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 symptom s on seedlings included necrosis of roots and thickening of roots and shoots. S. scabies strain 87-22 was more virulent on seedlings than wa s strain 84-34. A nonpathogen, S. lividans strain TK24, did not consis tently reduce the seedling height of any plant species and did not pro duce growth abnormalities or necrosis. Both supernatants of cultures o f S. scabies strains and thawtomin A, a phytotoxin produced by S. scab ies, reproduced the symptoms of pathogenic strains on seedlings. Conce ntrations of 50 to 100 mu M thaxtomin A greatly reduced the total leng th of radish seedlings and caused tissue necrosis and death, whereas c oncentrations of 10 to 25 mu M caused shoot and root stunting and thic kening. Cross sections of roots and shoots demonstrated that tissue th ickening was due to cell hypertrophy, rather than cell hyperplasia. Th axtomin A is a broad-spectrum phytotoxin and may be responsible for pl ant pathogenicity in S. scabies.