Attempts to modify sink strength revealed that the harvest index seems to be strongly dependent on transport processes. This hypothesis is confirmed by the following examples.
Expressing an apoplastic invertase that increases the tuber size when expressed in potato tubers, in source but not in sink tissues, leads to a very strong reduction of biomass production, because the carbohydrates that are usually transported as sucrose cannot leave the source organs.6 The same is achieved by inhibiting the expression of the sucrose transporter that is responsible for loading sucrose into the vascular system.7 It is there fore very likely that transport of photosyn-thates from the source to the sink organs is the limiting step in biomass production. Consequently the question would be whether we could increase production by raising the transport abilities. Unfortunately, the question cannot be answered yet, because to date increased transport capacity has not been achieved. But, nevertheless, there is good evidence for the hypothesis of transport limitation. Potato plants react very sensitively to the reduction of transporter activity, thereby demonstrating that export of sucrose from the sinks is likely to be a rate limiting step in the control of photosynthetic production. In transgenic plants showing antisense inhibition of transporter expression, photosynthesis is reduced and soluble sugars accumulate in the mesophyll (Table 15.5).
From the experiments described, we can conclude that biomass production can be influenced by means of genetic modification that alters sink strength, as demonstrated by expression of cytosolic invertase and glucoki-nase, or by modifying transport processes as shown for antisense inhibition of sucrose transporter expression. In the given examples, genetic modification negatively affects production, thereby demonstrating which steps are
Table 15.1. Soluble sugars in potato tubers in ^mol/g fresh weight ± standard deviation
Sucrose Glucose Fructose
Control: untransformed wild type (Var. Désirée). U-In2-34: plant No. 34 expressing cytosolic yeast invertase. U-In2-17: plant No. 17 expressing cytosolic invertase.
candidates for rate limiting steps. This gives an idea of what would be needed to increase biomass production, but so far there are no convincing strategies for increasing photosyn-thetic carbon fixation in transgenic plants.
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