Because protocols for transformation of rice are now well established (Shimamoto et al 1989. Christou et al 1991), it is possible to introduce single alien genes that can selectively modify yield-determining processes. Starch biosynthesis plays a pivotal role in plant metabolism, both as a transient storage metabolite of leaf tissue and as an important energy and carbon reserve for sink organs such as seed, roots, tubers, and fruits. Thus, starch is the critical determinant of the sink strength of developing sink organs as well as the source. Several enzymatic steps are involved in starch biosynthesis in plants. ADP glucose pyrophosphorylase (ADPGPP) is a critical enzyme in regulating starch biosynthesis in plant tissues. Even in storage organs with high levels of ADPGPP, its activity is still limiting. This limitation appears to be primarily at the level of allosteric regulation of the enzyme, at least in sink tissues. It should be possible to affect starch production in storage tissues positively by regulated expression of the gene encoding this enzyme (Kishore 1994). Starch levels and dry matter accumulation were enhanced in potato tubers of plants transformed with the glgC"' gene from Escherichia coli encoding ADPGPP (Stark et al 1992). The transformed potato plants had tubers with higher dry matter and starch content, under both growth chamber and field conditions. The plants were similar in growth and development to the nontransgenic controls. The nature of starch produced by the tubers containing the glgC16 gene was similar to that of the controls. This gene has been introduced into rice and transgenic plants are being evaluated.
In several crop species, incorporation of the "stay-green" trait or slower leaf senescence has been a major achievement of breeders (Evans 1993). In some genotypes with slower senescence (stay green), Rubisco degradation is slower, which results in longer duration of canopy photosynthesis and higher yields. The onset of senescence is controlled by a complement of external and internal factors. Plant hormones such as ethylene and abscisic acid promote senescence, whereas cytokinins are senescence antagonists. Therefore, overproduction of cytokinins can delay senescence. The ipt gene from Agrobacterium tumefaciens encoding an isopentenyl transferase (Akiyoshi et al 1984) was fused with the senescence-specific promoter SAG 12 (Gan and Amasino 1995) and introduced into tobacco plants. Leaf and floral senescence in the transgenic plants was markedly delayed, biomass and seed yield increased, but other aspects of plant growth and development were normal. This approach appears to have great potential for improving crop yields by slowing senescence and Rubisco degradation and thus increasing canopy photosynthesis. The ipt gene has been introduced into rice and transgenic plants are being evaluated.
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