Physiological and genomic characterisation of Luteimonas fraxinea sp. nov., a bacterial species associated with trees tolerant to ash dieback

A group of isolates of the genus Luteimonas was characterised, which represented a specific component of the healthy core microbiome of Fraxinus excelsior in forest districts with a high infection rate of H. fraxineus, the causal agent of ash dieback. Based on phylogenomic and phenotypic analyses, a clear differentiation from related Luteimonas species was shown. Comparisons of the overall genome relatedness indices with the closest phylogenetic neighbours resulted in values below the recommended species cut-off levels. In addition, differences in several physiological and chemotaxonomic traits allowed a clear demarcation from the type strains of closely related species. Conclusively, the strain group was considered to represent a novel species in the genus Luteimonas, for which the name Luteimonas fraxinea sp. nov. is proposed, with strain D4P002(T) (=DSM 113273(T) = LMG 32455(T)) as the type strain. A functional analysis of the genome revealed features particularly associated with attachment, biofilm production and motility, indicating the ability of D4P002T to effectively colonise ash leaves. In nursery trials, ash seedlings inoculated with this strain showed suppression of the pathogen over a period of three years. This effect was accompanied by a significant shift in the bacterial microbiome of the plants. Altogether, the exclusive occurrence in the microbiome of tolerant ash trees, the genetic background and the results of the inoculation experiment suggest that strain D4P002(T) may suppress the penetration and spreading of H. fraxineus in or on ash leaves via colonisation resistance or trigger a priming effect of plant defences against the pathogen. (C) 2022 Elsevier GmbH. All rights reserved.

Automated summary

A group of isolates of the genus Luteimonas, identified as a specific component of the healthy core microbiome of Fraxinus excelsior in forest districts with a high infection rate of H. fraxineus, showed clear differentiation from related Luteimonas species based on phylogenomic and phenotypic analyses. The isolates, identified as a novel species Luteimonas fraxinea sp. nov., exhibited features associated with attachment, biofilm production, and motility, indicating their ability to colonize ash leaves effectively. In nursery trials, inoculation with the novel species suppressed the pathogen and resulted in significant changes in the bacterial microbiome of the plants. These findings suggest that the novel species may suppress the penetration and spreading of H. fraxineus in or on ash leaves through colonization resistance or priming effect of plant defenses against the pathogen.