Due to the low solubility of Fe in soils, nitrogen-fixing bacteria have acquired several different strategies for the acquisition of Fe from the environment. Since the utilization of Fe by rhizospheric bacteria has been discussed in Chapter 8, this chapter will focus on the nitrogen-fixing bacteria. Several species of rhizobacteria are commonly found associated with plant roots where they derive benefit from organic compounds released from the root and enhance plant growth through nitrogen fixation. Organisms of the genus Azospirillum are important plant growth promoting bacteria that are colonized principally on the root surface and have been extensively characterized (Steenhoudt and Vanderleyden, 2000). Several endophytic diazotrophic bacteria are found and the ones most commonly isolated include Acetobacter diazotrophicus, Herbaspirillum seropedicea, and Azoarcus sp. (Elmerich et al., 1992). Iron-uptake activities of the associated bacteria other than Azospirillum have not been established.
Under Fe limited conditions, the uptake of Fe from the soil by Azospirillum is attributed to siderophores (see Table 9-2). Derivatives of dihydroxybenzoic acid (DHBA) are produced as siderophores of Azospirillum with Azospirillum lipoferum producing 2, 3-DHBA and 3, 5-DHBA leucine and lysine conjugates. The production of 3, 5-DHBA in Azospirillum lipoferum appears to be regulated by both Fe3+ and molybdate because production of this siderophore is increased under molybdate as well as Fe starvation (Saxena et al., 1989).
Azotobacter vinelandii produces several catecholate siderophores (Table 9-2) and these molecules may have multiple roles in bacterial and plant nutrition. Of considerable interest is the observation that the catecholate siderophore forms complexes with Mo5+ as well as with Fe3+ (Duhme-Klair, 2003; Duhme-Klair et al., 2003). Not only do molybdate and transition metals have a role in accumulation of protochelin by Azotobacter vinelandii (Cornish and Page, 2000) but also a siderophore derivative serves as a selective signal for molybdate (Jedner et al., 2001). The ferri-siderophores, loaded with Fe, of Azotobacter vinelandii are used by Agrobacterium tumefaciens and Erwinia carotovora (Page and Dale, 1986). Thus, the Azotobacter siderophores, unlike those of Pseudomonas putida (Leong and Expert, 1989), do not inhibit phytopathogenic bacteria. It has been reported that Azotobacter enhances the growth response of oat (Singh et al., 2000) and the shelf-life of tomato (Chaurasia et al., 2001). It would be important to learn if siderophores produced by Azotobacter have a role in each of these activities.
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