Disentangling the effects of tree species, soil nutrients, and site on pathogen-suppressive Streptomyces in a tropical dry forest

Background/Question/Methods:

Antibiotic-producing soil bacteria are an underexplored microbial functional group with the potential to influence plant community composition. Actinobacteria in the genus Streptomyces produce diverse antibiotics and can influence plant fitness by inhibiting soil-borne pathogens and reducing disease. Streptomyces may moderate the effects of pathogens on plant communities, yet little is known about their distribution and ecology in nonmanaged ecosystems, especially tropical forests. Our objective is to characterize variation in pathogen inhibitory capacities among Streptomyces communities associated with five tree species in tropical dry forests of Costa Rica. Previous work demonstrated extensive variation in Streptomyces across the landscape and correlations between pathogen inhibition and soil nutrients at broad spatial scales. In the current study we sampled soil-borne Streptomyces communities from focal tree species in two sites (n = 5 trees/species/site) in order to determine the relative contributions of tree species and soil type to differences in pathogen inhibitory capacity. A culture-based functional assay was used to measure Streptomyces densities, inhibitor densities, and inhibition zone size. We addressed the questions: (1) Are Streptomyces communities host-differentiated in their capacities to suppress soil-borne pathogens? (2) Are tree species effects on Streptomyces communities consistent between sites that differ in soil fertility, texture, and parent material?

Results/Conclusions:

Streptomyces densities from soil collected beneath adult trees ranged from 4.00 x 10³ to 2.33 x 10⁶ colony forming units (CFUs) per gram soil. The proportion of pathogen-suppressive Streptomyces within each community ranged from less than 1% to over 50%, which suggests differences in the potential for disease suppression in these soils. There were significant effects of tree species (p = 0.017), site (p < 0.0001), and the species by site interaction (p < 0.0001) on the density of inhibitory Streptomyces among samples. These results suggest that adult trees may vary in the capacity to select for pathogen-suppressive Streptomyces communities, and that the degree of functional differentiation in host-associated Streptomyces is likely to depend on both tree species and soil type. Variation in the frequency and intensity of pathogen-suppressive Streptomyces reflects variation in the potential for disease suppression to develop in association with different tree species, which could have important implications for seedling recruitment and community composition in tropical forests.