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Role of Starch in Acclimation to Elevated CO2
Ludewig, F., Sonnewald, U., Kauder, F., Heineke, D., Geiger, M., Stitt, M., Muller-Rober, B.T., Gillissen, B., Kuhn, C. and Frommer, W.B.  1998.  The role of transient starch in acclimation to elevated atmospheric CO2FEBS Letters 429: 147-151.

What was done
The authors grew normal and transformed potato plants in environmental chambers with atmospheric CO2 concentrations of 400 or 1000 ppm for approximately seven weeks to elucidate the mechanism responsible for triggering photosynthetic acclimation or down regulation that sometimes occurs in plants exposed to elevated CO2 concentrations.  The transformed plants contained specific antisense RNA that effectively reduced the activity of ADP-glucose pyrophosphorylase, the key regulatory enzyme involved in the synthesis of starch, by an average of 82% relative to untransformed normal plants.  Thus, transformed plants lack the ability to accumulate starch, and can, therefore, be used to determine the role, if any, of starch in photosynthetic acclimation to elevated CO2.

What was learned
Elevated CO2 increased shoot fresh weight by 70% and doubled tuber yield in both transformed and normal untransformed plants relative to their respective controls grown at 400 ppm CO2.  At this lower level of CO2, there was little difference in photosynthetic rates between potato plants; but at 1000 ppm CO2, normal plants exhibited photosynthetic increases of approximately 20%, while the transformants displayed photosynthetic reductions as great as 30%.  Additionally, elevated CO2 had no significant effect on rubisco activity in any of the potato plants tested, indicating that the observed depression of photosynthesis in transformed plants was not due to the down regulation of this important photosynthetic enzyme.

Elevated CO2 increased leaf starch concentrations about 10-fold in normal plants but to a much lesser extent in transformed ones, as predicted.  In antisense plants, the significantly smaller increases in starch content were correlated with large reductions in photosynthesis.  Consequently, photosynthetic acclimation to elevated CO2 in these transformants was not caused by the accumulation of starch, nor by the down regulation of rubisco, but rather by the inability to synthesize starch as a photosynthetic end product.  Moreover, in untransformed normal plants, elevated CO2 increased the expression of ADP-glucose pyrophosphorylase, indicating that increased starch accumulation may allow for increased photosynthetic rates in elevated CO2.

What it means
As the CO2 content of the air rises, most plants respond by increasing their photosynthetic rates, which results in greater leaf carbohydrate concentrations, including starch.  It has been suggested that starch accumulation may signal photosynthetic acclimation in response to growth in elevated CO2 conditions.  However, in the present study, transformed potatoes lacked the ability to accumulate starch, yet they still displayed reduced rates of photosynthesis.  This observation suggests that rates of starch synthesis can influence rates of photosynthesis.  Thus, as the CO2 content of the air continues to rise, the enhanced ability of plants to synthesize starch from increased photosynthetic products should allow them to maintain their higher rates of photosynthesis, which ultimately should stimulate their growth and biomass production.

Reviewed 15 February 1999