5
siderophores (Armstrong and Baalen 1979). Siderophores with functional groups
other than C (=O) N-(OH) R, lysine derivative (e.g., mycobactin), ornithine derivative (e.g., pyoverdine), and histamine derivative (e.g., anguibactin) are also
recognized.
1.4 Fungal Siderophores and Iron Transport
The siderophores are closely associated with metals, especially iron. Their biosynthesis is regulated by the internal concentration of iron (Renshaw et al. 2002). A
wide range of living organisms including the plants compete for iron through siderophores (Ahmed and Holmstrom 2014). The primary mechanism is through the
suppression of the plant diseases induced by the biological control agents via the
production of antimicrobial secondary metabolites such as siderophores, antibiotics, volatile substances, and many more. The living organisms utilize more than one
mechanism for the optimal iron transport. Among them, two mechanisms are widely
observed: (i) transportation through siderophores that are specific to each organism
and (ii) adherence to iron in the form of ferric oxide. The second mechanism is
simple and favors a much faster iron transport.
The iron-chelated siderophores are transported inside the cell by siderophore
iron transporters. The iron acquisition of the organism depends on specific siderophore iron transporters. There are ten siderophore iron transporters in Aspergillus
species. MirA and MirB are the siderophore iron transporters found in A. nidulans.
MirB transports triacetyl fusarinine C in A. nidulans and A. fumigatus. The siderophores with the transporters are internalized and the iron from triacetyl fusarinine C
is released by esterase B. The iron from the iron-siderophore complex is also
released by reductase (Howard 1999). In fungi, the iron transportation is mainly
through siderophores and most of the fungi secrete more than one type of siderophores. The iron transportation is achieved through a mechanism, the siderophoremediated transport system (SMTS) that requires energy. The transported iron is
utilized in enhancing the fungal growth. SMTS helps the fungi to compete effectively with the growth of other microorganisms by limiting the availability of iron.
Though the system is believed to be useful in acquiring other metals, iron is the only
documented essential element acquired by fungi. SMTS consist of four major transportation mechanisms (Zheng and Nolan 2012; Haas 2003; Luca and Wood 2000):
(a) Vehicle transport is facilitated by the penetration of iron-siderophore complex
into the fungal cell and subsequent intracellular breakup by fungal reductase.
(b) The siderophores mediated taxicab transport that results in ligand exchange and
subsequent iron transfer.
(c) Hydraulic acquirement is facilitated by the influx of the iron-mediated siderophores and consequent breakup by fungal intracellular mechanisms.
(d) The iron-siderophore complex triggers the reductive procurement that results in
reduction of iron adjacent to fungal membrane and subsequent uptake of Fe
2+
.
1 Basics of Fungal Siderophores: Classification, Iron Transport and Storage…
siderophores (Armstrong and Baalen 1979). Siderophores with functional groups
other than C (=O) N-(OH) R, lysine derivative (e.g., mycobactin), ornithine derivative (e.g., pyoverdine), and histamine derivative (e.g., anguibactin) are also
recognized.
1.4 Fungal Siderophores and Iron Transport
The siderophores are closely associated with metals, especially iron. Their biosynthesis is regulated by the internal concentration of iron (Renshaw et al. 2002). A
wide range of living organisms including the plants compete for iron through siderophores (Ahmed and Holmstrom 2014). The primary mechanism is through the
suppression of the plant diseases induced by the biological control agents via the
production of antimicrobial secondary metabolites such as siderophores, antibiotics, volatile substances, and many more. The living organisms utilize more than one
mechanism for the optimal iron transport. Among them, two mechanisms are widely
observed: (i) transportation through siderophores that are specific to each organism
and (ii) adherence to iron in the form of ferric oxide. The second mechanism is
simple and favors a much faster iron transport.
The iron-chelated siderophores are transported inside the cell by siderophore
iron transporters. The iron acquisition of the organism depends on specific siderophore iron transporters. There are ten siderophore iron transporters in Aspergillus
species. MirA and MirB are the siderophore iron transporters found in A. nidulans.
MirB transports triacetyl fusarinine C in A. nidulans and A. fumigatus. The siderophores with the transporters are internalized and the iron from triacetyl fusarinine C
is released by esterase B. The iron from the iron-siderophore complex is also
released by reductase (Howard 1999). In fungi, the iron transportation is mainly
through siderophores and most of the fungi secrete more than one type of siderophores. The iron transportation is achieved through a mechanism, the siderophoremediated transport system (SMTS) that requires energy. The transported iron is
utilized in enhancing the fungal growth. SMTS helps the fungi to compete effectively with the growth of other microorganisms by limiting the availability of iron.
Though the system is believed to be useful in acquiring other metals, iron is the only
documented essential element acquired by fungi. SMTS consist of four major transportation mechanisms (Zheng and Nolan 2012; Haas 2003; Luca and Wood 2000):
(a) Vehicle transport is facilitated by the penetration of iron-siderophore complex
into the fungal cell and subsequent intracellular breakup by fungal reductase.
(b) The siderophores mediated taxicab transport that results in ligand exchange and
subsequent iron transfer.
(c) Hydraulic acquirement is facilitated by the influx of the iron-mediated siderophores and consequent breakup by fungal intracellular mechanisms.
(d) The iron-siderophore complex triggers the reductive procurement that results in
reduction of iron adjacent to fungal membrane and subsequent uptake of Fe
2+
.
1 Basics of Fungal Siderophores: Classification, Iron Transport and Storage…