Pollen Partners: The Symbiotic Microbes of Pinus radiata Pollen

Pollen, a crucial source of nutrients and energy for pollinators. It also provides a unique habitat for ecological microbiota. Previous research on the microbiome of pollen has largely focussed on angiosperm systems, with limited research into coniferous gymnosperms. This study characterises the pollen microbiome associated with one of the worlds most widely grown tree species, Pinus radiata. Trees were sampled from locations across Canterbury, New Zealand, with repeated collections in 2020 and 2021. Metabolomic analysis revealed the main compounds present on P. radiata pollen to be amino acids (principally proline), and carbohydrates (fructose, glucose, and sucrose). Although phenolic compounds such as {rho}-coumaric acid and catechin, and terpenoids such as dehydroabietic acid, were present at low concentrations, their strong bioactive natures mean they may be important in filtering of microbiome communities on pollen. Pinus radiata pollen was found to host a microbiome dominated by fungi; this directly contrasts with those for many angiosperm species. Geographic range and sampling years were evaluated as secondary drivers of microbiome assembly. Neither sampling location nor annual variation had a significant impact on the fungal component of the pine pollen microbiome, which was remarkably stable/conserved among samples. However, some bacterial taxa exhibited sensitivity to geographic distances and yearly variations, suggesting a secondary role for some. A core microbiome was identified in P. radiata pollen, characterized by a consistent presence of specific fungal and bacterial taxa across samples. While the dominant phyla, Proteobacteria and Ascomycota, align with findings from other pollen microbiome studies, unique core members were unidentified at genus level. This tree species-specific microbiome assembly emphasizes the crucial role of the host plant in shaping the pollen microbiome. These findings contribute to a deeper understanding of pollen microbiomes in gymnosperms, shedding light on the need to look further at their ecological and functional roles.