High-density mapping of the major FHB resistance gene Fhb7 derived from Thinopyrum ponticum and its pyramiding with Fhb1 by marker-assisted selection

Fusarium head blight in wheat: contemporary status and molecular approaches

Fusarium head blight (FHB) disease that occurs in wheat is caused by Fusarium graminearum and is a major risk to wheat yield. Although several research efforts focusing on FHB have been conducted in the past several decades, conditions have become more critical due to the increase in its virulent forms. In such a scenario, conferring complete resistance in plants seems to be difficult for handling this issue. The phenotyping for FHB and finding a solution for it at the genetic level comprises a long-term process as FHB infection is largely affected by environmental conditions. Modern molecular strategies have played a crucial role in revealing the host-pathogen interaction in FHB. The integration of molecular biology-based methods such as genome-wide association studies and marker-based genomic selection has provided potential cultivars for breeding programs. In this review, we aim at outlining the contemporary status of the studies conducted on FHB in wheat. The influence of FHB in wheat on animals and human health is also discussed. In addition, a summary of the advancement in the molecular technologies for identifying and developing the FHB-resistant wheat genetic resources is provided. It also suggests the future measures that are required to reduce the world's vulnerability to FHB which was one of the main goals of the US Wheat and Barley Scab Initiative.

Cytogenetic Analysis and Molecular Marker Development for a New Wheat-Thinopyrum ponticum 1Js (1D) Disomic Substitution Line With Resistance to Stripe Rust and Powdery Mildew

Thinopyrum ponticum (2n = 10x = 70), a member of the tertiary gene pool of wheat (Triticum aestivum L.), harbors many biotic and abiotic stress resistance genes. CH10A5, a novel disomic substitution line from a cross of T. aestivum cv. 7182 and Th. ponticum, was characterized by cytogenetic identification, in situ hybridization, molecular marker analysis, and morphological investigation of agronomic traits and disease resistance. Cytological observations showed that CH10A5 contained 42 chromosomes and formed 21 bivalents at meiotic metaphase I. Genome in situ hybridization (GISH) analysis indicated that two of its chromosomes came from the J^s genome of Th. ponticum, and wheat 15K array mapping and fluorescence in situ hybridization (FISH) revealed that chromosome 1D was absent from CH10A5. Polymorphic analysis of molecular markers indicated that the pair of alien chromosomes belonged to homoeologous group one, designated as 1J^s. Thus, CH10A5 was a wheat-Th. ponticum 1J^s (1D) disomic substitution line. Field disease resistance trials demonstrated that the introduced Th. ponticum chromosome 1J^s was probably responsible for resistance to both stripe rust and powdery mildew at the adult stage. Based on specific-locus amplified fragment sequencing (SLAF-seq), 507 STS molecular markers were developed to distinguish chromosome 1J^s genetic material from that of wheat. Of these, 49 STS markers could be used to specifically identify the genetic material of Th. ponticum. CH10A5 will increase the resistance gene diversity of wheat breeding materials, and the markers developed here will permit further tracing of heterosomal chromosome fragments in the future.

Molecular cytogenetic characterization of wheat-Elymus repens chromosomal translocation lines with resistance to Fusarium head blight and stripe rust

BACKGROUND

Fusarium head blight (FHB) caused by the fungus Fusarium graminearum Schwabe and stripe rust caused by Puccinia striiformis f. sp. tritici are devastating diseases that affect wheat production worldwide. The use of disease-resistant genes and cultivars is the most effective means of reducing fungicide applications to combat these diseases. Elymus repens (2n = 6x = 42, StStStStHH) is a potentially useful germplasm of FHB and stripe rust resistance for wheat improvement.

RESULTS

Here, we report the development and characterization of two wheat-E. repens lines derived from the progeny of common wheat-E. repens hybrids. Cytological studies indicated that the mean chromosome configuration of K15-1192-2 and K15-1194-2 at meiosis were 2n = 42 = 0.86 I + 17.46 II (ring) + 3.11 II (rod) and 2n = 42 = 2.45 I + 14.17 II (ring) + 5.50 II (rod) + 0.07 III, respectively. Genomic and fluorescence in situ hybridization karyotyping and simple sequence repeats markers revealed that K15-1192-2 was a wheat-E. repens 3D/?St double terminal chromosomal translocation line. Line K15-1194-2 was identified as harboring a pair of 7DS/?StL Robertsonian translocations and one 3D/?St double terminal translocational chromosome. Further analyses using specific expressed sequence tag-SSR markers confirmed that the wheat-E. repens translocations involved the 3St chromatin in both lines. Furthermore, compared with the wheat parent Chuannong16, K15-1192-2 and K15-1194-2 expressed high levels of resistance to FHB and stripe rust pathogens prevalent in China.

CONCLUSIONS

Thus, this study has determined that the chromosome 3St of E. repens harbors gene(s) highly resistant to FHB and stripe rust, and chromatin of 3St introgressed into wheat chromosomes completely presented the resistance, indicating the feasibility of using these translocation lines as novel material for breeding resistant wheat cultivars and alien gene mining.

Mining and genomic characterization of resistance to tan spot, Stagonospora nodorum blotch (SNB), and Fusarium head blight in Watkins core collection of wheat landraces

BACKGROUND

In the late 1920s, A. E. Watkins collected about 7000 landrace cultivars (LCs) of bread wheat (Triticum aestivum L.) from 32 different countries around the world. Among which 826 LCs remain viable and could be a valuable source of superior/favorable alleles to enhance disease resistance in wheat. In the present study, a core set of 121 LCs, which captures the majority of the genetic diversity of Watkins collection, was evaluated for identifying novel sources of resistance against tan spot, Stagonospora nodorum blotch (SNB), and Fusarium Head Blight (FHB).

RESULTS

A diverse response was observed in 121 LCs for all three diseases. The majority of LCs were moderately susceptible to susceptible to tan spot Ptr race 1 (84%) and FHB (96%) whereas a large number of LCs were resistant or moderately resistant against tan spot Ptr race 5 (95%) and SNB (54%). Thirteen LCs were identified in this study could be a valuable source for multiple resistance to tan spot Ptr races 1 and 5, and SNB, and another five LCs could be a potential source for FHB resistance. GWAS analysis was carried out using disease phenotyping score and 8807 SNPs data of 118 LCs, which identified 30 significant marker-trait associations (MTAs) with -log10 (p-value) > 3.0. Ten, five, and five genomic regions were found to be associated with resistance to tan spot Ptr race 1, race 5, and SNB, respectively in this study. In addition to Tsn1, several novel genomic regions Q.Ts1.sdsu-4BS and Q.Ts1.sdsu-5BS (tan spot Ptr race 1) and Q.Ts5.sdsu-1BL, Q.Ts5.sdsu-2DL, Q.Ts5.sdsu-3AL, and Q.Ts5.sdsu-6BL (tan spot Ptr race 5) were also identified. Our results indicate that these putative genomic regions contain several genes that play an important role in plant defense mechanisms.

CONCLUSION

Our results suggest the existence of valuable resistant alleles against leaf spot diseases in Watkins LCs. The single-nucleotide polymorphism (SNP) markers linked to the quantitative trait loci (QTLs) for tan spot and SNB resistance along with LCs harboring multiple disease resistance could be useful for future wheat breeding.

Plasticity in Triticeae centromere DNA sequences: a wheat × tall wheatgrass (decaploid) model

Centromeres mediate chromosome attachment to microtubules and maintain the integrity of chromosomes for proper segregation of the sister chromatids during cell division. Advances in the assembly of Triticeae genome sequences combined with the capacity to recover hybrid species derived from very distantly related species provides potential experimental systems for linking retrotransposon amplification and repositioning of centromeres via non-mendelian inheritance in partial amphiploid breeds. The decaploid tall wheatgrass (Thinopyrum ponticum) is one of the most successfully used perennial species in wheat breeding for generating translocation lines with valuable agronomic traits. We found that wheat centromere retrotransposons CRW and Quinta widely occur within the tall wheatgrass genome. In addition, one of the genome donors to Th. ponticum, Pseudoroegneria stipifolia (StSt), has been shown to have Abigail and a satellite repeat, CentSt. We also found two other centromeric retrotransposons, Abia and CL135 in Th. ponticum by ChIP-seq. Examination of partial amphiploid lines that were generated in the 1970s demonstrated extensive modification in centromere sequences using CentSt, Abigail and Abia as probes. We also detected that St-genome chromosomes were more enriched with Abigail and CentSt, whereas E-genome chromosomes were enriched with CRW and Quinta in tall wheatgrass and its closer relatives. It can be concluded that bursts of transposition of retrotransposons and repositioning of centromeres via non-mendelian segregation are common in partial amphiploids derived from interspecific hybrids. Practically speaking, our study reveals that the existence of homologous centromere functional sequences in both a donor and its receptor can substantially contribute to the successful transfer of alien genes into crop species. OPEN RESEARCH BADGES: This article has earned an Open Data Badge for making publicly available the digitally-shareable data necessary to reproduce the reported results. The data is available at https://www.ncbi.nlm.nih.gov/sra/SRR9089557; https://www.ncbi.nlm.nih.gov/sra/SRR9089558; https://www.ncbi.nlm.nih.gov/sra/SRR9089559; https://www.ncbi.nlm.nih.gov/sra/SRR9089560; https://www.ncbi.nlm.nih.gov/sra/SRR9089561; https://www.ncbi.nlm.nih.gov/sra/SRR9089562; https://www.ncbi.nlm.nih.gov/sra/SRR9089563; https://www.ncbi.nlm.nih.gov/sra/SRR9089564; https://www.ncbi.nlm.nih.gov/nuccore/MK999394; https://www.ncbi.nlm.nih.gov/nuccore/MK999395; https://www.ncbi.nlm.nih.gov/nuccore/MK999396.

The Associations between Leaf Morphology, Phenylalanine Ammonia Lyase Activity, Reactive Oxygen Species, and Fusarium Resistance in Selected Species of Wheat with Different Ploidy Levels

In wheat, resistance to Fusarium is conditioned by anatomical, morphological, and physiological traits. The aim of this study was to evaluate selected elements of constitutive barriers in common wheat, spelt, Polish wheat, emmer, and einkorn. The activity of the phenylalanine ammonia-lyase (PAL) enzyme and rate of reactive oxygen species (ROS) production were evaluated in the tissues of common wheat and spelt inoculated with Fusarium culmorum. Most of the relict wheat species were more abundant in morphological barriers than common wheat. F. culmorum penetrated constitutive barriers, which increased PAL activity and intensified ROS production 24 h after inoculation in wheat tissues. The lowest increase in PAL activity after inoculation was observed in cv. Sumai3, which resistance is based on limiting the spread of F. culmorum within the spike. Spelt line Tas 581 glumes were characterized by the highest concentration of ROS 24 h after inoculation. The ROS content remained high for five days. The results of this study indicate that high trichome density plays a key role in resistance to pathogens. In the resistant spelt line with effective constitutive barriers, PAL activity and ROS content were higher than those observed in susceptible wheats after inoculation with F. culmorum.

Isobaric tags for relative and absolute quantification-based proteomic analysis of defense responses triggered by the fungal pathogen Fusarium graminearum in wheat

Fusarium head blight (FHB) is a devastating disease worldwide that is predominantly caused by the fungal pathogen Fusarium graminearum. The aim of this work was to study differentially abundant protein species of near-isogenic lines A061-3 and A061-4 with the final goal of elucidating the molecular mechanisms of their differential resistance to F. graminearum. The objectives were accomplished using isobaric tags for relative and absolute quantification (iTRAQ) with mass spectrometry (MS). Lines A061-3 and A061-4 were resistant and susceptible to F. graminearum, respectively. At four post-inoculation points, 11,070 protein species were identified, of which 762 were differentially abundant. Gene Ontology enrichment analysis showed that most differentially abundant protein species participated in 18 biological processes after inoculation. Further analysis demonstrated that crucial metabolic pathways like plant-pathogen interaction had increased abundance. Real-time quantitative PCR (qRT-PCR) analysis revealed increased gene products of eight selected genes in plant-pathogen interaction. This investigation provides a basic bioinformatics-based characterization of differentially abundant protein species during early stages against F. graminearum. SIGNIFICANCE: FHB leads to severe yield loss and reduction in grain quality in wheat and other small grain cereals. Although extensive studies have focused on wheat resistance against F. graminearum, the molecular mechanism of FHB resistance in wheat remains to be further elucidated. In the present study, Kyoto Encyclopedia of Genes and Genomes analysis indicated that ten pathways were putatively associated with FHB resistance. Principal component analysis (PCA) and orthogonal partial least squares discriminant analysis (OPLS-DA) showed that a valuable set of differentially abundant protein species including pathogenesis-related protein species were identified for further discovery of candidate genes for FHB resistance. This investigation provides new insights into the molecular mechanisms associated with FHB resistance and as well as a foundation for future studies.

Genetic factors affecting Fusarium head blight resistance improvement from introgression of exotic Sumai 3 alleles (including Fhb1, Fhb2, and Fhb5) in hard red spring wheat

BACKGROUND

Fusarium head blight resistance genes, Fhb1 (for Type-II resistance), Fhb2 (Type-II), and Fhb5 (Type-I plus some Type-II), which originate from Sumai 3, are among the most important that confer resistance in hexaploid wheat. Near-isogenic lines (NILs), in the CDC Alsask (susceptible; n = 32) and CDC Go (moderately susceptible; n = 38) backgrounds, carrying these genes in all possible combinations were developed using flanking microsatellite markers and evaluated for their response to FHB and deoxynivalenol (DON) accumulation in eight environments. NILs were haplotyped with wheat 90 K iSelect assay to elucidate the genomic composition and confirm alleles' presence. Other than evaluating the effects of three major genes in common genetic background, the study elucidated the epistatic gene interactions as they influence FHB measurements; identified loci other than Fhb1, Fhb2, and Fhb5, in both recurrent and donor parents and examined annotated proteins in gene intervals.

RESULTS

Genotyping using 81,857 single nucleotide polymorphism (SNP) markers revealed polymorphism on all chromosomes and that the NILs carried < 3% of alleles from the resistant donor. Significant improvement in field resistance (Type-I + Type-II) resulted only among the CDC Alsask NILs, not the CDC Go NILs. The phenotypic response of NILs carrying combinations of Sumai 3 derived genes suggested non-additive responses and Fhb5 was as good as Fhb1 in conferring field resistance in both populations. In addition to Fhb1, Fhb2, and Fhb5, four to five resistance improving alleles in both populations were identified and three of five in CDC Go were contributed by the susceptible parent. The introgressed chromosome regions carried genes encoding disease resistance proteins, protein kinases, nucleotide-binding and leucine rich repeats' domains. Complex epistatic gene-gene interactions among marker loci (including Fhb1, Fhb2, Fhb5) explained > 20% of the phenotypic variation in FHB measurements.

CONCLUSIONS

Immediate Sumai 3 derivatives carry a number of resistance improving minor effect alleles, other than Fhb1, Fhb2, Fhb5. Results verified that marker-assisted selection is possible for the introgression of exotic FHB resistance genes, however, the genetic background of the recipient line and epistatic interactions can have a strong influence on expression and penetrance of any given gene.

Equipping Durum Wheat-Thinopyrum ponticum Recombinant Lines With a Thinopyrum elongatum Major QTL for Resistance to Fusarium Diseases Through a Cytogenetic Strategy

Prompted by recent changes in climate trends, cropping areas, and management practices, Fusarium head blight (FHB), a threatening disease of cereals worldwide, is also spreading in unusual environments, where bread wheat (BW) and durum wheat (DW) are largely cultivated. The scarcity of efficient resistance sources within adapted germplasm is particularly alarming for DW, mainly utilized for human consumption, which is therefore at high risk of kernel contamination by health-dangerous mycotoxins (e.g., deoxynivalenol = DON). To cope with this scenario, we looked outside the wheat primary gene pool and recently transferred an exceptionally effective FHB resistance QTL (Fhb-7EL) from Thinopyrum elongatum 7EL chromosome arm onto a Thinopyrum ponticum 7el₁L arm segment, containing additional valuable genes (including Lr19 for leaf rust resistance and Yp for yellow pigment content), distally inserted onto 7DL of BW lines. Two such lines were crossed with two previously developed DW-Th. ponticum recombinants, having 7el₁L distal portions on 7AL arms. Genomic in situ hybridization (GISH) analysis showed homologous pairing, which is enabled by 7el₁L segments common to the BW and DW recombinant chromosomes, to occur with 42-78% frequency, depending on the shared 7el₁L amount. Aided by 7EL/7el₁L-linked markers, 7EL+7el₁L tetraploid recombinant types were isolated in BC₁ progenies to DW of all cross combinations. Homozygous 7EL+7el₁L recombinant plants and null segregates selected in BC₂F₂ progenies were challenged by Fusarium graminearum spike inoculation to verify the Fhb-7EL efficacy in DW. Infection outcomes confirmed previous observations in BW, with >90% reduction of disease severity associated with Fhb-7EL presence vs. its absence. The same differential effect was detected on seed set and weight of inoculated spikes, with genotypes lacking Fhb-7EL having ∼80% reduction compared with unaffected values of Fhb-7EL carriers. In parallel, DON content in flour extracts of resistant recombinants averaged 0.67 ppm, a value >800 times lower than that of susceptible controls. Furthermore, as observed in BW, the same Fhb-7EL also provided the novel DW recombinants with resistance to Fusarium crown rot (∼60% symptom reduction) as from seedling infection with Fusarium culmorum. Through alien segment stacking, we succeeded in equipping DW with a very effective barrier against different Fusarium diseases and other positive attributes for crop security and safety.

Detection and validation of a novel major QTL for resistance to Fusarium head blight from Triticum aestivum in the terminal region of chromosome 7DL

A novel QTL for FHB resistance was mapped on wheat 7DL, being effective in multiple genetic backgrounds and environments, and comparable to Fhb1 in effect magnitude. Fusarium head blight (FHB) is one of the major fungal diseases affecting wheat production in many countries. The wheat line AQ24788-83 (AQ) possesses FHB resistance. The American wheat cultivar Luke is FHB susceptible. A Luke × AQ population consisting of 1652 advanced recombinant inbred lines (RILs) was developed, from which 272 RILs were randomly sampled and used to construct a linkage map. Another 154 RILs were selected for homogeneity in plant height (PH) and flowering date (FD). This selection strategy was adopted to reduce possible confounding effects on FHB assessment due to variation in PH and FD. The 272 and 154 RILs were genotyped applying simple sequence repeat (SSR), diversity arrays technology (DArT) and single-nucleotide polymorphism (SNP) markers. The two sets of RILs were evaluated for FHB resistance applying point inoculation in greenhouses; the 154 RILs were also evaluated applying spray inoculation in multiple field environments. The linkage map consisted of 2088 SSR, DArT, and SNP markers. A FHB resistance quantitative trait locus (QTL), designated as QFhb.cau-7DL, was detected on chromosome arm 7DL; this QTL was closely linked to the SSR marker gwm428 ( http://www.wheat.pw.usda.gov/ggpages/SSR/ ). QFhb.cau-7DL was significantly effective (α = 0.01) in every test trial, and its effectiveness was validated using three additional wheat crosses. Sumai 3 (donor wheat of the FHB resistance gene Fhb1) was used in one of these crosses. QFhb.cau-7DL was comparable to Fhb1 in effect magnitude, providing a great potential for improving FHB resistance.

Application of Brachypodium genotypes to the analysis of type II resistance to Fusarium head blight (FHB)

The resistance to Fusarium head blight (FHB) in wheat is mainly via the restrain of fungal expansion through spike rachis (type II resistance). In order to unravel the resistance mechanisms, Brachypodium distachyon 21 (Bd21), a monocotyledonous model plant, was previously proved to interact with F. graminearum, while the disease development in spike still needs to be explored in detail. Herein, it is found that the fungal spores mainly germinate on pistil of Bd21, then the hyphae rapidly extend to the bottom of floret and enter spike rachis, similar with the infection progress in wheat. However, structural difference of spike rachis was found between Brachypodium and wheat. It was found that the spread of the fungus through the rachis node of inoculated spikelets is an important index for the evaluation of type II FHB resistance in Brachypodium under optimal conditions at 28 °C and 50%-70% humidity. To verify the feasibility of this strategy, the transcription factor TaTGA2 was overexpressed in Bd21, and transgenic plants were found to show improved resistance to F. graminearum in both spikes and detached leaves, which was further supported by the increased disease severity when silencing TaTGA2 in the wheat cultivar "Sumai 3" or in tilling "Kronos" mutants. Except for Bd21, another 49 Brachypodium germplasms were further screened for FHB resistance, and three moderately susceptible germplasms, namely, PI 317418, W6-39284, and PI 254868, feasible for transformation, were determined to be better hosts than Bd21 when evaluating heterologous genes that positively regulate FHB resistance. The present study also observed variations in the levels of FHB resistance between coleoptiles and spikes or transgenic plants and natural germplasms.

Genetic Analysis of Fusarium Head Blight Resistance in CIMMYT Bread Wheat Line C615 Using Traditional and Conditional QTL Mapping

Fusarium head blight (FHB) is a destructive wheat disease present throughout the world, and host resistance is an effective and economical strategy used to control FHB. Lack of adequate resistance resource is still a main bottleneck for FHB genetics and wheat breeding research. The synthetic-derived bread wheat line C615, which does not carry the Fhb1 gene, is a promising source of FHB resistance for breeding. A population of 198 recombinant inbred lines (RILs) produced by crossing C615 with the susceptible cultivar Yangmai 13 was evaluated for FHB response using point and spray inoculations. As the disease phenotype is frequently complicated by other agronomic traits, we used both traditional and multivariate conditional QTL mapping approaches to investigate the genetic relationships (at the individual QTL level) between FHB resistance and plant height (PH), spike compactness (SC), and days to flowering (FD). A linkage map was constructed from 3,901 polymorphic SNP markers, which covered 2,549.2 cM. Traditional and conditional QTL mapping analyses found 13 and 22 QTL for FHB, respectively; 10 were identified by both methods. Among these 10, three QTL from C615 were detected in multiple years; these QTL were located on chromosomes 2AL, 2DS, and 2DL. Conditional QTL mapping analysis indicated that, at the QTL level, SC strongly influenced FHB in point inoculation; whereas PH and SC contributed more to FHB than did FD in spray inoculation. The three stable QTL (QFhbs-jaas.2AL, QFhbp-jaas.2DS, and QFhbp-jaas.2DL) for FHB were partly affected by or were independent of the three agronomic traits. The QTL detected in this study improve our understanding of the genetic relationships between FHB response and related traits at the QTL level and provide useful information for marker-assisted selection for the improvement of FHB resistance in breeding.

A Review of the Interactions between Wheat and Wheat Pathogens: Zymoseptoria tritici, Fusarium spp. and Parastagonospora nodorum

Zymoseptoria tritici is a hemibiotrophic pathogen which causes Septoria leaf blotch in wheat. The pathogenesis of the disease consists of a biotrophic phase and a necrotrophic phase. The pathogen infects the host plant by suppressing its immune response in the first stage of infection. Hemibiotrophic pathogens of the genus Fusarium cause Fusarium head blight, and the necrotrophic Parastagonosporanodorum is responsible for Septoria nodorum blotch in wheat. Cell wall-degrading enzymes in plants promote infections by necrotrophic and hemibiotrophic pathogens, and trichothecenes, secondary fungal metabolites, facilitate infections caused by fungi of the genus Fusarium. There are no sources of complete resistance to the above pathogens in wheat. Defense mechanisms in wheat are controlled by many genes encoding resistance traits. In the wheat genome, the characteristic features of loci responsible for resistance to pathogenic infections indicate that at least several dozen genes encode resistance to pathogens. The molecular interactions between wheat and Z. tritici, P. nodorum and Fusarium spp. pathogens have been insufficiently investigated. Most studies focus on the mechanisms by which the hemibiotrophic Z. tritici suppresses immune responses in plants and the role of mycotoxins and effector proteins in infections caused by P. nodorum and Fusarium spp. fungi. Trichothecene glycosylation and effector proteins, which are involved in defense responses in wheat, have been described at the molecular level. Recent advances in molecular biology have produced interesting findings which should be further elucidated in studies of molecular interactions between wheat and fungal pathogens. The Clustered Regularly-Interspaced Short Palindromic Repeats/ CRISPR associated (CRISPR/Cas) system can be used to introduce targeted mutations into the wheat genome and confer resistance to selected fungal diseases. Host-induced gene silencing and spray-induced gene silencing are also useful tools for analyzing wheat-pathogens interactions which can be used to develop new strategies for controlling fungal diseases.

Harnessing Genetic Diversity of Wild Gene Pools to Enhance Wheat Crop Production and Sustainability: Challenges and Opportunities

Wild species are extremely rich resources of useful genes not available in the cultivated gene pool. For species providing staple food to mankind, such as the cultivated Triticum species, including hexaploid bread wheat (Triticum aestivum, 6x) and tetraploid durum wheat (T. durum, 4x), widening the genetic base is a priority and primary target to cope with the many challenges that the crop has to face. These include recent climate changes, as well as actual and projected demographic growth, contrasting with reduction of arable land and water reserves. All of these environmental and societal modifications pose major constraints to the required production increase in the wheat crop. A sustainable approach to address this task implies resorting to non-conventional breeding strategies, such as “chromosome engineering”. This is based on cytogenetic methodologies, which ultimately allow for the incorporation into wheat chromosomes of targeted, and ideally small, chromosomal segments from the genome of wild relatives, containing the gene(s) of interest. Chromosome engineering has been successfully applied to introduce into wheat genes/QTL for resistance to biotic and abiotic stresses, quality attributes, and even yield-related traits. In recent years, a substantial upsurge in effective alien gene exploitation for wheat improvement has come from modern technologies, including use of molecular markers, molecular cytogenetic techniques, and sequencing, which have greatly expanded our knowledge and ability to finely manipulate wheat and alien genomes. Examples will be provided of various types of stable introgressions, including pyramiding of different alien genes/QTL, into the background of bread and durum wheat genotypes, representing valuable materials for both species to respond to the needed novelty in current and future breeding programs. Challenging contexts, such as that inherent to the 4x nature of durum wheat when compared to 6x bread wheat, or created by presence of alien genes affecting segregation of wheat-alien recombinant chromosomes, will also be illustrated.

Cytogenetic mapping of a major locus for resistance to Fusarium head blight and crown rot of wheat on Thinopyrum elongatum 7EL and its pyramiding with valuable genes from a Th. ponticum homoeologous arm onto bread wheat 7DL

A major locus for resistance to different Fusarium diseases was mapped to the most distal end of Th. elongatum 7EL and pyramided with Th. ponticum beneficial genes onto wheat 7DL. Perennial Triticeae species of the Thinopyrum genus are among the richest sources of valuable genes/QTL for wheat improvement. One notable and yet unexploited attribute is the exceptionally effective resistance to a major wheat disease worldwide, Fusarium head blight, associated with the long arm of Thinopyrum elongatum chromosome 7E (7EL). We targeted the transfer of the temporarily designated Fhb-7EL locus into bread wheat, pyramiding it with a Th. ponticum 7el₁L segment stably inserted into the 7DL arm of wheat line T4. Desirable genes/QTL mapped along the T4 7el₁L segment determine resistance to wheat rusts (Lr19, Sr25) and enhancement of yield-related traits. Mapping of the Fhb-7EL QTL, prerequisite for successful pyramiding, was established here on the basis of a bioassay with Fusarium graminearum of different 7EL-7el₁L bread wheat recombinant lines. These were obtained without resorting to any genetic pairing promotion, but relying on the close 7EL-7el₁L homoeology, resulting in 20% pairing frequency between the two arms. Fhb-7EL resided in the telomeric portion and resistant recombinants could be isolated with useful combinations of more proximally located 7el₁L genes/QTL. The transferred Fhb-7EL locus was shown to reduce disease severity and fungal biomass in grains of infected recombinants by over 95%. The same Fhb-7EL was, for the first time, proved to be effective also against F. culmorum and F. pseudograminearum, predominant agents of crown rot. Prebreeding lines possessing a suitable 7EL-7el₁L gene/QTL assembly showed very promising yield performance in preliminary field tests.

Marker-Assisted Development and Evaluation of Near-Isogenic Lines for Broad-Spectrum Powdery Mildew Resistance Gene Pm2b Introgressed into Different Genetic Backgrounds of Wheat

At present, most of released wheat cultivars or breeding lines in China are susceptible to powdery mildew (Pm) (caused by Blumeria graminis f. sp. tritici, Bgt), so there is an urgent need to rapidly transfer effective and broad-spectrum Pm resistance genes into elite cultivars/lines. Near-isogenic lines (NILs) with short target gene region are very important in molecular breeding and map-based cloning and can be developed by combining marker-assisted selection and conventional phenotypic identification. However, no Pm gene NILs were reported by using this method in the previous studies. A new broad-spectrum dominant resistance gene Pm2b, derived from the Chinese wheat breeding line KM2939, conferred high resistance to Pm at both the seedling and adult stages. In this study, with the aid of forward and background selection (FS and BS) using molecular markers, the Pm2b gene was introgressed into three elite susceptible commercial cultivars Shimai 15, Shixin 828, and Kenong 199 through the back-crossing procedure. With the appropriate backcrossing generations, selected population sizes and marker number for BS, the homozygous resistant BC3F2:3 NILs of Pm2b gene in the three genetic backgrounds with the highest recipient genome composition of about 99%, confirmed by simple sequence repeat markers and 660K single nucleotide polymorphic array, were developed and evaluated for the powdery mildew resistance and agronomic traits. The different resistance and similar or improved agronomic performance between Pm2b NILs and their corresponding recurrent parents indicated their potential value in the marker-assisted breeding of the Pm2b gene. Moreover, the development of four flanked diagnostic markers (CFD81, BWM25, BWM20, and BWM21) of the Pm2 gene can effectively assist the forward selection and accelerate the transfer and use of this resistance gene.

Molecular Cytogenetic Characterization of two Triticum-Secale-Thinopyrum Trigeneric Hybrids Exhibiting Superior Resistance to Fusarium Head Blight, Leaf Rust, and Stem Rust Race Ug99

Fusarium head blight (FHB), leaf rust, and stem rust are the most destructive fungal diseases in current world wheat production. The diploid wheatgrass, Thinopyrum elongatum (Host) Dewey (2n = 2x = 14, EE) is an excellent source of disease resistance genes. Two new Triticum-Secale-Thinopyrum trigeneric hybrids were derived from a cross between a hexaploid triticale (X Triticosecale Wittmack, 2n = 6x = 42, AABBRR) and a hexaploid Triticum trititrigia (2n = 6x = 42, AABBEE), were produced and analyzed using genomic in situ hybridization and molecular markers. The results indicated that line RE21 contained 14 A-chromosomes, 14 B-chromosomes, three pairs of R-chromosomes (4R, 6R, and 7R), and four pairs of E-chromosomes (1E, 2E, 3E, and 5E) for a total chromosome number of 2n = 42. Line RE62 contained 14 A-chromosomes, 14 B-chromosomes, six pairs of R-chromosomes, and one pair of translocation chromosomes between chromosome 5R and 5E, for a total chromosome number of 2n = 42. At the seedling and adult growth stages under greenhouse conditions, line RE21 showed high levels of resistance to FHB, leaf rust, and stem rust race Ug99, and line RE62 was highly resistant to leaf rust and stem rust race Ug99. These two lines (RE21 and RE62) display superior disease resistance characteristics and have the potential to be utilized as valuable germplasm sources for future wheat improvement.

Reevaluation of Two Quantitative Trait Loci for Type II Resistance to Fusarium Head Blight in Wheat Germplasm PI 672538

Fusarium head blight (FHB), mainly caused by Fusarium graminearum, is a destructive disease in wheat. A population consisting of 229 F₂ and F_2:3 plants derived from the cross PI 672538 × L661 was used to evaluate the reactions to FHB. The FHB resistance data distribution in the F₂ population indicates that some quantitative trait loci (QTLs) were controlling the FHB resistance in PI 672538. We further detected two major QTLs (Qfhs-2B, Qfhs-3B) from analysis of the resistance data and the PCR-amplified results using WinQTLCart 2.5 software. Qfhs-2B, flanked by Xbarc55-2B and Xbarc1155-2B, explained more than 11.6% of the phenotypic variation of the percentage of diseased spikelets (PDS), and Qfhs-3B, flanked by Xwmc54-3B and Xgwm566-3B, explained more than 10% of the PDS phenotypic variation in the F_2:3 population. In addition, Qfhs-3B was different from Fhb1 in terms of the pedigree, inheritance, resistance response, chromosomal location, and marker diagnosis. We also detected QTLs for other disease resistance indices, including the percentage of damaged kernels and 1,000-grain weight, in similar chromosomal regions. Therefore, the FHB resistance of PI 672538 was mainly controlled by two major QTLs, mapped on 2B (FhbL693a) and 3B (FhbL693b). PI 672538 could be a useful germplasm for improving wheat FHB resistance.

Multiple Minor QTLs Are Responsible for Fusarium Head Blight Resistance in Chinese Wheat Landrace Haiyanzhong

Fusarium head blight (FHB), caused by Fusarium graminearum Schwabe, is a devastating disease in wheat (Triticum aestivum L.). Use of host resistance is one of the most effective strategies to minimize the disease damage. Haiyanzhong (HYZ) is a Chinese wheat landrace that shows a high level of resistance to FHB spread within a spike (type II resistance). To map the quantitative trait loci (QTLs) in HYZ and identify markers tightly linked to the QTLs for FHB resistance, a population of 172 recombinant inbred lines (RILs) from a cross between HYZ and Wheaton (FHB susceptible) was genotyped using simple sequence repeats (SSRs) and single-nucleotide polymorphisms (SNPs) derived from genotyping-by-sequencing (GBS), and evaluated for percentage of symptomatic spikelets (PSSs) per spike in three greenhouse experiments. Six QTLs for type II resistance were identified in HYZ, indicating that multiple minor QTLs together can provide a high level of FHB resistance in wheat. The QTL with the largest effect on FHB resistance was mapped on the chromosome arm 5AS, and the other five from HYZ were mapped on the chromosomes 6B, 7D, 3B, 4B and 4D. In addition, two QTLs from Wheaton were mapped on 2B. Critical SNPs linked to the QTLs on chromosomes 5A, 6B, and 2B were converted into KBioscience competitive allele-specific PCR (KASP) assays, which can be used for marker-assisted selection (MAS) to pyramid these QTLs in wheat.

Disease Impact on Wheat Yield Potential and Prospects of Genetic Control

Wheat is grown worldwide in diverse geographical regions, environments, and production systems. Although many diseases and pests are known to reduce grain yield potential and quality, the three rusts and powdery mildew fungi have historically caused major crop losses and continue to remain economically important despite the widespread use of host resistance and fungicides. The evolution and fast spread of virulent and more aggressive race lineages of rust fungi have only worsened the situation. Fusarium head blight, leaf spotting diseases, and, more recently, wheat blast (in South America and Bangladesh) have become diseases of major importance in recent years largely because of intensive production systems, the expansion of conservation agriculture, undesirable crop rotations, or increased dependency on fungicides. High genetic diversity for race-specific and quantitative resistance is known for most diseases; their selection through phenotyping reinforced with molecular strategies offers great promise in achieving more durable resistance and enhancing global wheat productivity.