The yield from crop plants has been subject to constant improvement through conventional breeding and refinements in agricultural practice for many decades. In the case of the potato, which will form the major focus of this chapter, the harvest index (ratio of dry weight of harvestable organs to the dry weight of the entire plant) has been increased from 0.09 in wild species up to 0.81 in modern cultivars (Inouhe and Tanaka, 1978), with smaller, yet dramatic, improvements found in other crop species (Ellen, 1993; Hay, 1995).
Although such improvements have proved both dramatic and revolutionary they were also time consuming and slow. The emergence of molecular-assisted breeding (see Chapter 6) and plant transformation technologies (see Chapters 8 and 9) offers the possibility of manipulating metabolism using a more rapid, targeted approach. Indeed since the advent and widespread adoption of transgenesis approaches some 15 years ago gave rise to the discipline of molecular plant physiology, much information has been obtained concerning the potential to manipulate plant metabolism. In this chapter we intend to review the many previous studies of genetic manipulation of heterotrophic carbohydrate metabolism in plants.
It is clear that the successful manipulation of plant metabolism requires detailed understanding of the underlying factors that regulate it. For this reason we intend to describe the current understanding of the central pathways of carbohydrate metabolism. Some stress will be put on the sucrose to starch transition since this pathway has received great attention over the past few years. In the case of the potato (Solanum tuberosum) tuber all the genes believed to be directly involved in the sucrose to starch transition have been cloned, the final gaps being filled within the last couple of years (Veramendi et al., 1999; Tauberger et al., 2000; Fernie et al., 2002; Veramendi et al., 2002).
Furthermore, an impressive variety of transgenic lines have been generated where the activities of most of the individual genes have been modulated, alone or in combination. In addition, a large range of transgenic potato lines has been created where deregulated alternatives to endogeneous enzymes have been introduced. These studies have allowed the confirmation of many longstanding hypotheses which were previously based upon indirect methodologies all of which will be summarised within this chapter. In addition, this chapter will also cover the synthesis of fructans and the introduction of novel carbohydrates into plants and will review recent advances in understanding and influencing structural properties of starch. As stated above we intend to split this review into two major sections. The first of these is a description of the pathways operating in various agronomically important plants following the path of carbohydrate from sucrose transported from source tissues to its assimilation into storage carbohydrate in sink tissues.
The metabolism of the major forms of carbohydrate: sucrose, starch and fructans will be covered in some detail. The second section provides a review of strategies taken to manipiulate these pathways for commercial gain with particular prominence given to alteration of starch functionality and to increasing end product accumulation of starch and sucrose and finally, the manipulation of minor and novel sugars are reviewed.
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