Probable involvement of thaxtomin A in pathogenicity of Streptomyces scabies on seedlings

Inoculation with Streptomyces scabies significantly (P 0.05) reduced shoot height of seedlings in 11 of 14 plant species tested. Reduction in shoot height was 53 to 89% in crucifers and 35 to 67% in legumes, whereas effects on monocot seedlings were more variable. Other symptoms on seedlings included necrosis of roots and thickening of roots and shoots. S. scabies strain 87-22 was more virulent on seedlings than was strain 84-34. A nonpathogen, S. lividans strain TK24, did not consistently reduce the seedling height of any plant species and did not produce growth abnormalities or necrosis. Both supernatants of cultures of S. scabies strains and thaxtomin A, a phytotoxin produced by S. scabies, reproduced the symptoms of pathogenic strains on seedlings. Concentrations of 50 to 100 micromolars thaxtomin A greatly reduced the total length of radish seedlings and caused tissue necrosis and death, whereas concentrations of 10 to 25 micromolars caused shoot and root stunting and thickening. Cross sections of roots and shoots demonstrated that tissue thickening was due to cell hypertrophy, rather than cell hyperplasia. Thaxtomin A is a broad-spectrum phytotoxin and may be responsible for plant pathogenicity in S. scabies