Post-secretory synthesis of a natural analog of iron-gall ink in the black nectar of Melianthus spp.

Abstract

The black nectar of Melianthus flowers is thought to serve as a visual attractant to pollinators, but the chemical identity and synthesis of the black pigment are unknown. Here we report that the black nectar contains a natural analog of iron-gall ink, which humans have used since medieval times. Specifically, dark black nectar at anthesis contains high levels of ellagic acid and iron; synthetic solutions of ellagic acid and iron(III) recapitulate the black color of the nectar. Conversely, lightly colored nectars before and after anthesis contain significantly lower levels of ellagic acid and iron, but higher levels of gallic acid. We then explored the possibility of post-secretory synthesis of ellagic acid from gallic acid. Indeed, Melianthus nectar contains a peroxidase that oxidizes gallic acid to form ellagic acid. Reactions containing the nectar peroxidase, gallic acid, hydrogen peroxide, and iron can fully recreate the black color of the nectar. Visual modeling indicates that the black color is both visible and conspicuous to birds within the context of the flower. In summary, the black nectar of Melianthus is derived from an ellagic acid-Fe complex analogous to iron-gall ink and is likely involved in the attraction of passerine bird pollinators.