Factors Affecting Thaxtomin A Production by Streptomyces scabies in Egypt

Abstract

wenty isolates of Streptomyces scabies were isolated from scab lesions on potato tubers collected from three different regions in Behera Governorate during the 2009-2010 growing season. The isolates of S. scabies recovered were totally monomorphic for the morphological and the biochemical characteristics investigated. All isolates were non motile, gram positive, exhibited grey colonies, spiral spore chain and were melanin producers. Meantime the isolates showed positive reaction for catalase activity, starch hydrolysis, casein hydrolysis and gelatine liquefaction. Also, all isolates were tolerant and exhibited growth at NaCl 5, 6 & 7% and at 20, 25 & 30°C at pH 7 & 9. None of the tested isolates were able to grow at pH 5. More variations, however, were revealed for the pathogenicity of twenty isolates. Five out of twenty investigated isolates were found to be highly virulent. The rest of tested isolates were moderately virulent. Spunta was the most tolerant potato cultivar, while cvs. Mondial and Hermis were more susceptible. The highly virulent S. scabies (isolate ST5) was a highly producer of Thaxtomin A, while the moderately virulent isolate (ST10) was poorly toxin producer (1.26 mg/ml medium). The optimal conditions for Thaxtomin A production were 28ºC, pH 8 and 1% cellobiose in medium. The amino acids, i.e. tryptophan, phenylalanine and tyrosine exhibited an in vitro inhibitory effect on the toxin production in the amended oatmeal bran broth medium with highest inhibitory effect was recorded for tryptophan.

The Role of Thaxtomin A and Necrosis Protein in Virulence of Streptomyces scabies that Cause Potato Common Scab Disease

“…These results were in harmony with earlier studies that found variations in virulence among Streptomyces spp., notably S. scabies isolates (Wanner, 2004;Dees et al, 2013;Karagöz and Kotan, 2017;Shi et al, 2019). In former research conducted in Elbeheira Governorate of Egypt, El-Sheikh et al (2012) identified 20 isolates of S. scabies with five that were highly virulent and 15 were moderately virulent based on scab index on potato mini tubers. Also, Hussein et al (2019) isolated 12 isolates of S. scabies that differed in their virulence on potato plants.…”

“…Inocula suspensions were prepared at approximately 1 x 10 7 spore/ ml tryptone yeast broth medium, and 50 ml of inoculum of each isolate were mixed with 1.5 L of autoclaved vermiculite (Balcony garden, Egypt). This mixture was added to 20 potato mini tubers in small plastic bags, incubated for 14 days, and shaken many times during the incubation period (El-Sheikh et al, 2012). Clean potato mini tubers were served as control.…”

“…When the scab severity index was < 50%, the isolate was classified as weakly virulent, moderately virulent when the scab severity index was < 70% -≥ 50%, and highly virulent when the scab severity index was ≥ 70% (El-Sheikh et al, 2012).…”

“…are pathogenic and capable of infecting the underground parts of numerous plant species such as potato, sweet potato, beet, turnip, carrot, radish, and peanut (Wanner and Kirk, 2015;Li et al, 2019a). Streptomyces scabies is the most prevalent species and causes potato common scab, a severe and economically significant disease widely spread in potato growing regions in Egypt (El-Sayed et al, 2001;El-Sheikh, 2010;El-Sheikh et al, 2012;Abd El-Rahman et al, 2018).…”

“…Several analogues of thaxtomins were identified in Streptomyces spp., with thaxtomin A the predominant in S. scabies (Loria et al, 2008;King and Calhoun, 2009;Liu et al, 2021). Pathogenicity of S. scabies strains was positively linked with the production of thaxtomin A phytotoxin (El-Sayed et al, 2000;El-Sheikh, 2010;El-Sheikh et al, 2012). The crude extracts from nonpathogenic S. scabies strains did not release the toxin and did not affect the potato slices, while the crude extracts from pathogenic S. scabies strains induced necrotic lesions (El-Sheikh, 2010).…”