An integrated QTL mapping and transcriptome sequencing provides further molecular insights and candidate genes for stem strength in rapeseed (Brassica napus L.)

The stem's mechanical properties reflect its ability to resist lodging. In rapeseed (Brassica napus L.), although stem lodging negatively affects yield and generates harvesting difficulties, the molecular regulation of stem strength remains elusive. Hence, this study aimed to unravel the main loci and molecular mechanisms governing rapeseed stem strength. A mapping population consisting of 267 RILs (recombinant inbred lines) was developed from the crossed between ZS11 (high stem strength) and 4D122 (low stem strength), and two mechanical properties of stems including stem breaking strength and stem rind penetrometer resistance were phenotyped in four different environments. Four pleiotropic QTLs that were stable in at least two environments were detected. qSR.A07, the major one, was fine-mapped to a 490 kb interval between markers SA7-2711 and SA7-2760 on chromosome 7. It displayed epistatic interaction with qRPR.A09-2. Comparative transcriptome sequencing and analysis unveiled methionine/S-adenosylmethionine cycle (Met/SAM cycle), cytoskeleton organization, sulfur metabolism and phenylpropanoid biosynthesis as the main pathways associated with high stem strength. Further, we identified two candidate genes, BnaA07G0302800ZS and BnaA07G0305700ZS, at qSR.A07 locus. Gene sequence alignment identified a number of InDels, SNPs and amino acid variants in sequences of these genes between ZS11 and 4D122. Finally, based on these genetic variants, we developed three SNP markers of these genes to facilitate future genetic selection and functional studies. These findings offer important genetic resources for the molecular-assisted breeding of novel rapeseed stem lodging-resistant varieties.