Salicylic acid production occurs across a range of organisms and is an important signaling molecule in plants regulating many functions, including plant defense and abiotic stress responses. In the Salicaceae family, which includes Populus, the abundance and varieties of these compounds is greatly increased. Using black cottonwood, Populus trichocarpa, as a model organism to study root-microbial interactions, our study goal is to determine if salicylic acid and its metabolites directly select for specific microbial taxa due to group-specific physiological capabilities in salicylate mineralization or are inhibitory towards other exogenous soil microbiota. We conducted an experiment using 12 P. trichocarpa genotype clones that vary in higher-order salicylate concentration and composition (e.g., salicortin, salicin, tremuloidin, populin) to pinpoint the relative importance in plant chemotype versus extrinsic physicochemical factors influencing the rhizosphere microbiome. Five replicates per genotype (N=120) were grown in soils from two origins for 4 months and then harvested for root metabolomics (targeting salicylic acid related compounds) via gas chromatography/mass-spectrometry analyses, and amplicon-based sequencing of 16S and ITS rRNA genes to identify archaea, bacteria, and fungal communities in the rhizosphere. We used constrained ordinations to determine which salicylates predicted microbial community composition and further confirmed these effects by individual regression models for each OTU.
Results/Conclusions
Populin was the only higher-order salicylate which significantly differed among genotypes. However, mean salicylate production ranged from 3,376-9,450 ug g-1 FW-1 across all genotypes. Soil origin was the most influential driver in archaeal, bacterial (R2 = 0.52) and fungal (R2 = 0.40) community differentiation, whereas genotype was secondarily influential (R2 = 0.09). Specific metabolites correlated with diversity and taxon distributions. Tremuloidin and populin, unique salicylate derivatives to poplar, altered bacterial diversity in the rhizosphere and were the main predictors of community composition at the OTU-level. Majority of bacterial OTU abundances which varied across tremuloidin and populin gradients primarily had positive interactions (i.e., positive regression slopes) thus indicating salicylates utilization as potential mineralizable substrates versus compound inhibition; these OTUs were classified across a range of bacterial phyla (e.g., Actinobacteria, Bacteroidetes, Alpha-, Betaproteobacteria). Fungal diversity was nonresponsive to salicylates, whereas community composition varied across a salicortin and salicylic acid gradient, although further confirmation of OTU differences was not detected by individual regression models. These data suggest that metabolite production is a selective pressure for root-associated microbiota likely due to facilitative effects for bacterial consortia whereas fungi are less constrained by these specific plant metabolites.