Aluminum Tolerance in Sorghum and Maize - Translational Genomics for Crop Breeding: Abiotic Stress, Yield and Quality - page 90

mechanisms operating simultaneously. Although
further compelling evidence is still required,
additional mechanisms could potentially involve
exudation of phenolic compounds (Tolra et al.
2009; Kidd et al. 2001) and/or changes in cell-
wall pectin content and degree of methylation
(Eticha et al. 2005), which have started to receive
some attention in the literature.

for specific combining ability (SCA) in diallele
crosses (Magnavaca et al. 1987; Paterniani and
Furlani 2002; Concei¸ ao et al. 2009), in agree-
ment with the identification of Al tolerance QTL
showing partial dominance effects (Ninamango-
Cardenas et al. 2003).

Mapping of Al Tolerance QTL in Maize

In the first published study on Al-tolerance loci
in maize, Moon and colleagues (1997) used
somaclonal variation to generate an Al-sensitive
mutant from a highly Al-tolerant inbred line,
Cateto 100-6. Using a mapping population gen-
erated from these parents, two loci ( Alm1 ,on
the short arm of chromosome 6; and Alm2 ,on
the short arm of chromosome 10) contributing
to Al tolerance were identified (Sibov et al.
1999). Subsequently, Ninamango-Cardenas and
colleagues (2003) mapped Al-tolerance QTL
using a population of 168 F 3:4 families gener-
ated from a cross between a highly Al-tolerant
inbred commonly used as a tolerance donor in
the breeding programs (L1327, currently named
Cateto Al237 or Al237), and Al-sensitive inbred
line L53. Five QTL were detected on chromo-
somes 2, 6, and 8 that could explain 60% of the
variance in Al tolerance, measured as net seminal
root growth in a hydroponic system. For all but
one of the QTL, the tolerant allele was donated
by the tolerant parent.
Using the F 3 generation of a cross generated
from a different set of inbred lines, Concei¸ ao
and colleagues (2009) mapped five QTL that
together explain 41% of the variation in Al tol-
erance, in this case measured as root regrowth
after Al stress. This work detected SSR mark-
ers associated with Al tolerance that could be
considered as coincident with QTL previously
detected (see Table 6 in Conceicao et al. 2009),
with exception for the locus detected in chromo-
some 4 explaining 10% of the phenotypic varia-
tion. QTL were detected on chromosomes 5, 6,
and 8, on locations equivalent to those described
by Ninamango-Cardenas and colleagues (2003),
and the QTL on chromosome 10 is an equivalent

Genetics of Maize Al Tolerance

As can be inferred from its physiological charac-
teristics, the genetics of Al tolerance in maize is
also quite complex. High genetic variability for
Al tolerance has been reported in tropical and
temperate maize germplasm, using hydroponic-
based phenotyping (Rhue and Grogan 1977;
Magnavaca 1982; Furlani et al. 1986) and on acid
soils with different level of Al saturation (Bahia
Filho et al. 1978; Naspolini Filho et al. 1981),
as well as in sand culture irrigated with nutrient
solution (Garcia J unior et al. 1979). Although
most of the genetic studies agree that Al tol-
erance is a quantitative trait in maize, divergent
conclusions were reached using different genetic
materials. Rhue and colleagues (1978) and Gar-
cia Junior and Silva (1979) reported that Al tol-
erance is controlled at a single dominant locus,
in which the wide variability for this trait in
maize would be explained by a multiple allelic
series (Rhue et al. 1978) or by modifiers (Gar-
cia J unior and Silva 1979). However, Al toler-
ance in F 2 progenies showed continuous and
unimodal frequency distributions, typical for a
quantitatively inherited trait (Magnavaca 1982;
Magnavaca et al. 1987). Brondani and Paiva
(1996) described Al tolerance as a quantitative
trait but also reported on dominant allele interac-
tions. In addition to confirming the genetic com-
plexity of this trait, other studies have empha-
sized the contribution of additive gene effects to
the total genetic variation in maize Al tolerance
(Sawazaki and Furlani 1987; Pandey et al. 1994;
Borrero et al. 1995). Nevertheless, dominance
effects may contribute to Al tolerance in maize,
as revealed by significant mean square values