A quorum sensing-defective mutant of Pectobacterium carotovorum subsp. brasiliense 1692 is attenuated in virulence and unable to occlude xylem tissue of susceptible potato plant stems
L. MOLELEKI (1), R. Pretorius (1), C. Tanui (1), G. Mosina (1) (1) University of Pretoria, South Africa

Pectobacterium carotovorum subsp. 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 bacterial colonisation of vascular tissue and formation of aggregates, also known as biofilms. To understand the role of quorum sensing in vascular colonisation by Pcb1692, we generated a Pcb1692ΔexpI mutant strain. Inactivation of expI led to reduced production of plant cell wall degrading enzymes (PCWDE), 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 hyper-piliation, and loss of flagella and swimming motility in the mutant strain compared to the wild type Pcb1692. Furthermore, we noted that in the early stages of infection, Pcb1692 wild type cells have intact flagella which are shed at the later stages of infection. Confocal laser microscopy of Pcb1692-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, while 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 PCWDE were down-regulated in the mutant compared to the wild type strain.

Abstract Number: P7-193
Session Type: Poster