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Alternative
biotechnologies for controlling Striga:
the next generation
J.
Gressel
New biotechnologies will
be needed for Striga control.
The IR-maize seed treatment technology is an excellent stop-gap
technology; but it too can be perfected by controlled slow release
formulation of the herbicides used so as to prevent phytotoxic
concentration of herbicide near the crop root in low rainfall
conditions, or washout of the herbicide in high rainfall, as well as to
extend the duration of control needed for long season maize.
Other herbicide resistant genes could be introduced/stacked;
based on work with Orobanche and
transgenic crop; inhibitors of
EPSP-synthase (glyphosate) and dihydropteroate synthase (asulam) should
be effective. Additionally,
work on Orobanche seedlings and tissue cultures indicated that herbicides
affecting cellulose synthase as well as tubulin polymerization severely
inhibited the parasitic weed. Crops
with target site resistance should allow control. Stacked genes for herbicide resistances would allow the use
of herbicide mixtures, which would delay the inevitable evolution of
resistance. The promising
results of putting toxin genes behind a wound-inducible promoter in the
crop have been obtained with parasitic Orobanche
and could be tested with Striga
and the crops affecting it. Too
little is known about the pathway(s) of biosynthesis of Striga germination stimulation; whether one or more stimulants are
produced, and whether from the same precursors. Should there be a single precursor, and should the pathway
not be required for vital functions, it could be suppressed by RNAi or
antisense technologies. The
fact that random gene suppression by transposon tagging has not
elucidated a Striga resistant
mutant suggests that this may be a futile hope.
Native microorganisms pathogenic on Striga
have been proposed as biocontrol agents (mycoherbicides,
bioherbicides), but none seem to be sufficiently virulent for
controlling Striga in row crop situations.
Such organisms can be augmented with transgenes for
hypervirulence that would interfere with the Striga hormonal balance
and/or that secrete toxins toxic to the parasite, as has been done with
mycoherbicides for Orobanche. Additional genes could be introduced to produce
ethylene to stimulate Striga
germination. We proposed a
strategy to debilitate Striga
designed to lead to its self control.
This would entail developing transgenic Striga
with high copy number transposons carrying suppressed kev
genes, i.e. deleterious transposons (DTs) which will quickly spread
the gene to field populations because S.
hermonthica is an obligate outcrosser requiring exogenous
pollination. The kev genes could be turned on by a chemical inducer applied by the
farmer or a novel compound produced by a transgenic variant of the crop.
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