63
4.6.1 Role of Siderophores in Fungal Pathogenicity
In plant fungal pathogenesis, the roles of siderophores vary between the different
pathosystems, and the importance of the siderophores for fungal pathogenesis was
first reported in the pathogen of maize C. heterostroph (Lee et al. 2005; Oide et al.
2006). Later it was reported that iron metabolism mediated by siderophores is necessary for a complete virulence in the pathogen of rice C. miyabeanus, the wheat pathogen F. graminearum, and the Brassicaceae pathogen A. brassicicola (Table 4.1). Hof
et al. (2007) revealed that the inhibition of the synthesis of ferricrocin siderophore in
Magnaporthe grisea influences its pathogenicity of rice. However, siderophore apparently has no role in the virulence of basidiomycete maize pathogen Ustilago maydis.
Siderophores are also an important factor of virulence of the human pathogen
Aspergillus fumigatus (Hissen et al. 2005; Schrettl et al. 2007).
4.6.2 Role of Siderophores, in (ROS) Generation, and Fungal
Sexual Development
Siderophores, iron-scavengers, have a crucial role in oxidative defense. Papanikolaou
and Pantopoulos suggested that excess of intracellular iron can cause oxidative
stress through the Fenton reaction while on the other hand, heam is essential for the
function of many peroxidases, an important family of enzymes in the detoxification
of hydrogen peroxide. Thus, it is important in fungi to maintain the iron concentration carefully. Eisendle et al. (2006) showed that oxidative stress causes upregulation of intracellular ferricrocin siderophore in A. nidulans. Similarly, this increase is
also shown when exposed to excessive iron conditions, although it is not clear
whether this is due to the iron itself or due to the iron-mediated oxidative stress
resulting in the cell.
Deletion of siderophore synthetase sidC inhibits the production of ferricrocin,
resulting in different phenotypes, including inefficient use of iron, delayed germination in iron replete conditions, and inhibiting the formation of cleistothecia in
homothallic conditions. Additionally these phenotypes, the conidia of the sidC
mutant, show sensitivity to H 2 O 2 . In A. fumigatus, a similar relationship of siderophores has been reported in oxidative defense mechanism. In the null mutant siderophore, ΔsidA observed severe sensitivity to O 2 , while the absence of intracellular or
extracellular siderophore observed low sensitivity separately. This suggested that
both intra- and extracellular siderophores play a pivotal role in mediating oxidative
damage and showed redundancy in the system. The cross-talk between ROS detoxification and siderophores is not limited to Aspergillus species. In Alternaria alternata, the elimination of NPS6, NRPS essential for the biosynthesis of extracellular
siderophores, increases the sensitivity to H 2 O 2 .
In addition, the expression of NPS6 in A. alternata is regulated by NOX, YAP1, and
HOG1, genes involved in oxidative defense mechanisms (Chen et al. 2014). Deletion of
NPS6 in Cochliobolus miyabeanus, Fusarium graminearum, and Alternaria brassicicola also resulted in enhanced sensitivity to oxidative stress (Oide et al. 2006).
4 Siderophores: Mediated Iron Acquisition and Virulence of Brown Rot Disease…
4.6.1 Role of Siderophores in Fungal Pathogenicity
In plant fungal pathogenesis, the roles of siderophores vary between the different
pathosystems, and the importance of the siderophores for fungal pathogenesis was
first reported in the pathogen of maize C. heterostroph (Lee et al. 2005; Oide et al.
2006). Later it was reported that iron metabolism mediated by siderophores is necessary for a complete virulence in the pathogen of rice C. miyabeanus, the wheat pathogen F. graminearum, and the Brassicaceae pathogen A. brassicicola (Table 4.1). Hof
et al. (2007) revealed that the inhibition of the synthesis of ferricrocin siderophore in
Magnaporthe grisea influences its pathogenicity of rice. However, siderophore apparently has no role in the virulence of basidiomycete maize pathogen Ustilago maydis.
Siderophores are also an important factor of virulence of the human pathogen
Aspergillus fumigatus (Hissen et al. 2005; Schrettl et al. 2007).
4.6.2 Role of Siderophores, in (ROS) Generation, and Fungal
Sexual Development
Siderophores, iron-scavengers, have a crucial role in oxidative defense. Papanikolaou
and Pantopoulos suggested that excess of intracellular iron can cause oxidative
stress through the Fenton reaction while on the other hand, heam is essential for the
function of many peroxidases, an important family of enzymes in the detoxification
of hydrogen peroxide. Thus, it is important in fungi to maintain the iron concentration carefully. Eisendle et al. (2006) showed that oxidative stress causes upregulation of intracellular ferricrocin siderophore in A. nidulans. Similarly, this increase is
also shown when exposed to excessive iron conditions, although it is not clear
whether this is due to the iron itself or due to the iron-mediated oxidative stress
resulting in the cell.
Deletion of siderophore synthetase sidC inhibits the production of ferricrocin,
resulting in different phenotypes, including inefficient use of iron, delayed germination in iron replete conditions, and inhibiting the formation of cleistothecia in
homothallic conditions. Additionally these phenotypes, the conidia of the sidC
mutant, show sensitivity to H 2 O 2 . In A. fumigatus, a similar relationship of siderophores has been reported in oxidative defense mechanism. In the null mutant siderophore, ΔsidA observed severe sensitivity to O 2 , while the absence of intracellular or
extracellular siderophore observed low sensitivity separately. This suggested that
both intra- and extracellular siderophores play a pivotal role in mediating oxidative
damage and showed redundancy in the system. The cross-talk between ROS detoxification and siderophores is not limited to Aspergillus species. In Alternaria alternata, the elimination of NPS6, NRPS essential for the biosynthesis of extracellular
siderophores, increases the sensitivity to H 2 O 2 .
In addition, the expression of NPS6 in A. alternata is regulated by NOX, YAP1, and
HOG1, genes involved in oxidative defense mechanisms (Chen et al. 2014). Deletion of
NPS6 in Cochliobolus miyabeanus, Fusarium graminearum, and Alternaria brassicicola also resulted in enhanced sensitivity to oxidative stress (Oide et al. 2006).
4 Siderophores: Mediated Iron Acquisition and Virulence of Brown Rot Disease…