Plant species influence density and inhibitory phenotypes of soil Streptomyces

Background/Question/Methods

Establishment of native perennial prairie species on previously cultivated land is often poor. One hypothesis suggests that plant establishment and productivity are influenced significantly by microbially-mediated plant-soil feedbacks, specifically by the presence of plant pathogens, mutualists and pathogen antagonists. The effects of prior plant host on microbial antagonist communities and their capacities to inhibit plant pathogens are, however, poorly understood. We hypothesize that plant species vary in their capacities to support pathogen antagonists in soil, resulting in differences in the density of inhibitory microbes, the frequencies of inhibitors, and the intensity of pathogen inhibition.  To assess the effects of different plant species on soil microbial communities, 14 plant treatments, including native perennials, crops and invasive species were grown in a field with a history of corn-soy rotation (n=4 replicates per treatment). Bacterial and Streptomyces densities and pathogen inhibitory activity against three common soil-borne plant pathogens, Fusarium oxysporum, Rhizoctonia solani and Streptomyces scabies, were assessed for every plot after one growing season. Relationships among microbial community characteristics, along with corresponding soil edaphic and plant productivity measurements provide insight into the possible mechanisms by which plants influence soil microbial communities.

Results/Conclusions

Streptomyces densities differed significantly among plant hosts, ranging from 1.71 x 10⁶ to 2.46 x 10⁶ CFU per gram of soil (mean 2.06 x 10⁶ CFU/g). Both the density and proportion of Streptomyces isolates able to inhibit growth of soil-borne pathogens varied significantly among plant hosts.  Native prairie polycultures showed the highest densities of inhibitors.  No differences were observed among plant hosts in inhibition zone sizes, a measure of the strength of pathogen inhibition.  Inhibition zone sizes increased with the proportion of inhibitory isolates (R² = 0.49, p < .001).  Thus, rhizosphere communities that supported higher frequencies of inhibitors also supported Streptomyces that were more effective at inhibiting pathogens.  In summary, individual plant species differ significantly in their effects on soil microbial communities, and specifically their effects on pathogen-inhibitory Streptomyces populations.  Such variation is correlated with significant differences in root disease in agricultural settings, suggesting potential for previous plant host to alter future plant establishment.