Streptomyces – from Basic Microbiology to Role as a Plant Pathogen

Abstract

Streptomycetes are spore-forming Gram-positive bacteria found in soil in large numbers world-wide. More than 600 Streptomyces species have been described. The complex developmental morphology and molecular genetics of Streptomyces is connected to production of a wealth of secondary products, including more than 60% of antibiotics in industrial and pharmaceutical use. Streptomycetes have important roles in soil ecology as decomposers, though specific roles in microbial community structure and plant health are poorly understood. Some species are used as biocontrol agents while others have specific associations with potatoes as endophytes, pathogens or as part of plant rhizosphere communities. Very few species are plant pathogenic (ca. 1%), causing common scab disease on underground tubers in potatoes and root diseases in a broad range of host plants. Several unique aspects of Streptomyces as a plant pathogen are that (a) there is a main dominant pathogenicity determinant (thaxtomin); (b) only the developing underground stems, stolons and tubers are susceptible to potato common scab (CS); (c) Streptomyces does not incite a plant defense response; and (d) CS is not easily managed. The best available control is the use of resistant potato cultivars and there is wide variation in resistance (tolerance) among potato cultivars, though none is completely resistant. New molecular genetic tools, including the complete genome sequences of a number of plant pathogenic Streptomyces species and association mapping using the potato genome sequence, promise greater understanding of the genetics of CS tolerance and of regulation of thaxtomin production and contributory pathogenicity factors for better management of potato CS.