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location to the
Alm2
QTL described by Sibov
and colleagues (1999).
that could potentially lead to novel Al-tolerance
mechanisms are still lacking.
The study by Maron and colleagues (2008)
identified two genes encoding members of
the MATE family of transporters that showed
expression patterns consistent with a potential
role in Al tolerance. These were characterized
in a subsequent study (Maron et al. 2010).
One of these genes,
ZmMATE1
, was mapped
to the telomeric region of chromosome 6, co-
localizing with a major Al tolerance QTL that
explains 16.2% of the phenotypic variance for
Al tolerance. This genomic region was pre-
viously associated with Al tolerance in two
QTL studies (Sibov et al. 1999; Ninamango-
Cardenas et al. 2003).
ZmMATE1
encodes a 563
amino acid protein sharing significant identity to
SbMATE and AtMATE. Transient expression of
ZmMATE1::GFP fusions in Arabidopsis proto-
plasts indicated that the protein is localized to
the plasma membrane, while [
14
C]-Citrate efflux
studies in oocytes showed that ZmMATE1 is
able to mediate citrate transport. In addition,
ZmMATE1
expression in transgenic Arabidopsis
conferred a significant increase in Al tolerance
and root citrate exudation in response to Al.
Quantitative real-time PCR (qPCR) showed
that
ZmMATE1
expression is concentrated in
the root tips and is strongly up-regulated by
Al, with much higher Al-induced expression
in the Al-tolerant genotypes. Expression in the
absence of Al is also significantly higher in Al-
tolerant Al237 and C100-6 (used in the origi-
nal microarray study) compared to Al-sensitive
L53.
ZmMATE1
expression is up-regulated by
Al as early as one hour after exposure. It is inter-
esting to note that gene up-regulation is also
observed in L53, although the relative levels
of
ZmMATE1
expression in L53 are not very
high even after up-regulation by Al. Cloning the
cDNA from the parents of the mapping popu-
lation (Cateto Al237, Al-tolerant and L53, Al-
sensitive) revealed only six nucleotide differ-
ences in the coding region, of which two resulted
in amino acid substitutions. These small differ-
ences in protein sequence are unlikely to cause
Molecular Biology of Maize Al Tolerance
Few studies have attempted to elucidate the
molecular mechanisms underlying maize Al tol-
erance and, so far, these studies have not been
able to shed light on potential alternative tol-
erance mechanisms (i.e., other than root tip Al
exclusion based on root citrate release). A large
body of research on other abiotic stresses indi-
cates that stress can affect the expression of
numerous genes and that specific changes in
gene expression can play a key role in deter-
mining the resistance response. On this basis,
two large studies of transcriptional profiling in
maize roots have been performed, both based
on microarrays (Maron et al. 2008; Mattiello
et al. 2010). The two studies also used the same
maize line, Cateto 100-6, as the Al-tolerant geno-
type chosen for comparison. In the first study,
Maron and colleagues performed a detailed tem-
poral analysis of root gene expression under Al
stress. Al altered the expression of significantly
more genes in the Al-sensitive line, possibly as
a result of more severe toxicity symptoms. Nev-
ertheless, several Al-regulated genes exhibited
higher expression in the Al-tolerant line. Many
cell wall-related genes were found to be regu-
lated by Al, as well as genes previously shown to
respond to low phosphorus, another stress com-
montoacidsoils.
In a second study of global changes in gene
expression in response to Al, Mattiello and col-
leagues (2010) looked at the transcriptome of
maize roots growing in acid soils containing
toxic levels of Al. The genetic materials used
in this study were the highly Al-tolerant Cateto
100-6 and the same somaclonal mutant derived
from it by Moon and colleagues (1997). Inter-
estingly, several genes previously reported as
up-regulated by Al based on hydroponic exper-
iments were also identified in roots grown in
acid soil. Previously unidentified genes were also
detected, but follow-up physiological studies
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