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concomitantly present difficulties as the flip side
of the Al-tolerance breeding equation. Because
transgressive segregation can be exploited, eluci-
dating the molecular nature of Alt SB -interacting
genes may bring breeding efforts to a new
level, resulting in the development of highly
Al-tolerant lines. However, genetic background
effects can also reduce the efficiency of molec-
ular breeding strategies based solely on Alt SB ,
leading to incomplete transfer of Al tolerance
from donors to the recipient lines. Efforts are
now under way to identify and characterize addi-
tional Al-tolerance genes so that comprehen-
sive molecular breeding strategies can be formu-
lated and applied to breeding highly Al-tolerant
sorghum cultivars.
rapid Al-activated root citrate exudation, with
exudation rates usually being higher in the Al-
resistant maize genotypes studied (SA3: Pellet
et al. 1995; IAC-TAIUBA: Jorge and Arruda
1997; ATP-Y: Kollmeier et al. 2001; CMS36:
Cateto Colombia and Cateto 100-6:Pineros et al.
2002; Mariano and Keltjens 2003; Pineros et al.
2005; Pi neros et al. 2008b). However, in contrast
to the strong correlation between Al tolerance
and Al-activated root malate release observed in
more than 36 different wheat genotypes (Ryan
et al. 1995a; Ryan et al. 1995b), the correla-
tion between Al tolerance and Al-activated cit-
rate exudation in maize roots has usually been
studied with a single Al-tolerant genotype, com-
paring it with one or two Al-sensitive lines. In
fact, a comparative study using a panel of six
genotypes that capture the range of maize Al
tolerance and that differ significantly in their
genetic background (three Brazilian and three
North American genotypes) indicated that the
degree of Al tolerance among the genotypes
was not entirely correlated with the magni-
tude of Al-induced citrate release (Pi neros et al.
2005). Although they found a positive correla-
tion between root tip Al-exclusion (based on root
Al content) and Al tolerance, the authors of that
study also reported a significant lack of correla-
tion between differential Al tolerance and root
citrate exudation for the six maize genotypes,
with several of the Al-sensitive lines from Brazil
and North America also exhibiting significantly
high rates of citrate exudation upon exposure
to aluminum. Although this study identified cit-
rate as the only organic acid that was released in
an Al-activated manner, large constitutive exu-
dation rates of other potential Al-binding lig-
ands (e.g., malate and phosphate) were reported.
Consequently, it is quite likely that in con-
trast to other species like wheat, Al-induced cit-
rate release is only one of several mechanisms
operating simultaneously in the very tolerant
maize genotypes. Although root citrate efflux
plays a significant role, maize Al tolerance
appears as a complex quantitatively inherited
trait (see next section) with several physiological
Maize AlTolerance
Physiological Mechanisms of Maize Al
Tolerance
Root growth inhibition is one of the earliest tar-
gets and symptoms of Al toxicity, making rel-
ative root growth (i.e., the root growth ratio
between
Al/-Al growth conditions) a com-
mon and suitable phenotypic criteria to assess
Al tolerance. However, phenotypic comparisons
should be made with caution, as the experimen-
tal conditions employed (e.g., composition of the
nutrient solutions and Al 3 + activities) vary sig-
nificantly among different studies. Factors such
as amelioration of Al toxicity in high-ionic-
strength nutrient solutions (Magnavaca et al.
1987) should be taken into account when com-
paring the Al tolerance of a given genotype
relative to that reported in studies where toler-
ance has been assessed in less physiological,
simple-salt solutions. Even so, a general pat-
tern has emerged correlating the degree of maize
Al tolerance with lower levels of Al accumula-
tion in the root tips, a result that strongly sup-
ports the assumption that Al tolerance in maize
is being mediated by an Al-exclusion mecha-
nism. This hypothesized Al-tolerance mecha-
nism is also supported by the observation of
+
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