Beyond nectar sweetness: the hidden ecological role of non-protein amino acids

1) Studies from the last decade clearly demonstrate that nectar is much more than just a simple alimentary reward for pollinators. Considerable progress has been made in recent years with regard to the chemical ecology of nectar, demonstrating unequivocally that its chemistry is strongly involved in the interactions between plants and a larger variety of organisms - from bacteria to fungi and animals- than previously thought. As far as plant-pollinator relationships are concerned, attention has recently been directed towards secondary compounds, mainly alkaloids that are able to induce a wide range of behavioural and physiological responses in floral visitors. Nonetheless the study of some other secondary compounds, such as non-protein amino acids (NPAAs), has been neglected. The roles of these substances in nectar are still unknown although several ecological and physiological functions have been attributed to a number of NAAs in both animals and plants in several ecological and physiological contexts. 2) The existence of NPAAs in floral nectar was first reported in early surveys of nectar composition dating from the 1970s. Since that time, only a very few papers have mentioned the presence of these compounds in nectar, but without any hypothesis relating to their ecological role and their putative effect on flower visitors. This appears surprising considering that interaction with other organisms is a recognized function of NPAAs in plants. 3) Here, after exploring the multiple roles of NPAAs at the community level and focusing on the more recent advances in nectar chemistry and ecology, I review the small body of literature relating to the presence of NPAAs in nectar before outlining some ecological roles in the light of complex, nectar-mediated, plant-animal interactions that have been recognized recently for other nectar secondary compounds based also on data and information obtained for other biological systems (from arthropods to humans). 4) Synthesis. By integrating data obtained from both literature and field observations, I have arrived at the proposal that non-protein amino acids in nectar may contribute to the plant-insect network of interactions in a number of ways: by affecting the physiology of the nervous system of the insect, regulating nectar intake through phagostimulation and promoting muscle function during flight. Confirmation of these hypotheses are needed in order to reinforce the concept that plants are able to affect the foraging behaviour of insects through nectar chemistry, thereby eventually optimizing their pollination effectiveness.