Abstract |
The objective of this study was to elucidate the mechanisms of the three-way plant-B.
amyloliquefaciens MBI600-bacterial pathogen interaction. A holistic approach was
implemented; at first, the bactericidal activity of B. amyloliquefaciens MBI600 was
evaluated in vitro and in planta against various bacterial pathogens. MBI600 showed
antibacterial activity in vitro against both gram- and gram+ species. Among the tested
species, Clavibacter michiganensissubsp. michiganensis, Ralstonia solanacearum, and
Xanthomonas spp. were of high susceptibility, Erwinia amylovora of intermediate
susceptibility and E. chrysanthemi and Pseudomonas spp. of low/no susceptibility.
Foliar application of Serifel, the commercial formulation of MBI600, inhibited infection
of tomato plants by the soil-borne phytopathogen R. solanacearum. It also displayed
biocontrol activity against X. campestris pv. vesicatoria and reduced disease
symptoms caused by P. syringae pv. tomato, even if its population density was not
affected significantly. In support, wildfire disease development and population density
of its causing agent were impaired significantly in Nicotiana benthamiana leaves.
MBI600 produced and secreted bacillibactin, a catecholate siderophore, under iron
depletion growth conditions. In vitro and in planta tests suggested that production of
bacillibactin resulted in a broadening of the MBI600 target range among
phytopathogens.
The ability of MBI600 to colonize different plant was also evaluated. Results suggested
that ΜΒΙ600 effectively colonized, in a dose-dependent manner, tomato and corn
roots as well as leaves of tomato, N. benthamiana and lettuce plants following Serifel
application. Furthermore, microbiological tests and expression of genes involved in
rhizosphere competence revealed that colonization is facilitated by root exudates and
elicitorsfrom competitive species, which enhanced swarming of MBI600 and synthesis
of bactericidal antibiotics.
Finally, the activation of immune responses of tomato plants in the presence of
MBI600 or its metabolites was investigated based on the expression of defenserelated genes and transcriptomic analysis. Foliar application of Serifel activated
defense mechanisms in a dose-dependent manner; low dosage activated only SAresponsive genes (npr1, pr1b1) whereas use of suggested dosage resulted in the
activation of all JA/ET-responsive genes assessed (erf1, loxD, myc2), while the SA
signaling pathway was still active. These results suggested a synergy between SA and
JA/ET – responsive pathways, which was confirmed in experiments using MBI600
metabolites. Furthermore, MBI600 metabolites suppressed absisic acid and enhanced
auxin signaling pathways. They also activated components of Jasmonic acid (JA) and
ethylene (ET) biosynthesis and signaling pathways. In contrast to leaf application, CFCF
did not affect SA-responsive genes targeted in this study.
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