Aluminum Tolerance in Sorghum and Maize - Translational Genomics for Crop Breeding: Abiotic Stress, Yield and Quality - page 88

markers possible, and these markers are poten-
tially useful for identifying Al-tolerant acces-
sions in diverse germplasm collections. One
clear gain in such a strategy is the identifica-
tion of materials that may already be adapted
to the target environmental conditions. How-
ever, a better understanding of the distribution
of Al tolerance with regard to patterns of genetic
diversity in sorghum are needed to direct such
molecular breeding strategies. With this purpose,
Caniato and colleagues (2011) assessed popu-
lation structure and Al tolerance in a diverse
cultivated sorghum collection and, as expected,
observed a rather low frequency of Al tolerance
in sorghum (approximately 5%). From this sur-
vey the highly Al-tolerant line, SC566, was iden-
tified as a useful Alt SB donor (also, Magalhaes
et al. 2004 showed the presence of a functional
Alt SB allele in SC566). Genetic analysis based
on markers tightly linked to Alt SB and SbMATE
expression analysis confirmed an important role
for Alt SB in providing Al tolerance to most of
the Al-tolerant accessions. The fact that the vast
majority of the panel was composed of either Al-
sensitive (80%) or intermediately tolerant (14%)
accessions emphasizes the need for elucidating
a possible relationship between genetic diver-
gence and Al tolerance, as a guide for pre-
breeding efforts aimed at the identification of
novel sources of Al tolerance in sorghum. A
population structure analysis revealed clusters
that were consistent with both geographical and
racial origins as previously described by Deu and
colleagues (2006). Interestingly, Al tolerance
was not randomly distributed across the species-
diversity continuum, being more prevalent in cer-
tain genetically differentiated subgroups featur-
ing specific racial and geographical origins. In
general, subpopulations containing guinea types
from western and southern Africa and, to a lesser
extent, caudatum subpopulations are important
repositories of Al tolerance in sorghum. These
results indicate that efforts toward the identifica-
tion of novel Al-tolerance sources in sorghum
have to be undertaken in light of the species
genetic diversity, and that marker-assisted intro-

gression will be needed in the observed cases
where Al tolerance does not overlap with popu-
lation substructure.
The degree of dominance related to Al tol-
erance was assessed as the ratio between dom-
inance (d) and additive (a) effects based on F 1
hybrids generated by crossing 17 accessions to
one common Al-sensitive line, BR007. Although
Al tolerance has been attributed to either dom-
inant (Furlani and Bastos 1990) or partially
dominant Al-tolerance genes (Magalhaes et al.
2004; Caniato et al. 2011), the results indicated
additive gene action in 4 donors, whereas Al
tolerance in 11 out of the 17 sorghum acces-
sions was either a recessive or partially reces-
sive trait. Only two accessions including SC283
showed a degree of dominance (d/a) exceeding
0.3 and strict complete dominance was never
observed.

Implications for Molecular Breeding
Strategies Aimed at Improving Al
tolerance in Sorghum

Although Alt SB explains a large portion of
the phenotypic variation for Al tolerance in
some crosses, the current data supports early
reports indicating the presence of additional Al-
tolerance genes in the sorghum genome. Fur-
thermore, because highly Al-tolerant transgres-
sive segregants have been detected in progeny
derived from SC283, these other genes appar-
ently act additively to Alt SB and may thus be used
in recombination-based schemes for improving
sorghum Al tolerance. These additional genes
are still unknown and may encode completely
distinct Al-tolerance mechanisms to Al-activated
organic acid release. Another possibility is that
these genes may act epistatically to Alt SB and
enhance Al tolerance by the same Al-activated
citrate release pathway controlled by SbMATE.
If so, accessory genes may influence SbMATE
expression or interact with the transporter pro-
tein, changing its permeability properties. With
the goal of improving sorghum Al tolerance,
accessory loci may offer an opportunity and