Phenylphenalenones and diarylheptanoids – biosynthesis and ecological functions
Phenylphenalenones and related aromatic natural products are characteristic natural products occurring in the plant families Haemodoraceae, Pontederiaceae, Strelitziaceae, and Musaceae. The Haemodoraceae are a monocotyledonous plant family, which is almost distributed over the southern hemisphere from Australia, the South African Cape region to the northern and eastern parts of South America. Some Anigozanthos species ("kangaroo paw") are used as ornamental plants. The Musaceae, including the economically important banana varieties, originate from Southeast Asia and are now widely distributed in tropical and subtropical areas. Among the Pontederiaceae, the water hyacinth (Eichhornia crassipes) is an invasive plant on many tropical waterbodies, and the bird of paradise plant, Strelitzia reginae, is the most well-known Strelitziaceae species.
From a biosynthetic point of view, phenylphenalenones are a subgroup of diarylheptanoids. Curcumin, which is the most commonly known diarylheptanoid, is widely used as a food dye and spice, e.g. in curry. Diarylheptanoids with a seven-membered linear carbon chain are typical for the Zingiberaceae and alicyclic diarylheptanoids were found in several dicots.
Owing to their phenolic structure, phenylphenalenones and diarylheptanoids play a role in radical scavenging and UV protection. In banana plants, phenylphenalenones are part of the defense system and function as phytoalexins. Fungitoxic and antibacterial activity of phenylphenalenones have been demonstrated. Studies about specific ecological functions of phenylphenalenones are in progress. According to the constitutive high levels in the Haemodoraceae, several species of this family are of special interest as model systems for studying the biosynthesis, tissue-specific localization, and biological role of phenylphenalenones. Most of the Haemodoraceae species studied so far accumulate phenylphenalenones mainly in their underground plant parts, which often show bright coloration due to their natural product content. Therefore, in vitro root and cell cultures of Anigozanthos preissii, Wachendorfia thyrsiflora and Xiphidium caeruleum have been established and used for biosynthetic studies.