Populus is a commercially and ecologically important tree species and a potential biofuel feedstock source. Populus commonly occur in riparian areas throughout North America where they are considered a keystone species. Populus have become an important model host system to study plant-microbe interactions due to their amenability to laboratory and greenhouse experimental manipulation, as well as molecular biology tools enabled from genome sequencing. We conducted an experiment using 12 Populusgenotype clones that vary in higher-order salicylate concentration and composition (e.g., salicortin, salicin, tremuloidin, populin) to better understand how genotype, chemotype and phenotype of the host plant influence microbiome community assembly and community composition. To assess the relative importance of host genotype vs. soil conditions and origin, we planted 5 replicate cuttings per genotype (N = 120) in 2 soil types (2:1 sterile sand:soil inoculum) collected from two Oregon locations (Corvallis and Clatskanie). Root tissue and rhizosphere soils were collected during the destructive harvest at 4 months after transplant. Root tissue is being analyzed for salicylate metabolites, ectomycorrhizal root tip colonization, and biomass. Root endospheres and rhizosphere soils will be analyzed for bacterial, archaeal, and fungal community composition as determined by rRNA gene targeted amplicon-based Illumina MiSeq sequencing.
Across plant genotype, host salicylate concentration varied from 1221 – 10610 ug/g fresh weight (FW), tremuloidin varied from 17 – 225 ug/g FW, and populin ranged from 0 – 8.9 ug/g FW. Populus leaf chlorophyll content, leaf growth (number of new leaves since transplant), and net photosynthetic rate differed across genotypes and soil type (P < 0.01). Ectomycorrhizal colonization of root tips differed between genotypes (P = 0.01), but not soil type. Specifically, the genotype with the lowest root salicortin and salicin concentration had significantly greater ectomycorrhizal colonization (10 ± 1.5% colonized) compared to three other genotypes with greater salicylate metabolite concentrations. Preliminary analyses of 16S rRNA gene-based communities indicate that bacterial and archaeal community composition of rhizosphere soils are significantly influenced by soil type (Corvallis vs. Clatskanie soils; R2 = 0.34, P = 0.001), but not Populus genotype. Analyses are ongoing and additional data will investigate root endosphere microbial community composition via 16S rRNA and fungal ITS2 rRNA gene amplicon sequencing and population sizes via qPCR detection.