The Phytotoxin Thaxtomin A Is the Primary Virulence Determinant for Scab Disease of Beet, Carrot, and Radish Caused by <i>Streptomyces scabiei</i>

Abstract

Several species of <i>Streptomyces</i> cause common scab, a major disease of potato, primarily through the phytotoxic effects of the phytotoxin thaxtomin A. Several phytopathogenic <i>Streptomyces</i> species have also been implicated as the causative agents of scab diseases of taproot crops including beet, carrot, radish, parsnip, and turnip. But the molecular mechanisms employed by <i>Streptomyces</i> to infect these crops is unknown. In this work, we tested the hypothesis that thaxtomin A biosynthesis is also necessary for <i>Streptomyces</i>-caused scab of beet, carrot, radish, and turnip. Thaxtomin A induced plant stunting and cell death of all four of these species. <i>Streptomyces</i> mutants in which the transcriptional regulator of thaxtomin A biosynthesis is disrupted were nonvirulent on all four crops, and complementation of the transcriptional regulator rescued thaxtomin A biosynthesis and plant pathogenicity to wild-type levels. These results demonstrate that thaxtomin A is the primary virulence determinant of scab disease of these other crops.