Integrative ATAC-Seq and RNA-Seq Analysis Identifies a WD40 Repeat Protein, ObWPA, as a Significant Regulator of the Purple Coloration in Syringa oblata

Lilac (Syringa spp.) is a widely cultivated ornamental plant prized for its fragrant aroma and attractive flower colors. However, the molecular mechanisms governing its flower pigmentation remain poorly understood. In this study, we performed integrated transcriptomic and metabolomic analyses on purple (Syringa oblata) and white (Syringa oblata var. alba) lilacs at the P1 stage, the point of deepest pigmentation. Compared with W1, P1 has a total of 918 differentially expressed genes, including 614 up-regulated genes and 304 down-regulated genes. And S. oblata exhibited significant upregulation of key anthocyanin biosynthesis genes, including the rate-limiting enzyme gene ObDFR, ObF3’H and transcriptional regulators such as ObWPA, which encodes a WD40 repeat protein. This transcriptional activation was accompanied by a substantial accumulation of 27 anthocyanins, including Petunidin Chloride, Cyanidin Chloride, Delphinidin and so on, while the Petunidin-3-O-rutinoside, Petunidin-3-O-(6-O-p-coumaroyl)-glucoside and Malvidin-3-O-sambubioside-5-O-glucoside were only detected in S. oblata. Furthermore, ATAC-seq analysis revealed that, in comparison to white lilac, purple lilac exhibited 3522 and 805 genes with increased and decreased chromatin accessibility, respectively. Integrative analysis with the transcriptome identified 135 genes that were both more accessible and transcriptionally upregulated in purple lilac, including ObWPA, Ob0214386, and Ob0227194 which belong to WD40 members. Subsequent qRT-PCR validation confirmed ObWPA as the most significantly upregulated gene in purple lilac, a finding consistent with the specific chromatin accessibility detected in its promoter region. To validate its function, we knocked down ObWPA expression in purple lilac using Virus-Induced Gene Silencing (VIGS). This intervention resulted in a dramatic color shift from purple to white, concomitant with a significant decrease in key anthocyanin metabolites such as Cyanidin-3-(6-O-p-caffeoyl)-glucoside, Cyanidin Chloride, Pelargonidin, Cyanidin-3-O-rutinoside, Dihydrokaempferol, and Petunidin Chloride. Collectively, our findings demonstrate that ObWPA is an indispensable positive regulator of purple color formation in S. oblata.