Phytochrome Interacting Factor 3 Mediates Low Light Signaling to Regulate Isorhynchophylline Biosynthesis in Uncaria Rhynchophylla

Phytochrome interacting factors (PIFs) serve as crucial regulators in the light signal transduction pathway and also mediate light signals to regulate secondary metabolite synthesis in plants. However, the regulator role of PIFs in secondary metabolites often varies among different plants. Isorhynchophylline (IRN), an iconic secondary metabolite of Uncaria rhynchophylla, holds significant medicinal value. Low light induces the synthesis of IRN in previous research, but PIFs in U. rhynchophylla have not been studied to date. Building on this, we identified a PIF protein, UrPIF3, which possesses the typical conserved domains of the PIFs and is localized in the nucleus. Moreover, the expression level of UrPIF3 is consistently positively correlated with the expression of two key enzyme genes (UrSGD and UrSTR) in the IRN biosynthesis pathway, regardless of whether under low light or restoring light conditions. Yeast one-hybrid and dual-luciferase assays further demonstrated that UrPIF3 can directly upregulate UrSGD. Conversely, silencing UrPIF3 inhibits IRN synthesis, and significantly reduces the expression levels of UrSGD and UrSTR. In summary, our results suggest that under low light conditions, UrPIF3 can directly upregulate UrSGD and indirectly upregulate UrSTR, thereby promoting the synthesis of IRN.