Culture-dependent
and independent analyses of root-associated bacterial communities have
determined that members of the Flavobacterium genus are often highly
abundant in the rhizosphere of a wide array of plants. Although flavobacteria
are ubiquitous in root environments and positively correlated to plant disease
suppression, the appropriate explanation for their high persistence in
rhizosphere is still missing. Flavobacteria harbor a unique gliding motility
apparatus that is tightly associated with a recently characterized Bacteroidetes-specific
type IX (T9SS) protein secretion system; and therefore, we hypothesize that this
gliding motility/T9SS complex may confer a competitive advantage in the
rhizosphere. In order to test this
hypothesis, we established
mutant lines of root-associated Flavobacterium sp. F52 with
dysfunctional secretion and gliding motility. We then conducted a series of in-planta
experiments that demonstrated that these mutants exhibited significantly
lower rhizosphere survival (approx. 10 fold), plant root colonization (approx. 5
fold) and seed adhesion (approx. 7 fold) capacity than corresponding non-mutant
strains. Furthermore, application of the mutant strain to tomato roots
resulted in a significant increase in susceptibility towards the bacterial
phytopathogen Clavibacter michiganens (applied to the canopy) relative
to the wild type
strain, suggesting a potential role of flavobacteria in induced plant
resistance. Collectively these results provide an initial link between the
high abundance of flavobacteria in rhizosphere and their unique physiology,
indicating that the flavobacterial motility/secretion complex may play a central
role in root colonization and plant defense stimulation.
