| dc.description.abstract | Host-associated bacterial species have evolved numerous mechanisms to enable them to
interact and compete with other microorganisms and with their hosts. One such mechanism is the
bacterial Type VI Secretion System (T6SS). Many plant-beneficial bacteria have one or
moreT6SS, but their functions in plant-associated niches are poorly understood. T6SS are
bacterial machines studied predominantly in human and plant pathogens as a mechanism for
bacterial competition, killing, and the transmission of effector proteins critical for virulence into
prokaryotic and eukaryotic cells. T6SS may respond to environmental stimuli differently and
may have different spectra of influence on community members depending on how firing is
regulated, and the types of effectors delivered, suggesting that in strains with multiple T6SS,
each may have non-redundant functions. The present work focuses on Pseudomonas
chlororaphis 30-84, a well-studied wheat rhizosphere-colonizing plant-beneficial bacterial strain
with two distinct T6SS that may perform non-redundant functions. The role of each T6SS
(T6SS-1, T6SS-2) in microbial competition and persistence in the rhizosphere and anti-predation
was examined. Mutants deficient in T6SS function were less persistent in the rhizosphere in
natural field soil in repeat-harvest assays. Mutants deficient in T6SS function were also less
competitive than the wild type against other bacteria in competition assays in vitro and on wheat
roots, with T6SS-2 playing an important role against competing strains lacking their own T6SS.
Finally, mutants lacking both T6SS lost the ability to effectively protect themselves from resist
predation by several eukaryotic bacterivores. These data indicate important roles for the T6SS of
plant-beneficial bacteria in rhizosphere dynamics, interactions with eukaryotes, and overall soil
ecology. | |
