Enzyme-Specific Responses within Networks

Individual reactions in a pathway may affect the same process in different ways. Although antisense inhibition of each of several Calvin cycle enzymes ultimately restricts the rate of CO₂ assimilation, the mechanisms by which photosynthesis is affected differ for the different enzymes. This is revealed by considering the impact of the decrease in the rate of CO₂ assimilation on the two major photosynthetic end-products, sucrose and starch. In Rubisco antisense lines, the decrease in photosynthesis led to proportional decreases in the rate of sucrose and starch synthesis (Stitt and Schulze, 1994), whereas inhibition of CO₂ fixation due to decreased expression of aldolase (Haake et al., 1998), plastid fructose-1,6-bisphosphatase (Kossmann et al., 1994), or SBPase (Harrison et al., 1998) was accompanied by a far greater inhibition of starch synthesis and preferential retention of sucrose synthesis. In contrast, decreased expression of transketolase led to preferential retention of starch accumulation and a decrease in sucrose content, suggesting a shift in allocation in favor of starch relative to sucrose (Henkes et al., 2001).

The difference in assimilate partitioning may be explained in part by the position of the selected enzyme within the Calvin cycle relative to fructose 6-phosphate, the immediate precursor for starch synthesis. Transketolase operates downstream of fructose 6-phosphate, which is therefore likely to increase when expression of the enzyme is decreased, hence stimulating starch synthesis (Fig. 1.3). In contrast, aldolase and plastid fructose 1,6-bisphosphatase are both upstream of fructose 6-phosphate and decreased expression of either of these enzymes is likely to result in lower levels of this intermediate, reducing the availability of precursors for starch synthesis.

However, the availability of fructose 6-phosphate cannot provide the complete explanation because SBPase is also downstream of fructose 6-phosphate and yet a decrease in expression of this enzyme led to a preferential restriction of starch production rather than enhancement (Harrison et al., 1998). This apparent anomaly arises because erythrose 4-phosphate is a potent inhibitor of phosphoglucoisomerase, the enzyme catalyzing the conversion of fructose 6-phosphate to glucose-6-phosphate in the pathway of photosynthetic starch synthesis. Both aldolase and SBPase are involved directly in the catabolism of erythrose 4-phosphate, and decreased expression of either of these enzymes is likely to result in an increase in the level of this intermediate, leading to increased inhibition of starch synthesis. In contrast, decreased expression of transketolase presumably leads to lower erythrose 4-phosphate, which relieves inhibition of phosphoglucoisomerase and thus favors starch synthesis despite a decline in the concentration of 3PGA, an important activator of ADPglucose pyrophosphorylase, which might otherwise be predicted to restrict starch production. This implies that the metabolic consequences of adjusting the amount of a specific enzyme must be assessed on their own merits and that any similarity to the changes produced by different target enzymes should not be taken to imply that the manipulations are affecting the process through a common route.