Flavone From Plants—Uncommonly Common? A Comprehensive Review

Flavone is a very simple and unsubstituted flavonoid of restricted distribution in the plant kingdom. Historically it is known for a long time as major constituent of many Primula and Dionysia exudates produced by glandular hairs in unusually large amounts, often as a farinose (mealy) deposit on aerial plant parts. The ecological function of this unique farinose deposit and its main compound flavone is still not really understood. Concerning flavone, a knowledge gap exists about its biosynthetic origin, when compared to all of the flavonoid biosynthetic routes known to date. Thus, this review addresses not only the history of this compound and its occurrence in plants and in their respective compartments, but also discusses ideas on its putative biosynthesis. Furthermore, we provide up-to-date analytical data on this compound from a natural source, rather than from chemosynthesis, in a comprehensive way. Eco-functional aspects complement this study that is intended to be highly stimulating for future research in ecology and evolution.