Altering Carbohydrate Composition Starch

Completely different is the picture for approaches to modifying carbohydrate composition in an attempt to improve plant quality. There are many reports on successful modification of starch composition in potato, pea or other species, and also on the production of fructans in transgenic plants that are normally not capable of fructan production.

Starch is a glucan polymer that is synthesized from activated glucose units that are synthesized from G6P and ATP by ADP-glucosepyrophosphorylase. The activation of glucose is the rate limiting step in the pathway.8 Four different starch synthases are involved, one of them being tightly bound to the growing starch granule, the others being soluble enzymes. The a-(1,4)-glucan chain produced by the synthases is subject to modification by branching enzymes that introduce a-(1,6) branches that characterize the amy-lopectin portion of starch. Besides branching enzymes, debranching enzyme and dispropor-tioning enzyme, as well as starch phosphory-lases and the degradative amylases, are involved in the metabolism of starch. All these enzymes have been characterized and the genes are cloned from different species. Work in our institute focused on potato, which, behind corn and wheat one of the most important starch producing crop plants and yields a qualitatively interesting starch because of its high phosphate content.

By means of antisense reduction of enzyme activity of single or multiple enzymes, a set of starches with different properties has been generated.9 The antisense repression of branching enzyme in combination with repression of the so-called R1 enzyme, which is involved in starch modification in a way not yet completely understood,10 yields a starch that has a dramatically enhanced gelation capacity and is therefore expected to be superior in the production of films.

A very important feature of starch with respect to the production of films is the color of the starch gel. A very turbid gel can be obtained in plants with reduced activity of starch phosphorylases, but a completely colorless gel results from repression of granule bound starch synthase (GBSS), which leads to loss of the amylose portion of starch. The pure amylopectin starch obtained in these plants shows other interesting features like reduced retrogradation and is already a very interesting raw material for use as a food additive in production of soups and sauces. Not only the turbidity, but also the gel strength, can be manipulated. For example, repression of starch phosphorylases as well as repression of branching enzyme and R1 give turbid and very stiff gels, the latter of which can only be obtained by gelatinization of starch at high temperatures.

Table 15.2. Density in g/cm3 of potato tubers as direct measure of starch content

Density

Control

1,082

Inv

1,074

GK-41

1,066

GK-29

1,061

GK-38

1,057

Control: untransformed wild type (Var. Désirée). Inv: plant expressing cytosolic yeast invertase. GK-41, -29, -38: cytosolic invertase plants expressing glucokinase from Zymomonas mobilis.

The production of pure amylopectin starch by repression of GBSS that is essential for amylose synthesis has already been mentioned. It is also possible to reduce amylopectin synthesis and raise the amylose content of starch. This is achieved by repression of branching enzyme and R1 gene expression, whereas the repression of either one of the enzymes has no significant effect. Amylose is an interesting polymer because it is mostly linear and therefore interesting for film production.

One of the most intersting features of potato starch is the high content of covalently bound phosphate that adds chemical functionality to the polymer matrix. The way in which phosphate gets integrated into starch is not fully understood, but it is mainly associated with the amylopectin fraction of starch. Consequently, phosphate content can be lowered by repression of branching enzyme and R1, which leads to a higher amylose content. Astonishingly, the lowest phosphate content is observed for R1 repression that does not significantly reduce amylopectin production. The desired increase in phosphate content is achieved via inhibition of GBSS, linked to increased amylopectin content. But most

Table 15.3. Release of CO2 from potato tubers in nmol C/g fresh weight

Table 15.3. Release of CO2 from potato tubers in nmol C/g fresh weight

CO2 Production

Control

18

Inv

58

GK-41

71

GK-29

79

GK-38

81

Control: untransformed wild type (Var. Désirée). Inv: plant expressing cytosolic yeast invertase. GK-41, -29, -38: cytosolic invertase plants expressing glucokinase from Zymomonas mobilis.

interesting is the additional repression of branching enzyme, which leads to a more than 3-fold increase in the phosphate content of starch.

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