Meta-analysis of Resistance to Fusarium Head Blight in Tetraploid Wheat: Implications for Durum Wheat Breeding
Abstract Improvement of resistance to Fusarium head blight (FHB) is a continuous challenge for durum wheat (Triticum durum) breeding, where most germplasm are susceptible and low genetic variation is available for this trait. Research has focused on broadening the genetic basis by introducing alleles for FHB resistance from landraces and related species such as bread wheat (Triticum aestivum), cultivated emmer (Triticum dicoccum), wild emmer (Triticum dicoccoides) and Persian wheat (Triticum carthlicum) into durum wheat. We summarize and compare here QTL mapping studies carried out to date in tetraploid wheat. Thirteen QTL with small to moderate effects were repeatedly detected on 11 chromosomes with alleles improving FHB resistance deriving from relatives and from durum wheat itself. Comparison showed large overlaps of QTL positions with those identified in hexaploid wheat suggesting a common genetic basis for FHB resistance. FHB resistance breeding by allele introgression into durum wheat is feasible and QTL pyramiding in novel cultivars is a promising strategy for resistance breeding.
Keywords Durum wheat • Fusarium head blight • QTL • Resistance • Tetraploid wheat • Triticum durum
Durum wheat (Triticum durum) is the principal cultivated tetraploid wheat species. Its annual production accounts for ~5 % of the total wheat grown worldwide and it is used mainly for preparation of pasta and semolina (Taylor and Koo 2012). Durum wheat, as other small grain cereals, suffers from susceptibility to Fusarium head blight (FHB), a devastating disease that affects wheat growing regions throughout the world. FHB is caused by a broad range of fungi from the Fusarium genera (Xu and Nicholson 2009). FHB epidemics are a serious threat for wheat production as the disease leads not only to yield losses but also infests crops with potent mycotoxins hazardous for food safety (Pestka 2010). This is particularly alarming in durum wheat since it is predominantly intended for direct human consumption.
An increasing demand for pasta products has led to an expansion of durum wheat production zones from traditional warm and dry cropping areas to more humid regions with climatic conditions conducive to the disease. Solutions to prevent FHB damages are limited and the development of resistant cultivars is considered a sustainable and highly desired approach to reduce FHB damages (Bai and Shaner 2004).
FHB Resistance in Durum Wheat
Current durum cultivars are generally susceptible to FHB (Clarke et al. 2010; authors' unpublished results). Sources of resistance remain scarce despite efforts undertaken to discover FHB resistant lines: large collections of thousands of durum wheat accessions have been screened without identifying resistant lines (Elias et al. 2005). Surveys on material from CIMMYT and ICARDA identified only five lines from a Tunisian source with moderate resistance to FHB spread (Huhn et al. 2012) and four Syrian landraces with stable resistance (Talas et al. 2011). The lack of resistance found in durum wheat may be attributed to historically low exposure to FHB and to the limited breeding efforts put into this relatively modern crop, which led to a narrow genetic base compared to other wheat species (Ban et al. 2005; Oliver et al. 2008). It is also speculated that durum carries susceptibility factors and/ or suppressor genes that compromise FHB resistance (Stack et al. 2002; Garvin et al. 2009; Ghavami et al. 2011).
Studies have thus been directed at evaluating relatives of durum wheat in order to broaden the genetic basis for breeding and efforts targeted at transferring FHB resistance into durum wheat.
In hexaploid wheat (Triticum aestivum) more than 100 QTL for FHB resistance have been mapped and some have been successfully integrated in breeding programs through marker assisted selection (Buerstmayr et al. 2009). Yet, attempts to transfer resistance into tetraploid wheat have met limited success (Oliver et al. 2007; authors' unpublished results). One hypothetical explanation for the often disappointingly low effect of hexaploid wheat QTL alleles when transferred into durum wheat is that the D-genome, absent in tetraploid wheat, contributes resistanceinducing factors (Fakhfakh et al. 2011).
Consequently resistance has been sought in tetraploid close relatives of durum wheat, where transfer of resistance is not confounded by differences in ploidy levels. Moderate to good FHB resistant tetraploid accessions have been successfully identified (Oliver et al. 2008; Buerstmayr et al. 2003).