Ecological fitness of Bacillus subtilis BGS3 regarding production of the surfactin lipopeptide in the rhizosphere

Cyclic lipopeptides and particularly surfactins produced
by Bacillus species retain antibacterial, antiviral,
biofilm-forming and plant resistance-inducing
activities. In most cases, their role in biological
control of plant diseases was evoked on the basis
of in vitro assays or by using non-producing/
overproducing mutants but there is a need for more
direct evidence of an efficient lipopeptide biosynthesis
in the rhizosphere. In this work, we coupled
LC-MS quantification of the lipopeptides secreted by
cells colonizing tomato plants with the use of psrfA–
lacZ reporter system integrated within the BGS3
chromosome to study the expression of the surfactin
operon in planta. Results showed that a higher
level of psrfA induction was observed upon the
establishment of a stable BGS3 population on roots
and surfactins extracted from the rhizosphere were
produced in biologically significant quantities. Our
results also demonstrate that BGS3 efficiently utilizes
the main substrates from plant exudates to
produce surfactins. This synthesis is also efficient
in cells forming colonies and the production may be
favoured in bacteria developing slowly in the rhizosphere.
This provides a first understanding of how
environmental factors may influence lipopeptide
production by beneficial Bacillus strains.