Aluminum Tolerance in Sorghum and Maize - Translational Genomics for Crop Breeding: Abiotic Stress, Yield and Quality - page 93

flanking the target region and should also be
expanded to other maize breeding programs
aiming to improve maize yield on acidic soils
throughout Latin America, Africa, and Asia.
As mentioned above, despite a number of
studies indicating that Al tolerance in maize is
controlled by several loci with additive effects
(Magnavaca 1982; Sawazaki and Furlani 1987),
nonadditive effects have also been reported for
this trait (Magnavaca and Bahia Filho 1995;
Concei¸ ao et al. 2009). As hybrid development
is one of the main products of a maize breed-
ing program, it will be important to evaluate the
combining ability of qALT6 under field condi-
tions, in order to predict its contribution to grain
yield and yield stability on acid soils.

tural research will enable us to identify and tar-
get protein residues/motifs underlying MATE
and ALMT transport properties critical for cit-
rate or malate transport and Al activation, and
consequently in Al tolerance. This structural-
functional information will also be used to
develop a platform for bioengineering MATE or
ALMT proteins, as a novel way to enhance cereal
Al tolerance.
Functional studies in Xenopus oocytes have
demonstrated that TaALMT1, as well as the sub-
sequently identified homologues in Arabidopsis
and rape (Hoekenga et al. 2006; Ligaba et al.
2006), encode malate permeable transporters
whose activities can be specifically enhanced
by the presence of extracellular Al 3 + (Sasaki
et al. 2004; Pineros et al. 2008a). The remarkable
similarities between the functional characteris-
tics of these ALMT transporters and the root
organic acid exudation in response to Al strongly
indicate that ALMT-type transporters underlie
the exudation process characterized at the whole
root level. Recently other members of the ALMT
family, including two from maize (ZmALMT1
and ZmALMT2) have also been identified and
implicated not only in Al-tolerance responses
but in a variety of physiological processes such as
mineral nutrition, malate homeostasis, and guard
cell function (Kovermann et al. 2007; Pi neros
et al. 2008b; Meyer et al. 2010; Meyer et al.
2011; Ligaba et al. 2012). The molecular and
functional characterization of ZmALMT1 and
ZmALMT2, particularly their expression pat-
terns and the lack of transport enhancement upon
exposure to Al, suggest that they are likely to be
involved in mediating other mineral nutrition and
ion homeostasis processes, rather than mediating
Al-enhanced transport responses in maize.
SbMATE is a major sorghum Al-tolerance
gene encoding a plasma membrane transporter
from a different family of proteins, namely the
multidrug and toxin extrusion (MATE) family
of transporters. The MATE family is one of
five major multidrug resistance (MDR) trans-
porter families, and although they are widely dis-
tributed across all kingdoms of living organisms,

Structure-functionAnalysis of
Membrane Transporters Involved
inRoot Citrate Exudation and Al
Tolerance

Prior to the molecular identification of
SbMATE1 and ZmMATE1, earlier studies
implementing electrophysiological approaches
(e.g., patch clamp) had already identified candi-
date membrane transporters in the plasma mem-
brane of wheat and maize protoplasts (Ryan et al.
1997; Kollmeier et al. 2001; Pineros and Kochian
2001; Zhang et al. 2001; Pineros et al. 2002).
The activity of these anion transporters was
shown to be modulated by extracellular Al 3 + .
The subsequent identification of TaALMT1 (for-
merly named ALMT1) from wheat (Sasaki et al.
2004) and SbMATE from sorghum (Magal-
haes et al. 2007) represents a pivotal break-
through, as members from two different families
of membrane transporter proteins [ALMT (Al-
activated malate transporter) and MATE (mul-
tidrug and toxic compound efflux)] mediate the
Al-activated root organic acid efflux underly-
ing Al tolerance in a number of plant species.
It is interesting to note that members of these
two families have similar transport functions but
quite different structural properties. Our think-
ing is that the integration of functional and struc-