Roles of Iron in Plant Defence and Fungal Virulence
[Plant Signaling & Behavior 2:4, 300-302; July/August 2007]; ©2007 Landes Bioscience
David L. Greenshields
Department of Biology; University of Saskatchewan; Saskatoon, Canada
*Correspondence to: Yangdou Wei; Department of Biology; University of
Saskatchewan; 112 Science Place; Saskatoon, SK S7N 5E2, Canada;
Tel.: 1.306.966.4447; Fax: 1.306.966.4461; Email:firstname.lastname@example.org
Original manuscript submitted: 02/20/07
Manuscript accepted: 02/20/07
Previously published online as a Plant Signaling & Behavior E-publication:
iron, oxidative burst, reactive oxygen, plant
defence, fungal pathogen, siderophore
Targeted Alterations in Iron Homeostasis Underlie Plant
Liu G, Greenshields DL, Sammynaiken R, Hirji RN,
Selvaraj G, Wei Y
J Cell Sci 2007; 120:596–605
Iron is an essential component of various proteins and pigments for both plants and
pathogenic fungi. However, redox cycling between the ferric and ferrous forms of iron
can also catalyse the production of dangerous free radicals and iron homeostasis is
therefore tightly regulated. our work has indicated that monocot plants challenged
by pathogenic fungi redistribute cellular iron to the apoplast in a controlled manner to
activate both intracellular and extracellular defences. In the apoplast, the accumulation of
free, reactive ferric iron mediates defensive H2o2 production. Inside the cell, this efflux of
iron creates a state of iron depletion, which directs the transcription of pathogenesis‑re‑
lated genes in concert with H2o2. In this addendum, we describe differences between
the roles of iron in mediation of the oxidative burst in cereal and Arabidopsis responses
to fungal pathogens. Also, we discuss the implications of current work concerning fungal
iron uptake on host defence strategies.
Iron AnD the oxIDAtIve burst In PLAnts
The oxidative burst, a localized release of reactive oxygen species (ROS) by host
plant cells following pathogen recognition, is well-documented.1,2 In the model plant
Arabidopsis thaliana, the respiratory burst oxidase homologues (RBOH) AtrbohD and
AtrbohF are responsible for the majority of hydrogen peroxide (H2O2) production
following inoculation with the avirulent bacterium Pseudomonas syringae pv tomato or
the virulent oomycete Hyaloperonospora parasitica.3 Since the RBOHs are NADPH
oxidases that produce superoxide (O2·-), this H2O2 arises via dismutation rather than being
a direct product of the RBOHs. Likewise, Nicotiana benthamiana NbrbohA and NbrbohB,
which are similar to AtrbohF and AtrbohD, respectively, are required for H2O2 produc-
tion in response to Phytophthora infestans.4 In addition to RBOHs, type III peroxidases
have recently been shown to generate significant levels of H2O2 in Arabidopsis suspension
cultures in response to pathogen-derived elicitors.5 Significantly, silencing of peroxidises,
but not RBOHs, led to increased Arabidopsis susceptibility.3,5 In monocots, the role of
individual ROS generators is less clear. In wheat and barley, H2O2 is produced at defen-
sive cell wall appositions (CWAs); fortified papillae of crossed linked phenolics, suberin,
callose and proteins.6 O2·- however, is produced in epidermal cells only in association with
successful fungal penetration.7,8 Recently, Trujillo et al9 found that silencing the barley
AtrbohF homologue HvrbohA led to increased penetration resistance against the powdery
mildew fungus Blumeria graminis f. sp. hordei, suggesting that O2·- is required for cellular
accessibility in that system. Unlike O2·-, H2O2 is produced at wheat and barley CWAs
in response to successful or defeated host fungi as well as nonhost fungi, and is therefore
linked to basal resistance, which is active in all plants against all pathogens.
Basal resistance-linked H2O2 production can be seen as early as 3 h after inoculation,
and in the wheat-powdery mildew pathosystem it is always associated with the accumula-
tion of ferric iron.10 This finding shows interesting parallels between iron-mediated H2O2
production in plants and animals. Iron accumulation at CWAs is not specific to wheat; we
found similar CWA-associated ferric iron in pathogen challenged barley, oat, corn, sorghum
and millet, suggesting that iron accumulation is a universal phenomenon in cereals. This
CWA-associated iron is important in the context of the oxidative burst because when the
iron is blocked from accumulating, either with the iron chelator deferoxamine (DFO) or
by blocking iron-laden vesicle like bodies from arriving at CWAs with the actin filament
disruptor cytochalasin A, there is a concomitant loss of H2O2 production at the CWA.
300 Plant Signaling & Behavior 2007; Vol. 2 Issue 4
www.landesbioscience.com Plant Signaling & Behavior 301
While basal resistance in both cereals and Arabidopsis involves the
elaboration of a CWA and H2O2 production,11 the source of the H2O2
differs. We have now shown that basal resistance-linked H2O2 produc-
tion is mediated by CWA iron accumulation in cereals, but we found
no iron accumulation at Arabidopsis CWAs (Fig. 1). The broad loss of
resistance to both virulent and avirulent pathogens in peroxidase-si-
lenced Arabidopsis5 suggests that peroxidase may fill the role that iron
plays in cereals. Interestingly, the Arabidopsis AtrbohD-dependent
oxidative burst is activated by ROS and goes on to limit the spread
of pathogen-induced cell death,12 suggesting that the knee-jerk
production of H2O2 by peroxidases may prime a more targeted
ROS production pathway. It would be interesting to investigate a
possible interaction between iron-mediated H2O2 production and
RBOH-dependent cellular accessibility in cereals.
Iron AnD FunGAL PAthoGenesIs
Having found important roles for iron in cereal defence mecha-
nisms, we also began to investigate the role of iron uptake in the
fungal pathogen Fusarium graminearum (Greenshields et al., In
Press). In order to scavenge host iron for survival, fungal pathogens
have evolved at least two iron acquisition systems. One system is
hinged on the secretion and subsequent uptake of ferric iron-specific
siderophores and the other system uses cell wall iron reductases to
free bound or insoluble ferric iron by reducing it to ferrous iron for
uptake.13 Recently, Oide et al.14 described the role of the nonribosomal
peptide synthase NPS6 in extracellular siderophore production
and showed that it was required for full virulence in the ascomy-
cetes Cochliobolus heterostrophus, C. miyabeanus, F. graminearum and
Alternaria brassicicola. Interestingly, siderophore production is not
required for virulence in the basidiomycetes Ustilago maydis15 and
Microbotryum violaceum,16 but loss of the ferroxidase/permease
system of reductive iron uptake leads to a reduction in U. maydis
virulence.17 On the surface, it seems convenient to attribute these
different modes of infection-related iron uptake to the taxonomic
distance between ascomycetes and basidiomycetes. However, neither
Saccharomyces cerevisiae nor Candida albicans can produce or secrete
siderophores13 and B. graminis f. sp. hordei spores show abundant
ferric reductase activity.18 Also, B. graminis f. sp. hordei conidia
express the multicopper oxidase gene FET3,19 which is required
for full virulence in U. maydis.17 In light of our data showing the
induction of wheat PR genes following DFO treatment, it is tempting
to consider that pathogen produced siderophores may in fact work
as pathogen-associated molecular patterns (PAMPS) in triggering host
defences. All known plant pathogenic fungi that require siderophore
production for virulence are necrotrophs,14 which are not as sensitive
to recognition by the host as biotrophs.20 Also, U. maydis, the only
biotrophic pathogen characterised with respect to iron uptake, uses the
reductive uptake system, thus avoiding secretion of potentially recogni-
sable siderophores. Similarly, what little evidence that exists suggests
that biotrophic B. graminis f. sp. hordei also uses the reductive iron
uptake system.18,19 It will be interesting to explore this relationship
further and to understand how fungal siderophores are recognised
and handled by host plants.
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Figure 1. Differential iron accumulation at wheat and Arabidopsis cell wall
appositions (CWA). A blue ring of ferric iron (Fe3+) can be seen surrounding
the Blumeria graminis f. sp. tritici penetration attempt on the wheat epidermis
(top), but is absent from the attack site on the Arabidopsis leaf (bottom). Fixed
leaves were stained for iron using the Prussian blue technique. C, conidium;
Iron in Plant Disease
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Iron in Plant Disease
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