Transcriptome and Regional Association Analyses Reveal the Effects of Oleosin Genes on the Accumulation of Oil Content in <i>Brassica napus</i>

Rapeseed stores lipids in the form of oil bodies. Oil bodies in the seeds of higher plants are surrounded by oleosins. Adjusting oleosin protein levels can prevent the fusion of oil bodies and maintain oil body size during seed development. However, oil contents are affected by many factors, and studies on the complex molecular regulatory mechanisms underlying the variations in seed oil contents of <i>B. napus</i> are limited. In this study, a total of 53 <i>BnOLEO</i> (<i>B. napus</i> oleosin) genes were identified in the genome of <i>B. napus</i> through a genome-wide analysis. The promoter sequences of oleosin genes consisted of various light-, hormone-, and stress-related cis-acting elements, along with transcription factor (TF) binding sites, for 25 TF families in 53 <i>BnOLEO</i> genes. The differentially expressed oleosin genes between two high- and two low-oil-content accessions were explored. <i>BnOLEO3-C09</i>, <i>BnOLEO4-A02</i>, <i>BnOLEO4-A09</i>, <i>BnOLEO2-C04</i>, <i>BnOLEO1-C01,</i> and <i>BnOLEO7-A03</i> showed higher expressions in the high-oil-content accessions than in low-oil-content accessions, at 25, 35, and 45 days after pollination (DAP) in two different environments. A regional association analysis of 50 re-sequenced rapeseed accessions was used to further analyze these six <i>BnOLEO</i> genes, and it revealed that the nucleotide variations in the <i>BnOLEO1-C01</i> and <i>BnOLEO7-A03</i> gene regions were related to the phenotypic variations in seed oil content. Moreover, a co-expression network analysis revealed that the <i>BnOLEO</i> genes were directly linked to lipid/fatty acid metabolism, TF, lipid transport, and carbohydrate genes, thus forming a molecular network involved in seed oil accumulation. These favorable haplotypes can be utilized in molecular marker-assisted selection in order to further improve seed oil contents in rapeseed.