A HD-Zip transcription factor is a candidate upstream regulator of FAD2 associated with the oleic/linoleic acid ratio variation in Camellia chekiangoleosa kernels

The oil of Camellia chekiangoleosa is valued for its health‑beneficial fatty acid composition, particularly its high content of unsaturated fatty acids. FAD2 catalyzes the conversion of oleic acid to linoleic acid, but its transcriptional regulation remains poorly understood. Here, we integrated fatty acid metabolomics and transcriptomics across six kernel developmental stages with yeast one‑hybrid (Y1H) screening to investigate the regulatory network. The oleic/linoleic acid (O/L) ratio peaked at stage S5 and subsequently declined, and FAD2 expression showed a strong correlation with the O/L ratio variation (r = 0.73) but weak correlations with absolute oleic acid or linoleic acid contents. Although FAD2 was not assigned to a fatty acid-associated WGCNA module, its stage-specific expression pattern suggests that it may function as an important regulatory node controlling oleic-to-linoleic acid conversion. Y1H screening identified a homeobox‑leucine zipper protein (HB) as a candidate transcription factor that binds the FAD2 promoter. A non‑canonical HD-Zip-related binding site (5’‑GAAAAATAATTAAAT‑3’) was predicted by JASPAR, and molecular docking supported the structural feasibility of HB interaction with this predicted binding region. Furthermore, HB expression was positively correlated with the O/L ratio (r = 0.627). Collectively, these findings propose a working model in which HB may participate in the transcriptional regulation of FAD2 and provide new insights into the upstream regulatory network controlling fatty acid composition in C. chekiangoleosa .