The biocontrol agent Rhodococcus erythropolis disrupts virulence of plant and human Gram-negative pathogens by catabolizing their N-acyl-homoserine lactones. This quorum-quenching activity requires the expression of the qsd (quorum-sensing signal degradation) operon, which encodes the lactonase QsdA and the fatty acyl-CoA ligase QsdC, involved in the catabolism of lactone ring and acyl chain moieties of signaling molecules, respectively. Here, we demonstrate the regulation of qsd operon expression by a TetR-like family repressor, QsdR. This repression was lifted by adding the pathogen quorum signal or by deleting the qsdR gene, resulting in enhanced lactone degrading activity. Using interactomic approaches and transcriptional fusion strategy, the qsd operon derepression was elucidated: it is operated by the binding of the common part of signaling molecules, the homoserine lactone ring, to the effector-receiving domain of QsdR, preventing a physical binding of QsdR to the qsd promoter region. To our knowledge, this is the first evidence revealing quorum signals as inducers of the suitable quorum-quenching pathway, confirming this TetR-like protein as a lactone sensor. This regulatory mechanism designates the qsd operon as encoding a global disrupting pathway for degrading a wide range of signal substrates, allowing a broad spectrum anti-virulence activity mediated by the rhodococcal biocontrol agent. Understanding the regulation mechanisms of qsd operon expression led also to the development of biosensors useful to monitor in situ the presence of exogenous signals and quorum-quenching activity.

Development of protection tools targetingDickeya species is an important issue in the potato production. Here, we present the identification and the characterization of novel biocontrol agents. Successive screenings of 10,000 bacterial isolates led us to retain 58 strains that exhibited growth inhibition properties against several Dickeya sp. and/or Pectobacterium sp. pathogens. Most of them belonged to the Pseudomonas and Bacillus genera. In vitro assays revealed a fitness decrease of the tested Dickeya sp. and Pectobacterium sp. pathogens in the presence of the biocontrol agents. In addition, four independent greenhouse assays performed to evaluate the biocontrol bacteria effect on potato plants artificially contaminated with Dickeya dianthicola revealed that a mix of three biocontrol agents, namely, Pseudomonas putida PA14H7 and Pseudomonas fluorescens PA3G8 and PA4C2, repeatedly decreased the severity of blackleg symptoms as well as the transmission of D. dianthicola to the tuber progeny. This work highlights the use of a combination of biocontrol strains as a potential strategy to limit the soft rot and blackleg diseases caused by D. dianthicola on potato plants and tubers. The Pectobacterium and Dickeya pectinolytic bacteria are phytopathogens responsible for several macerating diseases on a wide range of crop and ornamental plants (1-3). The damage caused by these pathogens remains an important issue in many countries worldwide. In Europe, the pectinolytic pathogens on potato crops include Pectobacterium atrosepticum, Pectobacterium carotovorum subsp. carotovorum, Dickeya dianthicola, and Dickeya solani (4-8). Recently, new Pectobacterium taxa, i.e., P. wasabiae and P. carotovorum subsp. brasiliense, have been characterized (2, 9, 10). Pectobacterium sp. populations are qualified as endemic, as their presence has been observed for almost a century in Europe, while the presence of Dickeya spp. as a cause of the symptoms in potato fields was sporadic until the 2000s and the Dickeya sp. damages were probably exclusively due to the species D. dianthicola. In contrast, D. solani seems to have emerged in the 2000s (3).Pectobacterium and Dickeya potato pathogens induce blackleg on stems and soft rot on tubers (11,12). A polyphyletic pathogen population may be isolated from a single sample of symptomatic plant tissues (13). The pathogens penetrate in host plants through natural pores or wounds (i.e., the lenticels, the elongation zones of roots, or insect wounds) as well as mechanical wounds. Insects can act as vectors; surface water and aerosols may also contribute to the dissemination of the pathogens (12, 14-21). Once the pathogens have infected the plant, they may propagate throughout the whole plant and son tubers by the vascular vessels. Losses due to Pectobacterium and Dickeya, related to stem and tuber rot, are major causes for downgrading and rejection of potatoes during seed potato certification. The certification is based on visual inspections in fields or on lots, ranging in Europe from 0 to 4% blackl...

Soft-rot bacteria Pectobacterium and Dickeya use N -acyl homoserine lactones (NAHSLs) as diffusible signals for coordinating quorum sensing communication. The production of NAHSLs was investigated in a set of reference strains and recently-collected isolates, which belong to six species and share the ability to infect the potato host plant. All the pathogens produced different NAHSLs, among which the 3-oxo-hexanoyl- and the 3-oxo-octanoyl- l -homoserine lactones represent at least 90% of total produced NAHSL-amounts. The level of NAHSLs varied from 0.6 to 2 pg/cfu. The involvement of NAHSLs in tuber maceration was investigated by electroporating a quorum quenching vector in each of the bacterial pathogen strains. All the NAHSL-lactonase expressing strains produced a lower amount of NAHSLs as compared to those harboring the empty vector. Moreover, all except Dickeya dadantii 3937 induced a lower level of symptoms in potato tuber assay. Noticeably, aggressiveness appeared to be independent of both nature and amount of produced signals. This work highlights that quorum sensing similarly contributed to virulence in most of the tested Pectobacterium and Dickeya , even the strains had been isolated recently or during the past decades. Thus, these key regulatory-molecules appear as credible targets for developing anti-virulence strategies against these plant pathogens.