Unlocking the Taggiasca olive orchard microbiota: metabarcoding approach to reveal the bacterial and fungal above- and below-ground communities

The olive tree (Olea europaea L.) is one of the most important crops in the Mediterranean region, essential for in terms of economic and agricultural significance. In Liguria, North- West Italy, the Taggiasca cultivar is grown in a distinctive landscape characterized by steep hills and terraced slopes. This region produces high-quality Taggiasca extra-virgin olive oil (bearing the P.D.O mark) and fermented olives, both renowned for their superior quality. In our study, the main objective was to unravel the bacterial and fungal communities of above- (barks, leaves, olive fruits) and below-ground (rhizospheres, soils) compartments with the use of a metabarcoding approach for a better understanding of the plant-associated niches and secondly, to identify the core microbiota within compartments. 24 trees were sampled across three di_erent areas of the orchard according to an altitude gradient to monitor the potential microbiota shifts. Overall, relative abundance of the fungal and bacterial populations showed that the most represented phyla were Ascomycota and Firmicutes respectively, with the main fungal genera being Aureobasidium and Cladosporium and the most abundant bacterial ones represented by Levilactobacillus and Sphingomonas. Beta-diversity based on Bray- Curtis dissimilarity showed significant di_erences among compartments, with olives and leaves samples being mostly characterized by the genera Aureobasidium and Cladosporium and Levilactobacillus and Pediococcus, for fungi and bacteria respectively, with no significant di_erence among areas. Barks and rhizospheres were mainly characterized by the presence of Pseudophaemoniella, Penicillium and Nocardioides, Actinomycetospora for fungi and bacteria respectively, with no significant di_erence among areas. To determine the core microbiota, Venn diagrams were generated based on 70% shared ASVs. Comparing all areas, some of the ASVs common to all compartments were Aureobasidium pullulans, Veronaea botryose, Cladosporium herbarum and Blastococcus, Solirubrobacter, Streptomyces for fungi and bacteria at the lowest taxonomic level, respectively. This study represents an e_ort towards a better understanding of the complex microbial consortia that characterizes the Taggiasca olive tree microbiota. It also represents a focus into the investigation of the variability among endophytic and epiphytic populations of the di_erent compartments, as well as their connections with the fermented fruits, given the relevance of the previously mentioned microorganisms in the fermentative process.