Efficient Biostimulation of Native and Introduced Quorum-Quenching Rhodococcus erythropolis Populations Is Revealed by a Combination of Analytical Chemistry, Microbiology, and Pyrosequencing

Abstract

Degradation of the quorum-sensing (QS) signals known as N-acylhomoserine lactones (AHL) by soil bacteria may be useful as a beneficial trait for protecting crops, such as potato plants, against the worldwide pathogen Pectobacterium. In this work, analytical chemistry and microbial and molecular approaches were combined to explore and compare biostimulation of native and introduced AHL-degrading Rhodococcus erythropolis populations in the rhizosphere of potato plants cultivated in farm greenhouses under hydroponic conditions. We first identified gamma-heptalactone (GHL) as a novel biostimulating agent that efficiently promotes plant root colonization by AHL-degrading R. erythropolis population. We also characterized an AHLdegrading biocontrol R. erythropolis isolate, R138, which was introduced in the potato rhizosphere. Moreover, root colonization by AHL-degrading bacteria receiving different combinations of GHL and R138 treatments was compared by using a cultivationbased approach (percentage of AHL-degrading bacteria), pyrosequencing of PCR-amplified rrs loci (total bacterial community), and quantitative PCR (qPCR) of the qsdA gene, which encodes an AHL lactonase in R. erythropolis. Higher densities of the AHLdegrading R. erythropolis population in the rhizosphere were observed when GHL treatment was associated with biocontrol strain R138. Under this condition, the introduced R. erythropolis population displaced the native R. erythropolis population. Finally, chemical analyses revealed that GHL, gamma-caprolactone (GCL), and their by-products, gamma-hydroxyheptanoic acid and gamma-hydroxycaproic acid, rapidly disappeared from the rhizosphere and did not accumulate in plant tissues. This integrative study highlights biostimulation as a potential innovative approach for improving root colonization by beneficial bacteria. N-Acylhomoserine lactones are intercellular signals used by numerous alpha-, beta-, and gammaproteobacteria to regulate gene expression at the population and community levels (18,38). The mechanism connecting cell population to gene expression via AHLs is termed quorum sensing (QS) (17). Pectobacterium carotovorum is a causative agent of blackleg and soft rot diseases in several crops, including potato plants and tubers. Over the 2004-2009 period, quality refusals oscillated between 2 and 5% and 3 and 8% of the total seed tuber production in France and Netherlands, respectively; blackleg disease, which is caused by Pectobacterium and Dickeya, represented 10 to 30% and 60 to 80% of the causes of refusal in the same countries. In Pectobacterium, production of virulence factors such as pectinolytic and cellulolytic enzymes and harpins is positively controlled by AHLs (23, 29). In Pectobacterium carotovorum, inhibition of AHL synthesis or degradation of the AHLs produced results in the absence of the expression of the QS-regulated genes and consequently in a decrease of the virulence symptoms on potato plants (23,31).During the past decade, several approaches have been proposed to disrupt QS regulatio...

Exploiting Quorum Sensing To Confuse Bacterial Pathogens

“…Both treatment conditions resulted in an increase in the R. erythropolis population and a concomitant biasing of the bacterial community, including a decrease in the resident Agrobacterium population. Furthermore, artificial introduction of the biocontrol strain in combination with biostimulation treatment appeared to displace native AHL-degrading populations (352). Although overall the results demonstrate that beneficial bacteria can be selected for by using biostimulation approaches, they also indicate the potential impact that such selection could have on soil microbial communities and point out the need for more rigorous evaluation of global effects following QSI-based treatments.…”

“…Although that study aimed to interrupt QS in a biocontrol agent, which would likely not be the goal of QSI therapies, it still highlights the unanticipated effects that quorum quenching can have on other interspecies and interkingdom interactions. This issue was more directly illustrated by Cirou et al when they used biostimulation both to increase the native population of the AHL-degrading bacterium Rhodococcus erythropolis in the hydroponically grown potato rhizosphere and to facilitate rhizosphere colonization following introduction of the biocontrol isolate R. erythropolis strain R138 (352). Both treatment conditions resulted in an increase in the R. erythropolis population and a concomitant biasing of the bacterial community, including a decrease in the resident Agrobacterium population.…”

Transcriptome of the quorum-sensing signal-degrading Rhodococcus erythropolis responds differentially to virulent and avirulent Pectobacterium atrosepticum

“…Only the lactonaseencoding gene qsdA was identified in R. erythropolis (Uroz et al, 2008), whereas the genetic determinants coding for the reductase and amidase activities are still unknown. QsdA was showed to be involved in the cleavage of NAHLs and related lactones, such as gammaheptanolactone and gamma-hexanolactone, which are assimilated as a sole carbon and energy source by R. erythropolis (Uroz et al, 2008;Barbey et al, 2012;Cirou et al, 2012). Because of its ability to cleave NAHLs, R. erythropolis is proposed as a plant-protecting and antibiofouling agent (Cirou et al, 2007;Faure and Dessaux, 2007;Oh et al, 2012).…”

Biocontrol of the Potato Blackleg and Soft Rot Diseases Caused by Dickeya dianthicola

“…These biocontrol agents could be also evaluated in combination with the quorum-quenching Rhodococcus erythropolis (55)(56)(57) or with other proposed strategies using plant defense stimulation, predators, or competitors (28)(29)(30)(31)(32)(33)(34)(39)(40)(41)(42)(43)(44)(45)(46)(47)(48). In regard to the practicality of using such findings, this strategy used only 3 treatments by the biocontrol agents.…”

“…The quorum-quenching bacteria identified so far belong to the Delftia, Ochrobactrum, Bacillus, and Rhodococcus genera. The colonization of potato plant roots by the quorum-quenching Rhodococcus was shown to be enhanced using the growth-stimulating agents gamma-caprolactone and gamma-heptalactone (55)(56)(57)(58)(59)(60)(61).…”