Future Prospects

The effects on starch, brought about by altering the starch metabolic protein expression per se, in transgenic plants are presented in Table 1. Apart from these, environmental factors such as elemental concentration, sugars, temperature, photoperiod, etc., have been implicated in control of starch synthesis in plants. When potatoes were transformed with a bacterial gene for polyphosphate kinase, the principal enzyme in many bacteria responsible for the synthesis of inorganic polyphosphate from ATP, the partitioning of photoassimilate between starch and soluble sugars were altered [61]. Soluble sugar levels increased at the expense of starch in these transgenic plants. Antisense repression of the hexokinases, the first enzyme in hexose synthesis, has been shown to affect the starch content in various plants and plant organs [62, 63]. These results suggest either the existence of defined controls all through the pathway, beginning from the time carbon is fixed during photosynthesis to the time starch is finally stored in the sink cells, or that perturbations in hexose pathway leading to the production of ADPglucose result in starch degradation.

The promoters of barley sbea and isoamylase have been shown to contain motifs that are starch and sugar inducible [46, 58]. Experimental results such as these indicate a plausible role for signal transduction factors in the cascade of metabolites leading to the accumulation of starch. Gene promoter analysis and the use of heterologous promoters from the starch synthetic pathway to drive the genes involved in starch synthesis would provide valuable information on signal transduction factors and the regulation of genes involved in starch

Alteration in starch biosynthetic enzyme expression in transgenic plants

Plant

Enzyme* Targeted

Transgenic mechanism

Changes made

References

Solanum tuberosum

AGPase

Antisense RNA mediated suppression

Reduction in starch quantity

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