Abstract:
The complexity of plant microbial communities provides a rich model for investigating
biochemical and regulatory strategies involved in interbacterial competition. Within these
niches, the soft rot Enterobacteriaceae (SRE) represents an emerging group of plant–
pathogens causing soft rot/blackleg diseases resulting in economic losses worldwide
in a variety of crops. A preliminary screening using next-generation sequencing of
16S rRNA comparatively analyzing healthy and diseased potato tubers, identified
several taxa from Proteobacteria to Firmicutes as potential potato endophytes/plant
pathogens. Subsequent to this, a range of molecular and computational techniques
were used to determine the contribution of antimicrobial factors such as bacteriocins,
carbapenem and type VI secretion system (T6SS), found in an aggressive SRE
(Pectobacterium carotovorum subsp. brasiliense strain PBR1692 – Pcb1692) against
these endophytes/plant pathogens. The results showed growth inhibition of several
Proteobacteria by Pcb1692 depends either on carbapenem or pyocin production.
Whereas for targeted Firmicutes, only the Pcb1692 pyocin seems to play a role in
growth inhibition. Furthermore, production of carbapenem by Pcb1692 was observably
dependent on the presence of environmental iron and oxygen. Additionally, upon
deletion of fur, slyA and expI regulators, carbapenem production ceased, implying
a complex regulatory mechanism involving these three genes. Finally, the results
demonstrated that although T6SS confers no relevant advantage during in vitro
competition, a significant attenuation in competition by the mutant strain lacking a
functional T6SS was observed in planta.
IMPORTANCE
Soft rot Enterobacteriaceae (SRE) represents important phytopathogens causing
soft rot/blackleg diseases in a variety of crops leading to huge economic losses
worldwide. These pathogens have been isolated alongside other bacteria from different
environments such as potato tubers, stems, roots and from the soil. In these
environments, SREs coexist with other bacteria where they have to compete for
scarce nutrients and other resources. In this report, we show that Pectobacterium
carotovorum subsp. brasiliense strain PBR1692 – Pcb1692, which represents one of
the SREs, inhibits growth of several different bacteria by producing different antimicrobial
compounds. These antimicrobial compounds can be secreted inside or outside the
plant host, allowing Pcb1692 to effectively colonize different types of ecological niches.
By analyzing the genome sequences of several SREs, we show that other SREs likely
deploy similar antimicrobials to target other bacteria.
