Identification, evaluation and selection of a bacterial endophyte able to colonize tomato plants, enhance their growth and control Xanthomonas vesicatoria, the causal agent of the spot disease

Beneficial endophytes are key factors in plant productivity and disease control; therefore, research is ongoing to characterize the potential of endophytes as microbial inoculants to promote plant growth and act as biocontrol agents. In this study, we searched for and identified a set of endophytes occurring naturally in a few important crops. Beneficial endophytes were shown to antagonize several important fungi and bacteria, in particular those affecting tomato (Solanum lycopersicum), including Rhizoctonia solani, Alternaria alternata, Clavibacter michiganensis pv. michiganensis, Ralstonia solanacearum, and Xanthomonas vesicatoria. We characterized the most promising as potential inoculants of tomato, aiming to foster plant growth and control the bacterial spot disease caused by Xanthomonas vesicatoria. Among the selected endophytes, we identified a novel streptomycetes, taxonomically related to Streptomyces avermitilis and a pseudomonad, identified as Pseudomonas granadensis. The beneficial effects of these two bacteria, used as single inoculants or as a combination of both, were seen in a significant increase in root and shoot length (approximately 31% and 34% for the streptomycetes and 18% and 16% for the pseudomonad) and dry root biomass (90% for the streptomycetes and 70% for the pseudomonad). Additionally, both inoculants reduced disease progression and severity following inoculation with X. vesicatoria. There was no significant difference between plants treated with single inoculants and plants treated with both. The penetration and efficient colonization of tomato tissues by a green fluorescent protein-tagged culture of the streptomycetes was observed by confocal microscopy, confirming its endophytic nature.