Transfer of a major qtl for resistance to fusarium head blight from Thinopyrum elongatum onto durum wheat 7al chromosome arm and its pyramiding with other useful genes from Th. ponticum

Fusarium Head Blight (FHB), also called scab, a major wheat disease worldwide, has in recent years become a threat also in uncommon cultivation areas of bread and durum wheat. The lack of efficient and durable sources of resistance within adapted durum wheat germplasm is particularly alarming. In fact, durum wheat is used almost exclusively for human consumption, and Fusarium attacks, besides causing severe yield losses, pose a high risk of kernel contamination with health-dangerous mycotoxins, notably deoxynivalenol (DON). To cope with this scenario, looking outside of the primary genepool and exploring exotic variability harboured within related Triticeae species, appears as a worth strategy. We recently mapped an exceptionally effective QTL for FHB resistance (provisional designation Fhb-7EL) onto the telomeric portion of the 7EL chromosome arm of diploid Thinopyrum elongatum, and pyramided it with other useful genes from the 7el1L arm of decaploid Th. ponticum (Lr19, Yp, yield related traits) onto the 7DL arm of bread wheat- Thinopyrum recombinant lines (Ceoloni et al. TAG, in press). Two such FHB resistant recombinants, having distal 70% of their 7DL replaced by chromatin of the two Thinopyrum species in slightly different relative proportions, were crossed with two previously developed durum wheat- Th. ponticum recombinants, having 23% and 28% of their 7AL arm replaced by 7el1L chromatin, respectively. In the shared 7el1L region between the bread wheat (6x) and durum wheat (4x) parental recombinant types, including in all cases the Lr19 gene, homologous pairing evidently occurred with high frequency (over 70%), as proved by GISH-based analysis of meiotic pairing of 5x F1\u2019s. As a result, new 7EL-7el1L recombinant types could be isolated in the BC1 generation to durum wheat. Selection for desired recombinants, i.e. those involving chromosome 7A and with a total Thinopyrum spp. (7EL+7el1L) chromatin not exceeding the 28% of the arm, was carried out by a panel of user-friendly PCR-based markers. To accelerate reduction of chromosome number to the euploid 2n=28, in vitro culture of BC2 embryos was carried out. Selected heterozygous recombinant plants, most of them having reached a stable chromosomal condition, were selfed and in the BC2F2 progeny homozygous genotypes were isolated. The latter were challenged by Fusarium graminearum spike inoculation, to verify the efficacy of the Fhb-7EL QTL once inserted into a durum wheat background. Compared to highly susceptible control plants, inoculation outcomes confirmed what previously observed at the bread wheat level, with remarkable reduction of disease severity, averaging over 90%, invariably associated with presence of the Fhb-7EL QTL. Thus, what is considered to be by far the most effective resistance to FHB available, is now readily exploitable in breeding to provide protection against this threatening disease, hence substantially enhancing food security and safety of the bread and durum wheat crop, in Italy and worldwide

A major qtl for resistance to fusarium head blight and Crown rot of wheat on Thinopyrum elongatum chromosome 7e: cytogenetic mapping and assembling into bread wheat with valuable genes from Th. Ponticum

Among wheat relatives, the Thinopyrum genus represents one of the richest sources of valuable genes/QTL for wheat improvement. One notable and still unexploited trait is the exceptionally effective resistance to Fusarium Head Blight (FHB) originating from a diploid member of the genus, Thinopyrum elongatum. Up to date, the resistance was only associated to the long arm of Th. elongatum chromosome 7E (7EL), while detailed genetic mapping of the responsible gene/QTL was still lacking. We targeted the transfer of the temporarily designated Fhb- 7EL locus into bread wheat, by pyramiding it with other valuable genes/QTL (Lr19 for leaf rust resistance, yield-related traits) and included in a Th. ponticum 7el1L segment, stably inserted into the wheat 7DL arm of line T4. Mapping of the Fhb-7EL QTL was here based on a bioassay with Fusarium graminearum, the main causal agent of FHB, of different 7EL-7el1L bread wheat recombinant lines. Nine such recombinant types were successfully obtained without resorting to any genetic pairing promotion, but relying on the 7EL-7el1L close homoeology. Pairing between the two critical arms was in fact observed by Genomic In Situ Hybridization (GISH) at meiotic metaphase I of F1 plants between the 7E(7D) substitution line and the translocation line T4 (70% distal 7el1L on 7DL), which resulted in 14% 7EL-7el1L recombination frequency. The Fhb-7EL locus was mapped to the telomeric portion of 7EL, associated with marker loci XBE405003 Xsdauk66 and Xcfa2240. FHB resistant recombinants, with useful combinations of more proximally located 7el1L genes/QTL, could be selected. The transferred Fhb-7EL locus was shown to reduce disease severity at the spike level and fungal biomass in the grains of infected recombinants by over 95%. The same Fhb-7EL QTL was, for the first time, proved to be effective also against F. culmorum and F. pseudograminearum, predominant agents of Fusarium Crown Rot (FCR). Yield performance in preliminary field tests of the pre-breeding lines possessing a suitable 7EL-7el1L gene/QTL assembly showed to be very promising. Given the expected inheritance as a unit of the composite Thinopyrum segment in cross progeny with wheat, either of the several co-dominant PCR-based markers identified in the course of the work will enable easy tracking of the novel gene/QTL assembly in transfer programs into adapted cultivars