Transcriptome and miRNA sequencing analyses reveal the regulatory mechanism of -linolenic acid biosynthesis in Paeonia rockii

Paeonia rockii is a promising woody oil crop because its seeds are rich in polyunsaturated fatty acids especially alpha-linolenic acid (ALA). ALA is an essential fatty acid that the human body cannot synthesize and is the direct synthetic precursor of eicosapentaenoic and docosahexaenoic acids, which play crucial roles in the development of the blood vessels, brain and nervous system of humans. However, the mechanisms underlying the dynamic changes in ALA during seed development are unknown. In this study, we found that the fatty acid content gradually increased with P. rockii seed development, with ALA being the main unsaturated acid component (37-44%). The content of ALA reached the peak value of 306.26 mg/g DW 20 days before the seeds had fully maturated. Seeds from three different developmental stages were selected for transcriptome and miRNA sequencing analyses to explore the molecular mechanism of ALA accumulation in P. rockii seeds. A total of 39 differentially expressed genes were screened for their involvement in ALA biosynthesis, among which FAD2/8, GPAT, PDAT, LACS, LPAAT, and KAS II might be the key structural genes of ALA accumulation. The differential expression of these genes was dependent on the regulation of five miRNAs (mdm_miR156b, novel miR_91, novel miR_133, novel miR_291, and novel miR_405) and four transcription factors (AP2, SNL2, TGA-like, and SPL). This study reveals the mechanism behind the dynamic changes of ALA contents in P. rockii during seed development, and also provides an important theoretical basis for the breeding of excellent varieties of P. rockii.