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Like other micronutrients and macronutrients, iron is also necessary for living organisms. Iron
is the fourth most abundant element in the earth. Iron oxides, comprising minerals such as
hematite, magnetite and limonite, are the most abundant of metal oxides in soil (Schwertmann
and Taylor, 1989). Iron is required for a large variety of metabolic process, virtually for all
organisms (Crichton and Charloteaux, 1987) except Lactobacilli (Archibald, 1983). Soil with
neutral pH contains iron in the form of insoluble ferric hydroxide polymer which is not available biologically (Linhsay, 1979). Most microorganisms have efficient high affinity iron uptake
system to fulfill their requirements. In this process, siderophore – low molecular weight iron
(III) chelating agents are synthesized (Neilands, 1981). Siderophores chelate insoluble iron
and solubilize iron and ferric siderophore complex are taken up by the cell through specific
membrane receptors (Neilands, 1982). Tang et al. (1990) observed that the number of nodule
initials were depressed when the iron concentration was low but when initiation had occurred
nodules developed normally in vitro.
The mechanism by which a plant avoids iron (chlorosis) is both more diverse and less
investigated than the siderophore-mediated iron uptake system of microorganisms. Three
strategies of iron assimilation have been identified in plant (Bienfait, 1989; and Sugiura
et al., 1981). Strategy I is found in non-graminaceous monocots and all dicots. It involves
acidification of the rhizosphere, thus increasing iron solubility by approximately 103 per pH
unit. The Fe3+ ion reduces to Fe2+ ion and uptake of Fe (II) occurs. Strategy II is observed
in graminaceous monocot. And this strategy involves secretion of iron-chelating agent
(phytosiderophore) of mugineic acid family, whereas Strategy III involves uptake of microbial
Fe (III) siderophores.
Various reports showed that Rhizobium strains produce siderophore. Sinorhizobium
meliloti produce siderophores like carboxylates such as rhizobactin (Smith et al., 1985),
Bradyrhizobium (cowpea) produce catechol and citrate (Modi et al., 1985; and Guerinot
et al., 1990). Berraho et al. (1997) reported that growth and siderophore production of
each strain was influenced by metal as well as the culture medium. Earlier, Ames-Gottfred
et al. (1989) observed that growth of Rhizobium leguminosarum bv. trifolii on CAS agar
plates accompanied by the halo production and this production is greater with addition of
mannitol. Roy and Chakraborty (2000) observed that Rhizobium spp. (cicer arietinum) in
the presence of metal produced a threefold higher level of siderophore than their control.
Berraho et al. (1997) stated that iron under limited condition, secreted salicylic acid and
2, 4 DHBA as phenolate type siderophore in Rhizobium ciceri. Duhan and Dudeja (1998)
evaluated the effect of iron acquisition on rhizobial siderophores and induced nodulation
pigeon pea, as also evidenced by the work of Duhan et al. (1998), who established the
involvement of siderophore production with nitrogen fixation. The aim of the present paper
is to study the qualitative and quantitative analysis of siderophore production by Rhizobium
strains and evaluate the effect of iron on siderophore production.
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