Aluminum Tolerance in Sorghum and Maize - Translational Genomics for Crop Breeding: Abiotic Stress, Yield and Quality - page 85

Al-tolerance genes from the fine mapping of
Al-tolerance loci in sorghum (Magalhaes et al.
2007) and barley (Furukawa et al., 2007). Our
group identified the sorghum MATE via the posi-
tional cloning of the major sorghum Al-tolerance
locus, Alt SB . This gene, SbMATE , encodes a
plasma membrane citrate transporter responsi-
ble for the root citrate exudation in response to
Al stress (Magalhaes et al. 2007). More recently
we identified a maize ortholog, ZmMATE1 ,as
a major Al-tolerance gene in maize (Maron
et al. 2010). The identification of these genes
in sorghum and maize, along with HvMATE
(Furukawa et al. 2007), AtMATE (Hoekenga
et al. 2006), OsFRDL4 (Yokosho et al. 2011),
ScFRDL2 (Yokosho et al. 2010), and VuMATE1
(Yang et al. 2011), which are root citrate efflux
transporters involved in barley, Arabidopsis, rice,
rye, and rice bean Al tolerance, indicates a broad
role for this subgroup of plant MATEs in crop Al
tolerance.
The identification of these plant Al-tolerance
genes has provided the materials for more
efficient and effective molecular breeding of
enhanced crop Al tolerance. In this chapter, we
will describe the characterization of the major
sorghum and maize Al-tolerance genes, SbMATE
and ZmMATE1 , and how we are using the infor-
mation from this research to develop molecular
breeding pipelines for improved sorghum and
maize adaptation to acid soils.

develop sorghum cultivars adapted to acid soils.
In contrast to barley and, to a lesser extent, wheat,
inheritance of Al tolerance in sorghum has not
been historically explained by a simple genetic
model. A high degree of general combining abil-
ity (GCA) for Al tolerance has been reported
(Boye-Goni et al. 1985; Borgonovi et al. 1987a;
Gourley et al. 1990; Flores et al. 1991), which
suggests that genes with additive effects could
be controlling sorghum Al tolerance. However,
specific combining ability (SCA) effects were
also found to be significant, although less impor-
tant than GCA effects. This fact, along with an
early report of a bimodal frequency distribution
in progeny derived from the cross of Al-tolerant
and Al-sensitive sorghum cultivars (Furlani and
Bastos 1990), strongly suggested the existence
of dominant genes with major phenotypic effects
in some of the sources of sorghum Al tolerance.
Borgonovi et al. (1987b) summarized the find-
ings of a number of field and hydroponic-based
studies on Al tolerance and stated that, in gen-
eral, sorghum Al tolerance is controlled by a
few major genes with dominant effects, prob-
ably one partially dominant gene, and several
minor genes with some additive effects. Mater-
nal effects for Al tolerance have not been com-
monly observed in sorghum. Heritability associ-
ated with the trait was reported as high for Al
tolerance assessments based on Al inhibition of
root growth (Boye-Goni and Marcarian 1983;
Borgonovi et al. 1987a), indicating that a large
proportion of the variability may be explained
by genetic factors (Borgonovi et al. 1997a). The
observed high degree of broad-sense heritability
(Furlani and Bastos 1990) indicates that assess-
ment of Al tolerance, either for breeding or
genetic mapping purposes, should be effective
in the F 2 generation.
SC283 (IS7173), a sorghum cultivar belong-
ing to the guinea race collected in Tanzania,
is the most widely accepted Al-tolerance stan-
dard in this species. This cultivar has consis-
tently exhibited exceptional values for Al tol-
erance in the United States (Duncan et al.
1983; Duncan 1988; Foy et al. 1993) and Brazil

Sorghum AlTolerance

Historical Aspects of Aluminum
Tolerance in Sorghum

As Al tolerance is not frequently found in wheat,
Garvin and Carver (2002) have suggested that
this trait represents a derived state rather than
an inherent characteristic of the crop. This also
appears to pertain to sorghum, since among
world populations of sorghum screened up to
1993, only 5% of the lines showed apprecia-
ble tolerance to Al (Foy et al. 1993). Nonethe-
less, extensive genetic variability for Al tolerance
exists and has long been utilized by breeders to