Identification of a novel solid stem QTL in hexaploid wheat contributing resistance to wheat stem sawfly.

Stem solidness is a heritable quantitative trait that provides resistance against wheat stem sawfly (WSS) infestation in durum and hexaploid wheat. Stem solidness arises from the action of multiple genetic regions contributing to temporal stem pith production, identified in ancient landraces and cultivars of wheat. However, uniform expression of stem pith across all internodes and throughout wheat's lifecycle remain unobserved. In the present study, we dissected the genetic architecture of the solid stem trait by leveraging a cultivar from the International Maize and Wheat Improvement Center, 'CIMMYT-9263'. A recombinant inbred mapping population comprising 146 lines was created by crossing 'CIMMYT-9263' with a Montana adapted, photoperiod-insensitive, WSS susceptible variety, 'Hi-Line'. Multi-year phenotypic assessment of the solid stem trait at stem elongation (Zadoks 32) and maturity (Zadoks 77), along with WSS field stem cutting (%), revealed QTL regions contributing to host plant resistance. We report a major QTL region on the long arm of chromosome 3A spanning the physical distance 730,477,755 - 739,555,941 bp, named, Qss.msub.3AL-2. The allele Qss.msub.3AL-2.b contributed from the 'CIMMYT-9263' controls for increased stem pith production and reduced stem cutting. Markers associated with the Qss.msub.3AL-2.b solid stem allele were used to create Kompetetive Allele Specific Primers (KASP) for marker assisted selection (MAS). A small effect QTL on chromosome 3D was also identified controlling for temporal stem solidness. The two solid stem QTL identified in the present study will contribute to creating cultivars carrying multiple solid stem alleles for improving WSS resistance.