Science, doi:10.1126/science.aaa8764

Credit: SCIENCE

The microbial communities existing among plants contain not only pathogenic species but also potentially beneficial ones. How plants signal to recruit beneficial species has not been clarified by studies that have examined the microbes present adjacent to plant roots (rhizosphere) and within the roots (endophytic compartment). Lebeis et al. reasoned that phytohormones that promote plant defense to pathogens could be at play, so they compared the bacterial root microbiomes present with wild-type Arabidopsis thaliana to those present with mutants that cannot produce salicylic acid (SA), jasmonic acid or ethylene or that lack the ability to respond to these phytohormones. Endophytic bacterial communities were less diverse, both at the level of individual species and at higher taxonomic orders, than were those from the rhizosphere, and they had altered compositions (both in number and identity) in the presence of the mutant plants. Comparing the composition patterns across the different mutants, the authors concluded that defense phytohormones, especially SA, modulate taxonomic groups of bacteria at the family level in the root. SA does so by affecting the whole population at the family level, not just a few dominant strains. Lebeis et al. proposed that SA is required to modulate the composition of a normal root microbiota. Similar conclusions were supported by the next set of experiments, in which the authors exposed sterile roots of wild-type and mutant plants to a synthetic community (SymCom) of 38 documented bacterial strains across four phyla, and saw increased abundance of endophytic bacterial isolates after eight weeks. Again, the mutants deficient in SA differed from the wild-type plants. Application of exogenous SA also altered the compositions of the bacterial communities. These results suggest that SA influences the structure of the microbial community on the plant root perhaps through a direct recruitment, which could prove useful for agricultural applications.