Durum wheat and septoria tritici blotch: genes and prospects for breeding

Abstract

Durum wheat (Triticum durum) is among the most important food crops of the Mediterranean Basin, encompassing regions in Southern Europe and North Africa, as well as the Northern Great Plains of the U.S. and parts of Southern Asia. Particularly in North Africa it is the prime economical and dietary crop for small holder farmers in marginal areas and has greatly contributed to the existing genetic diversity of bread wheat. Despite its standing as a staple crop, primarily in North Africa and Southern Europe, the overall vulnerability of durum wheat germplasm to fungal diseases is well known and frequently reported. Among those is septoria tritici blotch (STB), the major foliar disease of wheat in Europe that is caused by Zymoseptoria tritici. However, the scientific community has limited attention for this crop and mostly focused on bread wheat. Therefore, very little is known about the genetic basis of the resistance to STB in tetraploid wheat. In bread wheat, 21 Stb major genes and manifold quantitative trait loci (QTLs) have been identified and intensively deployed in breeding programs, whereas Stb genes are neither recognized nor mapped in the largely under- investigated tetraploid wheats. One of the reasons is the reported dichotomy of the pathogenicity of Z. tritici isolates towards bread and durum wheat. This has been an additional hurdle hampering breeding for resistance in durum wheat because well characterized Z. tritici isolates that are pathogenic on bread wheat cannot be used in durum wheat phenotyping assays (and vice versa). On top of that, specificity in either of these wheat systems has been questioned for a long time and therefore hindered effective breeding strategies. Thus, deciphering the genetics of the wheat-Z. tritici interaction, specifically for tetraploid wheats, greatly contributes to enhancing our understanding of this important pathosystem and thereby to more effective breeding strategies in this important cereal staple crop. Chapter 1 is the introduction of the thesis and provides an historical overview of the emergence of the current forms of durum wheat and their vulnerability to Z. tritici. This fungal pathogen has evolved in close association with wheat, thereby deploying an arsenal of effector genes, and has a very strategic life style which generated abundant diversity. Therefore, Z. tritici has evolved as a major pathogen of wheat. The chapter concludes with an overview of the thesis. Chapter 2 describes the map-based cloning and functional analysis of the first Z. tritici effector gene AvrStb6 that interacts in a gene-for-gene manner with the first cloned and widespread major resistance gene Stb6. An even more important discovery represents the new exclusive paternal parenthood (EPP) epidemiological model. This shows that host resistance indeed precludes the developement of biomass of avirulent strains, but cannot stop sexual reproduction. Hence, the avirulence genes of avirulent parents are maintained in natutal populations, which extends the longevitiy of resistant wheat cultivars. The EPP model confirms many observations in agricultural and natural environments and is therefore most likely applicable to several other pathosystems. Chapter 3 unveils the genetic basis of resistance to Z. tritici in the cultivated emmer wheat (Triticum dicoccum) accesion PI41025. Mapping populations generated from crosses between PI41025 and the contemporary cv. Ben were used to unravel the reistance in the former accession. This resulted in a first QTL confering wide-spectrum resistance to Z. tritici in durum wheat. The QTL was mapped on chromosome 3AL, was derived from PI41025 and designated as Stb22q. In addition, another novel locus was mapped on chromosome 5A of cv. Ben, which provides an isolate-specific resistance, hence with a limited efficacy. Chapter 4 takes the reader to more recent times by focusing on the resistance to Z. tritici in a suite of Tunisian durum wheat landraces. The oustanding landrace accession „Agili39‟ was crossed to the contemporary high-yielding cv. Khiar that is very susceptible to Z. tritici. The analyses of the resistance in the generated recombinant inbred population revealed that the broad spectrum resistance of „Agili39‟ results from the natural pyramiding of several minor effect QTLs. Nonetheless, QTLs on chromosome 2BL and 2BS exerted a strong effect on „Agili39‟ resistance. The latter was exclusivly associated with adult plant resistance, whereas the former co-locolizes with Stb9 that has a very low efficacy in bread wheat, but is crucial in „Agili 39‟. Chapter 5 brings the reader to the present time by investigating STB resistance in contemporary durum wheat cultivars, which are preferred by farmers due to their high- yielding potential. Recombinant inbred populations were subsequently generated from crosses between cvs. Simeto and Levante and cvs. Kofa and Svevo and were tested with four Z. tritici isolates under greenhouse conditions and with one strain in the field. The analyses of the generated data showed that the STB resistance in these cultivars results from the synergic effect of several minor effect QTLs on several new genomic locations that collectively provide an acceptable level of STB resistance. Chapter 6 is the final piece of this thesis and is a general discussion that puts the results in a boarder prespective and places all generated data in an overarching context. The newly elucidated epidemiological model applies for bread and durum wheat. Along with the newly discovered Stb genes and QTLs, this will lead to more effective (durum) wheat breeding programs that aim for resistance to Z. tritici.