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

<i>Streptomyces scabies</i> is the first identified common scab-causing pathogen, producing thaxtomin A as a principal virulence factor. Interestingly, <i>S. scabies</i> 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 <i>S. scabies</i> acts as a dual-function regulator, activating thaxtomin biosynthesis while repressing IAA biosynthesis, both derived from l-tryptophan (Trp). BldDss directly bound to the <i>txtR</i> and <i>txtE</i> promoters to promote thaxtomin biosynthesis. <i>bldDss</i> deletion led to knockdown-virulence of <i>S. scabies</i> on potato tuber slices and radish seedlings, and unexpectedly promoted lateral root development in <i>Arabidopsis thaliana</i>. In <i>S. scabies</i>, the <i>iaaH</i>-<i>iaaM</i> 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 <i>iaaH</i>-<i>iaaM</i> promoter. This study demonstrates BldD's dual regulatory function in a plant-pathogenic <i>Streptomyces</i> species for the first time.