54
lular and intracellular siderophores. They can participate in the transport or storage of
ferrous iron (Winkelmann 2007). Siderophores are either excreted to mobilize extracellular or intracellular iron produced mainly for iron storage. They are specific
growth inhibitors of several phytopathogenic fungi, including Phytophthora parasitica, Pythium ultimum, Fusarium oxysporum vera dianthi, and Sclerotinia sclerotiorum.
Siderophores act as plant growth regulators (Verma et al. 2011; Yadav et al.
2011), biological control agents (Verma et al. 2011), and bioremediation agents.
Iron is the essential component for various vital processes (photosynthesis,
enzyme cofactor, redox reagent, respiration, synthesis of nucleosides and amino
acids) of the plant. For avoiding deficiency of iron, various plants seem to rely on
the excretion of phytosiderophores (i.e., chelate compounds, common in grasses
that sequester iron) by the roots and secretion of siderophores by a group of microbes
to facilitate the Fe complex uptake under iron deficiency conditions, which binds
with high affinity for iron (Mino et al. 1983; Neilands 1995; Takagi 1976). Although
iron is one of the most abundant elements on earth, bioavailability is less in aerobic
conditions (in the presence of oxygen and at neutral pH), primarily because ferric
iron (concomitant oxidization of Fe
2+
to Fe
3+
) reacts with oxygen to form insoluble
ferric hydroxides. Conversely, excess iron or incorrect storage of iron is deleterious
as “free iron” (reduced ferrous) catalyzes the production of cell-damaging reactive
oxygen species via the Fenton reaction. To maintain iron homeostasis, the delicate
balance of sufficient iron supply while preventing iron-induced toxicity and subsequent cell-damage regulated strategies for the careful control of iron uptake, utilization, and storage have evolved in a diversity of organisms.
To date, siderophore secretion systems have been described in only a few microorganisms. Siderophores are exported from the microbial cell by efflux pumps. As for
fungi, little was known until recently about the role of iron in the interactions between
fungi and hosts, and about the general regulatory mechanisms that govern iron homeostasis. In fungi, siderophore uptake is mediated by high-affinity transporters from the
major facilitator family, which have strong specificity for their substrates (Haas 2014).
However, a successful microbe must be able to acquire iron from its host’s iron-limiting environment through the expression of high-affinity iron absorption systems. Two
main systems compromised by fungi are the assimilation of reductive iron and the
absorption of iron assisted by siderophores, in plants. Furthermore, its importance for
the acquisition of iron for interactions between fungi and hosts and the regulatory
mechanisms that influence this aspect will be examined. Recent reports have shown
that some fungal siderophores act as determinants of virulence (Eisendle et al. 2006;
Oide et al. 2006) and play a role in maintaining symbiotic fungal plant interactions
(Johnson 2008). Conversely, in other fungi, the components of the reductive iron
assimilation pathway are necessary for virulence (Ramanan 2000).
Siderophores have been biochemically characterized from various fungi and
their structures elucidated. The majority of fungal siderophores identified to date
belong to Zygomycotina, Ascomycotina, and Deuteromycotina. Generally, fungi
produce a hydroxamate type of siderophore. Exceptions are the carboxylate-type
siderophore rhizzoferrin produced by several Mucorales and the catecholate pistillarin produced by the marine species Penicillium bilaii. Most widely studied fungi
S. Ashraf et al.
lular and intracellular siderophores. They can participate in the transport or storage of
ferrous iron (Winkelmann 2007). Siderophores are either excreted to mobilize extracellular or intracellular iron produced mainly for iron storage. They are specific
growth inhibitors of several phytopathogenic fungi, including Phytophthora parasitica, Pythium ultimum, Fusarium oxysporum vera dianthi, and Sclerotinia sclerotiorum.
Siderophores act as plant growth regulators (Verma et al. 2011; Yadav et al.
2011), biological control agents (Verma et al. 2011), and bioremediation agents.
Iron is the essential component for various vital processes (photosynthesis,
enzyme cofactor, redox reagent, respiration, synthesis of nucleosides and amino
acids) of the plant. For avoiding deficiency of iron, various plants seem to rely on
the excretion of phytosiderophores (i.e., chelate compounds, common in grasses
that sequester iron) by the roots and secretion of siderophores by a group of microbes
to facilitate the Fe complex uptake under iron deficiency conditions, which binds
with high affinity for iron (Mino et al. 1983; Neilands 1995; Takagi 1976). Although
iron is one of the most abundant elements on earth, bioavailability is less in aerobic
conditions (in the presence of oxygen and at neutral pH), primarily because ferric
iron (concomitant oxidization of Fe
2+
to Fe
3+
) reacts with oxygen to form insoluble
ferric hydroxides. Conversely, excess iron or incorrect storage of iron is deleterious
as “free iron” (reduced ferrous) catalyzes the production of cell-damaging reactive
oxygen species via the Fenton reaction. To maintain iron homeostasis, the delicate
balance of sufficient iron supply while preventing iron-induced toxicity and subsequent cell-damage regulated strategies for the careful control of iron uptake, utilization, and storage have evolved in a diversity of organisms.
To date, siderophore secretion systems have been described in only a few microorganisms. Siderophores are exported from the microbial cell by efflux pumps. As for
fungi, little was known until recently about the role of iron in the interactions between
fungi and hosts, and about the general regulatory mechanisms that govern iron homeostasis. In fungi, siderophore uptake is mediated by high-affinity transporters from the
major facilitator family, which have strong specificity for their substrates (Haas 2014).
However, a successful microbe must be able to acquire iron from its host’s iron-limiting environment through the expression of high-affinity iron absorption systems. Two
main systems compromised by fungi are the assimilation of reductive iron and the
absorption of iron assisted by siderophores, in plants. Furthermore, its importance for
the acquisition of iron for interactions between fungi and hosts and the regulatory
mechanisms that influence this aspect will be examined. Recent reports have shown
that some fungal siderophores act as determinants of virulence (Eisendle et al. 2006;
Oide et al. 2006) and play a role in maintaining symbiotic fungal plant interactions
(Johnson 2008). Conversely, in other fungi, the components of the reductive iron
assimilation pathway are necessary for virulence (Ramanan 2000).
Siderophores have been biochemically characterized from various fungi and
their structures elucidated. The majority of fungal siderophores identified to date
belong to Zygomycotina, Ascomycotina, and Deuteromycotina. Generally, fungi
produce a hydroxamate type of siderophore. Exceptions are the carboxylate-type
siderophore rhizzoferrin produced by several Mucorales and the catecholate pistillarin produced by the marine species Penicillium bilaii. Most widely studied fungi
S. Ashraf et al.