Sucrose cycling, Rubisco expression, and prediction of photosynthetic acclimation to elevated atmospheric CO2
Photosynthetic acclimation to elevated CO2 cannot presently be predicted due to our limited understanding of the molecular mechanisms and metabolic signals that regulate photosynthetic gene expression. We have examined acclimation by comparing changes in the leaf content of RuBP carboxylase/oxygenase (Rubisco) with changes in the transcripts of Rubisco subunit genes and with leaf carbohydrate metabolism. When grown at 1000 mm3 dm–3 CO2, 12 of 16 crop species at peak vegetative growth had a 15–44% decrease in leaf Rubisco protein, but with no specific association with changes in transcript levels measured at midday. Species with only modest reductions in Rubisco content (10–20%) often had a large reduction in Rubisco small subunit gene mRNAs (> 30%), with no reduction in large subunit gene mRNAs. However, species with a very large reduction in Rubisco content generally had only small reductions in transcript mRNAs. Photosynthetic acclimation also was not specifically associated with a change in the level of any particular carbohydrate measured at midday. However, a threshold relationship was found between the reduction in Rubisco content at high CO2 and absolute levels of soluble acid invertase activity measured in plants grown at ambient or high CO2. This relationship was valid for 15 of the 16 species examined. There also occurred a similar, albeit less robust, threshold relationship between the leaf hexose/sucrose ratio at high CO2 and a reduced photosynthetic capacity ≥ 20%. These data indicate that carbohydrate repression of photosynthetic gene expression at elevated CO2 may involve leaf sucrose cycling through acid invertase and hexokinase.