Quorum quenching strategies of endophytic Bacillus thuringiensis KMCL07 against soft rot pathogen Pectobacterium carotovorum subsp. carotovorum

Abstract

Phytopathogens are global threats to agriculture, causing substantial economic losses and decreased crop productivity. Developing a control strategy without emerging resistance or creating environmental and health hazards is necessary. The majority of potential pathogens of crops are gram-negative and they communicate through Acyl homoserine lactones (AHLs)-mediated quorum sensing (QS) systems to establish their pathogenicity. By synthesizing small signal molecules, they collectively respond, regulate the expression of virulence factors, biofilm development, secondary metabolite production, and interactions with the host and other microbes in a population-density-dependent manner. Targeting QS mechanisms has been put forward as an attractive approach for conventional infection control.

The quorum quenching endophytic Bacillus thuringiensis strain KMCL07 cell free lysate (CFL) was used to attenuate the virulence of the soft-rot Pectobacterium carotovorum subsp. carotovorum (Pcc) by targeting its QS system. The CFL inhibition ability of Pcc on the AHL signal molecules were tested using a biosensor strain (Chromobacterium subtsugae), which showed a significant (p < 0.001) reduction in the production of AHL signalling molecules without inhibiting Pcc growth. Pcc pathogenicity is related to the expression of various virulence traits like the secretion of extracellular enzymes, motility, and biofilm. The test results showed a significant degree (p < 0.0001) of inhibition in the production of virulence-causing extracellular enzymes (Pel, Cel, and Prt) when Pcc was treated with CFL. Soft rot in-vitro assays revealed that CFL, irrespective of different families, showed a significant level (p ≤ 0.0001) of reduction in disease severity and effectively reduced tissue maceration under different temperature ranges (25°, 30°, and 40 °C). LC-MS analysis confirmed the hydrolytic degradation of QS signalling molecules (3-oxo-C6-HSL and 3-oxo-C8-HSL) by CFL indicating the presence of lactonase enzyme activity. These results suggest that CFL can degrade a wide range of AHL molecules, and control soft rot in a wide variety of hosts and temperatures without affecting the host. Applying cell free lysates (CFLs) from endophytic bacteria to control soft rot pathogens can be an environmentally friendly way to improve plant health. CFLs protect plants by preventing the establishment of pathogenic organisms.