Interactions with microorganisms are important for plant wellness and productivity. Assembly and maintenance of rhizosphere communities are primarily driven by root exudates that provide carbon and nutrients to soil bacteria. Soil harbors a sizeable pool of inactive (dormant) bacteria that can resuscitate under specific environmental conditions and provide a reservoir of metabolic diversity. Thus, resuscitation of bacteria and their recruitment to the rhizosphere may be important for plant responses to stress. We hypothesized that phytohormones resuscitate dormant bacterial taxa and signal their recruitment to the rhizosphere. To begin to test this hypothesis, we first examined changes in growth, resuscitation, and behavior (motility and biofilm production) of several environmental bacterial strains when exposed to the stress phytohormones salicylic acid (SA), jasmonic acid (JA), abscisic acid (ABA), and the growth phytohormone auxin (IAA). We tested strains that associate with plants (both pathogenic and beneficial), and compared them to strains that are common in soils but not known to associate with plants. We also conducted an in situ experiment in which we added SA to rhizosphere soil and measured changes in microbiome activity using 16S rRNA (transcript) sequencing. In addition, we are using mass spectrometry to observe the root exudate chemical composition under different hormone-induced plant stresses and over time.
Results/Conclusions
After exposure to phytohormones, many plant-associated bacteria had a relative increase in growth rate and biomass production, and could also be resuscitated from a state of starvation-induced dormancy. There also were changes in motility and biofilm production in some of bacterial strains when they were exposed to phytohormones. In general, growth and resuscitation responses were very consistent within and across strains to SA, JA, and IAA, but responses to ABA were more variable and strain-specific. We found that phytohormone treatment to rhizosphere soil stimulated activity of bacteria commonly associated with plants, including as Burkholderia and Bacillus taxa. Overall, our results suggest that phytohormones produced by plants under biotic and abiotic stress can resuscitate bacteria that are potentially beneficial. Furthermore, these observations point to an important role of dormant soil bacteria as a reservoir of functions significant for plant wellness, and an evolutionarily conserved relationship between soil bacteria and plants that is mediated through phytohormones.