Utilization of microbial siderophores by plants

Numerous studies have investigated the ability of plants to use microbial siderophores as iron sources for growth (Table 8-2), and have examined the possible mechanisms by which plants might obtain iron from these compounds, either by exchange with phytosiderophores or by the iron reductase that is expressed by Strategy I plants. However, these experiments have been controversial for several reasons. Since siderophores are labile and are subject to degradation after their addition to soil, almost all of the studies on plant use of microbial siderophores relied on hydroponic experiments in which radiolabeled siderophores were added to nutrient solutions at known concentrations generally ranging from 1 to 100 ^.M. Root uptake rates and transport of iron to the plants shoots were then ascertained by measuring the quantity and distribution of iron in the plant. Iron provided by microbial siderophores is often found to be associated with the root tissues, which may be explained in part by uptake of iron by microorganisms that are associated with the root surface (Crowley et al., 1992).

3.3.1 Strategy I plants

Iron uptake from siderophores by Strategy I plants has been examined for the hydroxamate siderophores ferrichrome, ferrioxamine B, rhodotorulic acid, and the mixed ligand siderophore pseudobactin produced by Pseudomonas spp. Other siderophores that have been studied and shown to be relatively effective for delivery of iron to plants include rhizoferrin, a citrate based siderophore produced by the fungus Rhizopus, and fusarinines, also produced by fungi. As a basis for comparison, iron uptake from siderophores is normally compared to that measured with supply of iron from synthetic chelates such as EDTA and HEDTA (Walter et al., 1994; Johnson et al., 2002). In general, iron acquisition from these compounds requires relatively high micromolar concentrations in hydroponic solutions where the entire root system is bathed in a uniform concentration of the siderophore. The utilization of siderophores that are used by extracellular reduction is sensitive to the presence of excess deferrated siderophore and to ferrous trapping agents such as BPDS (Yehuda et al., 2000). This appears to be the most common mechanism with siderophores that have a high redox potential and that can be reduced to donate ferrous iron to the transport system of the plant. Ferrioxamine B on the other hand appears to be taken up directly and is only very slowly reduced by the Strategy I extracellular reductase (Wang et al., 1993). With ferrichrome and ferrioxamine B, concentrations between 10 to 50 ^M siderophore are required to support growth depending on the plant species.

The classical method for determining whether siderophores are used by reductive release of iron from iron chelates by Strategy I dicots involves the use of a ferrous trapping agent such as bipyridyl or BPDS which inhibit uptake of the ferrous iron that has been cleaved from ferric-ion-specific chelates and siderophores. Using these methods, reductase activity is minimal with siderophores such as ferrioxamine B, and only a small portion of the iron is translocated to the plant shoots within the short time frame over which these experiments are usually conducted (Crowley et al., 1992; Johnson et al., 2002). Other research using dual radiolabeled siderophore and iron has shown that in some cases plants may take up the intact iron-chelate, after which the entire complex eventually is transported to the shoot (Manthey et al., 1996). It has been suggested that there may be a transport system for this compound in some plants (Wang et al., 1993; Johnson et al., 2002). However, uptake might also occur via breaks in the apoplast at the sites of lateral root emergence where the siderophore iron complex may enter the xylem and undergo transport by mass flow in the transpiration stream to the plant leaves.

One interesting approach that was used to examine reductive removal or iron from siderophores and the role of root associated microorganisms employed a synthetically produced siderophore NBD-desferrioxamine B (NBD-DFO). This siderophore is an analog of the natural siderophore ferrioxamine B, but has the property of fluorescing when it is deferrated (Bar-Ness et al., 1992). In this manner, the deferration process can be observed visually using a microscope. In experiments with this compound, iron uptake occurred by separation of iron from the chelate in the apoplast with no transport of the chelate across the cell membrane. Uptake rates were significantly affected by the presence or absence of root microflora, and was lower in the presence of an antibiotic agent, which suggests that some of the iron uptake was associated with use of the siderophore by rhizosphere microorganisms or perhaps that microorganisms reductively removed iron from the chelate which was then captured and transported by the plant iron transport system.

One of the most effective siderophores for supply of iron to plants is rhizoferrin, a siderophore produced by Rhizopus arrhizus (Yehuda et al., 2000). This siderophore is readily reduced by the ferric chelate reductase of tomato and cucumber and supplies iron as effectively as the synthetic chelate EDTA. Rhizoferrin also supplies iron via ligand exchange to Strategy II monocots and supports the growth of corn and barley in iron deficient media. These findings are significant in that most studies on iron uptake mechanisms from siderophores have used only a few model compounds that have high affinity for iron. Overall, very few siderophores have been purified and used in experiments with plants, so it is possible that production and accumulation of siderophores such as rhizoferrin could contribute to plant iron uptake. However, more research is needed to determine whether the amounts of these low affinity siderophores may accumulate to concentrations that would support plant growth.

3.3.2 Strategy II plants

The primary mechanism by which iron is taken up by Strategy II monocots involves the production of phytosiderophores that are released into the rhizosphere to solubilize iron. Phyto siderophores are effective iron sources for some microorganisms and thus may suppress siderophore production of bacteria that use the compounds as alternative sources of iron.

On the other hand, there have been several studies that show that some microbial siderophores can mobilize iron from soils which can then undergo ligand exchange with phytosiderophores. The efficacy of this mechanism depends on the concentrations of the competing chelators, their stability constants for iron and other competing ions, the composition of the solution with respect to competing ions, and the pH and redox conditions in the root medium.

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