Diversity, specificity, co-occurrence and hub taxa of the bacterial–fungal pollen microbiome

Abstract

Flower pollen represents a unique microbial habitat, however the factors driving microbial assemblages and microbe-microbe interactions remain largely unexplored. Here we compared the structure and diversity of the bacterial-fungal microbiome between eight different pollen species (four wind-pollinated and four insect-pollinated) from close geographical locations, using high-throughput sequencing of the 16S the rRNA gene fragment (bacteria) and the internal transcribed spacer 2 (ITS2, fungi). Proteobacteria and Ascomycota were the most abundant bacterial and fungal phyla, respectively. Pseudomonas (bacterial) and Cladosporium (fungal) were the most abundant genera. Both bacterial and fungal microbiota were significantly influenced by plant species and pollination type, but showed a core microbiome consisting of 12 bacterial and 33 fungal genera. Co-occurrence analysis highlighted significant inter- and intra-kingdom interactions, and the interaction network was shaped by four bacterial hub taxa: Methylobacterium (two OTUs), Friedmanniella and Rosenbergiella. Rosenbergiella prevailed in insect-pollinated pollen and was negatively correlated with the other hubs, indicating habitat complementarity. Inter-kingdom co-occurrence showed a predominant effect of fungal on bacterial taxa. This study enhances our basic knowledge of pollen microbiota, and poses the basis for further inter- and intra-kingdom interaction studies in the plant reproductive organs.

The Xylella fastidiosa-Resistant Olive Cultivar “Leccino” Has Stable Endophytic Microbiota during the Olive Quick Decline Syndrome (OQDS)

“…Inference of co-occurrence networks is a computational method able to detect potential interactions between microbes based on their relative abundances in the samples [36]. This method is increasingly used to characterize the plant-associated microbiota, for example, in the rhizosphere [61,[67][68][69] or in the pollen habitat [70]. Here we showed that X. fastidiosa is potentially interacting with several species of bacteria and fungi in the leaf endosphere of Xf -infected "Leccino".…”

“…To identify significant patterns of correlated bacteria in the Xf -infected leaves of "Leccino" trees, the fungal and bacterial OTUs with at least 30 total reads and occurring in at least 3 samples were subjected to a co-occurrence analysis, using the software CoNet [103], which is available as add-on in Cytoscape 3.7 [104]. Non-rarefied, absolute abundance dataset was used, as recommended to avoid the false-positive issues when using relative abundance data [70]. A preliminary check of the dataset indicated a total of 1378 possible interactions.…”

Defence compounds in pollen: why do they occur and how do they affect the ecology and evolution of bees?

“…The pollen coat covering the outer layer of the pollen grain (the pollenkitt) is rich in lipids and carbohydrates (Heslop-Harrison & Heslop-Harrison, 1985;Pacini & Hesse, 2005), potentially creating an attractive habitat for microbial colonists. Indeed, recent investigations have demonstrated that the surface of pollen grains is colonised by a diverse community of microorganisms (F€ urnkranz et al, 2012;Ambika Manirajan et al, 2016;Kim et al, 2018;Manirajan et al, 2018); however, the impact of this microbiota on plant fitness is unknown. In nectar, colonisation by bacteria and yeast affects carbohydrate, amino-acid and secondary metabolite composition (Herrera et al, 2008;Peay et al, 2012;Vannette & Fukami, 2016;Rering et al, 2018), with negative or positive impacts on pollinator attraction (Vannette et al, 2012;Schaeffer et al, 2019).…”

“…Finally, the recent discovery of the complex pollen microbiota demands investigation of its role in pollination (Manirajan et al 2018, McFrederick & Rehan 2019. Indirect evidence points toward a role of pollen-colonising microorganisms in pollination.…”

Towards a better understanding of the role of nectar-inhabiting yeasts in plant–animal interactions

“…As the main reward for their services, flowers offer nectar to the visiting pollinators providing them with sugars and other nutrients [17,18]. Given the nutrientrich nature of nectar and other floral rewards, microorganisms are commonly found in the flowers of a wide diversity of plant species worldwide [7,8,[19][20][21][22][23][24][25][26][27][28][29]. Flowerinhabiting microbes can alter the chemical composition of their habitat by consuming the available nutrients and/ or releasing metabolic by-products [30][31][32], which, in turn, may affect pollinators' foraging behavior and have an impact on the reproductive success of the plant (see below).…”