Phenylphenalenones and diarylheptanoids – biosynthesis and ecological functions
Phenylphenalenones are characteristic natural products occurring in the monocotyledonous plant families Haemodoraceae, Pontederiaceae, Strelitziaceae, and Musaceae. The genera of the Haemodoraceae are native to many regions of the southern hemisphere. Some Australian Anigozanthos species ("kangaroo paw") are used as ornamental plants. The fruits of the Musaceae (bananas, plantains) are important food products.
Structure and biosynthesis
From a biosynthetic point of view, phenylphenalenones are a subgroup of diarylheptanoids. Linear diarylheptanoids are typical natural products of the Zingiberaceae and alicyclic diarylheptanoids were found in several dicots.
Curcumin, which is the most commonly known diarylheptanoid, is a widely used food colorant and spice, e.g. as a curry component. Most of the Haemodoraceae species accumulate phenylphenalenones in their roots, which often are colored due to their natural product content. According to the high levels of phenylphenalenones, in vitro plants, root and cell cultures of the Haemodoraceae have been used as model systems for biosynthetic studies (Brand et al., Planta 2006, 224, 413-428; Munde et al., Phytochemistry 2011, 72, 49-58; Munde et al., Phytochemistry 2013, 91, 165-176).
Ecological function and mode of action
In banana plants, phenylphenalenones are part of the defence system and function as phytoalexins. Plant pathogenic fungi, insects, and parasitic nematodes are causative agents of major pests in banana crop cultivation. Fungitoxic activities of phenylphenalenones have been reported (Otálvaro et al., J. Nat. Prod. 2007, 70, 887–890).
The nematode Radopholus similis attacks the roots of Musa plants, finally resulting in plants toppling and yield losses up to 75%. Roots of the resistant Musa variety “Yangambi km5” locally accumulate high levels of nematotoxic phenylphenalenones, mainly anigorufone, precisely at the infestation site but not in the healthy tissue. In contrast, roots of a susceptible variety exhibited large, continuous lesions containing subtoxic levels of phenylphenalenones. Anigorufone was shown to be the most toxic compound of that type. Raman and laser desorption ionization-mass spectrometric imaging techniques visualized that anigorufone accumulated in lipid droplets within the body of the nematode. The lipid droplets extend, converge and finally kill the nematode, likely by displacing its inner organs and causing an eventual metabolic dysfunction (Hölscher et al., Proc. Natl. Acad. Sci. USA 2014, 111, 105-110).
Another study has elaborated the contribution of individual functional groups to the antioxidant effect of the phenylphenalenones. H-Atom transfer from the phenolic hydroxyl, a captodative effect of the hydroxy ketone unit, and the presumed involvement of the phenyl ring in the terminal step of the radical reaction were disclosed to be relevant factors of the antioxidant activity of phenylphenalenones (Duque et al., Org. Lett. 2013, 15, 3542-3545).