The ability of plants to utilize microbial siderophores as iron sources for growth is highly controversial and has been investigated using a variety of experimental approaches using hydroponic and soil culture. Although there are considerable differences in the quantities of siderophores that are produced by various strains and species of bacteria, there is no evidence to date that inoculation with bacterial strains that produce high quantities of siderophores is beneficial to plant iron nutrition. Experiments in hydroponic culture show that inoculation of Strategy II plants with soil microorganisms can be detrimental to plants by causing rapid degradation of phytosiderophores which are not replaced by concomitant production of microbial siderophores at a commensurate level that can support plant growth (Duijff et al., 1994; Walter et al., 1994). This rapid degradation of phytosiderophores in hydroponics is thought to be an artifact of hydroponic culture where the microorganisms have rapid access to phytosiderophores that are released into the solution. In soils, the spatial separation of microorganisms from zones of high phytosiderophore release behind the root tip where bacterial populations are relatively low allows phytosiderophores to accumulate to locally high concentrations (Von Wiren et al., 1993). Similar disappointments with inoculation have been observed in field experiments. In an experiment with two lines of oat, one capable of producing phytosiderophore and the second an iron inefficient cultivar, there was no benefit of inoculating the plants with siderophore producing bacterial strains (Alexander and Zuberer, 1993). In this case, the population densities of the siderophores producing strains were relatively low, ranging between 104 to 106 cells per gram of root. In this experiment, it was not clear whether the oat cultivars were inefficient in use of the microbial siderophores, or whether the population densities of the bacteria were too low to produce siderophores at concentrations that would be relevant to provide iron to the plants.
While plant iron nutrition may not be directly improved by soil inoculation with siderophore producing microorganisms, plant growth promoting bacteria that stimulate plant growth through the production of hormones or destruction of plant produced ethylene rely on the use of siderophores for rhizosphere competence in iron-limiting soils (Forlani et al., 1995; Bevivino et al., 1998; Cattelan et al., 1999). The enzyme, 1-aminocyclopropane-1-carboxylate deaminase (ACC), is responsible for the degradation of the precursor of ethylene which can inhibit plant growth. Bacteria that produce this enzyme also frequently produce siderophores. Auxins are produced by many different bacteria including Enterobacter, Klebsiella, and others (Haahtela et al., 1990). These bacteria also produce siderophores which presumably increases their rhizosphere competence. In the search for bacteria and fungi that can be used as biofertilizers or for biocontrol of disease, siderophore production is commonly included as a criterion for characterizing potential soil and seed inoculants. However, it is clear that this trait alone does not necessarily confer plant-beneficial properties since many bacteria and fungi that are inhibitory to plant growth also produce siderophores (Gardner et al., 1984; Kremer et al., 1990).
Along with various bacteria that are being developed for biofertilizers, mycorrhizae are commonly advocated as soil inoculants in areas where severe disturbance has reduced the indigenous mycorrhizal inoculum in the soil. Superior mycorrhizal fungal strains are also being identified with the aim of using these as soil inoculants to enhance plant growth. The role of mycorrhizae for enhancing plant iron nutrition through siderophore production or utilization is still not clear, however. Mycorrhizal sorghum plants were shown to take up iron at higher concentrations than nonmycorrhizal plants (Caris et al., 1998). It is still not clear whether the mycorrhizal fungi take up iron directly, or instead enhance the recovery of siderophores that are released by plant roots and their associated microflora. In the study by Caris and coworkers, mycorrhizae did not enhance iron uptake by peanuts, but did enhance iron uptake by sorghum, which produces phytosiderophores. It is thus of interest whether mycorrhizae are able to take up iron from the phytosiderophore-iron complexes and transport this iron to the plant roots for utilization by the plant.
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