A sterile plant culture system of <i>Uncaria rhynchophylla</i> as a biosynthetic model of monoterpenoid indole alkaloids

<i>Uncaria</i> plants, belonging to the Rubiaceae family, develop characteristic hooks at their leaf axils. In the Japanese Pharmacopoeia, the hooks from three <i>Uncaria</i> species, including <i>U. rhynchophylla</i>, are collectively defined as "Uncaria Hook" and are widely used as medicinal materials. The pharmacological properties of the diverse bioactive metabolites in <i>U. rhynchophylla</i>, particularly monoterpenoid indole alkaloids (MIAs), have been extensively studied. In this study, we aimed to establish sterile cultures of <i>U. rhynchophylla</i> as models for investigating MIA biosynthesis. LC-MS/MS-based untargeted metabolomic analysis revealed that the metabolomic profiles of stems from cultured plants showed strong similarity to those of medicinal parts from mature plants, specifically the hooks and stems. Furthermore, the analysis indicated that the contents of oxindole and indole alkaloids exhibited distinct variations depending on the plant part and developmental stage, both in sterile plant cultures and mature plants. Our findings demonstrate that <i>U. rhynchophylla</i> can be maintained under sterile conditions while stably producing MIAs. These cultured plants can serve as a model system not only for studying MIA biosynthetic pathways but also for ensuring quality control of Uncaria Hook in medicinal applications. This model system would contribute to the fundamental research by enhancing our understanding of the biosynthetic mechanisms and facilitating applications such as metabolic control of the contents of bioactive compounds in Uncaria Hook.