PrMYB6-PrbZIP19 module coordinates triacylglycerol assembly and fatty acid desaturation in Paeonia rockii seeds

How seed plants couple triacylglycerol assembly to fatty acid desaturation remains unresolved, despite the importance of both processes for oil quantity and quality. The oil-rich, α-linolenic-acid-enriched seeds of Paeonia rockii offer a system in which to investigate this coupling. Here, we identified the nuclear group F bZIP protein PrbZIP19, whose transcript abundance coincided with the period of rapid fatty acid accumulation in developing seeds. PrbZIP19 knockdown lowered seed total fatty acid content and shifted the C18:2/C18:3 profile. Conversely, its expression in Nicotiana benthamiana leaves and Arabidopsis increased lipid accumulation and changed the fatty acid profile. Promoter-binding assays identified PrDGAT2 and PrFAD3 as direct targets of PrbZIP19, with reporter assays revealing activation of the former and repression of the latter. A yeast two-hybrid library screen recovered PrMYB6, a nuclear transcription factor that interacts with PrbZIP19. PrMYB6 enhanced PrbZIP19-mediated activation of PrDGAT2 promoter activity and partially relieved PrbZIP19-mediated repression of PrFAD3, thereby strengthening lipid droplet formation and total fatty acid accumulation while modulating α-linolenic acid accumulation. Together, the data place the PrMYB6–PrbZIP19 partnership at the interface between TAG assembly and fatty acid desaturation, providing a transcriptional basis for linking oil accumulation to fatty acid composition in P. rockii seeds.