Rhizosphere interactions and siderophores

Abstract

Certain plant growth-promoting pseudomonads inhibit deleterious and pathogenic rhizosphere bacteria and fungi by producing siderophores. Properties of a siderophore transport system which might provide a competitive advantage under iron stress conditions include ability to utilize other organisms' siderophores, higher Fe(III) stability constant, faster kinetics of dissolution of Fe(III) minerals, more efficient transport system, and resistance to degradation. In order to determine the concentration and localization of siderophores in the rhizosphere monoclonal antibodies (Mabs) to ferric pseudobactin, the siderophore of Pseudomonas putida B10, have been developed. Several Mabs cross reacted differently with various pseudobactins. A growth medium has been developed for the study for siderophore-mediated rhizosphere interactions in the laboratory.

Heterogeneity of iron bioavailability on plants assessed with a whole-cell GFP-based bacterial biosensor

“…The results presented here do, however, complement other studies of iron bioavailability (Loper & Lindow, 1994 ;Loper & Henkels, 1999), and also models and hypotheses regarding the iron available in microenvironments encountered by microbes on plant surfaces (Loper & Lindow, 1994 ;Buyer & Sikora, 1991) in that they show that at least a subset of the cell population does indeed encounter low iron conditions, at least occasionally on plants. Overall, the general distribution pattern of measured ferric iron availability indicates that approximately 10 % of the microbes on leaves and roots in our study experienced substantial iron limitation at some point during colonization of the plant.…”

“…Siderophore-mediated biocontrol also apparently occurs among members of the microbial community in fish intestines (Sugita et al, 1998). Models of iron competition have been developed to explain such interactions, such as that formulated by Buyer & Sikora (1991), which describes how interactions which reduce the ferrated siderophore concentration in the vicinity of a cell can result in a reduction in the growth rate of that microbe. Other iron-regulated metabolites have also been implicated in the efficacy of biocontrol strains (Gill & Warren, 1988 ;Leeman et al, 1996).…”

“…The root tips of some plant species acidify the rhizosphere, reducing ferric iron to the utilizable ferrous form. Chemical reactions and microbial respiration (Hojberg & Sorensen, 1993) can result in fluctuations in pH which in turn could affect the solubility of iron oxides, ultimately affecting the bioavailability of this element locally (Ehrlich, 1990 ;Loper & Henkels, 1999 ;Buyer & Sikora, 1991 ;Marschner, 1995 ;Loper & Ishimaru, 1991). Microhabitats of bacteria may represent areas of vast chemical diversity, but this concept has not been sufficiently characterized.…”

The Role of Ligand Exchange in the Uptake of Iron from Microbial Siderophores by Gramineous Plants

“…The rate of this reaction and its effectiveness in supplying Fe from MS to plants is controlled by the following factors: (a) amount of PS released, (b) concentration of the FeMS, (c) stability constants of the various ligands involved with Fe3+ and other competitive metals, (d) kinetics of ligand exchange between Fe-siderophore and the free PS, and (e) efficiency of the FePS uptake system. A similar concept for the interactions between organisms in relation to Fe nutrition was described by Buyer and Sikora (1990), and in the present work we studied some of these aspects.…”

“…On the basis of their high-stability constants with ferric Fe and their low reduction potential, Bienfait (1989) concluded that MS are not readiIy reduced by the root plasma membrane reductase system. It was emphasized, however, that kinetics rather than equilibrium (e.g., reduction potential) should be considered (Buyer and Sikora, 1990;BarNess et al, 1991). Wang et al (1993) found a direct and specific utilization of FeFOB by cucumber plants.…”