Abstract
We are interested in the root microbiome of the fast-growing Eastern cottonwood tree, Populus
25 deltoides. There is a large bank of bacterial isolates from P. deltoides and there are 44 draft
26 genomes of bacterial endophyte and rhizosphere isolates. As a first step in efforts to understand
27 the roles of bacterial communication and plant-bacterial signaling in P. deltoides we focused on
28 the prevalence of acyl-homoserine lactone (AHL) quorum sensing signal production and
29 reception in members of the P. deltoides microbiome. We screened 129 bacterial isolates for
30 AHL production using a broad-spectrum bioassay that responds to many but not all AHLs, and
31 we queried the available genome sequences of microbiome isolates for homologs of AHL
32 synthase and receptor genes. AHL signal production was detected in 40% of 129 strains tested.
33 Positive isolates included α-, β- and γ-Proteobacteria. Members of the luxI family of AHL
34 synthases were identified in 18 of 39 Proteobacteria genomes including genomes of some
35 isolates that tested negative in the bioassay. Members of the luxR family of transcription factors,
36 that include AHL-responsive factors, were more abundant than luxI homologs. There were 72 in
37 the 39 Proteobacteria genomes. Some of the luxR homologs appear to be members of a
38 subfamily of LuxRs that respond to as yet unknown plant signals rather than bacterial AHLs.
39 Apparently, there is a substantial capacity for AHL cell-to-cell communication in Proteobacteria
40 of the P. deltoides microbiota and there are also Proteobacteria with LuxR homologs of the type
41 hypothesized to respond to plant signals or cues.
