Nectar bacteria stimulate pollen germination and bursting to enhance microbial fitness

• Published in: Current biology Vol. 31; no. 19; pp. 4373 - 4380.e6
• Main Authors: Christensen, Shawn M., Munkres, Ivan, Vannette, Rachel L.
• Format: Journal Article
• Language: English
• Published: England Elsevier Inc 11-10-2021
• Subjects: Animals; Bacteria; Bees; floral ecology; flower microbiome; Flowers; nectar; nectar yeast; plant microbiome; Plant Nectar; plant-microbe interactions; Pollen; pollen digestion; pollen tube; Pollination - physiology; plant-microbe interactions; floral ecology; flower microbiome; nectar; pollen tube; nectar yeast; pollen digestion; plant microbiome
• ISSN: 0960-9822; 1879-0445
• DOI: 10.1016/j.cub.2021.07.016

Many organisms consume pollen, yet mechanisms of its digestion remain a fundamental enigma in pollination biology,1–3 as pollen is protected by a recalcitrant outer shell.4–8 Pollen is commonly found in floral nectar,9,10 as are nectar microbes, which are nearly ubiquitous among flowers.11–13 Nectar specialist bacteria, like Acinetobacter, can reach high densities (up to 109 cells/mL), despite the fact that floral nectar is nitrogen poor.14–17 Here, we show evidence that the genus Acinetobacter, prevalent nectar- and bee-associated bacteria,12,18–20 can induce pollen germination and bursting, gain access to protoplasm nutrients, and thereby grow to higher densities. Although induced germination had been suggested as a potential method in macroscopic pollen consumers,2,21–23 and fungal inhibition of pollen germination has been shown,24–27 direct biological induction of germination has not been empirically documented outside of plants.28–32Acinetobacter pollinis SCC47719 induced over 5× greater pollen germination and 20× greater pollen bursting than that of uninoculated pollen by 45 min. 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 (Metschnikowia)33 neither induced nor benefited from pollen germination. 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,34,35 pollinator nutrient acquisition from pollen,2,3,21,36 and cues of pollen germination for plant reproduction.37–39
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•Nectar bacteria Acinetobacter induce pollen germination and bursting•Pollen germination and bursting increase protein in solution•Germinability of pollen benefits Acinetobacter, but not nectar yeast
Pollen is nutrient rich but protected by the pollen wall, which hinders pollen digestion. Nectar microbes are nitrogen limited and frequently encounter pollen. Christensen et al. show that nectar-dwelling Acinetobacter bacteria stimulate protein release by inducing pollen to germinate and burst, benefitting Acinetobacter.