100
7.1 Introduction
Plants and microbes have vast significance in our day-to-day life. Iron is considered
to be an abundant element in the soil (earth’s crust) and essential for all life processes such as respiration, DNA synthesis, tricarboxylic acid cycle and production
of various small molecules like amino acids, lipids, and sterols. Being an essential
element in earth’s crust, the bioavailability of iron is limited in the habitat (soil and
sea) owing to its low solubility. This property of iron results in its poor uptake by
plants, which eventually makes iron an essential nutrient for plant growth. In aqueous and oxygenated conditions, the iron is found in its supreme state, which accumulates in the form of minerals such as iron oxides and iron hydroxides and is not
ready to be utilized as such by organisms. In order to overcome this restricted process, the microbial flora of soil such as Pseudomonas spp., Enterobacter genus,
Bacillus spp. produces special iron-binding tiny carriers, called ‘siderophore’, that
help to scavenge iron from these mineral phases (oxides and hydroxides) by forming soluble iron (Fe
3+
) complexes which are readily taken up by the environment
through active transport mechanism (Philpott 2006).
Siderophore came from Greek words sidero that means ‘iron’ and phore that
means ‘carriers’, and in combination, it is termed as ‘iron carrier’. Siderophores are
small, low-molecular-weight (<10 kDa) iron-chelating compounds, secreted by
plants and microorganisms (bacteria and fungi) to maintain their iron requirement.
These are also produced by rhizospheric bacteria in iron-limiting conditions in
order to increase the plant growth by scavenging iron from the environment and
make it available to the cell near the plant roots (Sah and Singh 2015; Li et al. 2016).
7.2 Siderophore-Mediated Iron Transport
Iron transport in siderophore is an energy-dependent mechanism. Type and stereoselectivity of siderophore are specific factors in recognition and transport of ironsiderophore complexes in microbes. The complexation also depends on metal ion
coordination geometry as well as N-acyl residues present at the periphery of central
metal ion. For instance, the coordination of metal centre and configuration of ligand
affect the stability of complex. In case of Rhodotorula pilmanae, configuration of
macrocyclic rings of siderophore is favoured, whereas in contrast to this, in
Penicillium parvum, Neurospora crassa and Aspergillus quandricinctus, L-cisferrichrome is found to be a stable configuration. Further, the geometrical stability
of complex also depends on the types and number of N-acyl residues surrounding
the iron coordination centre (Huschka et al. 1986).
In spite of having specific transport mechanism of siderophore, many microbes
may utilize multiple transport system as well as more than one type of siderophore
at a time for efficient transport of metal ion. For example, microorganism like
Agaricus bisporus has variable transport systems for fusarinines and ferrichromes,
S. Bhardwaj et al.
7.1 Introduction
Plants and microbes have vast significance in our day-to-day life. Iron is considered
to be an abundant element in the soil (earth’s crust) and essential for all life processes such as respiration, DNA synthesis, tricarboxylic acid cycle and production
of various small molecules like amino acids, lipids, and sterols. Being an essential
element in earth’s crust, the bioavailability of iron is limited in the habitat (soil and
sea) owing to its low solubility. This property of iron results in its poor uptake by
plants, which eventually makes iron an essential nutrient for plant growth. In aqueous and oxygenated conditions, the iron is found in its supreme state, which accumulates in the form of minerals such as iron oxides and iron hydroxides and is not
ready to be utilized as such by organisms. In order to overcome this restricted process, the microbial flora of soil such as Pseudomonas spp., Enterobacter genus,
Bacillus spp. produces special iron-binding tiny carriers, called ‘siderophore’, that
help to scavenge iron from these mineral phases (oxides and hydroxides) by forming soluble iron (Fe
3+
) complexes which are readily taken up by the environment
through active transport mechanism (Philpott 2006).
Siderophore came from Greek words sidero that means ‘iron’ and phore that
means ‘carriers’, and in combination, it is termed as ‘iron carrier’. Siderophores are
small, low-molecular-weight (<10 kDa) iron-chelating compounds, secreted by
plants and microorganisms (bacteria and fungi) to maintain their iron requirement.
These are also produced by rhizospheric bacteria in iron-limiting conditions in
order to increase the plant growth by scavenging iron from the environment and
make it available to the cell near the plant roots (Sah and Singh 2015; Li et al. 2016).
7.2 Siderophore-Mediated Iron Transport
Iron transport in siderophore is an energy-dependent mechanism. Type and stereoselectivity of siderophore are specific factors in recognition and transport of ironsiderophore complexes in microbes. The complexation also depends on metal ion
coordination geometry as well as N-acyl residues present at the periphery of central
metal ion. For instance, the coordination of metal centre and configuration of ligand
affect the stability of complex. In case of Rhodotorula pilmanae, configuration of
macrocyclic rings of siderophore is favoured, whereas in contrast to this, in
Penicillium parvum, Neurospora crassa and Aspergillus quandricinctus, L-cisferrichrome is found to be a stable configuration. Further, the geometrical stability
of complex also depends on the types and number of N-acyl residues surrounding
the iron coordination centre (Huschka et al. 1986).
In spite of having specific transport mechanism of siderophore, many microbes
may utilize multiple transport system as well as more than one type of siderophore
at a time for efficient transport of metal ion. For example, microorganism like
Agaricus bisporus has variable transport systems for fusarinines and ferrichromes,
S. Bhardwaj et al.