Aluminum Tolerance in Sorghum and Maize - Translational Genomics for Crop Breeding: Abiotic Stress, Yield and Quality - page 92

significant changes to the functional character-
istics of the protein. This was confirmed by the
fact that Xenopus oocytes injected with cRNA
made from either allele showed the same trans-
port properties via electrophysiological analysis
(Maron et al. in preparation). These results sug-
gest that it is the level of expression of ZmMATE1
and not functional differences that underlie the
Al-tolerance QTL that ZmMATE1 represents in
this population. Moreover, eQTL mapping of
ZmMATE1 indicates that expression is controlled
mostly in cis. Recent data suggests that structural
variation in the ZmMATE1 locus is responsible
for the expression differential observed between
the parental lines (Maron et al. in preparation).
However, the expression pattern of ZmMATE1 in
maize NILs differed from the SbMATE expres-
sion in sorghum NILs. SbMATE expression often
was lower when different Alt SB alleles were intro-
gressed into a common Al-sensitive line, sug-
gesting the existence of regulatory factors acting
in trans on SbMATE (Melo et al. 2012).

Cateto was widely adopted by the early European
immigrants, representing the most widespread
maize racial group in South America. Hence
Cateto is classified as an ancient commer-
cial maize race because of its indigenous pre-
Columbian origin followed by its extensive com-
mercial use as maize varieties or in local hybrid
programs (Paterniani and Goodman 1977). Even
without a high-yielding performance, Cateto
exhibited high combining ability when crossed
with different races and considerable adaptation
to specific environments (Paterniani and Good-
man 1977). Thus, Al tolerance can be considered
an important adaptive trait carried by Cateto,
which may have contributed to its overall accep-
tance in Brazil, a region with large areas of acid
soils.
Based on our recent findings, one QTL
explaining 16% of the variation for Al tolerance
in a recombinant inbred line (RIL) population
derived from Cateto Al237 was co-localized with
a candidate gene, ZmMATE1 , which encoded
the maize root citrate efflux transporter (Maron
et al. 2010). More recently, the genetic map for
this RIL population was saturated with markers
generated via genotyping-by-sequencing (GBS,
Elshire et al. 2011), and this same genomic
region harboring ZmMATE1 , named qALT6 ,was
able to improve Al tolerance in maize NILs,
when transferred to an Al-sensitive line (L53)
using marker-assisted backcrossing (Guimaraes
et al. in preparation). Nevertheless, Cateto has
limited use in a modern maize breeding program
because of its divergence from improved mate-
rials. Despite the large genetic diversity present
in maize, this crop species has experienced one
of the most intensive breeding efforts of all culti-
vated crop species, mainly focused on adapted
materials, and this has contributed to widen-
ing the distance between breeding lines and
germplasm bank materials (Nass and Paterniani
2000). Thus, a pre-breeding program is the most
promising alternative for linking the introduc-
tion of Al-tolerance alleles from native Brazilian
races into elite lines of maize. This strategy could
be performed using marker-assisted selection

Molecular Breeding for Al Tolerance
in Maize

Al-tolerance studies in maize have a long history,
and genetic materials derived from Cateto have
been highlighted as Al-tolerant since the early
1980s, both under field conditions (Naspolini
Filho et al. 1981) and in nutrient solution (Mag-
navaca 1982; Furlani et al. 1986; Sawazaki and
Furlani 1987). Subsequently, two major Cateto-
derived inbred lines, Cateto Al237 and Cateto
100-6 (or C100-6), were selected as sources of
Al tolerance for QTL mapping (Sibov et al. 1999;
Ninamango-Cardenas et al. 2003), physiological
investigations (Moon et al. 1997; Pineros et al.
2005), and molecular studies in response to Al
stress (Maron et al. 2008; 2010; Mattiello et al.
2010). Thus, it has been shown that Cateto is
clearly an important source of Al tolerance in
tropical maize germplasm.
Cateto constitutes a group of landraces origi-
nally cultivated by the native peoples living in
coastal areas from Argentina to the Guianas.