Non-protein amino acids in nectar: are they tools to modulate insect behaviour and increase plant reproductive output?

Floral and extrafloral angiosperms’ nectar is undoubtedly recognized as a valuable energetic alimentary resource for a large variety of animals. Through nectar consumption insects, small mammals, birds, marsupials and reptiles establish more or less specialized interactions with plants. But according to recent studies, nectar mediates interactions that are much more complex than simply alimentary relations. From the more abundant nectar compounds, i.e. sugars and amino acids mainly responsible for its high alimentary value, attention of scientists is moving towards compounds that are not directly connected with its alimentary importance. Among these, secondary compounds appear to have a special role in regulating interaction with other organisms. Although very few is known about their ecological roles in nectar, recently it was proved that secondary compounds may affect the nectar feeders’ behaviour interacting with their neurobiology. We addressed a special focus on one class of secondary compounds: the non-protein amino acids. Their presence in floral nectar has been reported since long time but their ecological function has not been investigated. Data from several phylogenetically unrelated species indicate that they may represent a consistent part of the total amino acid content of floral nectar (25-45%) and generally the more abundant ones are γ-amino butyric acid (GABA) and β-alanine. The study of nectar chemistry in a restricted taxonomic context (i.e. the tribe Lithospermeae of the Boragniaceae family) revealed that GABA concentration is particularly high in species with specialist bee- and bumblebee-pollination whilst β-alanine increases in species pollinated by flies and passerine birds. This result suggested to test the effect of an artificial diet consisting of 20% sucrose solutions enriched with GABA and β-alanine on some species of Apoidea. Results show that some species of insects increase their mobility, flying activity and survival time when fed with the artificial diet and particularly with the solution enriched with GABA. Although we used high concentration of the non-protein amino acid (20-fold higher than the concentration occurring in nectar), these results push us to further consider the role of GABA in increasing the mobility of insects between flowers and their foraging activity and thus their pollination performances.