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(1998). Inheritance of race-specific necrotic and chlorotic reactions induced by Pyrenophora tritici-repentis in hexaploid wheats. Canadian Journal of Plant Pathology: Vol. 20, No. 4, pp. 401-407.

The objectives of this study were to determine the inheritance of aryloxyphenoxypropionate (APP) resistance in the wild oat population UM33 and to determine the genetic relationship between resistance in UM33 and another population, UM1, which has a different cross-resistance pattern. Reciprocal crosses were made between UM33 and a susceptible population UM5, and between UM33 and UM1. Initial screenings of F1and F2Is populations derived from crosses between UM33 and UM5 were conducted over a range of fenoxaprop-P rates to determine a discriminatory dosage. F2populations and F2-derived F3families were then screened at this dosage (1200 g ai ha−1) to determine segregation patterns. Results from reciprocal UM33 x UM5 F1dose-response experiments, and F2and F2-derived F3segregation experiments indicated that UM33 resistance to fenoxaprop-P was governed by a single, partially dominant nuclear gene system. To determine if resistance in UM1 and UM33 results from alterations at the same gene locus, 584 F2plants derived from reciprocal UM33 x UM1 crosses were screened with 150 g ha−1fenoxaprop-P. This dosage was sufficient to kill susceptible plants (UM5), but was not sufficient to kill plants with a resistance allele from either parent. None of the treated F2plants exhibited injury or death, indicating that UM1 and UM33 resistance genes did not segregate independently. From this it was concluded that resistance in both populations is encoded at the same gene locus.

The importance of various factors influencing the evolution of herbicide resistance in weeds is critically examined using population genetic models. The factors include gene mutation, initial frequency of resistance alleles, inheritance, weed fitness in the presence and absence of herbicide, mating system, and gene flow. Where weed infestations are heavy, the probability of selecting for resistance can be high even when the rate of mutation is low. Subsequent to the occurrence of a resistant mutant, repeated treatments with herbicides having the same mode of action can lead to the rapid evolution of a predominantly resistant population. At a given herbicide selection intensity, the initial frequency of resistance alleles determines the number of generations required to reach a specific frequency of resistant plants. The initial frequency of resistance alleles has a greater influence on the evolutionary process when herbicides impose weak selection, as opposed to very strong selection. Under selection, dominant resistance alleles increase in frequency more rapidly than recessive alleles in random mating or highly outcrossing weed populations. In highly self-fertilizing species, dominant and recessive resistance alleles increase in frequency at approximately the same rate. Gene flow through pollen or seed movement from resistant weed populations can provide a source of resistance alleles in previously susceptible populations. Because rates of gene flow are generally higher than rates of mutation, the time required to reach a high level of resistance in such situations is greatly reduced. Contrary to common misconception, gene flow from a susceptible population to a population undergoing resistance evolution is unlikely to slow the evolutionary process significantly. Accurate measurements of many factors that influence resistance evolution are difficult, if not impossible, to obtain experimentally. Thus, the use of models to predict times to resistance in specific situations is markedly limited. However, with appropriate assumptions, they can be invaluable in assessing the relative effectiveness of various management practices to avoid, or delay, the occurrence of herbicide resistance in weed populations.

Most tetraploid durum wheat (Triticum turgidum L var. durum) cultivars are susceptible to Fusarium head blight (FHB). This study reports novel quantitative trait loci (QTL) associated with FHB resistance. A backcross recombinant inbred line (BCRIL) population was developed from the cross BGRC3487/2*DT735, and 160 lines were evaluated for resistance to Fusarium graminearum Schwabe (teleomorph Gibberella zeae (Schwein. Petch) in field trials over 3 years (2008–2010) and to a F. graminearum 3-acetyl-deoxynivalenol (3-ADON) chemotype in greenhouse trials. The population was genotyped with 948 polymorphic loci using DArT and microsatellite markers. Eleven QTL were associated with FHB resistance under field conditions on chromosomes 2A, 3B, 5A, 5B, 7A, and 7B. Two of these, QFhb.usw-3B from BGRC3487 and QFhb.usw-7A2, were consistently detected over environments. The QFhb.usw-3B QTL was in a similar position to a resistance QTL in hexaploid wheat. The combination of the two QTL reduced field index by 53.5%–86.2%. Two QTL for resistance to the 3-ADON chemotype were detected on chromosomes 1B and 4B. Both BGRC3487 and DT735 could provide new sources of FHB resistance and the combination of QTL reported here could be valuable tools in breeding FHB-resistant durum wheat.

Resistance to fenoxaprop-P and other aryloxyphe- noxypropionate and cyclohexanedione herbicides in the wild oat population, UM1, is controlled by a single, partially dominant, nuclear gene. In arriving at this conclusion, par- ents, F, hybrids, and F2 plants derived from reciprocal crosses between UM1 and a susceptible wild oat line, UM5, were treated with fenoxaprop-Povera wide range of dosages. Based on these experiments, a dosage of 400 g ai ha-' fenoxaprop-P was selected to discriminate between three response types. At this dosage, susceptible plants were killed and resistant plants were unaffected, whereas plants charac- terized as intermediate in response were injured but recov- ered. Treated F2 plants segregated in a 1:2:1 (R, I, S) ratio, indicative of single nuclear gene inheritance. This was con- finned by sellmg F2 plants and screening several F3 families. Families derived from internediate F2 plants segregated for the three characteristic response types, whereas those de- rived from resistant F2 plants were uniformly resistant. Chi- square analysis indicated the F2 segregation ratios fit those expected for a single partially dominant nuclear gene system. In addition, F2 populations from both crosses were screened with a mixture of fenoxaprop-Pand sethoxydim. The dosages of both herbicides (150 g ai ha-' fenoxaprop-P and 100 g ha-1 sethoxydim) were sufficient to control only susceptible plants. Treated F2 populations segregated in a 3:1 (R:S) pattern, thereby confirming that resistance to the two chemi- cally unrelated herbicides results from the same gene altera- tion. Nomenclature: Fenoxaprop-P (R-2-(4-((6-chloro-2- benzoxazolyl)oxy)phenoxy)propanoic acid, ethyl ester; sethoxydim, (2-(1-(ethoxyimino)butyl)-5-(2-(ethylthio)- propyl)-3-hydroxy-2-cyclohexen-1-one); wild oat, Avena fa- tua L. #3 AVEFA. Additional index words. Fenoxaprop-p-ethyl, aryloxyphe- noxypropionate, cyclohexanedione, ACCase, herbicide resis- tance, weed genetics, AVEFA.

The inheritance of resistance to dicamba in wild mustard was deternined by making reciprocal crosses be- tween a resistant (R) population derived from a field treated repeatedly with auxin-type herbicides, and a known suscep- tible (S) population. The resulting F1 hybrids were selfed to produce F2 populations and backcrossed to the S parent. At the three- to four-leaf stage, parental, F1, F2, and backcross populations were screened for resistance to dicamba at three dosages (50, 200, and 400 g ai ha-1). F1 progeny survived all dosages and exhibited levels of injury similar to the R paren- tal population. F2 populations segregated in a 3:1 ratio of R to S phenotypes. Progeny of backcrosses segregated in a 1:1 (R:S) ratio. Responses of the F1, F2, and backcross popula- tions to treatment with dicamba indicate that resistance is determined by a single, completely dominant nuclear allele. Nomenclature: Dicamba, 3,6-dichloro-2-methoxybenzoic acid; wild mustard, Brassica kaber(DC.) L. C. Wheeler, Sinapis arvensis L. #3 SINAR. Additional index words. Genetics, herbicide resistance, auxin, Sinapis arvensis, SINAR.

In F2progeny, derived from F1hybrids, shoot growth of seedlings, measured 4 d after germination, distinguished susceptible (S) and resistant (R) phenotypes. Chisquare values indicated that the F2data fit a 3:1 (S:R) ratio for both populations and all trifluralin concentrations in which S and R phenotypes could be differentiated. Results indicate that trifluralin resistance in these green foxtail populations is controlled by a single, nuclear recessive gene. This study is the first to demonstrate recessive gene control of herbicide resistance in a weed species. The highly selfed nature and prolific seed production of green foxtail may have facilitated evolution of the recessive trait.

Total RNA from wheat spikes infected with F. graminearum Fg2 was extracted and the mRNA was purified. Switching Mechanism at 5' end of the RNA Transcript (SMART) technique and CDS Ill/3' primer were used for first-strand cDNA synthesis using reverse transcriptase by RT-PCR. Primer extension polymerase chain reaction was used to construct the double-strand cDNA that was digested by proteinase K, then by Sfi I and fractionated. cDNAs longer than 0.5 kb were collected and ligated to λTriplEx2 vector followed λ phage packaging reaction and library amplification. The qualities of both unamplified and amplified cDNA libraries were strictly checked by conventional titer determination. One hundred and sixty five plaques were randomly picked and tested using PCR with universal primers derived from the sequence flanking the vector. A high quality cDNA library from wheat spikes that have been infected by F. graminearum was successfully constructed.

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 F(1) 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.

Frost at flowering can cause significant damage to cereal crops. QTL for low temperature tolerance in reproductive tissues (LTR tolerance) were previously described on barley 2HL and 5HL chromosome arms. With the aim of identifying potential LTR tolerance mechanisms, barley Amagi Nijo × WI2585 and Haruna Nijo × Galleon populations were examined for flowering time and spike morphology traits associated with the LTR tolerance loci. In spring-type progeny of both crosses, winter alleles at the Vrn-H1 vernalization response locus on 5H were linked in coupling with LTR tolerance and were unexpectedly associated with earlier flowering. In contrast, tolerance on 2HL was coupled with late flowering alleles at a locus we named Flt-2L. Both chromosome regions influenced chasmogamy/cleistogamy (open/closed florets), although tolerance was associated with cleistogamy at the 2HL locus and chasmogamy at the 5HL locus. LTR tolerance controlled by both loci was accompanied by shorter spikes, which were due to fewer florets per spike on 5HL, but shorter rachis internodes on 2HL. The Eps-2S locus also segregated in both crosses and influenced spike length and flowering time but not LTR tolerance. Thus, none of the traits was consistently correlated with LTR tolerance, suggesting that the tolerance may be due to some other visible trait or an intrinsic (biochemical) property. Winter alleles at the Vrn-H1 locus and short rachis internodes may be of potential use in barley breeding, as markers for selection of LTR tolerance at 5HL and 2HL loci, respectively.

Choice of an appropriate promoter is critical to express target genes in intended tissues and developmental stages. However, promoters capable of directing gene expression in specific tissues and stages are not well characterized in monocot species. To identify such a promoter in wheat, this study isolated a partial sequence of the wheat small subunit of RuBisCO (TarbcS) promoter. In silico analysis revealed the presence of elements that are characteristic to rbcS promoters of other, mainly dicot, species. Transient expression of the TarbcS:GUS in immature wheat embryos and tobacco leaves but not in the wheat roots indicate the functionality of the TarbcS promoter fragment in directing the expression of target genes in green plant tissues.

A major fusarium head blight (FHB) resistance gene Fhb1 (syn. Qfhs.ndsu-3BS) was fine mapped on the distal segment of chromosome 3BS of spring wheat (Triticum aestivum L.) as a Mendelian factor. FHB resistant parents, Sumai 3 and Nyubai, were used as sources of this gene. Two mapping populations were developed to facilitate segregation of Qfhs.ndsu-3BS in either a fixed resistant (Sumai 3*5/Thatcher) (S/T) or fixed susceptible (HC374/3*98B69-L47) (HC/98) genetic background (HC374 = Wuhan1/Nyubai) for Type II resistance. Type II resistance (disease spread within the spike) was phenotyped in the greenhouse using single floret injections with a mixture of macro-conidia of three virulent strains of Fusarium graminearum. Due to the limited heterogeneity in the genetic background of the crosses and based on the spread of infection, fixed recombinants in the interval between molecular markers XGWM533 and XGWM493 on 3BS could be assigned to discrete “resistant” and “susceptible” classes. The phenotypic distribution was bimodal with progeny clearly resembling either the resistant or susceptible parent. Marker order for the two maps was identical with the exception of marker STS-3BS 142, which was not polymorphic in the HC/98 population. The major gene Fhb1 was successfully fine mapped on chromosome 3BS in the same location in the two populations within a 1.27-cM interval (S/T) and a 6.05-cM interval (HC/98). Fine mapping of Fhb1 in wheat provides tightly linked markers that can reduce linkage drag associated with marker-assisted selection of Fhb1 and assist in the isolation, sequencing and functional identification of the underlying resistance gene.

Background Sucrose transporters (SUTs) play important roles in regulating the translocation of assimilates from source to sink tissues. Identification and characterization of new SUTs in economically important crops such as wheat provide insights into their role in determining seed yield. To date, however, only one SUT of wheat has been reported and functionally characterized. The present study reports the isolation and characterization of a new SUT, designated as TaSUT2, and its homeologues (TaSUT2A, TaSUT2B and TaSUT2D) in hexaploid wheat (Triticum aestivum L.). Results TaSUT2A and TaSUT2B genes each encode a protein with 506 amino acids, whereas TaSUT2D encodes a protein of 508 amino acids. The molecular mass of these proteins is predicted to be ~ 54 kDA. Topological analysis of the amino acid sequences of the three homeologues revealed that they contain 12 transmembrane spanning helices, which are described as distinct characteristic features of glycoside-pentoside-hexuronide cation symporter family that includes all known plant SUTs, and a histidine residue that appears to be localized at and associated conformationally with the sucrose binding site. Yeast SUSY7/ura3 strain cells transformed with TaSUT2A, TaSUT2B and TaSUT2D were able to uptake sucrose and grow on a medium containing sucrose as a sole source of carbon; however, our subcellular localization study with plant cells revealed that TaSUT2 is localized to the tonoplast. The expression of TaSUT2 was detected in the source, including flag leaf blade, flag leaf sheath, peduncle, glumes, palea and lemma, and sink (seed) tissues. The relative contributions of the three genomes of wheat to the total expression of TaSUT2 appear to differ with tissues and developmental stages. At the cellular level, TaSUT2 is expressed mainly in the vein of developing seeds and subepidermal mesophyll cells of the leaf blade. Conclusion This study demonstrated that TaSUT2 is a new wheat SUT protein. Given that TaSUT2 is localized to the tonoplast and sucrose is temporarily stored in the vacuoles of both source and sink tissues, our data imply that TaSUT2 is involved in the intracellular partitioning of sucrose, particularly between the vacuole and cytoplasm.

Background In allopolypoid crops, homoeologous genes in different genomes exhibit a very high sequence similarity, especially in the coding regions of genes. This makes it difficult to design genome-specific primers to amplify individual genes from different genomes. Development of genome-specific primers for agronomically important genes in allopolypoid crops is very important and useful not only for the study of sequence diversity and association mapping of genes in natural populations, but also for the development of gene-based functional markers for marker-assisted breeding. Here we report on a useful approach for the development of genome-specific primers in allohexaploid wheat. Findings In the present study, three genome-specific primer sets for the waxy (Wx) genes and four genome-specific primer sets for the starch synthase II (SSII) genes were developed mainly from single nucleotide polymorphisms (SNPs) and/or insertions or deletions (Indels) in introns and intron-exon junctions. The size of a single PCR product ranged from 750 bp to 1657 bp. The total length of amplified PCR products by these genome-specific primer sets accounted for 72.6%-87.0% of the Wx genes and 59.5%-61.6% of the SSII genes. Five genome-specific primer sets for the Wx genes (one for Wx-7A, three for Wx-4A and one for Wx-7D) could distinguish the wild type wheat and partial waxy wheat lines. These genome-specific primer sets for the Wx and SSII genes produced amplifications in hexaploid wheat, cultivated durum wheat, and Aegilops tauschii accessions, but failed to generate amplification in the majority of wild diploid and tetraploid accessions. Conclusions For the first time, we report on the development of genome-specific primers from three homoeologous Wx and SSII genes covering the majority of the genes in allohexaploid wheat. These genome-specific primers are being used for the study of sequence diversity and association mapping of the three homoeologous Wx and SSII genes in natural populations of both hexaploid wheat and cultivated tetraploid wheat. The strategies used in this paper can be used to develop genome-specific primers for homoeologous genes in any allopolypoid species. They may be also suitable for (i) the development of gene-specific primers for duplicated paralogous genes in any diploid species, and (ii) the development of allele-specific primers at the same gene locus.