BldDss Modulates the Functional Role of Streptomyces scabies through Directly Regulating the Biosynthesis of Phytotoxin Thaxtomin and Phytohormone Indole-3-Acetic Acid.

Streptomyces scabies is the first identified common scab-causing pathogen, producing thaxtomin A as a principal virulence factor. Interestingly, S. scabies also produces the phytohormone indole-3-acetic acid (IAA), but how it fine-tunes the production of phytotoxins and phytohormones remains poorly understood. Here, we found that BldDss from S. scabies acts as a dual-function regulator, activating thaxtomin biosynthesis while repressing IAA biosynthesis, both derived from l-tryptophan (Trp). BldDss directly bound to the txtR and txtE promoters to promote thaxtomin biosynthesis. bldDss deletion led to knockdown-virulence of S. scabies on potato tuber slices and radish seedlings, and unexpectedly promoted lateral root development in Arabidopsis thaliana. In S. scabies, the iaaH-iaaM operon was mainly responsible for IAA biosynthesis, and such production was largely dependent on Trp. Furthermore, BldDss repressed IAA biosynthesis by directly binding to the iaaH-iaaM promoter. This study demonstrates BldD's dual regulatory function in a plant-pathogenic Streptomyces species for the first time.