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(Borgonovi et al. 1987a; Borgonovi et al. 1987b;
Furlani et al. 1987), and was consistently scored
as the most tolerant genotype both in field (acid
soil) trials and in hydroponic studies (Duncan
et al. 1983). Furthermore, SC283 was found to
be the most Al-tolerant cultivar in a screening
of 391 sorghum genotypes evaluated by Furlani
and colleagues (1987). Accordingly SC283 is
an outstanding genotype for inheritance studies
and for molecular mapping of sorghum Al tol-
erance. In a study by Furlani and Bastos (1990),
the inheritance of Al tolerance in crosses involv-
ing SC283 and two Al-sensitive sorghum lines
was consistent with the action of a single dom-
inant gene. Accordingly, dominant gene action
was clearly observed in this study, since F
1
fam-
ilies derived from any of the susceptible/tolerant
(SC283) crosses were associated with pheno-
typic values several times larger than either of
the Al-susceptible parents and closer to the Al-
tolerant parent.
subsequently developed in sorghum, with mark-
ers located in the syntenic region where major
Al-tolerance loci, which are likely orthologous
(Tang et al. 2000), reside in the Triticeae group
4 chromosomes. Although significant macrosyn-
teny was observed, the major Al-tolerance locus
in
Sorghum bicolor
, designated
Alt
SB
, was found
to reside elsewhere, mapping to the terminal
region of sorghum chromosome 3.
Genetic Diversity for Al Tolerance
in Sorghum
Breeding advances depend on the existence
of genetic variation that can be identified and
manipulated to generate improved cultivars.
For example, considering known Al-tolerance
donors in barley, there is little potential for
Al-tolerance improvement based on nonallelic
additive genes, a result of the presence of a
single Al-tolerance locus with multiple alle-
les (Minella and Sorrells 1992). Evaluating the
range of genetic diversity controlling Al toler-
ance in sorghum based on the
Alt
SB
locus was
therefore the key motivation in the Caniato and
colleagues (2007) study. This effort included a
genetic characterization of
Alt
SB
-based Al tol-
erance in sorghum accessions of different ori-
gins, which was overlaid with a broader genetic
diversity study using SSR (simple sequence
repeat) markers to elucidate genetic relation-
ships. Although a major Al-tolerance gene exists
in sorghum, similar to what was found in barley,
in sorghum there seems to be potential for Al-
tolerance improvement by exploiting additive or
codominant effects of distinct Al-tolerance loci.
This potential was emphasized by the observa-
tion that some sources showing variable degrees
of Al tolerance did not appear to rely primarily
on
Alt
SB
. Furthermore, a highly Al-tolerant trans-
gressive segregant was in fact detected. How-
ever, an allelic series at the
Alt
SB
locus was
also observed using near-isogenic lines (NILs),
where
Alt
SB
alleles from different donors had
been introgressed by marker-assisted backcross-
ing (MABC) into an Al-sensitive line. Multiple
The Alt
SB
Locus in Sorghum
Previous studies reporting the presence of sin-
gle major Al-tolerance loci in the long arm of
wheat chromosome 4D (
Alt
BH
, Riede and Ander-
son 1996) and barley chromosome 4H (Tang
et al. 2000), linked to the RFLP marker locus
Xbcd1230
, prompted efforts to use comparative
mapping between sorghum and species in the
Triticeae tribe to quickly identify Al-tolerance
loci in sorghum (Magalhaes et al. 2004). As
reported by Furlani and Bastos (1990), bimodal
frequency distributions for Al tolerance in a
SC283-derived population was also observed
when F
2:3
families derived from a BR007 (highly
Al-sensitive line from the Embrapa Maize and
Sorghum breeding program) x SC283 cross was
evaluated for Al tolerance in nutrient solution set
at pH 4.0 and containing 27 μMAl
3
+
activity.
In the study by Magalhaes and colleagues, the
segregation ratio for Al tolerance and sensitivity
conformed to that expected for a single major Al-
tolerance locus, which nonetheless behaved in a
semidominant fashion. A comparative map was
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