BglC of Streptomyces scabiei is a scopolin-hydrolyzing β-glucosidase interfering with the host defense mechanism

Abstract

The beta-glucosidase BglC fulfills multiple functions in both primary metabolism and induction of pathogenicity of Streptomyces scabiei, the causative agent of the common scab disease of root and tuber crops. Indeed, this enzyme hydrolyzes cellobiose and cellotriose to directly feed glycolysis with glucose, but also modifies the intracellular concentration of these cello-oligosaccharides which are the virulence elicitors. The inactivation of bglC also led to unexpected phenotypes such as the constitutive overproduction of thaxtomin A, the main virulence determinant of S. scabiei. In this work we revealed a new target substrate of BglC, the phytoalexin scopolin. Removal of the glucose moiety of scopolin generates scopoletin, a potent inhibitor of thaxtomin A production. The hydrolysis of scopolin by BglC presents substrate inhibition kinetics which contrasts with the typical Michaelis–Menten saturation curve previously observed for the degradation of its natural substrate cellobiose. Our work therefore reveals that BglC targets both cello-oligosaccharide elicitors emanating from the hosts of S. scabiei, and the scopolin phytoalexin generated by the host defense mechanisms, thereby occupying a key position to fine-tune the production of the main virulence determinant thaxtomin A.