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We therefore tried to determine if there
was indeed an association between the
preharvest sprouting resistance suscepti-
bility of wheat cultivars and their tolerance
to aluminium toxicity or if this was mere
coincidence.
How did we conduct the
research?
A total of 41 wheat cultivars were screened
for their preharvest sprouting resistance
tolerance/susceptibility over a period of ten
years. This screening was done in a rain
simulator where the ripe wheat ears were
subjected to a simulated rain treatment and
scored according to the sprouting of the ker-
nels that took place in the ears. The various
cultivars were grouped into resistant, mod-
erate or susceptible groups.
The same cultivars were screened in the
laboratory for aluminium tolerance, based
on the haematoxylin staining method. Ac-
cording to this method cultivars could be
classed as tolerant (where the tip of the
root was not stained, indicating successful
regrowth after staining) versus suscepti-
ble (where the root tip was stained, which
means that no regrowth of the root took
place after staining).
In addition to the laboratory screening of
aluminium tolerance, molecular markers
were used to determine the presence of
different forms of the aluminium tolerance
gene
ALMT1
. These different forms confer
varying levels of aluminium tolerance in
each cultivar.
What did we find?
The preharvest sprouting resistance of the
cultivars in the study varied from good (in
35% of the cultivars) to susceptible (in 21%
of the cultivars) (
Graph 2
). The laboratory
screening of aluminium tolerance indicated
that 34% of the cultivars were tolerant and
54% of the cultivars were susceptible.
In the molecular laboratory four types of
the
ALMT1
gene were identified. Type I
and Type II confer aluminium susceptibility,
while Type V is an indication of aluminium
tolerance. Type IV shows moderate toler-
ance to aluminium.
A very good correlation (R
2
= 0,81) was
observed between the laboratory screen-
ing of aluminium and the marker validation.
This means that the
ALMT1
marker data
(
Photo 4
) showed similar results to the re-
search that was conducted with the visual
screening of aluminium tolerance.
In
Table 1
the preharvest sprouting resist-
ance data and the aluminium results for
the wheat cultivars used in this study are
shown. The cultivar Matlabas had a mixed
ALMT1
genotypic profile, showing both
Type II and Type IV alleles. The Type IV al-
lele is dominant, so this means that the culti-
var has a moderate tolerance to aluminium.
This was also confirmed by the phenotypic
screening.
The results further indicated that there is a
significant negative correlation (R
2
= -0,68)
between the preharvest sprouting resist-
ance characteristics of the cultivars and
their aluminium tolerance or susceptibility.
This means that in 94% of cultivars tested,
cultivars with ‘n good preharvest sprouting
resistance tolerance (preharvest sprouting
resistance score lower than 3), had poor al-
uminium tolerance. Similarly, in 75% of the
cultivars tested, cultivars with poor prehar-
vest sprouting resistance tolerance (prehar-
vest sprouting resistance score higher than
5), showed good aluminium tolerance.
The reason for this is still speculative. One
of the explanations is the fact that the major
genes responsible for preharvest sprout-
ing resistance tolerance and aluminium
tolerance are positioned on the same chro-
mosome. This creates a competition for
‘genetic space’ which complicates the nor-
mal crossing process in breeding. The fact
that these two important traits occur on the
same wheat chromosome, and also at simi-
lar positions, suggests that most of the time
a natural crossover event will knock each
other out.
To make progress in traditional breeding
programmes, thousands of lines will need
to be phenotypically screened in order to
try and find a line that contains both traits.
Without the dedicated selection for prehar-
vest sprouting resistance and aluminium
tolerance with molecular markers, this will
be very difficult, if not impossible.
Why is this important in
the breeding process?
There is a significant negative correlation
between preharvest sprouting resistance
and aluminium sensitivity in South African
wheat cultivars, which was confirmed by
molecular data.
Graph 1: Illustration to indicate susceptibility and resistance to preharvest sprouting
resistance (lower values indicate resistance to preharvest sprouting resistance).
Graph 2: Screening for preharvest sprouting resistance and aluminium in the
laboratory and through marker assisted selection.