title: The impact of the pathogen Rhizoctonia solani and its beneficial counterpart Bacillus amyloliquefaciens on the indigenous lettuce microbiome.
abstract: Lettuce belongs to the most commonly raw eaten food worldwide and its microbiome plays an important role for both human and plant health. Yet, little is known about the impact of potentially occurring pathogens and beneficial inoculants of the indigenous microorganisms associated with lettuce. To address this question we studied the impact of the phytopathogenic fungus Rhizoctonia solani and the biological control agent Bacillus amyloliquefaciens FZB42 on the indigenous rhizosphere and phyllosphere community of greenhouse-grown lettuce at two plant stages. The rhizosphere and phyllosphere gammaproteobacterial microbiomes of lettuce plants showed clear differences in their overall and core microbiome composition as well as in corresponding diversity indices. The rhizosphere was dominated by Xanthomonadaceae (48%) and Pseudomonadaceae (37%) with Rhodanobacter, Pseudoxanthomonas, Dokdonella, Luteimonas, Steroidobacter, Thermomonas as core inhabitants, while the dominating taxa associated to phyllosphere were Pseudomonadaceae (54%), Moraxellaceae (16%) and Enterobacteriaceae (25%) with Alkanindiges, Pantoea and a group of Enterobacteriaceae unclassified at genus level. The preferential occurrence of enterics in the phyllosphere was the most significant difference between both habitats. Additional enhancement of enterics on the phyllosphere was observed in bottom rot diseased lettuce plants, while Acinetobacter and Alkanindiges were identified as indicators of healthy plants. Interestingly, the microbial diversity was enhanced by treatment with both the pathogen, and the co-inoculated biological control agent. The highest impact and bacterial diversity was found by Rhizoctonia inoculation, but FZB42 lowered the impact of Rhizoctonia on the microbiome. This study shows that the indigenous microbiome shifts as a consequence to pathogen attack but FZB42 can compensate these effects, which supports their role as biocontrol agent and suggests a novel mode of action.
authors.affiliation: Institute of Environmental Biotechnology, Graz University of Technology Graz, Austria ; Institute of Plant Sciences, University of Graz Graz, Austria.
authors.forename: Armin
authors.initials: A
authors.lastname: Erlacher
authors.affiliation: Institute of Environmental Biotechnology, Graz University of Technology Graz, Austria ; Institute of Plant Sciences, University of Graz Graz, Austria.
authors.forename: Massimiliano
authors.initials: M
authors.lastname: Cardinale
authors.affiliation: Leibniz-Institute of Vegetable and Ornamental Crops Grossbeeren, Germany.
authors.forename: Rita
authors.initials: R
authors.lastname: Grosch
authors.affiliation: Institute of Plant Sciences, University of Graz Graz, Austria.
authors.forename: Martin
authors.initials: M
authors.lastname: Grube
authors.affiliation: Institute of Environmental Biotechnology, Graz University of Technology Graz, Austria.
authors.forename: Gabriele
authors.initials: G
authors.lastname: Berg
keywords: 16S rRNA gene pyrosequencing; Gammaproteobacteria; Lactuca sativa; lettuce microbiome; phyllosphere; rhizosphere; soil-borne pathogens