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Evidence That Thaxtomin C Is a Pathogenicity Determinant of Streptomyces ipomoeae, the Causative Agent of Streptomyces Soil Rot Disease of Sweet Potato

March 2012 , Volume 25 , Number  3
Pages  393 - 401

Dongli Guan,1 Brenda L. Grau,1 Christopher A. Clark,2 Carol M. Taylor,3 Rosemary Loria,4 and Gregg S. Pettis1,2

1Department of Biological Sciences, Louisiana State University, Baton Rouge, LA 70803, U.S.A.; 2Department of Plant Pathology and Crop Physiology, Louisiana State University Agricultural Center, Louisiana Agricultural Experiment Station, Baton Rouge, LA 70803, U.S.A; 3Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803, U.S.A.; 4Plant Pathology Department, University of Florida, Gainesville, FL 32611, U.S.A.

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Accepted 26 October 2011.

Streptomyces ipomoeae is the causal agent of Streptomyces soil rot of sweet potato, a disease marked by highly necrotic destruction of adventitious roots, including the development of necrotic lesions on the fleshy storage roots. Streptomyces potato scab pathogens produce a phytotoxin (thaxtomin A) that appears to facilitate their entrance into host plants. S. ipomoeae produces a less-modified thaxtomin derivative (thaxtomin C) whose role in pathogenicity has not been examined. Here, we cloned and sequenced the thaxtomin gene cluster (txt) of S. ipomoeae, and we then constructed targeted txt mutants that no longer produced thaxtomin C. The mutants were unable to penetrate intact adventitious roots but still caused necrosis on storage-root tissue. These results, taken in context with previous histopathological study of S. ipomoeae infection, suggest that thaxtomin C plays an essential role in inter- and intracellular penetration of adventitious sweet potato roots by S. ipomoeae. Once inside the plant host, the pathogen uses one or more yet-to-be-determined factors to necrotize root tissue, including that of any storage roots it encounters.

© 2012 The American Phytopathological Society