Biology Reference
In-Depth Information
Alt
SB
alleles were found to encode a wide range
of Al-tolerance phenotypes. In summary, the
Caniato and colleagues (2007) study indicated
that both allelic and non-allelic heterogeneity
are important factors for breeding for Al-tolerant
sorghum. Although
Alt
SB
is ikelytobean
important player relative to other loci in pro-
viding Al tolerance, recombination-based breed-
ing strategies thus emerge as a potentially useful
approach to exploiting transgressive segregation
in sorghum acid soil breeding programs.
sorghum is conferred by non-orthologous loci.
Quantitative RT-PCR showed that
SbMATE
is
more highly expressed in the first centimeter of
the root specifically in the Al-tolerant genotypes
and
SbMATE
expression increases over time of
exposure to Al
3
+
. This incremental increase in
SbMATE
expression in response to Al correlated
closely with the observed increase in root
citrate exudation and the Al-tolerance induction,
both over time in Al, strongly suggesting that
SbMATE is an organic acid transporter that
confers Al tolerance via the
Alt
SB
locus. Cellular
localization studies using SbMATE::GFP fusion
proteins and electrophysiological analysis in
Xenopus laevis
oocytes indicated that SbMATE
functions as a plasma membrane anion efflux
transporter responsible for citrate release into the
rhizosphere. Finally, genetic complementation
experiments where
SbMATE
was expressed
in a highly Al-sensitive Arabidopsis T-DNA
knockout mutant in which an Arabidopsis
homolog of the wheat
ALMT1
gene is disrupted,
demonstrated that expression of
SbMATE
resulted in a significant increase in Al tolerance
as well as Al-activated root citrate exudation.
These findings indicate that MATE transporter
functions in sorghum as an Al-activated citrate
transporter that confers Al tolerance via the
Alt
SB
locus. Initially characterized as microbial
drug transporters, MATE transporters are in
fact polyspecific, and different plant MATEs
have been shown to transport a range of organic
substrates (discussed in Magalhaes 2010).
Molecular and Physiological Basis of Al
Tolerance Conferred by Alt
SB
A positional cloning strategy was applied to
elucidate the molecular nature of the Al-
tolerance gene underlying the
Alt
SB
locus in
sorghum, which was accompanied by a charac-
terization of the physiological mechanism con-
trolled by the underlying gene (Magalhaes et al.
2007). Data obtained with Al-tolerant (ATF10B)
and Al-sensitive (ATF8B) NILs derived from
BR007
SC283 provided support for an Al-
exclusion mechanism based on Al-activated
citrate release from Al-tolerant ATF10B root
apices, with a general exclusion zone extend-
ing at least 20 mm from the root apex (Magal-
haes 2002). Interestingly, both Al tolerance and
Al-activated citrate release were found to be Al
inducible, significantly increasing over time of
exposure to Al (4-6 days; Magalhaes et al. 2007).
High-resolution
×
mapping
using
differ-
ent
mapping
populations
derived
from
BR007
SC283 led to the identification of
a 24.6-Kb region that harbored only three open
reading frames (ORFs; Magalhaes et al. 2007).
Only one of these ORFs, with high sequence
similarity to Arabidopsis and rice genes encod-
ing multidrug and toxic compound extrusion
(MATE) family proteins (Brown et al. 1999),
was highly expressed specifically in root apices
of the Al-tolerant NIL, ATF10B. This
Sorghum
bicolor
homolog in the MATE family, desig-
nated
SbMATE
, is distinct from wheat
ALMT1
,
thus confirming that Al tolerance in wheat and
×
The Relationship between Population
Structure and Al Tolerance in Sorghum
The cloning of
SbMATE
as the major gene
responsible for the
Alt
SB
locus opened up
new, gene-based avenues for molecular breed-
ing strategies aimed at improving Al tolerance
in sorghum. Al tolerance can be targeted tradi-
tionally by marker-assisted backcross programs
to introgress
Alt
SB
into cultivars that are sensitive
to Al toxicity. However, the identity of
SbMATE
now
makes
the
development
of
functional
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