Oral Technical Session: Bacterial-Host Interactions

4-O

Additional components involved in thaxtomin induction and regulation by Streptomyces scabies.
I. FRANCIS (1), Y. Zhang (1), M. J. Kim (1), R. Loria (1)
(1) University of Florida, Gainesville, FL, U.S.A.

Streptomyces scabies is the major scab-causing pathogen on several important root and tuber crops and is responsible for considerable annual losses worldwide. A family of nitrated dipeptidic phytotxins called thaxtomins have been identified as the major virulence factors. Thaxtomin A, the predominant form, acts as a cellulose synthesis inhibitor in expanding plant tissue, therefore enabling the bacterium to infect living plants, a rare trait among the commonly saprophytic streptomycetes. Toxin production is induced by cellobiose, the smallest subunit of cellulose, and also a well-known inducer of cellulases and other cell wall-degrading enzymes. In our search to elucidate the mechanisms used by the pathogen to detect a nearby growing plant and subsequently respond by inducing thaxtomin production, we identified three ABC-transporters involved in cellobiose uptake, of which one appeared to be specific for scab-causing streptomycetes. Several cell wall-degrading enzymes seemed to be upregulated by cellobiose, among which a cluster containing a gene encoding a putative expansin-like protein, normally found in plant cell walls where it loosens linkages between cellulose microfibrils during cell expansion, and a reported virulence factor of Clavibacter michiganensis. Furthermore, a cluster containing a metallophosphatase appears to be part of the regulatory network leading to thaxtomin production and therefore virulence.