The structure of the culturable root bacterial endophyte community of Nicotiana attenuata is organized by soil composition and host plant ethylene production and perception

P>A plant's bacterial endophyte community is thought to be recruited from the rhizosphere, but how this recruitment is influenced by the plant's phytohormone signaling is unknown. Ethylene regulates plant-microbe interactions; here, we assess the role of ethylene in the recruitment of culturable endophytic bacteria from native soils. We grew wild-type Nicotiana attenuata plants and isogenic transformed plants deficient in ethylene biosynthesis (ir-aco1) or perception (35S-etr1) in four native soils and quantified the extent of culturable bacterial endophyte colonization (by plate counting) and diversity (by amplified rDNA restriction analysis and 16S rDNA sequencing). The endophyte community composition was influenced by soil type and ethylene signaling. Plants grown in organic (vs mineral) soils harbored a more diverse community and plants impaired in ethylene homeostasis harbored a less diverse community than wild-type plants. Wild-type and ethylene signaling-impaired plants fostered distinct bacteria in addition to common ones. In vitro re-colonization by common and genotype-specific isolates demonstrated the specificity of some associations and the susceptibility of 35S-etr1 seedlings to all tested bacterial isolates, suggesting an active process of colonization driven by plant- and microbe-specific genes. We propose that soil composition and ethylene homeostasis play central roles in structuring the bacterial endophyte community in N. attenuata roots.