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

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K. Gasic and S.S. Korban
related to an up-regulation in expression of genes encoding for Fe 2+ uptake, and possibly
of that TcIRT1-G.
Nicotianamine (NA) is a metal chelator ubiquitously present in higher plants.
In graminaceous plants, NA is a biosynthetic precursor of phytosiderophores, and it is
therefore a crucial component in iron acquisition. In addition to their roles in
phytosiderophore secretion from roots, three rice nicotianamine synthase genes,
OsNAS1, OsNAS2 and OsNAS3, have been found to play important roles in longdistance
transport of iron in rice plants (Inoue et al., 2003). In maize, iron (III)-
phytosiderophores are taken up by roots via YS1 transporters (Curie et al., 2001), belonging
to the OPT oligopeptide transporter family (Fig. 1). Schaaf et al. (2004) have
reported that ZmYS1 encodes a proton-coupled broad-range metal-phytosiderophore
transporter that additionally transports Fe- and Ni-nicotianamine. These biochemical
properties indicate a novel role for YS1 transporters for heavy metal homeostasis in
plants.
Comparative analysis of gene expression profiles in Arabidopsis (Wintz et al.,
2003) revealed specific transcriptional regulation by metals of genes contrasting with
known wide-substrate specificities of encoded transporters. It was reported that two
ZIP genes, ZIP2 and ZIP4, were involved in copper transport; while, AtOPT3, a member
of the oligopeptide transporter gene family with significant similarities to the maize
iron–phytosiderofore YS1, was regulated by metals. Moreover, heterologous expression
of AtOPT3 could rescue yeast mutants deficient in metal transport.
Moreau et al. (2002) reported on the identification, characterization, and localization
of GmZIP1, the first soybean member of the ZIP family of metal transporters.
GmZIP1 encoded a symbiosis-specific zinc transporter in soybean. An antibody raised
against GmZIP1 specifically localized the protein to the peribacteroid membrane, an
endosymbiotic membrane in nodules resulting from the interaction of the plant with its
microsymbiont. It was found that GmZIP1 possessed eight putative transmembrane
(TM) domains together with a histidine-rich extra-membrane loop.
The two zinc transporters identified in rice showed many similarities in function,
but they differed in expression pattern, ionic selectivity, and optimum pH for
activity (Ramesh et al., 2003). One gene was widely expressed under all conditions,
while the other gene was mainly induced upon exposure of plants to zinc deficiency and
also to higher levels in roots than in leaves.
Another family of iron transporters, with homology to Nramp genes (Natural
resistance associated macrophage proteins, TC 2.A.55), has been identified in plants.
Thomine et al. (2000) carried out functional studies and characterized three members of
the Nramp gene family in Arabidopsis. Their findings suggested that AtNramp genes
encoded multispecific metal transport systems in plants that could transport Fe, Mn,
and Cd 2+ (Fig. 1). Following gene expression analyses of AtNramp in Arabidopsis it
was reported that Nramp genes play roles in constitutive metal transport.