Physiology and Molecular Biology of Stress ... - KHAM PHA MOI

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K. Gasic and S.S. Korban
Iron chelate reductases or the ferric reductase oxidase (FRO) protein family are
known to be responsible for the ability of plants to change the redox state of Fe +3 in the
soil as a first step in iron uptake (Fig. 2). These have been detected in Arabidopsis
(Robinson et al., 1997; Robinson et al., 1999), pea (Pisum sativum) (Waters et al., 2002),
and rice (Gross et al., 2003).
Nicotianamine (NA), a non-proteinaceous amino acid, is ubiquitously present
in higher plants, and it is known to be involved in chelation of metals. Nicotianamine
aminotransferase (NAAT) catalyzesthe amino group transfer of NA in the biosynthetic
pathway of phytosiderophores, and it is essential for iron acquisition in graminaceous
plants. In addition to its role in long-distance metal transport, NA is proposed to be
involved in the regulation of metaltransfer within plant cells (Pich et al., 2001; Takahashi
et al., 2003). On the other hand, nicotianamine synthase (NAS) is an enzyme that is
critical for the biosynthesis of the mugineic acid family of phytosiderophores in
graminaceous plants and for homeostasis of metal ions in nongraminaceous plants.
Mizuno et al. (2003) have revealed that maize has two types of NAS proteins, based on
their expression pattern and subcellular localization. A total of three NAS genes have
been isolated from maize, one genomic clone, ZmNAS3, and two cDNA clones, ZmNAS1
and ZmNAS2. Both ZmNAS1 and ZmNAS2 are expressed only in Fe-deficient roots.
This is in agreement with the reported increased secretion of phytosiderophores. In
contrast, ZmNAS3 is expressed under Fe-sufficient conditions, and it is negatively
regulated by Fe-deficiency, thus suggesting a role different than that of a precursor in
phytosiderophore production. Although ZmNAS1-green fluorescent protein (sGFP)
and ZmNAS2-sGFP have been localized in the cytoplasm of onion epidermal cells,
ZmNAS3-sGFP is distributed throughout the cytoplasm of these cells.
The hyperaccumulation of zinc (Zn) and cadmium (Cd) is a constitutive property
of the metallophyte A. halerii. Recently, Weber et al. (2004) have used Arabidopsis
GeneChips to identify those genes that are more active in roots of A. halerii than A.
thaliana under controlled conditions. Two genes showing highest levels of expression
in A. halerii roots code for a nicotianamine (NA) synthase and a putative Zn 2+
uptake system. In addition, roots of A. halerii also show higher levels of both NA
synthase and NA. Expression of NA synthase in Zn 2+ -hypersensitiveSchizo-
saccharomyces pombe cells has demonstrated that formation of NA can confer
Zn 2+ tolerance. Taken together, these observations suggest active roles of NA in plant
Zn homeostasis and NA synthase in hyperaccumulation of Zn in A. halerii.
On the other hand, algae respond to heavy metals by induction of several
antioxidants, including diverse enzymes such as superoxide dismutase, catalase, glutathione
peroxidase, and ascorbate peroxidase, as well as synthesis of low-molecular
weight compounds such as carotenoids and glutathione (Pinto et al., 2003). Boominathan
and Doran (2003) demonstrated that metal-induced oxidative stress occurred in tissues
of the hyperaccumulator T. caerulescens even though growth was unaffected in the
presence of heavy metals. They also suggested that superior antioxidative defenses,
particularly catalase activity, might play important roles in the hyperaccumulator phe-