The biochemical and molecular basis for photosynthetic acclimation to elevated atmospheric CO2

A max, maximum CO2 assimilation rate CAB, genes encoding chlorophyll a/b binding proteins Ci, intercellular CO2 concentration PGK, the gene encoding 3‐phosphoglycerate kinase PRK, the gene encoding phosphoribulokinase PSAB, the gene encoding the 83 kDa apoprotein of the PSI reaction centre PSBA, the gene encoding the D1 protein of photosystem II RBCS, genes encoding the Rubisco small subunit protein RBCL, the gene encoding the Rubisco large subunit protein Rubisco, ribulose‐1,5‐bisphosphate carboxylase/ oxygenase SBP, the gene encoding sedoheptulose‐1,5‐bisphosphatase There have been many recent exciting advances in our understanding of the cellular processes that underlie photosynthetic acclimation to rising atmospheric CO2 concentration. Of particular interest have been the molecular processes that modulate photosynthetic gene expression in response to elevated CO2 and the biochemical processes that link changes in atmospheric CO2 concentration to the production of a metabolic signal. Central to this acclimation response is a reduction in ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) protein content. Studies indicate that this reduction results from species‐dependent variation in the differential use and temporal control of molecular processes. We present a model for the control of Rubisco protein accumulation that emphasizes the role of subunit message translation as well as the abundance of subunit messages as components of the acclimation response. Many studies indicate that photosynthetic acclimation to elevated CO2 results from adjustments in leaf carbohydrate signalling. The repression of photosynthetic gene expression is considered to occur primarily by hexokinase functioning as a hexose flux sensor that ultimately affects transcription. Leaf hexoses may be produced as potential sources of signals primarily by sucrose cycling and secondarily by starch hydrolysis. An increased rate of sucrose cycling is suggested to occur at elevated CO2 by enhanced provision of sucrose to leaf acid invertases. Additionally, sink limitations that accentuate photosynthetic acclimation may result from a relative decrease in the export of leaf sucrose and subsequent increase in cellular sucrose levels and sucrose cycling.