Aluminum Tolerance in Sorghum and Maize - Translational Genomics for Crop Breeding: Abiotic Stress, Yield and Quality - page 94

their transport properties and functional roles
remain largely unknown (Omote et al. 2006;
Moriyama et al. 2008). In contrast to the low
number of MATE genes per species found in
other kingdoms, there are a large number of
plant MATE genes (40 to 60 family members
in the Arabidopsis, rice, and Medicago truncat-
ula genomes) suggesting a wide variety of dif-
ferent biological roles in planta (reviewed by
Yazaki 2005). Although the transport properties
of most plant MATEs remain unknown, recent
studies have begun to characterize several plant
MATEs, suggesting diverse physiological roles
such as the transport of cationic flavonoid, xeno-
biotic, and alkaloid substrates (Debeaujon et al.
2001; Diener et al. 2001; Li et al. 2002; Otani
et al. 2005; Marinova et al. 2007; Morita et al.
2009). Since the discovery of SbMATE, ortho-
logues involved in mediating organic acid efflux
in response to Al stress have also been identi-
fied in barley, maize, rice bean, rice, and Ara-
bidopsis (Furukawa et al. 2007; Liu et al. 2009;
Maron et al. 2010; Yokosho et al. 2011). Addi-
tional plant MATE transporters permeable to cit-
rate have also been identified that appear to be
involved in Fe translocation in the xylem (as an
Fe-citrate complex) and not Al tolerance (Durrett
et al. 2007; Yokosho et al. 2009).
As shown in Figure 6.1, phylogenetic analy-
sis of the plant MATEs that have been function-
ally characterized to date reveals a unique sub-
group of functionally characterized plant MATEs
that transport citrate (highlighted in red in Fig-
ure 6.1b). These transporters include those that
have been shown to be involved in sorghum,
maize, barley, rye, rice bean, Arabidopsis, and
rice Al tolerance, as well as rice and Arabidop-
sis MATEs localized to the root pericycle and
involved in the loading of citrate into the xylem
for Fe translocation to the shoot. Structurally,
comparison of the amino acid sequences of all
of the citrate-permeable plant MATEs in Fig-
ure 6.1b indicates that these proteins are mem-
bers of the NorM-like subset (COG0534) of
the MatE superfamily (Pfam01554), sharing a

Fig. 6.1. Phylogenetic analysis of MATE-type trans-
porters for all plant MATE transporters that have been func-
tionally characterized to date. Plant members colored in red
represent MATE's that have been shown to mediate citrate
transport. The asterisks indicate members mediating citrate
release in response to aluminum stress. The tree was built
using protein sequences with Geneious Tree Builder soft-
ware. For a color version of this figure, please refer to the
color plate.

common predicted secondary structure consist-
ing of about 500 to 700 amino acids, containing
12 transmembrane helices with a long ( 100

residues) cytoplasmic N-tail, and a distinc-
tive long cytoplasmic loop between the second
and third transmembrane domain. The presence
of the characteristic long cytoplasmic N-tails
suggests that these MATEs may interact with
other proteins (Moriyama et al. 2008).
Functionally, results from electrophysiologi-
cal and 14 C efflux studies have established that
when expressed in X . oocytes, the subgroup of
SbMATE-like plant transporters mediates H +
(and possibly Na + )-coupled citrate efflux. These
transport characteristics present intriguing ques-
tions regarding substrate recognition, energy
coupling, and transport mechanism, given that
most other experimentally characterized MATE
transporters across all kingdoms show prefer-
ence for organic cation substrates (see Table I in
Omote et al. 2006). Furthermore, while expres-
sion of any of the members of this subgroup of
SbMATE-like transporters in heterologous sys-
tems has resulted in a constitutive electrogenic

∼