The biology and structural distribution of surface flavonoids

Flavonoids are a large group of phenolic compounds present in most plants. Flavonoids can accumulate in vacuolar compartments, or be secreted, for example as part of root exudates. Most intriguing is the accumulation of flavonoids on the surface of leaves and flowers. These flavonoids are known as surface flavonoids, and their biosynthesis, distribution and functions are poorly understood. Earlier reports indicated that they may provide an adaptive feature for species survival in harsh environmental conditions, and for protection against herbivores and pathogens. Intriguingly, however, their distribution across the plant kingdom does not seem to support these hypotheses as they have only been described in a few plant species, and have also been isolated from leaf and flower surfaces of plants growing in normal environmental conditions. Research efforts towards the structural identification of these compounds are ongoing, especially within the dicotyledons. Results consistently show that surface flavonoids are mostly aglycones, and are often highly substituted with methoxy groups. Outstanding questions remain regarding the transport pathway/mechanism of surface flavonoids to leaf surfaces, their ecological significance, and the possible pharmaceutical benefits derivable from these compounds. This review addresses the structural variations, distribution, functions, pharmacological properties, and directions of the research on surface flavonoids.