Cereals processing technology

68 Cereals processing technology
4.8 Examples of transformed rice and maize
4.8.1 Herbicide resistance
Understanding the expression of simple reporter and selectable marker genes in
transgenic plants is important in predicting the behaviour of agronomically
useful genes introduced into crops. Consequently, it is imperative to assess gene
expression in large seed populations. In this respect, Zhong et al. (1996) studied
expression of the bar, potato proteinase inhibitor II and uidA (gus) genes,
confirming their co-integration, co-inheritance and co-expression in 286 first
generation (T1) plants and in 11,000 second seed generation (T2) maize plants.
Similar, Brettschneider et al. (1997) followed the inheritance and expression of
transgenes to the fourth seed generation in several inbred lines and sexual
hybrids of maize. In field studies, Oard et al. (1996) assessed expression of the
bar gene, giving resistance to the herbicide glufosinate, in the commercial rice
cultivars Gulfmont, IR72 and Koshihikari. They confirmed that the bar gene
was effective in conferring field-level resistance to the herbicide in rice,
although, importantly, variation amongst transgenic lines required traditional
breeding selection procedures to identify plants with high levels of herbicide
resistance. These workers also emphasised the need to generate several
independent transgenic lines of each cultivar for transgene assessments.
4.8.2 Insect resistance
Recent studies have reported the development of transgenic plants containing
agronomically useful genes, in addition to those for herbicide resistance. Since
insects cause substantial crop losses world-wide, it follows that engineering
plants for insect resistance has, and will continue, to receive high priority. Stemboring
insects are common pests in maize and rice, and resistance against these
insects has been achieved, primarily, by the introduction and expression of
modified or synthetic versions of the Bt -endotoxin, a natural insecticidal toxin
from Bacillus thuringiensis. For example, Wunn et al. (1996) and Cheng et al.
(1998) introduced the cryIA(b) gene into rice cultivars, including the indica
cultivar IR58, to confer resistance to yellow stem borer (Scirpophaga incertulas)
and striped stem borer (Chilo suppressalis), while Alam et al. (1998) were the
first to engineer a lowland deep water rice for stem borer resistance using the
same gene. In addition to giving resistance to stem-boring insects, expression of
the cryIA(b) gene also inhibited feeding of the leaf-folding insects Cnaphalo
crocis and Marasmia patnalis on transgenic rice (Wunn et al. 1996).
Comparisons have been made of the expression of the Bt cryIA(b) gene driven
by different promoters, including the constitutive 35S and Actin-1 promoters,
with tissue-specific promoters from pith tissue and the pep-carboxylase (PEPC)
promoter from chlorophyllous tissue of maize. The latter promoter gave high
levels of transgene expression in leaves and stems of rice (Datta et al. 1998).
Modified versions of the cryIA(b) gene have been used in rice transformation
to give plants which induced 100% mortality in feeding yellow stem borers (Wu