Genetic Dissection of Root Growth Angle Reveals Breeding Targets for Drought-Resilient Durum Wheat

Understanding the genetic basis of root system architecture is essential to improve drought adaptation in crops. Root Growth Angle (RGA) plays a key role in determining root depth, thereby influencing water and nutrient uptake. We characterised RGA at the seminal root stage in 1,236 tetraploid wheat (Triticum turgidum L.) accessions, including modern cultivars and landraces, using a high-throughput filter paper-based phenotyping platform. A genome-wide association study employing the 90K Illumina iSelect SNP array, coupled with subpopulation-specific analyses, revealed seven quantitative trait loci (QTLs) in cultivars and thirteen in landraces on chromosomes 2A (QRGA.ubo.2A.2), 6A (QRGA.ubo.6A.2), and 7A (QRGA.ubo.7A.2). Haplotype analysis identified alleles associated with steep (narrow) and shallow (wide) RGA. Narrow RGA haplotypes were linked to improved grain yield under terminal drought by enabling deeper soil exploration, and were more frequent in germplasm selected under rainfed conditions. Conversely, shallow RGA alleles were enriched in accessions from irrigated environments. Multi-omics analysis pinpointed conserved signalling components such as phosphorus response regulators (e.g., PHR1, CLE14) and auxin/ROS-related genes (e.g., ALMT1) as central to RGA plasticity. Meta-QTL integration revealed pleiotropic effects of RGA loci on yield, biomass, and grain quality. Our findings deliver validated KASP® markers and haplotype tools for marker-assisted selection, and highlight landraces as a rich reservoir of adaptive alleles. We propose a breeding framework combining deep rooting traits with plastic root responses to enhance durum wheat resilience under water-limited conditions.