High Throughput Phenotyping of Maize (Zea mays L.) Roots in the Field Unravels QTLs Controlling Root Architecture.

Numbers, angles and the branching pattern of crown and brace roots of (Zea mays L.) were assessed visually at flowering. The measurement of these traits allowed a reliable prediction of rooting depth and rooting penetration. We show the utility of these traits for water and nitrogen acquisition under optimal as well as under water and nitrogen deficient conditions in different soil types. While steep rooting angles are important for water and nitrogen acquisition in sandy soils, branching is of greater importance in more heavy soils. In a third step we assessed the genetic basis of root architectural traits using linkage and association mapping in 25 related maize populations (Nested association mapping population: 5000 recombinant inbred lines). Overall 400 quantitative trait loci for 13 traits were identified by linkage mapping. LOD scores ranged from 2.5 to 9.6 at a heritability of up to 0.39. Major QTL clusters for root numbers (chromosomes 1, 2, 5, 7, 8, 9), angles (1, 2) and branching (1, 3, 5, 7, 8, 9) were identified. The positions of 2 clusters for root angles and 4 clusters for root numbers were confirmed by association mapping. Moreover we present several putative candidate genes controlling root architecture at loci with QTL clusters. In the future phenotypic selection using the scoring method in combination with marker assisted selection will permit researchers and breeders to tailor root systems for adaptation to target environments.