Polymethoxyflavone (PMF) Accumulation and PMF Biosynthesis-Related Gene Expression in Tachibana (Citrus tachibana Tanaka) Fruit Treated Using a Combination of Gibberellic Acid and Prohydrojasmon and Post-harvest Deep-Ultraviolet Light-Emitting Diode Irradiation

Citrus fruits such as Tachibana (Citrus tachibana Tanaka) contain various flavonoids that perform biological functions important to human health. Considering the growing public health consciousness, combined treatment with gibberellic acid and prohydrojasmon (GP), which is used in Japan, may be used to delay flavonoid degradation and maintain high polymethoxyflavone (PMF) contents in the flavedo of citrus fruits. However, the effects of GP treatment on fruit growth, PMF accumulation, and flavonoid biosynthesis-related gene expression in Tachibana fruit remain unclear. Post-harvest ultraviolet (UV) light irradiation also induces production of flavonoids; however, it has rarely been applied to citrus fruits. In this study, we aimed to investigate the effects of combined treatment with GP during cultivation and post-harvest deep-UV light-emitting diode (DUV-LED) irradiation on accumulation of PMFs (nobiletin, tangeretin, and sinensetin) and expression of flavonoid biosynthesis-related genes in Tachibana fruit. Nobiletin, tangeretin, and sinensetin contents in the flavedo of a single fruit significantly increased after GP treatment and 1 day after DUV-LED irradiation. The fruits treated with GP and DUV-LED exhibited the highest PMF content and significantly upregulated expression of CitFNS, CitCHI, CitF3'H, CitOMT2, and CitOMT3. Moreover, DUV-LED irradiation significantly induced CitOMT1 expression. These results suggest that the combination of GP treatment during cultivation and post-harvest DUV-LED irradiation can promote PMF accumulation in Tachibana fruit. This study may serve as a basis for efficient extraction and utilization of functional ingredients, such as nobiletin, from Tachibana.