Increasing human population and climate change demand greater agricultural productivity. Wheat is a strategic crop, essential for Western countries. It is the most important source of protein, providing 20% of the calories consumed daily by humans, and its consumption is increasing, mainly in developing countries. Although the yield of primary crops, including wheat, in 2022 was double that of 2000, it is estimated that the global demand for wheat production will increase by 60% in 2050 to meet the requirements of a growing population (http://www.fao.org). Europe is the first-world producer of bread wheat and within the European Union; Spain ranks in the fifth position considering cereal production, mainly wheat, only behind France, Germany, Romania, and Poland (http://www.fao.org). Thus, the economic relevance of wheat as a strategic crop in agriculture is clear, but several biotic (diseases) and abiotic (higher temperatures, soil erosion, salinity, water availability, etc.) stresses can compromise the future of this crop. A solid strategy, based on the development of new approaches that allow the adaptation of wheat to climate change and are respectful of the principles of sustainable agriculture, is needed to face the challenge of feeding the expected human population (Hamblin, 1995). Plant breeders are playing a major role in worldwide efforts to understand gene functions and interactions, so the introduction of tolerant genes to both biotic and abiotic stresses can increase the quality and productivity of a key crop such as wheat (Mamrutha et al., 2019; Mondal et al., 2021; Pérez-Méndez et al., 2022).

Durum wheat (Triticum turgidum, 2n = 4x = 28) is a globally relevant species for agriculture and also has an enormous impact in Spain since it is one of the most cultivated cereals together with bread wheat and barley. Despite its high productivity, annual fluctuations can be associated with biotic and abiotic stresses. One of the most important diseases affecting durum wheat is Septoria tritici blotch (STB) caused by the fungus Zymoseptoria tritici (Desm.) (Quaedvlieg et al., 2011), previously known as Septoria tritici (teleomorph Mycospaherella graminicola (Fuckel) J. Schröt.), which triggers severe losses in yield that can reach 50% in susceptible cultivars (Fones and Gurr, 2015). Severe epidemics occur both in areas with frequent rains and in others where rainfall does not exceed 250 mm during the crop cycle. The wide range of temperatures in which the fungus can develop, the use of susceptible varieties, and the rapid evolution and adaptation of Z. tritici to diverse agricultural conditions have allowed a wide geographical distribution of the disease (Fones and Gurr, 2015).

The introgression of resistance genes in the framework of breeding programs is considered the main protection against this disease, and several sources of resistance have yielded adequate protection (Rubiales et al., 2001). Wheat resistance to STB can be qualitative (isolate-specific), depending on major genes with a strong effect according to a gene-for-gene interaction, or quantitative (isolate-non-specific) displaying a partial phenotype controlled by various genes with moderate to small effects (Kema et al., 2000; Brading et al., 2002; Brown et al., 2015). Quantitative resistance is quite significant for its effectiveness against all genotypes of the pathogen and its durability (Brown et al., 2015; Porras et al., 2021). When no sufficient genetic variability or resistant varieties are available, the incorporation of new alleles into the wheat germplasm is considered essential. Thus, genetic crosses between donor species and the crop itself can be a powerful tool to transfer chromosome segments carrying desirable genes from the alien species into wheat.

Breeders have been using related species as genetic donors with the target of widening the genetic basis of this crop obtaining, for example, wheat cultivars better adapted to specific agro-climatic conditions, carrying resistance to pests (Lukaszewski, 2000; Shubing and Honggang, 2005). Indeed, wide-crossing in plant breeding programmes is such an important tool that, sometimes the results are the starting point for new crops (O’mara, 1953; Martin and Sanchez-Mongelaguna, 1982) and marked a milestone in a breeding framework.

Diploid wild barley Hordeum chilense Roem. et Schultz (2n = 2x = 14 H^chH^ch) exhibits valuable agronomic and quality traits for wheat improvement, and it has already been crossed with some species of the tribe Triticeae (grasses), particularly with both durum and bread wheat (Martin et al., 1996; Calderón et al., 2012). This wild species also has enormous potential as a donor of other genetic features such as drought and salt tolerance as well as resistance to pests and diseases (Gallardo and Fereres, 1989; Bothmer et al., 1995; Martin et al., 1996). Particularly, H. chilense contains interesting genes for biotic and abiotic stress resistance, such as resistance to STB, conferred mainly by the 4H^ch chromosome (Rubiales et al., 2000). The transfer of H. chilense chromosome 4 in durum wheat has been already developed (Calderón et al., 2012), but, in a plant breeding framework, the size of full introgressed chromosomes from relatives must be reduced to diminish the drag linkage of undesirable genes.

Much of the work aimed to promote interspecific chromosome associations to transfer genetic variability from wild relatives into wheat has relied on the use of the ph1b mutant (Sears and Okamoto, 1958). Homoeologous recombination between chromosomes from related but different genomes can occur in the absence of the Ph1 locus, resulting in the generation of interspecific recombinant chromosomes (Sears and Okamoto, 1958; Othmeni et al., 2019). In the absence of the Ph1 locus, chromosome rearrangements in bread wheat involving wheat and relative H. chilense and H. vulgare chromosomes have already been obtained (Rey et al., 2015a, 2015). In this work, we used the ph1 mutant and the previously developed H. chilense chromosome 4H^ch substitution line (Calderón et al., 2012), both in durum wheat background, to carry out genetic crosses to facilitate durum wheat-wild barley interspecific recombination. A set of durum wheat–H. chilense chromosome 4H^ch translocation lines were obtained and characterised using molecular markers and fluorescence in situ hybridisation. We also analysed these lines for the incidence of STB under controlled conditions in growth chambers to assess resistance to STB to an H. chilense chromosome 4H^ch segment.

The effective introgression of genes from alien species like H. chilense into cultivated species such as durum wheat entails that target genes or the chromosome segment carrying them must be incorporated into wheat chromosomes as recombinant segments or translocations. Examples of an important source of chromosome introgressions are centric-break fusion events found several decades ago in bread wheat–rye translocations (e.g., 1BL-lRS and lAL-lRS), which have importantly contributed to global wheat production (Pena et al., 1990). Genetic introgressions from H. chilense chromosome 4H^ch, which contains resistance to STB (Rubiales et al., 2000), have been previously developed in the durum wheat background (Calderón et al., 2012). In this work, genetic crosses between the durum wheat line carrying a disomic 4H^ch(4B) chromosome substitution and durum wheat carrying the Ph1 deletion were developed to promote interspecific recombination between wheat and H. chilense. Recombination events between bread wheat and H. chilense lines in the absence of the Ph1 locus have been previously promoted for both fundamental and applied purposes, which is to shed light into the mechanism of this Ph1 locus or in a plant breeding framework to specifically introgress H. chilense chromosome 7H^ch to increase the carotenoid content in bread wheat (Rey et al., 2015a). To the best of our knowledge, it is the first time that the ph1 mutant has been used in genetic crosses between H. chilense and wheat in the tetraploid durum background. The use of the ph1 mutants in this work has enabled the possibility of promoting recombination events between 4A wheat and 4H^ch H. chilense homoeologous chromosomes, providing the opportunity to introgress part of 4H^ch chromosome in durum wheat and the development of a series of genetic introgressions for this H. chilense chromosome for durum breeding purposes. The durum wheat–H. chilense introgression lines for chromosome 4H^ch developed in this work might facilitate the possibility of transferring resistance to STB in durum wheat as the 4H^ch chromosome was previously targeted as a potential source to introduce this type of resistance in wheat (Rubiales et al., 2000).

The interaction between Z. tritici and durum wheat has been poorly studied, and no Stb genes have been identified in durum wheat so far (Somai-Jemmali et al., 2017). This lack of identified and characterised Stb resistance genes is a challenge for plant breeders to find durable sources of resistance to STB in durum wheat. Resistance to STB is generally expressed through the reduction of the foliar area covered with pycnidia and necrosis (Porras et al., 2021). In this work, we have obtained several durum wheat–H. chilense translocation lines including chromosome 4H^ch that present a moderate resistance to STB. The reduction of the diseased in the 4H^ch H. chilense introgression lines in durum wheat is attributed to the contribution of chromosome 4H^ch compared with both control lines, the cultivar Langdon, which is the genetic background of the H. chilense introgressions developed, and the cultivar SY Leonardo, which is routinely used as a susceptible control to STB. Both durum cultivars displayed elevated susceptibility with abundant development of the fungus and extensively coalesced lesions (DS 4 and 5, respectively) in contrast to the DS scores 1 and 2 expressed by the 4H^ch disomic substitution line, 4h^ch-DS, or the average DS score of 3 displayed by the translocation lines 4H^chS-T1, 4H^chS-T2, and 4H^chL-T4. Thus, our results demonstrate the contribution of chromosome 4H^ch for introgressing resistance to STB in durum wheat.

The combination of cytogenetic tools with a molecular analysis offers a reliable method of selection of H. chilense introgressions in the durum wheat background during a plant breeding programme. These combined approaches provide the possibility to detect and analyse a high number of plant lines carrying chromosome modifications in a suitable frame of time, and for this, they have been extensively used for the identification and characterisation of plant material for decades (Nalam et al., 2006; Faris et al., 2008; Wang et al., 2009; Calderón et al., 2012). In this work, genomic in situ hybridisation contributed to elucidate the exact chromosomal composition of all translocation lines obtained. All the H. chilense–wheat translocations obtained were disomic except 4H^chS-T3, which was monosomic. Disomic 4H^chS-T1, 4H^chS-T2, and 4H^chL-T4 translocation lines showed moderate resistance to STB, with limited production of pycnidia and small necrosis lesions, although the monosomic 4H^chS-T3 line displayed moderately susceptible reaction with limited pycnidia development (DS 3-4). In contrast, the disomic line 4H^ch-DS showed a resistant behaviour to STB displaying hypersensitive flecking (DS1) and necrotic lesions with no presence of pycnidia (DS 2). These results confirmed an apparent dosage effect on STB resistance as it was previously described in Agropyron–wheat derivates (Rillo et al., 1970) and agree with the presence of different Stb genes along the H. chilense chromosome 4H^ch.

The similar pattern obtained by the cytogenetic analysis and molecular markers in 4H^chS-T1 and 4H^chS-T2 translocation lines suggested that the recombination events occurring in each of these translocation lines were extremely close to each other along the 4H^chS chromosome. The fact of being the same recombination event is discarded as both 4H^chS-T1 and 4H^chS-T2 translocation lines were obtained from different genetic crosses between the original 4H^chdisomic line and the ph1 mutants.

All the durum wheat–H. chilense translocation lines obtained in this work retained the distal region of H. chilense chromosome 4H^chL, which might contain some homology to bread wheat chromosome 4AL, where QTL7 and Stb7 and Stb12 resistance genes are located (Brown et al., 2015). All the durum wheat–H. chilense translocation lines developed in this work (4H^chS-T1, 4H^chS-T2, 4H^chS-T3, and 4H^chL-T4) displayed moderate resistance to STB compared with the control lines. However, some other gene(s) might also be located on the distal region of chromosome 4H^chS, as the 4H^ch H. chilense disomic substitution (4H^chDS) line could be considered the most resistant one, showing a heterogeneous behaviour against the disease but with the lowest DS scores (1–2). In addition, no remarkable differences in the STB resistance were found between 4H^chS-T1 and 4H^chS-T2 translocation lines, indicating that putative resistance genes might not be located in the small segment of chromosome 4H^chS in which these two durum wheat–H. chilense translocation lines might be different. Thus, that putative additional gene(s) should be included in the most distal 4H^chS chromosome region. Consequently, our results suggest that H. chilense chromosome 4H^chL might contain genes conferring resistance to STB on the distal 4H^chL chromosome arm. These genes could be homoeologous to those Stb7 and Stb12 resistance genes or STB7 QTL described previously in the 4A bread wheat chromosome (Brown et al., 2015), but, interestingly enough, additional gene(s) conferring stronger resistance to STB might also be included in the distal region of the H. chilense chromosome 4H^chS. To sum up, this present study sheds light on the interaction of Z. tritici and durum wheat, giving some clues to identify possible Stb genes in durum wheat. Therefore, the plant material obtained in this work constitutes a valuable germplasm to introduce resistance to STB in durum wheat.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding authors. Zymoseptoria tritici isolated is deposited in the public NBCI database under the MZ026796 accession number.

ZC: Writing – original draft, Investigation. M-CC: Investigation, Writing – review & editing. CM-R: Investigation, Writing – review & editing. JS: Investigation, Writing – review & editing. PP: Conceptualization, Funding acquisition, Investigation, Project administration, Resources, Supervision, Writing – original draft, Writing – review & editing.