Phloretin, an Apple Phytoalexin, Affects the Virulence and Fitness of Pectobacterium brasiliense by Interfering With Quorum-Sensing

Abstract

The effects of phloretin a phytoalexin from apple, was tested on Pectobacterium brasiliense (Pb1692), an emerging soft-rot pathogen of potato. Exposure of Pb1692 to 0.2 mM phloretin a concentration that does not affect growth, or to 0.4 mM a 50% growth inhibiting concentration (50% MIC), reduced motility, biofilm formation, secretion of plant cell wall-degrading enzymes, production of acyl–homoserine lactone (AHL) signaling molecules and infection, phenotypes that are associated with bacterial population density-dependent system known as quorum sensing (QS). To analyze the effect of growth inhibition on QS, the activity of ciprofloxacin, an antibiotic that impairs cell division, was compared to that of phloretin at 50% MIC. Unlike phloretin, the antibiotic hardly affected the tested phenotypes. The use of DH5α, a QS-negative Escherichia coli strain, transformed with an AHL synthase (ExpI) from Pb1692, allowed to validate direct inhibition of AHL production by phloretin, as demonstrated by two biosensor strains, Chromobacterium violaceaum (CV026) and E. coli (pSB401). Expression analysis of virulence-related genes revealed downregulation of QS-regulated genes (expI, expR, luxS, rsmB), plant cell wall degrading enzymes genes (pel, peh and prt) and motility genes (motA, fim, fliA, flhC and flhD) following exposure to both phloretin concentrations. The results support the inhibition of ExpI activity by phloretin. Docking simulations were used to predict the molecular associations between phloretin and the active site of ExpI, to suggest a likely mode of action for the compound’s inhibition of virulence.

Disalicylic Acid Provides Effective Control of Pectobacterium brasiliense

“…Further assessments of the effects of the two compounds on growth, motility, biofilm formation, accumulation of signal molecules and Pb1692 infection were carried out using nonlethal concentrations that induced no more than 50% growth inhibition relative to the water-treated control (1 mM or less). In previous work involving the antibiotic ciprofloxacin, these concentrations were shown to have no effect on bacterial virulence [33,34].…”

Untargeted Metabolomics of Alternaria solani-Challenged Wild Tomato Species Solanum cheesmaniae Revealed Key Metabolite Biomarkers and Insight into Altered Metabolic Pathways

“…Flavonoids, having the widest distribution as secondary metabolites, primarily respond to biotic stresses in plants [ 62 ]. Upregulated flavonoids included pinocembrin (5,7-dihydroxyflavanone), pinocembrin chalcone, and apiforol, a tetrahydroxyflavan that influences plant defense by mediating pathogen signaling responses, membrane permeability, quorum sensing, and pathogen virulence [ 63 ]. Chalcones phloretin and isoliquiritigenin are strong antioxidants that showed an increase [ 64 ].…”

Expression of PmACRE1 in Arabidopsis thaliana enables host defence against Bursaphelenchus xylophilus infection

“…In particular, the levels of individual flavonoids or isoflavonoids, i.e., neohesperidin, homoeriodictyol, naringenin chalcone, nobiletin, vestitol and phloretin, were increased in the PmACRE1 transgenic line, and neohesperidin has been reported to be related to defence against brown leaf spot disease in grapes [ 45 ]. Phloretin was documented as a phytoalexin that affects the virulence and fitness of Pectobacterium brasiliense in apple [ 46 ], while vestitol is a phytoalexin that is induced in the leaves of Lotus corniculatus in response to inoculation with Helminthosporium turcicum Pass [ 47 ].…”