A quorum sensing‐defective mutant of Pectobacterium carotovorum ssp. brasiliense 1692 is attenuated in virulence and unable to occlude xylem tissue of susceptible potato plant stems

Abstract

Pectobacterium carotovorum ssp. brasiliense 1692 (Pcb1692) is an important emerging pathogen of potatoes causing blackleg in the field and soft rot during post-harvest storage. Blackleg diseases involve the bacterial colonization of vascular tissue and the formation of aggregates, also known as biofilms. To understand the role of quorum sensing in vascular colonization by Pcb1692, we generated a Pcb1692ΔexpI mutant strain. Inactivation of expI led to the reduced production of plant cell wall-degrading enzymes (PCWDEs), the inability to produce acyl homoserine lactone (AHL) and reduced virulence in potato tubers and stems. Complementation of the mutant strain with the wild-type expI gene in trans successfully restored AHL and PCWDE production as well as virulence. Transmission electron microscopy and in vitro motility assays demonstrated hyperpiliation and loss of flagella and swimming motility in the mutant strain compared with the wild-type Pcb1692. Furthermore, we noted that, in the early stages of infection, Pcb1692 wild-type cells had intact flagella which were shed at the later stages of infection. Confocal laser microscopy of PcbΔexpI-inoculated plants showed that the mutant strain tended to aggregate in intercellular spaces, but was unable to transit to xylem tissue. On the contrary, the wild-type strain was often observed forming aggregates within xylem tissue of potato stems. Gene expression analyses confirmed that flagella are part of the quorum sensing regulon, whereas fimbriae and pili appear to be negatively regulated by quorum sensing. The relative expression levels of other important putative virulence genes, such as those encoding different groups of PCWDEs, were down-regulated in the mutant compared with the wild-type strain.

Interplay of classic Exp and specific Vfm quorum sensing systems on the phenotypic features of Dickeya solani strains exhibiting different virulence levels

“…Twenty-six per cent of the P. atrosepticum SCRI1043 genes are under classic AHL-QS control, including genes coding for PCWDEs and different secretion systems . In P. carotovorum, the AHL-QS system controls similar phenotypes and is necessary for full virulence (Barnard and Salmond, 2007;Moleleki et al, 2017;Pirhonen et al, 1993).…”

Quorum Sensing in Gram-Negative Plant Pathogenic Bacteria

“…The LuxI/R QS also regulates Type 1 (T1SS) and Type 2 (T2SS) secretion systems in Pa that are responsible for secretion of PCWDE [26]. In Pcb, QS is important for pathogenicity, production of PCWDE and cell aggregation in xylem tissues [17].…”

“…Antibiotics give Pcc a competitive advantage over other bacteria coexisting during infection [46]. It is important to note that QS regulates motility in Pcc [11] and Pcb [17] and a cluster of genes for amino acids metabolism i.e. ilvGMEDA, ilvIH, ilvBN and leuABCD, signal transduction and lipid metabolism in Pa [18] Furthermore, the xylAB and xylFGH operons for xylose/xylulose metabolism as well as genes for anaerobic formate metabolism and operons for assimilation of hydrogen (hyp and hyb) are influenced by QS in Pa [18].…”

Horizontally acquired quorum sensing regulators recruited by the PhoP regulatory network expand host-adaptation repertoire in the phytopathogenPectobacterium carotovorum

“…Among gram-negative bacteria, one specific group commonly referred to as soft rot Pectobacteriaceae (SRP) (13, 14) (formerly known as soft rot Enterobacteriaceae), has increasingly gained attention over the last few decades as causative agents of wilt/blackleg diseases in economically important crops worldwide (15, 16). This group is most prominently represented by Pectobacterium and Dickeya genera.…”