Identification, synthesis and regulatory function of the N-acylated homoserine lactone signals produced by Pseudomonas chlororaphis HT66

Abstract

BackgroundPseudomonas chlororaphis HT66 isolated from the rice rhizosphere is an important plant growth-promoting rhizobacteria that produce phenazine-1-carboxamide (PCN) in high yield. Phenazine production is regulated by a quorum sensing (QS) system that involves the N-acylated homoserine lactones (AHLs)—a prevalent type of QS molecule.ResultsThree QS signals were detected by thin layer chromatography (TLC) and high-performance liquid chromatography–mass spectrometry (HPLC–MS/MS), which identified to be N-(3-hydroxy hexanoyl)-l-homoserine lactone (3-OH-C6-HSL), N-(3-hydroxy octanoyl)-l-homoserine lactone (3-OH-C8-HSL) and N-(3-hydroxy decanoyl)-l-homoserine lactone (3-OH-C10-HSL). The signal types and methods of synthesis were different from that in other phenazine-producing Pseudomonas strains. By non-scar deletion and heterologous expression techniques, the biosynthesis of the AHL-signals was confirmed to be only catalyzed by PhzI, while other AHLs synthases i.e., CsaI and HdtS were not involved in strain HT66. In comparison to wild-type HT66, PCN production was 2.3-folds improved by over-expression of phzI, however, phzI or phzR mutant did not produce PCN. The cell growth of HT66∆phzI mutant was significantly decreased, and the biofilm formation in phzI or phzR inactivated strains of HT66 decreased to various extents.ConclusionIn conclusion, the results demonstrate that PhzI–PhzR system plays a critical role in numerous biological processes including phenazine production.