Sink Capacity

The fact that sucrose accumulation in the mesophyll cell occurs when photosynthesis is saturated implies that the removal of photo-synthates from the sources, but not the source capacity, is limiting. So, consequently, the question is: "Can we increase sink demand?" We attempted to increase sink strength by expressing an enzyme that metabolizes sucrose in sink organs to enhance the sucrose gradient between source and sink. This was achieved by expressing an invertase from baker's yeast in the cytosol of potato tubers (Fig. 15.2). To our surprise, the consequence was a strong reduction of sink strength and a strong increase in the concentration of glucose (Table 15.1).4

We concluded that the phosphorylation of glucose, which is necessary to allow partitioning into starch, is limiting in the tubers.

Consequently we added a glucokinase from Zymomonas mobilis to overcome this limitation. The result was an even stronger reduction in starch synthesis in the tubers. Table 15.2 shows the density of the tubers, which is a direct measure of starch content.5 The reason for the further reduction in starch accumulation is that additional G6P was not diverted into starch formation but into glyco-lysis and respiration. Measuring CO2 release from the tubers of invertase/glucokinase plants revealed a 4-fold increase in respiration as compared to the untransformed wild type (Table 15.3). Currently we do not know the reason for the partitioning into glycolysis, but it is to be supposed that glucose itself or a metabolite acts as a signal to increase respiration.

A completely different result with respect to sink strength was achieved when the invertase was targeted to a different compartment: Expression of a chimeric gene construct that led to apolastic localization of the inver-tase caused a significant increase in the fresh

* Sucrose

* Sucrose

Fig 15.2. Regulation of carbohydrate metabolism in potato tubers (sink tissue). Metabolites: ADP: adenosine diphosphate; ATP: adenosine triphosphate; F-6-P: fructose-6-phosphate; G-1-P: glu-cose-1-phosphate; G-6-P: glucose-6-phosphate; P: phosphate; Pi: inorganic phosphate; PPi: pyrophosphate; UDP: uridine diphosphate; UTP: uridine triphosphate; UDP-G: UDP-glucose. Enzyme: SuSy: sucrose-synthase.

weight of individual tubers, whereas total yield per plant was unchanged (Table 15.4).4 This demonstrates that manipulating the capacity of a single sink organ like a potato tuber does not result in increased biomass production of the whole plant, but it is a means toward influencing the harvest index. Furthermore, it shows that the harvest index is influenced by the sink strength of an individual sink organ, which itself can be modulated by changing osmotic potential of cells or the surrounding apolastic space.

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