|Ali, Hazem Abd El-Rahman Obiadalla: Understanding of Carbon Partitioning in Tomato Fruit |
Tomato was one of the first plants to be genetically modified utilising recombinant DNA techniques. It might have been expected, therefore, that it would become a model system for the study of many aspects of plant biology. Although there have been some studies examining carbohydrate metabolism in tomatoes utilising molecular biological techniques (Ohyama et al., 1995; Klann et al., 1996; Chengappa et al., 1999; D‘Aoust et al., 1999; Nguyen-Quoc et al., 1999), in recent years other species, such as Arabidopsis and potato, have been much more widely used. The reasons for this are that potato, for example, produces a large, commercially important storage organ, whilst Arabidopsis has a short life cycle and a fully sequenced genome. It has been demonstrated that the tomato fruit are an excellent model for the investigation of the regulation of sink activity and strength (Ho, 1996).
Tomato is, however, interesting in its own right as it produces a fruit, which has a very different metabolism to either a leaf or a potato tuber. Leaves have the capacity to fix carbon through photosynthesis and, therefore, produce starch in the chloroplast directly. Potato tubers on the other hand rely on sucrose, which is exported from leaves via the phloem for a source of carbon. The sucrose has to be metabolised and transported over the amyloplast membrane before being converted to, among other things, starch. Tomato fruits initially contain chloroplasts that are photosynthetically active, but these differentiate to non-photosynthetic chromoplasts during the ripening process. They can, therefore, at least initially fix carbon, but they also receive carbon in the form of sucrose from the phloem. This raises several questions, not least about whether it is carbon fixed in the fruit or in the leaf that is most important for the growth and development of the fruit, and how this alters during fruit development.
Because tomato is a very close relative of potato it is normally possible to repress the activity of specific enzymes using cDNA‘s isolated from potato. Many cDNA‘s from potato have been isolated for genes involved in carbohydrate metabolism, which could act as a resource for studies in tomato. In addition, the recent isolation of many expressed sequence tags (EST) from tomato allows the possibility of using genetic engineering techniques to repress the activity of many enzymes in tomato. As tomato is a diploid species that can be crossed, it is also possible to combine the reduction in activities of multiple enzymes simultaneously, something that is difficult in potato.
The conversion of sucrose to starch has been relatively well studied in tomato fruits (Robinson et al., 1988; Yelle et al., 1988; Schaffer and Petreikov, 1997a). It has been
16found that sucrose concentrations are lower than both glucose and fructose (Damon et al., 1988; Klann et al., 1996; Schaffer and Petreikov, 1997a), whilst the wild type (WT) tomato relative Lycopersicon chemielewskii accumulates higher levels of sucrose than the other soluble sugars (Yelle et al., 1988). The reason for this accumulation of sucrose in the wild relative is due to a reduction in the activity of acid invertase (Klann et al., 1996)
There are two aims for this work. The first one was to examine whether the Micro-Tom tomato cultivar was a suitable candidate to act as a model system for the study of carbohydrate metabolism in tomato fruit generally and the second to elucidate the role of three enzymes are thought to influence the accumulation of starch in early development stage of tomato fruits (cp-FBPase, AGPase and GWD protein) by antisense technique.
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