Nectar bacteria stimulate pollen germination and bursting to enhance their fitness

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Shawn M. Christensen, Ivan Munkres and Rachel L. Vannette, University of California, Davis, Davis, CA

Background/Question/Methods
For many flower visitors, pollen is the primary source of non-carbon nutrition, containing substantial protein as well as trace nutrients. However, pollen is protected by a recalcitrant outer shell, which hinders digestion and nutrient extraction. Digestive methods of pollen consumers remain unclear. Nectar-dwelling microbes are nearly ubiquitous among flowers and can reach high densities, despite the fact that floral nectar itself is nitrogen poor. To address the question of whether and how these microbes may use pollen for nutrition, we utilized time-course microscopic imaging, microbial growth assays, and enzyme assays.
Results/Conclusions
We found that a common genus of nectar-dwelling bacteria, Acinetobacter, exploits and benefits from pollen nutrition by rapidly inducing pollen germination (15 min) and bursting (45 min). Acinetobacter pollinis SCC477 induced a germination rate over five times higher and a bursting rate twenty times higher than that of uninoculated pollen by 45 minutes. This phenotype is density-dependent, with lower concentrations of A. pollinis SCC477 resulting in a longer lag time before germination. A. pollinis SCC477 grows to nearly twice the density when provided with germinable pollen versus ungerminable pollen, indicating that this phenotype plays an important role in rapid growth. Pectinase activity did not explain the phenotype. We conclude that Acinetobacter both specifically causes and benefits from induced germination, and that pectin degradation is not solely responsible. Our results suggest that further study of microbe-pollen interactions may inform many aspects of pollination ecology, including microbial ecology in flowers, mechanisms of pollinator nutrient acquisition from pollen, and cues of pollen germination for plant reproduction.