Comprehensive Transcriptome and Metabolome Analysis Reveals the Potential Mechanism Influencing Flower Color Formation in Macadamia integrifolia

Color serves as a crucial visual signal for attracting pollinating insects and directly affects the fruit set rate in woody crops. This study investigated the molecular mechanisms underlying flower color formation in macadamia. The results demonstrated that darker flower colors were associated with higher fruit set rates: the rates for purple, pink, pinkish-white, and white flowers were 2.78, 1.99, 1.35, and 1.31, respectively. High-throughput sequencing identified 1359 differentially accumulated metabolites, including benzoic acid, 4-hydroxybenzaldehyde, and isorhamnetin. Transcriptional regulators such as ERF, MYB, and WRKY were significantly up-regulated in darker flowers. KEGG analysis revealed two key metabolic pathways, in which genes including HCT (shikimate hydroxycinnamoyl transferase) and F3GalTase (flavonol 3-O-galactosyltransferase), as well as related metabolites such as p-coumaric acid, chlorogenic acid, and myricetin, showed higher expression levels in darker flowers. Anthocyanin content was highest in pink and pinkish-purple varieties (462.79 and 446.35 μg/g, respectively), and lower in white and light pink varieties (140.52 and 167.97 μg/g). In conclusion, flower color intensity is positively correlated with both fruit set rate and anthocyanin content. Genes involved in the flavonoid and phenylpropanoid pathways, along with transcription factors such as WRKY and MYB, collectively regulate flower color formation. This study provides a theoretical basis for macadamia flower color breeding.