Effects of AttM lactonase on the pathogenicity of Streptomyces scabies

The biosynthesis of phytotoxin thaxtomin A (TXT) constitutes the major pathogenicity determinant in Streptomyces scabies, the most widely studied phytopathogen causing scab disease in potato and other root crops. It is recognized that S. scabies regulates its pathogenicity via γ-butyrolactone (GBL)-dependent quorum sensing (QS) signalling. AttM, from Agrobacterium tumefaciens C58 strain, has recently been proposed to have GBL-assimilative capacity. Here, we presented the introduction of A. tumefaciens-derived attM gene into S. scabies using the Escherichia coli-Streptomyces shuttle vector pIJ8600 via intergeneric conjugation, followed by the investigation of secondary metabolism (mycelium growth, TXT production and pathogenicity) in S. scabies attM exconjugants (S.s/attM) in comparison with their wild-type parent strain (S.s/WT). Among the resultant S.s/attM exconjugants, attM was found to be integrated into S. scabies chromosome as analysed by Southern blotting. Moreover, S.s/attM failed to evoke the disease symptoms in planta and displayed altered morphological differentiation in contrast to S.s/WT. The abolishment of TXT production in S.s/attM substantiated the loss of pathogenicity and also implied that attM, when constitutively expressed in S. scabies, could paralyse its GBL signalling pathway. Altogether, lactonase-coding gene attM would be useful in a quorum quenching strategy for plant protection via suppressing TXT production and pathogenicity of S. scabies.

This study provides an efficient means to introduce the lactonase gene attM from Agrobacterium tumefaciens into Streptomyces scabies for evaluating the role of γ-butyrolactone (GBL) in thaxtomin A production and pathogenicity, etc. Our results showed that pathogenicity was abrogated in attM-expressing S. scabies exconjugants. Although there are gene knockout approaches to inactivating GBL signalling and thus pathogenicity in S. scabies, they are not only time consuming due to refractory host but also possibly incomplete in view of gene redundancy. Our work is the first report for a kind of lactonase affecting pathogenicity and/or virulence of scab-causing Streptomyces species.