Presence of yeasts in floral nectar is consistent with the hypothesis of microbial-mediated signaling in plant-pollinator interactions

Olfactory floral signals are significant factors in plant-pollinator mutualisms. Recently, unusual fermentation odors have been described in the nectar and flowers of some species. Since yeasts are common inhabitants of many angiosperms nectars, this raises the possibility that nectar yeasts may act as causal agents of fermentation odors in flowers and, therefore, as possible intermediate agents in plant signaling to pollinators. A recent field study has reported that nectar yeasts were quite frequent in floral nectar across three different regions in Europe and America, where they reached high densities (up to 10(5) cells/mm(3)). Yeast incidence in floral nectar differed widely across plant host species in all sampling sites. A detailed study currently in progress on one of the species surveyed in that study (Helleborus foetidus, Ranunculaceae) has detected that, in addition to interespecific differences in yeast incidence, there is also a strong component of variance in yeast abundance that takes place at the subindividual level (among flowers of the same plant, among nectaries of the same flower). If yeast metabolism is eventually proved to contribute significantly to floral scent, then multilevel patchiness in the distribution of nectar yeasts (among species, among individuals within species, and among flowers and nectaries of the same individual) might contribute to concomitant multilevel variation in plant signaling and, eventually, also in pollination success, pollen flow and plant fitness.