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Novel
determinants of osmotic stress tolerance using the monocotyledonous
resurrection plant Xerophyta
viscosa
S.G. Mundree
& J.A.Thomson
Water
has become a major limiting factor in world agriculture.
In general, most crop plants are highly sensitive to even mild
dehydration stress. There are, however, a few genera of plants unique to
sourthern Africa called “resurrection plants” which can tolerate
extreme water loss or desiccation. The unique ability of these plants to
withstand severe water loss (greater than 90%) makes them an ideal
system to study osmotic stress tolerance. Xerophyta
viscosa, a representative of the monocotyledonous resurrection
plants, was used to isolate genes that are functionally important in
osmotic stress tolerance, and those that are differentially expressed
during dehydration stress, respectively.
A cDNA library, based on mRNA isolated during various stages of
dehydration was inserted in Lambda Phage Zap II vector (Stratagene).
The advantage of using the above vector is that it allowed for
the rescue of phagemids containing the cDNA inserts by helper
phage-mediated in vivo
excission. The rescued
phagemids were used to infect a specific strain of Escherichia
coli (srl: Tn 10 mutant) which was grown under severe osmotic
stress. With IPTG present
in the medium, bacterial cells containing cDNA that conferred osmo-tolerance,
emerged as colonies on minimal medium plates supplemented with 1.2 M
sorbitol. Nine colonies containing cDNA which conferred functional
sufficiency to the osmotically-stressed bacterial cells were identified.
Two of the cDNA have been characterised extensively viz. ALDRXV4,
an aldose reductase homologue, and XVSAP1, a stress-associated protein.
The second strategy involved the differential screening of 192 randomly
selected cDNA clones from above cDNA library. Of the 192 cDNA screened,
30 were found to be up-regulated. cDNA
have been sequenced and characterised.
The identities of these proteins included: XCDH, a dehydrin;
XVLEA, a lea-like homogue; XVGS, galactinol synthase, XVCBP, EF-hand
calcium-binding protein; XVHSP90, a heat shock 90 protein; and XVPER, a
peroxiredoxin. All six of
the cDNA have been found to be expressed during dehydration streess in
the resurrection plant and they have also been implicated in other
stresses, particularly those that have an osmotic stress component.
Evaluation of cassava clones for yield, farmer acceptability and disease
and pest resistance
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