Genetic analyses of native Fusarium head blight resistance in two spring wheat populations identifies QTL near the B1, Ppd-D1, Rht-1, Vrn-1, Fhb1, Fhb2, and Fhb5 loci.

KEY MESSAGE: QTL analyses of two bi-parental mapping populations with AC Barrie as a parent revealed numerous FHB-resistance QTL unique to each population and uncovered novel variation near Fhb1. Fusarium head blight (FHB) is a destructive disease of wheat worldwide, leading to severe yield and quality losses. The genetic basis of native FHB resistance was examined in two populations: a recombinant inbred line population from the cross Cutler/AC Barrie and a doubled haploid (DH) population from the cross AC Barrie/Reeder. Numerous QTL were detected among the two mapping populations with many being cross-specific. Photoperiod insensitivity at Ppd-D1 and dwarfing at Rht-B1 and Rht-D1 was associated with increased FHB susceptibility. Anthesis date QTL at or near the Vrn-A1 and Vrn-B1 loci co-located with major FHB-resistance QTL in the AC Barrie/Reeder population. The loci were epistatic for both traits, such that DH lines with both late alleles were considerably later to anthesis and had reduced FHB symptoms (i.e., responsible for the epistatic interaction). Interestingly, AC Barrie contributed FHB resistance near the Fhb1 locus in the Cutler population and susceptibility in the Reeder population. Analyses of the Fhb1 candidate genes PFT and TaHRC confirmed that AC Barrie, Cutler, and Reeder do not carry the Sumai-3 Fhb1 gene. Resistance QTL were also detected at the expected locations of Fhb2 and Fhb5. The native FHB-resistance QTL detected near Fhb1, Fhb2, and Fhb5 do not appear to be as effective as Fhb1, Fhb2, and Fhb5 from Sumai-3. The presence of awns segregated at the B1 awn inhibitor locus in both populations, but was only associated with FHB resistance in the Cutler/AC Barrie population suggesting linkage caused the association rather than pleiotropy.

Mapping the A Genome for QTL Conditioning Resistance to Fusarium Head Blight in a Wheat Population with Triticum timopheevii Background.

Development and use of resistant wheat cultivars is the most practical and economical approach for the control of Fusarium head blight (FHB). In the present study, a population of recombinant inbred lines derived from the cross between 'AC Brio' (a Canadian bread wheat cultivar moderately susceptible to FHB) and 'TC 67' (an FHB-resistant cultivar derived from Triticum timopheevii) was used to map quantitative trait loci (QTL) for FHB resistance using microsatellite molecular markers. Multiple interval mapping detected several QTL for FHB resistance on the chromosomes 5AL and 6A. The QTL detected in the marker interval of cfd6.1-barc48 on chromosome 5AL explained 10.9, 5.2, and 7.8% of phenotypic variation for disease incidence (type I resistance), disease severity (a combination of type I and type II resistance), and Fusarium-damaged kernels (FDK) (type IV resistance) under field conditions, respectively. The second QTL mapped to 5AL, in the marker interval of cfd39-cfa2185, explained 19.4 and 20.6% of phenotypic variation for FDK under field conditions and disease severity in the greenhouse (type II resistance), respectively. The QTL located on chromosome 6A conferred resistance to disease incidence and severity under field conditions and to disease severity in the greenhouse, explaining 6.8 to 11.8% of phenotypic variation for these traits. Several QTL for agronomic traits were also mapped in this study, including one and two QTL to the chromosomes 2A and 5AL, respectively, all for plant height, and two QTL to chromosome 6A for plant height and flowering date, respectively. The 5AL QTL for FHB resistance mapped in the marker interval of cfd39-cfa2185 in the present study is a novel QTL that originated from T. timopheevii and is reported here for the first time. Further validation of this QTL is required for wheat breeding programs to enhance resistance levels to FHB.

Fusarium Head Blight Resistance QTL in the Spring Wheat Cross Kenyon/86ISMN 2137.

Fusarium head blight (FHB), caused by Fusarium graminearum, is a very important disease of wheat globally. Damage caused by F. graminearum includes reduced grain yield, reduced grain functional quality, and results in the presence of the trichothecene mycotoxin deoxynivalenol in Fusarium-damaged kernels. The development of FHB resistant wheat cultivars is an important component of integrated management. The objective of this study was to identify QTL for FHB resistance in a recombinant inbred line (RIL) population of the spring wheat cross Kenyon/86ISMN 2137. Kenyon is a Canadian spring wheat, while 86ISMN 2137 is an unrelated spring wheat. The RIL population was evaluated for FHB resistance in six FHB nurseries. Nine additive effect QTL for FHB resistance were identified, six from Kenyon and three from 86ISMN 2137. Rht8 and Ppd-D1a co-located with two FHB resistance QTL on chromosome arm 2DS. A major QTL for FHB resistance from Kenyon (QFhb.crc-7D) was identified on chromosome 7D. The QTL QFhb.crc-2D.4 from Kenyon mapped to the same region as a FHB resistance QTL from Wuhan-1 on chromosome arm 2DL. This result was unexpected since Kenyon does not share common ancestry with Wuhan-1. Other FHB resistance QTL on chromosomes 4A, 4D, and 5B also mapped to known locations of FHB resistance. Four digenic epistatic interactions were detected for FHB resistance, which involved eight QTL. None of these QTL were significant based upon additive effect QTL analysis. This study provides insight into the genetic basis of native FHB resistance in Canadian spring wheat.

Diversity of Fusarium head blight populations and trichothecene toxin types reveals regional differences in pathogen composition and temporal dynamics.

Analyses of genetic diversity, trichothecene genotype composition, and population structure were conducted using 4086 Fusarium graminearum isolates collected from wheat in eight Canadian provinces over a three year period between 2005 and 2007. The results revealed substantial regional differences in Fusarium head blight pathogen composition and temporal population dynamics. The 3ADON trichothecene type consistently predominated in Maritime provinces (91%) over the sampled years, and increased significantly (P<0.05) between 2005 and 2007 in western Canada, accounting for 66% of the isolates in Manitoba by the end of the sampling period. In contrast, 3ADON frequency was lower (22%, P<0.001) in the eastern Canadian provinces of Ontario and Québec and did not change significantly between 2005 and 2007, resulting in two distinct longitudinal clines in 3ADON frequency across Canada. Overall, genetic structure was correlated with toxin type, as the endemic population (NA1) was dominated by 15ADON isolates (86%), whereas a second population (NA2) consisted largely of 3ADON isolates (88%). However, the percentage of isolates with trichothecene genotypes that were not predictive of their genetic population assignment (recombinant genotypes) increased from 10% in 2005 to 17% in 2007, indicating that trichothecene type became an increasingly unreliable marker of population identity over time. In addition, there were substantial regional differences in the composition of recombinant genotypes. In western and maritime provinces, NA2 isolates with 15ADON genotypes were significantly more common than NA1 isolates with 3ADON genotypes (P<0.001), and the reverse was true in the eastern provinces of Québec and Ontario. Temporal trends in recombinant genotype composition also varied regionally, as the percentage of 15ADON isolates with NA2 genetic backgrounds increased approximately three fold in western and Maritime provinces, while the opposite trends were observed in Québec and Ontario. The results indicate that F. graminearum population dynamics in Canada have been influenced by a complex adaptive landscape comprising different regional selective pressures, and do not reflect a simple model of dispersal and integration following the introduction of a novel pathogen population. In addition, we identified F. graminearum strains that produce the recently discovered A-trichothecene mycotoxin (NX-2) for the first time in Canada, representing a significant expansion of the known range of NX-2 producing strains in North America.

Molecular Phylogenetic Analysis, Trichothecene Chemotype Patterns, and Variation in Aggressiveness of Fusarium Isolates Causing Head Blight in Wheat.

Certain Fusarium species cause Fusarium head blight (FHB) in wheat and other small grains. Differences in characteristics of the pathogen species/isolates used in breeding programs may affect reaction of host genotypes, leading to erroneous results. To clarify differences among Fusarium isolates from different geographical zones, the phylogenetic, chemotypic, and pathogenic abilities of 58 isolates collected from three wheat-producing countries (Canada, Mexico, and Iran) were investigated. Phylogenetic relationships among the isolates were characterized using the Tri101 gene sequence. All Canadian and Iranian isolates clustered in one group and were identified as F. graminearum lineage 7 (=F. graminearum sensu stricto) within the F. graminearum (Fg) clade. The isolates from Mexico were identified as either F. graminearum lineage 3 (=Fusarium boothii) within the Fg clade or Fusarium crookwellense. A polymerase chain reaction assay based on the Tri12 gene identified three trichothecene chemotypes of 15-ADON, 3- ADON, and NIV, with 15-ADON being the most common. All F. boothii isolates from Mexico were of the 15-ADON chemotype, while all F. crookwellense isolates were determined to be NIV producers. While we did not find the NIV chemotype among the Canadian isolates, 25.6% of the Iranian isolates were determined to be NIV producers. High level of variation in aggressiveness was also observed among and within the species tested: F. graminearum sensu stricto isolates were the most aggressive, followed by those of F. boothii, and lastly by F. crookwellense. The differences observed among the isolates may explain why wheat lines/cultivars demonstrate different reactions to FHB in different geographical zones.

QTL-specific microarray gene expression analysis of wheat resistance to Fusarium head blight in Sumai-3 and two susceptible NILs.

Fusarium head blight, predominantly caused by Fusarium graminearum (Schwabe) in North America, is a destructive disease that poses a serious threat to wheat (Triticum aestivum L.) production around the world. cDNA microarrays consisting of wheat ESTs derived from a wheat - F. graminearum interaction suppressive subtractive hybridization library were used to investigate QTL-specific differential gene expression between the resistant Chinese cultivar Sumai-3 and two susceptible near isogenic lines (NILs) following inoculation with F. graminearum. Stringent conditions were employed to reduce the false discovery rate. A total of 25 wheat unigenes were found to express differentially in response to F. graminearum infection. Genes encoding pathogenesis-related (PR) proteins such as beta-1,3-glucanase (PR-2), wheatwins (PR-4), and thaumatin-like proteins (PR-5) showed a significant upregulation in genotypes having the Sumai-3 3BS region. For these three genes, the gene activity was significantly less in the genotype (NIL-3) lacking the Sumai-3 3BS segment. Significant upregulation of phenylalanine ammonia-lyase was detected only in the resistant Sumai-3, indicating the importance of both the 2AL and 3BS regions in the activation of effective defense responses to infection by F. graminearum. Differences in gene expression between the resistant Sumai-3 and the susceptible NILs were found to be mainly quantitative in nature.

An adaptive evolutionary shift in Fusarium head blight pathogen populations is driving the rapid spread of more toxigenic Fusarium graminearum in North America.

Analysis of Fusarium head blight (FHB) pathogen diversity revealed that 3ADON producing Fusarium graminearum are prevalent in North America and identified significant population structure associated with trichothecene chemotype differences (F(ST)>0.285; P<0.001). In addition, we identified a trichothecene chemotype cline in Canada and documented a recent and significant shift in FHB pathogen composition by demonstrating that the 3ADON chemotype frequency in western Canada increased more than 14-fold between 1998 and 2004. On average, isolates from 3ADON populations produced significantly (P<0.05) more trichothecene and had significantly (P<0.005) higher fecundity and growth rates than isolates from the 15ADON population. These results indicate that selection is driving the rapid spread of an introduced pathogen population that is more toxigenic and potentially more vigorous. The discovery of this previously unrecognized pathogen diversity has significant implications for food safety and cereal production in North America.

Microarray analysis of Fusarium graminearum-induced wheat genes: identification of organ-specific and differentially expressed genes.

A wheat cDNA microarray consisting of 5739 expressed sequence tags (ESTs) was used to investigate the transcriptome patterns of the glume, lemma, palea, anther, ovary and rachis dissected from infected wheat spikes after inoculation with the fungus Fusarium graminearum, the causal agent of fusarium head blight (FHB) disease. Stringent conditions were employed to reduce the false discovery rate. The significance analysis of microarrays (SAM) was used to identify transcripts that showed a differential response between fungal-challenged vs. control plants. To verify the microarray data, Northern blot analysis was carried out on randomly selected up-regulated clones. We observed 185 (3.2%) up-regulated and 16 (0.28%) down-regulated ESTs in the six organs constituting the wheat spike. Many up-regulated ESTs (46.67%) showed no homology with sequences of known functions, whereas others showed homology with genes involved in defence and stress responses, the oxidative burst of H(2)O(2), regulatory functions, protein synthesis and the phenylpropanoid pathway. The monitoring of genes in specific organs avoided the averaging of expression values over multiple organs that occurs when using data from the whole spike. Our data allowed us to uncover new up-regulated genes expressed in specific organs. The study revealed that each organ had a defined and distinctive transcriptome pattern in response to F. graminearum infection.

Genetic characterization of QTL associated with resistance to Fusarium head blight in a doubled-haploid spring wheat population.

Fusarium head blight (FHB) is one of the most important fungal wheat diseases worldwide. Understanding the genetics of FHB resistance is the key to facilitating the introgression of different FHB resistance genes into adapted wheat. The objectives of the present study were to detect and map quantitative trait loci (QTL) associated with FHB resistance genes and characterize the genetic components of the QTL in a doubled-haploid (DH) spring wheat population using both single-locus and two-locus analysis. A mapping population, consisting of 174 DH lines from the cross between DH181 (resistant) and AC Foremost (susceptible), was evaluated for type I resistance to initial infection during a 2-year period in spray-inoculated field trials, for Type II resistance to fungal spread within the spike in 3 greenhouse experiments using single-floret inoculation, and for resistance to kernel infection in a 2001 field trial. One-locus QTL analysis revealed 7 QTL for type I resistance on chromosome arms 2DS, 3AS, 3BS, 3BC (centromeric), 4DL, 5AS, and 6BS, 4 QTL for type II resistance on chromosomes 2DS, 3BS, 6BS, and 7BL, and 6 QTL for resistance to kernel infection on chromosomes 1DL, 2DS, 3BS, 3BC, 4DL, and 6BS. Two-locus QTL analysis detected 8 QTL with main effects and 4 additive by additive epistatic interactions for FHB resistance and identified novel FHB resistance genes for the first time on chromosomes 1DL, 4AL, and 4DL. Neither significant QTL by environment interactions nor epistatic QTL by environment interactions were found for either type I or type II resistance. The additive effects of QTL explained most of the phenotypic variance for FHB resistance. Marker-assisted selection for the favored alleles at multiple genomic regions appears to be a promising tool to accelerate the introgression and pyramiding of different FHB resistance genes into adapted wheat genetic backgrounds.

Survival of Gibberella zeae in Fusarium-Damaged Wheat Kernels.

The survival of Gibberella zeae in Fusarium-damaged kernels was investigated under field conditions at Glenlea, Morden, Portage la Prairie, and Winnipeg, Manitoba, Canada. Fusarium-damaged kernels were either left on the soil surface or buried at 5 or 10 cm and monitored for 24 months. G. zeae was isolated after 24 months from Fusarium-damaged kernels under all conditions, with isolation frequency ranging from 85 to 99% of kernels. Perithecia developed on Fusarium-damaged kernels from all locations and treatments, but ascospores developed only in perithecia on kernels located at the soil surface. A similar experiment was conducted under controlled conditions to test survival of the fungus in kernels left at the surface or buried at 5 cm in heat-treated and nontreated soil at -10, 2, and 20°C. The fungus survived in 76 to 100% of kernels. When kernels were incubated at 20°C, G. zeae was recovered from 83 and 76% of kernels in heat-treated and nontreated soil, respectively. Perithecia developed on kernels incubated at 20 and 2°C, but ascospores developed only in perithecia on Fusarium-damaged kernels at 20°C on the soil surface. As survival of G. zeae in Fusarium-damaged kernels did not decrease after 24 months, rotations of at least 2 years are necessary to avoid infection of new crops by G. zeae originating from Fusarium-damaged kernels.