Abundance and diversity of the nectar microbiome in Rhododendron catawbiense varies with elevation

Background/Question/Methods

The plant nectar microbiome (NMB) primarily composed of fungi and bacteria can qualitatively and quantitively affect floral rewards and ultimately impact plant-pollinator interactions and plant reproductive success. Evidence suggests that changes in microenvironmental conditions across spatial gradients can induce changes in the floral nectar microbiome, leading to microbiome variation within species. Specifically, changes in biotic and abiotic conditions across an elevation gradient (i.e. pollinator community composition, temperature, UV exposure, nutrient availability) have the potential to induce variation NMB composition and abundance. However, this has been little explored. For instance, we could expect that the diversity and abundance of the NMB will decrease with elevation as the growing conditions become less favorable (e.g. lower temperatures). In this study, we evaluate spatial variation in nectar microbiome composition, diversity, and abundance in populations of Rhododendron catawbiense at high (H) and low (L) elevations separated by over 1000ft in Roan Mountain, TN. Nectar samples were collected, plated, quantified and isolated. Fungal ITS sequences were obtained from individual colonies by Sanger sequencing and directly from nectar samples by Nanopore NGS.

Results/Conclusions

Preliminary results suggest that elevation can affect fungal abundance and composition in the NMB. We found 10 fungal species inhabiting the nectar of R. catawbiense (H= 4; L= 9). We also observed variation in nectar yeast abundance with more than 30x as many CFUs (colony forming units) (H= 1.89 ±10.64; L=38.43 ±1.39.12) and twice as many RPKMs (reads per kilobase matched) (H= 2.8x107 ± 2.46x107; L=5.5x107 ±3.6x107) on average at the lower elevation. Interestingly, R. catawbiense NMB in the lower elevation had a broader diversity of species (Simpson Diversity Index: H= 0.52; L= 0.95), perhaps as a result of more favorable growing conditions. This trend is largely associated with higher abundance of Metschnikowiaceae yeast OTUs and lower abundance of potentially pathogenic Basidyomycete OTUs. Overall, results indicate that elevation can mediate changes in the composition and abundance of microorganisms in the NMB, which in turn can lead to differences in pollinator community composition and plant reproductive success. These results highlight the need evaluate within-species variation in NMB at large spatial scales and its potential consequences for plant reproductive success, plant-pollinator interactions and plant community dynamics. This is particularly important in the face of human-mediated environmental disturbances that can alter plant-microbe interactions.