Integrating unconditional and conditional QTLs to dissect the genetic basis of stem mechanical strength in Brassica napus L

Stem mechanical strength (SMS) plays an important role in resisting stem dislodging. However, the genetic regulatory mechanisms underlying SMS in rapeseed remain unclear. In this study, a recombinant inbred line population containing 189 lines was used to investigate four SMS-related traits, namely stem breaking force (SBF), stem diameter (SD), stem weight (SW) and stem breaking strength (SBS). Accordingly, four conditional traits were also generated, namely SBF|SD, SBF|SW, SW|SD and SBS|SW. Quantitative trait locus (QTL) mapping for four unconditional SMS-related traits detected seven major QTLs, four of which were novel loci, with phenotypic contributions ranging from 10.41 to 27.22%. QTL mapping of conditional traits detected five major QTLs (including four novel loci), which explained 11.57 to 38.73% of the phenotypic variation. Comparative analyses between unconditional and conditional QTLs revealed that all 63 QTLs potentially govern biological processes (BPs) or unknown traits (UTs), which then influence SMS-related traits via 12 pathways. SBF was regulated by 13 unconditional QTLs via the QTL-BP-SD-SBF, QTL-BP-UT SD -SW-SBF, QTL-BP-SD-SW-SBF, and QTL-BP-UT SD&SW -SBF pathways, and by six conditional QTLs via the QTL-BP-SBF|SD-SBF and QTL-BP-SBF|SW-SBF pathways. SD was regulated by 18 unconditional QTLs via the QTL-BP-SD pathway, and SW by three regulatory pathways including QTL-BP-SD-SW, QTL-BP-UT SD -SW and QTL-BP-SW|SD-SW pathways. Finally, SBF potentially influences SMS via the SBF-SMS and SBF-SBS-SMS pathways. There were two additional regulatory pathways for SBS, namely QTL-BP-UT SW -SBS and QTL-BP-SBS|SW-SBS. In addition, 12 promising candidate genes were identified through multiple methods. These results contribute to our knowledge about the genetic regulatory mechanisms underlying SMS in Brassica napus .