Invisible third-parties behind mutualism: A review on plant-pollinator interactions altered by nectar-dwelling yeasts and bacteria

Because they are invisible to the naked eye, nectar-dwelling microbes have been largely ignored by researchers studying plant-pollinator interactions, despite their potential roles as nectar robbers. However, recent progress in molecular techniques has allowed us to tackle these invisible microbes, and several intriguing reports are now being published. First, this review provides basic information on nectar microbes: 1) they are common among diverse plant species; 2) some species are generalist nectivores, detected in various plants; 3) microbes consume sugar and amino acids, altering the nectar composition; 4) they are transferred by pollinators or air flow; and 5) their immigration into nectar is restricted by several factors. Second, based on these facts and a few examples of deleterious effects on plant reproduction, we discuss possible plant-pollinator-microbe interactions and their evolutionary consequences, with suggestions for future investigations. We also emphasize that microbes do not necessarily act as robbers; there is room for cooperation to evolve because both plants and microbes depend on pollinators for their dispersal, as suggested by plants harboring alcohol- or heat- producing yeasts, probably to attract pollinators. We hope that our review will lead to a better understanding of the biological interactions altered by invisible third parties.