3.5 ǀ Sensing of plant Fe status
Until now, the mechanisms of Fe sensing in plants have mostly been
investigated under Fe deficient conditions rather than under Fe excess.
In both rice and Arabidopsis (Arabidopsis thaliana ), a group of
proteins containing the hemerythrin motif are involved in sensing Fe
deficiency. In rice, Hemerythrin motif-containing RING- and zinc-finger
protein 1/2 (OsHRZ1/2), directly bind to Fe and functions as a molecular
sensor for Fe status (Kobayashi et al., 2013). OsHRZ1/2 proteins
negatively affect Fe deficiency responses, and knockdown of these
proteins renders plants less susceptible to Fe deficiency, probably by
destabilizing a basic helix-loop-helix (bHLH) transcription factor
OsPRI1 (Zhang et al., 2017). Likewise, a homologue of OsHRZ1/2 in
Arabidopsis, BRUTUS, negatively affects the stability of PRI1 homologues
(Arabidopsis bHLH105/115) and Fe deficiency responses (Selote et al.,
2015), indicating that the Fe sensing mechanisms via the hemerythrin
motif-containing proteins are conserved among higher plant species.
A recent study showed that knockdown of OsHRZ2 leads to susceptibility
to Fe excess stress in rice. The OsHRZ2-knockdown plants were associated
with greater growth retardation, leaf bronzing and shoot Fe
concentrations compared with the wild-type plants under Fe excess
condition (Aung, Kobayashi, Masuda & Nishizawa, 2018b).
Correspondingly, the expression of Fe deficiency-inducible genes
associated with Fe uptake, such as OsNAS1/2, mugineic acid transporter
OsTOM1, and Yellow Stripe-Like 15 (OsYSL15) that encodes a Fe
transporter, were higher in the OsHRZ2 knockdown plants than the
wild-type under the Fe excess (Aung et al., 2018b). Therefore, OsHRZ2,
and probably OsHRZ1 as well, are at least partly involved in sensing
excess Fe, and precise sensing of Fe status by these proteins is likely
to be important for tolerance. Other components involved in sensing Fe
status have been elucidated, such as the Fe-binding, graminaceous
species-specific transcription factor iron deficiency-responsive
elements factor 1 (IDEF1) (Kobayashi et al., 2007, 2012) and short
peptides IRON MAN (IMA) that are ubiquitously found in flowering plant
lineage (Grillet et al., 2018; Kobayashi, Nagano & Nishizawa, 2021).
IDEF1 is classified into the ABI3/VP1 transcription factor family which
binds to Fe2+ ion via its His-Asp repeats and Pro-rich
regions (Kobayashi et al., 2012), and the overexpression of rice
homologue (OsIDEF1) which leads to enhanced tolerance to Fe deficiency
(Kobayashi et al., 2007). Overexpression of Fe deficiency-inducible IMA
homologues in rice (OsIMA1/2) enhances the expression of many Fe
deficiency-inducible Fe uptake-related genes in roots (Kobayashi et al.,
2021). In Arabidopsis, IMA1-overexpression lines hyperaccumulate Fe and
exhibit foliar necrotic symptoms under the standard Fe conditions,
indicating Fe toxicity (Grillet et al., 2018). It remains to be
clarified whether these Fe sensing mechanisms are functional under the
Fe excess condition and linked with the tolerance in rice.
A recent GWAS based study on root shortening induced by excess Fe and a
subsequent mutant study revealed that S-nitrosoglutathione reductase
(GSNOR) is involved in sensing Fe status at the root tip (Li et al.,
2019). GSNOR alleviates Fe-dependent production of nitric oxide and
hydrogen peroxide, and resultant inhibition of root meristem growth.
Naturally occurring polymorphism in the promoter region of GSNOR gene
was associated with its expression level, and the allele of GSNOR that
yielded higher expression conferred tolerance. Consistently, the
knockout of rice homologues of GSNOR led to higher susceptibility to Fe
excess, in terms of root elongation (Li et al., 2019). It remains to be
determined if naturally occurring alleles of GSNOR in rice could be used
to increase the tolerance of currently grown rice cultivars and enhance
biomass production and grain yield.