, has been mapped to the short arm and its specificity is different from that of Pm34.