Several of our major crop species have gone through two great leaps in yield increase in the last several decades: increasing harvest index by reducing the height (making use of the semi-dwarf genes) and utilization of heterosis by producing hybrids. Reduced rates of yield increase have been observed in a number of major food crops in the last 10-15 years (Ministry of Agriculture, 1996). Increasing yield potential has therefore been a common concern in essentially all crop breeding programmes.
Two approaches have been reported in the literature. The first approach is called 'wild OTLs', in which efforts are devoted to bringing OTLs for yield increase from the wild relatives to enhance the yield of cultivars. The argument for such an approach is that only a portion of the genes that ever existed in the wild species was brought to cultivation in the processes of domestication, leaving most of the genes unused. With the help of molecular marker technology, it should therefore be possible to identify genes that can increase the yield of cultivated plants. Xiao et al. (1996), for example, reported two OTLs from a wild rice that showed significant effects in increasing the performance of an élite rice hybrid. This has generated considerable interest in identifying genes for agronomic performance from wild relatives that are potentially useful for varietal improvement.
The second approach is to modify certain physiological processes by genetic engineering. Gan and Amasino (1995) reported a system conceived to delay leaf senescence by autoregulated production of cytokinin. The construct was designed by fusing a senescence-specific promoter isolated from Arabidopsis with a DNA fragment from Agrobacterium encoding isopentenyl transferase (IPT), an enzyme that catalyses the rate-limiting step in cytokinin biosynthesis. The strategy for such a system is that the gene would be turned on at the onset of senescence, leading to the synthesis of cytokinin, and the production of cytokinin would in turn inhibit the process of senescence, thus repressing the expression of this construct itself. Such a system would, therefore, be able to produce cytokinin for delaying senescence, at the same time preventing overproduction of cytokinin, because overproduction of this hormone is detrimental to the plant. Transgenic tobacco plants carrying this construct showed a significant delay in leaf senescence, bringing about a large increase in the number of flowers, number of seeds and biomass, indicating the possibility of increasing plant productivity by delaying leaf senescence. It is interesting, therefore, to determine if this system can provide a general strategy for yield increase in crop improvement.
There are many opportunities for biotechnology to contribute to sustainable food production and to achieve higher yields, better quality and less dependence on chemicals, making crop production more environmentally friendly.
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