Effect of copper on the quorum sensing system of Pseudomonas capeferrum WCS358

Abstract

In agriculture, copper has largely been utilized as sulphate or oxychloride salts for the control of phytopathogen fungi. However, due to its non degradability, copper tends to accumulate in soils resulting in negative effects for plants, animals or microorganisms. Although copper is one of the most studied heavy metals, its effect on microbial interactions mediated by Quorum Sensing between rhizosphere microorganisms has not been evaluated up to date. Quorum sensing (QS) systems are cell-to-cell signaling mechanisms that control the microbial physiology in response to extracellular small signal molecules. Pseudomonas capeferrum WCS358 is a plant growth-promoting rhizobacterium. The QS system of P. capeferrum WCS358 is regulated by the signals N-acyl homoserine lactones and it is composed of the AHL synthase PpuI, the AHL receptor PpuR, the regulator RsaL and a second AHL receptor, PpoR. The aim of the present work is the evaluation of the effect of copper on the expression of ppuI, ppoR and rsaL. Transcriptional fusion plasmids for the mentioned gene promoters based on the pMP220 promoter probe vector were introduced independently into WCS358. The resulting strains were cultivated in solid and liquid medium with and without copper and β-galactosidase activity was mesured. The activities of the ppuI and ppoR promoters were considerably lower in the presence of copper in liquid or solid medium, while the promoter level of rsaL was lower in the presence of copper in liquid medium but it remained constant in solid medium. Results presented in this work show that copper modifies the expression of ppuI, ppoR and rsaL, suggesting a concomitant modification of the phenotypes controlled by the QS system in P. capeferrum WCS358.