Natural genetic variation in nodal root growth angle and anatomy underlies drought tolerance in bread wheat

Root growth angle (RGA) and anatomical attributes are pivotal for the acquisition and transport of water and nutrient during drought stress conditions. Here, we aimed to identify natural genetic variation and genomic loci associated with nodal RGA and anatomical traits in response to drought stress. RGA and anatomy traits were evaluated from nodal roots of the main shoot and tiller under well watered and drought conditions in field. Marker-trait association were identified using a genome-wide association study (GWAS) coupled with linkage disequilibrium in a diverse set of 200 winter wheat genotypes. We observed a highly heritable change in RGA and anatomy under drought stress in wheat in a genotype-dependent manner. Narrow RGA and smaller cortex diameter of nodal root at the expense of increased stele diameter under was the main adaptive response under drought stress. Increased stele diameter was caused by more and larger metaxylem vessel under drought stress than the control conditions. In addition, we observed a high phenotypic correlation in root anatomical traits of nodal roots between the main shoot and tillers, indicating possible overlapping genetic control. Next, we detected 24 significant marker-trait association for RGA and anatomy traits resolved into 14 loci, indicating quantitative inheritance. Linkage disequilibrium and haplotype analysis revealed that the favorable phenotype at detected loci was represented by minor allele in the studied population. We identified 161 candidate genes and some of them were involved in root morphogenesis and differentiation. Of these, two promising candidate genes highly expressed in roots, namely, TraesCS2A02G109700 and TraesCS1B02G363700 are orthologs to auxin-responsive genes, IBR3 and IBR10, respectively. They encode acyl-CoA dehydrogenase-like proteins that are required for root hair elongation and are potential targets for functional studies. The QTLs detected in this study are reliable and can be used for marker-assisted selection of favorable root features for breeding drought-tolerant wheat cultivars.