Saturation and mapping of a major Fusarium head blight resistance QTL on chromosome 3BS of Sumai 3 wheat

Identification and Expressional Analysis of siRNAs Responsive to Fusarium graminearum Infection in Wheat

The outbreak of Fusarium head blight (FHB) poses a serious threat to wheat production as it leads to both significant yield losses and accumulation of several mycotoxins including deoxynivalenol (DON) in the grains, which are harmful to human and livestock. To date, hundreds of FHB-resistance-related quantitative trait loci (QTLs) have been reported, but only a few of them have been cloned and used for breeding. Small interfering RNAs (siRNA) have been reported in plants to mediate host defense against pathogens, but they have rarely been reported in wheat-FHB interaction. In order to identify the key siRNAs that can potentially be used in the improvement of resistance to FHB, siRNAs from the spikes of an FHB-resistant variety Sumai 3 and an FHB-susceptible variety of Chinese Spring (CS) were sequenced after F. graminearum infection and mock inoculation, respectively. The expression patterns of the siRNAs of interest were analyzed. A total of 4019 siRNAs of high-confidence were identified, with 131 being CS-specific, 309 Sumai 3-specific and 3071 being common in both varieties. More than 87% of these siRNAs were 24 nt in length. An overall down-regulation trend was found for siRNAs in the spikes of both varieties after being infected with F. graminearum. The expression patterns for Triticum aestivum Dicer-like 3 (TaDCL3) that synthesizes 24 nt siRNAs were validated by qRT-PCR, which were positively correlated with those of the siRNAs. A total of 85% of the differentially expressed genes putatively targeted by the siRNAs were significantly up-regulated after infection, showing a negative correlation with the overall down-regulated expression of siRNAs. Interestingly, the majority of the up-regulated genes are annotated as disease resistance. These results suggested that the inhibition of siRNA by F. graminearum up-regulated the disease resistance genes, which were putatively suppressed by siRNAs through RNA-directed DNA methylation (RdDM). Consequently, the resistant capability to F. graminearum infection was enhanced. This study provides novel clues for investigating the function of siRNA in wheat-F. graminearum interaction.

WFhb1-1 plays an important role in resistance against Fusarium head blight in wheat

Fusarium head blight (FHB) is a severe disease of wheat (Triticum aestivum L.). Qfhb1 is the most important quantitative trait locus (QTL) for FHB resistance. We previously identified wheat gene WFhb1-1 (aka WFhb1-c1) as a candidate for FHB resistance gene. Here we report that WFhb1-1 has been cloned. The gene (GenBank # KU304333.1) consists of a single exon, encoding a putative membrane protein of 127 amino acids. WFhb1-1 protein produced in Pichia pastoris inhibits growth of both F. graminearum and P. pastoris in culture. Western Blotting with anti- WFhb1-1 antibody revealed a significant decrease (p < 0.01) in WFhb1-1 accumulation, 12 hours post Fusarium inoculation in non-Qfhb1-carrier wheat but not in Qfhb1-carrier wheat. Overexpressing WFhb1-1 in non-Qfhb1-carrier wheat led to a significant decrease (p < 0.01) in Fusarium-damaged rachis rate, Fusarium-diseased kernel rate and DON content in harvested kernels, while silencing WFhb1-1 in Qfhb1-carrier wheat resulted in a significant increase (p < 0.01) in FHB severity. Therefore, WFhb1-1 is an important FHB resistance gene with a potential antifungal function and probably a key functional component of Qfhb1 in wheat. A model regarding how WFhb1-1 functions in FHB resistance/susceptibility is hypothesized and discussed.

Meta-Analysis of the QTLome of Fusarium Head Blight Resistance in Bread Wheat: Refining the Current Puzzle

Background: Fusarium Head Blight (FHB) is a worldwide devastating disease of bread wheat (Triticum aestivum L.). Genetic resistance is the most effective way to control FHB and many QTL related to this trait have been mapped on the wheat genetic map. This information, however, must be refined to be more efficiently used in breeding programs and for the advance of the basic research. The objective of the present study was to in-depth analyze the QTLome of FHB resistance in bread wheat, further integrating genetic, genomic, and transcriptomic data, aiming to find candidate genes. Methods: An exhaustive bibliographic review on 76 scientific papers was carried out collecting information about QTL related to FHB resistance mapped on bread wheat. A dense genetic consensus map with 572,862 loci was generated for QTL projection. Meta-analysis could be performed on 323 QTL. Candidate gene mining was carried out within the most refined loci, containing genes that were cross-validated with publicly available transcriptional expression data of wheat under Fusarium infection. Most highlighted genes were investigated for protein evidence. Results: A total of 556 QTL were found in the literature, distributed on all sub-genomes and chromosomes of wheat. Meta-analysis generated 65 meta-QTL, and this refinement allows one to find markers more tightly linked to these regions. Candidate gene mining within the most refined meta-QTL, meta-QTL 1/chr. 3B, harvested 324 genes and transcriptional data cross-validated 10 of these genes, as responsive to FHB. One is of these genes encodes a Glycosiltransferase and the other encodes for a Cytochrome P450, and these such proteins have already been verified as being responsible for FHB resistance, but the remaining eight genes still have to be further studied, as promising loci for breeding. Conclusions: The QTLome of FHB resistance in wheat was successfully assembled and a refinement in terms of number and length of loci was obtained. The integration of the QTLome with genomic and transcriptomic data has allowed for the discovery of promising candidate genes for use in breeding programs.

Host to a Stranger: Arabidopsis and Fusarium Ear Blight

Fusarium ear blight (FEB) is a devastating fungal disease of cereal crops. Outbreaks are sporadic and current control strategies are severely limited. This review highlights the use of Arabidopsis to study plant-FEB interactions. Use of this pathosystem has identified natural variation in Fusarium susceptibility in Arabidopsis, and native plant genes and signalling processes modulating the interaction. Recent breakthroughs include the identification of plant- and insect-derived small molecules which increase disease resistance, and the use of a host-induced gene silencing (HIGS) construct to silence an important Fusarium gene to prevent infection. Arabidopsis has also been used to study other fungi that cause cereal diseases. These findings offer the potential for translational research in cereals which could yield much-needed novel control strategies.

Mutations in the Arabidopsis homoserine kinase gene DMR1 confer enhanced resistance to Fusarium culmorum and F. graminearum

BACKGROUND

Mutation of Arabidopsis DMR1, encoding homoserine kinase, leads to elevation in homoserine and foliar resistance to the biotrophic pathogens Hyaloperonospora arabidopsidis and Oidium neolycopersici through activation of an unidentified defence mechanism. This study investigates the effect of mutation of dmr1 on resistance to the ascomycete pathogens Fusarium graminearum and F. culmorum, which cause Fusarium Ear Blight (FEB) disease on small grain cereals.

RESULTS

We initially found that the dmr1-2 mutant allele confers increased resistance to F. culmorum and F. graminearum silique infection, and decreased colonisation of rosette leaves. Meanwhile the dmr1-1 allele supports less rosette leaf colonisation but has wild type silique resistance. Three additional dmr1 alleles were subsequently examined for altered F. culmorum susceptibility and all showed increased silique resistance, while leaf colonisation was reduced in two (dmr1-3 and dmr1-4). Amino acid analysis of dmr1 siliques revealed homoserine accumulation, which is undetectable in wild type plants. Exogenous application of L-homoserine reduced bud infection in both dmr1 and wild type plants, whilst D-homoserine application did not. Delayed leaf senescence was also observed in dmr1 plants compared to wild type and correlated with reduced Fusarium leaf colonisation.

CONCLUSIONS

These findings suggest that common Arabidopsis DMR1 mediated susceptibility mechanisms occur during infection by both obligate biotrophic oomycete and hemi-biotrophic fungal pathogens, not only in vegetative but also in reproductive plant tissues. This has the potential to aid the development of cereal crops with enhanced resistance to FEB.

Durum wheat (Triticum Durum Desf.) lines show different abilities to form masked mycotoxins under greenhouse conditions

Deoxynivalenol (DON) is the most prevalent trichothecene in Europe and its occurrence is associated with infections of Fusarium graminearum and F. culmorum, causal agents of Fusarium head blight (FHB) on wheat. Resistance to FHB is a complex character and high variability occurs in the relationship between DON content and FHB incidence. DON conjugation to glucose (DON-3-glucoside, D3G) is the primary plant mechanism for resistance towards DON accumulation. Although this mechanism has been already described in bread wheat and barley, no data are reported so far about durum wheat, a key cereal in the pasta production chain. To address this issue, the ability of durum wheat to detoxify and convert deoxynivalenol into D3G was studied under greenhouse controlled conditions. Four durum wheat varieties (Svevo, Claudio, Kofa and Neodur) were assessed for DON-D3G conversion; Sumai 3, a bread wheat variety carrying a major QTL for FHB resistance (QFhs.ndsu-3B), was used as a positive control. Data reported hereby clearly demonstrate the ability of durum wheat to convert deoxynivalenol into its conjugated form, D3G.

Identification of functional genic components of major fusarium head blight resistance quantitative trait loci in wheat cultivar Sumai 3

Fusarium head blight (FHB) is a devastating disease worldwide, affecting wheat and other small grains. To identify key wheat genes involved in FHB pathogenesis, 406 FHB-related wheat expressed sequence tags functionally identified in Sumai 3 were investigated for their association with FHB-resistance quantitative trait loci (QTL) Fhb1 and Fhb_6BL in 2010 and 2011. A total of 47 candidate genes were identified by bulk analysis, near-isogenic screening and expression QTL mapping, and were finally mapped to their carrier chromosomes with Chinese Spring nulli-tetra deficiency lines. One gene, designated WFhb1_c1 (wheat Fhb1 candidate gene 1), was both functionally associated with and physically located within Fhb1 and was found to be weakly similar (E = 5e+0) to an Arabidopsis gene encoding pectin methyl esterase inhibitor. Two other genes, designated WFI_6BL1 and WFI_6BL2 (wheat-Fusarium interaction genes 6BL1 and 6BL2), were functionally associated with Fhb_6BL but physically mapped on chromosomes 7D and 5A, respectively. WFI_6BL1 was annotated as a 13- lipoxygenase gene and WFI_6BL2 might encode a PR-4-like protein. Our data suggested that i) Fhb1 seems to contribute to FHB resistance by reducing susceptibility in the first 60 h, ii) Fhb_6BL makes its contribution via the jasmonate-mediated pathways, and iii) wheat seems to activate its defense mechanism in the biotrophic phase of FHB pathogenesis.

Identification, characterization and mapping of differentially expressed genes in a winter wheat cultivar (Centenaire) resistant to Fusarium graminearum infection

Fusarium head blight (FHB), predominantly caused by Fusarium graminearum, is a destructive disease that poses a serious threat to wheat (Triticum aestivum L.) production around the world. A suppression subtractive hybridization (SSH) cDNA library was constructed from F. graminearum infected spikes of a resistant Belgian winter wheat variety Centenaire, exhibiting Type II resistance to FHB. Forty-three differentially expressed transcripts were identified and classified in different categories according to their predicted function, including proteins involved in defense response, signaling, transport of molecules, metabolism and proteins with unknown function. Time-course gene expression analysis between the FHB resistant genotype Centenaire and the susceptible genotype Robigus was carried out on twelve selected genes in order to validate the SSH screening. Real-time quantitative polymerase chain reaction showed that the selected transcripts were differentially expressed between the resistant and the susceptible genotype at three-time points (24, 48 and 72 h) after inoculation with the pathogen, and mostly, the transcripts accumulation rates were higher in the FHB-resistant as compared to the susceptible one. Thirty identified differentially expressed loci were mapped on the corresponding wheat chromosomes either by in silico analysis or by PCR-based mapping strategy, and fifteen of these loci were located within or nearby chromosomal regions known to have quantitative trait loci for FHB resistance in winter wheat cultivars. This work emphasizes the differential gene expression between the FHB-resistant winter wheat Centenaire and the susceptible Robigus and highlights the putative genes and mechanism involved in the disease resistance reaction.

Expression of a radish defensin in transgenic wheat confers increased resistance to Fusarium graminearum and Rhizoctonia cerealis

Fusarium head blight (scab), primarily caused by Fusarium graminearum, is a devastating disease of wheat (Triticum aestivum L.) worldwide. Wheat sharp eyespot, mainly caused by Rhizoctonia cerealis, is one of the major diseases of wheat in China. The defensin RsAFP2, a small cyteine-rich antifungal protein from radish (Raphanus sativus), was shown to inhibit growth in vitro of agronomically important fungal pathogens, such as F. graminearum and R. cerealis. The RsAFP2 gene was transformed into Chinese wheat variety Yangmai 12 via biolistic bombardment to assess the effectiveness of the defensin in protecting wheat from the fungal pathogens in multiple locations and years. The genomic PCR and Southern blot analyses indicated that RsAFP2 was integrated into the genomes of the transgenic wheat lines and heritable. RT-PCR and Western blot proved that the RsAFP2 was expressed in these transgenic wheat lines. Disease tests showed that four RsAFP2 transgenic lines (RA1-RA4) displayed enhanced resistance to F. graminearum compared to the untransformed Yangmai 12 and the null-segregated plants. Assays on Q-RT-PCR and disease severity showed that the express level of RsAFP2 was associated with the enhanced resistance degree. Two of these transgenic lines (RA1 and RA2) also exhibited enhanced resistance to R. cerealis. These results indicated that the expression of RsAFP2 conferred increased resistance to F. graminearum and R. cerealis in transgenic wheat.