Nectar bacteria stimulate pollen germination and bursting to enhance microbial fitness

Many organisms consume pollen, yet mechanisms of its digestion remains a
fundamental enigma in pollination biology, as pollen is protected by a
recalcitrant outer shell. Pollen is commonly found in floral nectar, as
are nectar microbes, which are nearly ubiquitous among flowers. Nectar
specialist bacteria, like Acinetobacter, can reach high densities (up to
109 cells/mL), despite the fact that floral nectar is nitrogen poor. Here,
we show evidence that the genus Acinetobacter, a prevalent nectar- and
bee-associated bacteria, can induce pollen germination and bursting, gain
access to protoplasm nutrients, and thereby grow to higher densities.
Although induced germination has been suggested as a potential method in
macroscopic pollen consumers, and fungal inhibition of pollen germination
has been shown, direct biological induction of germination has not been
empirically documented outside of plants. Acinetobacter pollinis SCC477
induced over 5x greater pollen germination and 20x greater pollen bursting
than that of uninoculated pollen by 45 minutes. When provided with
germinable pollen, A. pollinis stimulates protein release and grows to
nearly twice the density compared to growth with ungerminable pollen,
indicating that stimulation of germination benefits bacterial fitness. In
contrast, a common nectar-inhabiting yeast did not affect pollen
germination nor benefit from it. We conclude that Acinetobacter both
specifically causes and benefits from inducing pollen germination and
bursting. Further study of microbe-pollen interactions may inform many
aspects of pollination ecology, including floral microbial ecology,
pollinator nutrient acquisition from pollen, and cues of pollen
germination for plant reproduction.