Study of molecular mechanisms associated with the induction of phytotoxins in Streptomyces scabies

Abstract

The members of the Streptomyces genus are mainly known for their high potential to produce secondary metabolites such as antibiotics, anti-cancer drugs, volatile organic compounds, herbicides. Mostly saprophytic, few exceptions have a phytopathogenic lifestyle, with S. scabies as a model species. Research on the pathogenicity of this bacteria - the predominant causal agent of the common scab (CS) disease – has begun more than 20 years ago. The phytotoxin thaxtomin A is the main virulence factor directly related to its pathogenic lifestyle, however many molecular mechanisms still remain misunderstood and ignored. Based on computational regulon prediction and proteomics, novel gene/protein candidates involved in the development of the virulent behaviour of S. scabies have been identified. In particular, a new hypothetical signalling pathway suggesting a tight link between the cello-oligosaccharides transport and the access to the host reservoir of starch has been proposed. The principal aim of this Master thesis was to investigate the role of two actors supposed to be involved in this pathway - the MalR regulator and the cytoplasmic component of the AfsQ1-AfsQ2 TCS -, evaluating the effects of their deletion through the PCR-targeting strategy. During this Master thesis we showed that these two important regulators certainly participate in the pathogenic lifestyle of S. scabies. The absence of MalR prevents S. scabies from perceiving the cellobiose as a signal to trigger virulence, while afsQ1 deletion resulted in increased toxin production rates, in a significant impairment of A. thaliana growth and in more severe pitting and necrosis of potato tuber slices. A further objective of this work aimed to generate new strains of S. scabies with improved thaxtomin production yields. Indeed, the properties of this phytotoxin made it a prime candidate as bioherbicide, however, nowadays the production costs are still too high for a large-scale application. Earlier works have shown that the mutation of either cebR or bglC genes resulted in an overproduction of thaxtomin A, therefore we have investigated the possibility that combining the deletion of both genes would turn in even higher production rates. The new generated strains are capable of producing significant toxin rates, however, although significant, did not exceed those of the single mutants.