Rhizobacterial diversity of Portuguese olive cultivars in the Douro valley and their potential as plant growth promoters.

Aims: This study investigated the bacterial communities in the rhizosphere of two traditional Portuguese olive cultivars, Cobrançosa and Negrinha de Freixo, in relation to soil properties. Additionally, we aimed to isolate and identify bacteria with potential for biocontrol and other plant growth-promoting traits from these rhizosphere communities.

Methods and results: Bacterial communities in the olive rhizosphere were investigated using a metabarcoding approach and the soil physicochemical properties of the olive groves were also analyzed. Higher bacterial richness was associated with Negrinha de Freixo growing in soil with high organic matter content and water-holding capacity. In contrast, the soils of the Cobrançosa grove presented higher pH and electric conductivity. Negrinha de Freixo rhizosphere was enriched with ASVs (Amplicon Sequence Variants) belonging to Bacillus, Gaiella, Acidothermus, Bradyrhizobium, and uncultured Xanthobacteraceae. On the other hand, the Cobrançosa rhizosphere was characterized by higher relative abundance of Streptomyces and Sphingomonas. Bacterial isolation from the rhizosphere and screening for plant growth-promoting activities were also performed. Six bacteria strains, predominantly Bacillus isolated from Negrinha de Freixo, demonstrated antagonistic activities against the olive fungal pathogen Colletotrichum gloeosporoides and other plant growth promotion (PGP) traits.

Conclusions: Our findings demonstrate that the structure of rhizosphere bacterial communities associated with olive trees is shaped by both plant cultivar and soil-related factors. The higher number of bacterial species in the rhizosphere of Negrinha de Freixo was related to a higher organic matter content and a greater abundance of isolates with plant growth promotion traits, particularly Bacillus strains.