Root-Associated Streptomyces Isolates Harboring melC Genes Demonstrate Enhanced Plant Colonization

Streptomycetaceae assemble into the internal, root endophytic compartment of a wide variety of plants grown in soils worldwide, suggesting their ability to survive during root microbiome assembly. A previous study found that among four nonpathogenic, root-isolated Streptomyces strains (303, 299, CL18, and 136), only 303 and 299 colonized endophytic root tissue of the majority of Arabidopsis thaliana roots when inoculated with 34 other bacterial isolates. Here we demonstrate that 303 and 299 also colonize significantly more in singly inoculated A. thaliana seedlings. The genomes of melanin-producing 303 and 299 each contain two copies of the gene encoding tyrosinase (melC2 and melD2), an enzyme necessary for melanin biosynthesis in Streptomyces. These genes were not found in the genomes of 136 or CL18. Tyrosinase activity was detected in 303 and 299 whole cell and supernatant protein extracts, suggesting functional intracellular and extracellular enzymes. . Because tyrosinase oxidizes phenolic compounds and Streptomyces colonization of A. thaliana appears to be influenced by the phenolic compound salicylic acid (SA), we measured direct sensitivity of Streptomyces isolates to the phenolic compounds catechol, ferulic acid (FA), and SA in vitro. While both 303 and 299 showed higher numbers of surviving colonies than CL18 and 136 in the presence of catechol, only 303 demonstrated a higher number of surviving colonies when isolates were challenges with FA and SA. Finally, when seedlings were singly inoculated with a collection of related plant-associated Streptomyces isolates, colonization was significantly higher in isolates possessing two tyrosinase gene copies than isolates with zero or one gene copy. Overall, we describe a connection between microbial tyrosinase activity and increased seedling colonization of nonpathogenic Streptomyces isolates in A. thaliana. We propose tyrosinase activity in Streptomyces partially protects against harmful plant-produced phenolic compounds as they transition into an endophytic lifestyle.