Cello-oligosaccharides released from host plants induce pathogenicity in scab-causing Streptomyces species

Thaxtomin, a phytotoxic dipeptide that inhibits cellulose synthesis in expanding plant cells, is a pathogenicity determinant in scab-causing Streptomyces species. Cellobiose and cellotriose, the smallest subunits of cellulose, stimulated thaxtomin production in a defined medium, while other oligosaccharides did not. Cellobiose upregulated transcription of thaxtomin biosynthetic genes. Streptomyces scabies, Streptomyces acidiscabies, and Streptomyces turgidiscabies did not hydrolyze cellulose, suggesting that these cello-oligosaccharides are plant-derived. Cellotriose was released from rapidly growing plant seedlings growing in vitro. These data support a model in which scab-causing pathogens upregulate thaxtomin production in response to cellotriose released from thaxtomin-sensitive plant tissue.