Resistance Gene Pyramiding in Wheat (Triticum aestivum L.): From Marker-Assisted Selection to Functional Validation

Gene pyramiding is widely used in wheat breeding to improve disease resistance; however, combining multiple resistance alleles does not necessarily provide stronger protection or long-term durability. Molecular markers, gene cloning and genome editing have greatly facilitated the tracking and pyramiding of resistance alleles. These approaches define the allelic composition of a line rather than the level of resistance it will express. This review examines why the effectiveness of resistance gene combinations remains difficult to predict despite advances in molecular tools. Evidence from leaf rust, stripe rust, stem rust, powdery mildew, eyespot and Fusarium head blight shows that the observed effects of combined resistance loci depend on the genetic background, gene interactions and environmental conditions, as well as on the methods used to assess disease development. We discuss how genetic characterization, expression analysis, functional assays and multi-environment phenotyping can provide complementary evidence for evaluating resistance gene combinations and how pathogen evolution and deployment strategies affect their durability. As genome editing expands the possibilities for combining resistance alleles, understanding their functional compatibility and their performance across genetic backgrounds remains essential for developing effective and durable resistance in wheat. The central conclusion of this review is that no marker profile, and no edited allele, is by itself evidence that a cultivar will be resistant in the field. Markers and gene editing shorten the route to a combination, but the resistance of that combination still must be validated phenotypically in the target backgrounds and environments.