Cereals processing technology

Biotechnology, cereal and cereal products quality 69
et al. 1997). Synthetic truncated genes based on the cryIA(b) gene, have also
been introduced into rice (Ghareyazre et al. 1997). The latter authors targeted
low tillering aromatic rices which are particularly difficult to improve by
conventional breeding because of loss of quality characteristics upon sexual
hybridisation. The cryIA(c) gene has also been assessed in rice transformation
for stem borer resistance (Cheng et al. 1998, Nayak et al. 1997); the cry2A Bt
gene also conferred resistance to yellow stem borer and rice leaf folder insects in
the indica rices Basmati 370 and M7 (Maqbool et al. 1998). A recent example of
the transformation of maize for insect resistance is that of Fearing et al. (1997)
who introduced the cryIA(b) gene into six commercial cultivars and four backcross
generations. They reported the highest concentration of insecticidal protein
to be at anthesis in transformed plants. Other genes conferring insect resistance
which have been evaluated in rice, including the Cowpea trypsin inhibitor
(CpTi) gene, which increased resistance of transgenic rice to striped stem borer
and the pink stem borer (Xu et al. 1996), and the snowdrop lectin (GNA) gene.
The latter was directed against sap-sucking insects, such as the brown plant
hopper, through the use of a rice sucrose synthase promoter to drive GNA
expression in the phloem of transgenic plants (Sudhakar et al. 1998).
Nematodes cause severe crop losses in some areas, including rice cultivated
in Africa. In attempts to reduce nematode damage, a cysteine proteinase
inhibitor (oryzacystatin-I Delta D86) gene was introduced into four elite African
rice cultivars (ITA212, IDSA6, LAC23, WAB56-104), resulting in a 55%
reduction in egg production by the nematode Meloidogyne incognita in the roots
of transgenic plants (Vain et al. 1998).
4.8.3 Disease resistance and environmental stress
Viral and fungal diseases reduce crop productivity. The insertion of viral coat
protein genes into transgenic plants is a well-established procedure for
conferring viral resistance, this approach being exploited in rice for resistance
to rice dwarf virus (Zheng et al. 1997). Rice has also been engineered for
resistance to sheath blight incited by the fungus Rhizoctonia solani. Thus,
introduction of a 1.1 kb fragment of a rice chitinase gene linked to the CaMV35S
promoter resulted in transgenic plants in which resistance to the fungus
correlated directly with chitinase activity (Lin et al. 1995). It will be interesting
to determine whether chitinase gene expression confers cross-protection to other
fungal pathogens. More recently, the introduction of the stilbene synthase gene,
which is thought to be involved in the sythesis of a phytoalexin, provided
protection in rice to infection by the fungus Pyricularia oryzae (Stark-Lorenzen
et al. 1997).
One of the challenges facing biotechnologists is to modify plants so as to
increase net carbon gain (Ku et al. 1999). C4 plants, such as maize and several
weed species, have evolved a biochemical mechanism to overcome oxygen
inhibition of photosynthesis. In an initial assessment of the feasibility of
improving photosynthesis in C 3 plants, the intact gene of phosphoenolpyruvate