Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 1, 1999
Publication Date: N/A
Interpretive Summary: Some biological control agents are thought to suppress plant pathogens by competing with them for limiting resources in natural environments, such as soil or plant surfaces. Unfortunately, scientists have little understanding of the composition of the habitats where this resource competition is thought to occur. In this study, we used a new biological sensor to evaluate the availability of iron to a bacterium living on root surfaces. Iron is abundant in soil, but it is very insoluble; and soluble iron is thought to be a limiting resource to the growth of microorganisms in soil and on roots. In fact, competition for the limited amounts of soluble iron available in soils is one way that biocontrol agents are thought to suppress plant pathogens. Our study showed, for the first time, that levels of soluble iron that are available to a bacterium on root surfaces are influenced by the presence of other bacteria in that environment. This study is important because it demonstrates the utility of the new biological sensor method to directly measure resource competition between two microorganisms in nature, which has not been done previously. It also demonstrates that microorganisms can alter the availability of important chemical resources (like iron) to their coinhabitants.
Technical Abstract: The goal of this study was to determine if siderophores produced by rhizosphere microorganisms alter the levels of iron available to a strain of Pseudomonas putida in this natural habitat. We used a previously-described transcriptional fusion (pvd-inaZ) between an iron regulated promoter (pvd) and the ice nucleation reporter gene (inaZ) to detect alterations in iron availability to P. putida. Ice nucleation activity (INA) expressed from the pvd-inaZ fusion by P. putida N1R or N1R Pvd-, a derivative deficient in the production of a pyoverdine siderophore, was inversely related to the concentration of ferric citrate in a culture medium. In culture, INA expressed by N1R Pvd- (pvd-inaZ) was reduced in the presence of the ferric complex of pseudobactin-358, a pyoverdine siderophore produced by P. putida WCS358 that can be utilized as a source of iron by N1R Pvd-. In the rhizosphere of cucumbers, N1R Pvd- (pvd-inaZ) expressed INA, indicating that iron availability was sufficiently low in that natural habitat to allow transcription of the iron regulated pvd promoter. Coinoculation with WCS358 or N1R significantly decreased INA expressed by N1R Pvd- (pvd-inaZ) in the rhizosphere, whereas coinoculation with a pyoverdine-deficient mutant of WCS358 did not reduce INA expressed by N1R Pvd- (pvd-inaZ). These results indicate that iron availability to N1R Pvd- (pvd-inaZ) in the rhizosphere was enhanced by the presence of another strain of P. putida that produces a pyoverdine that N1R Pvd- (pvd-inaZ) could utilize as a source of iron.