The term ‘photosynthetic control’ describes the short- and long-term mechanisms that regulate reactions in the
photosynthetic electron transport (PET) chain so that the rate of production of ATP and NADPH is coordinated with
the rate of their utilization in metabolism. At low irradiances these mechanisms serve to optimize light use
efficiency, while at high irradiances they operate to dissipate excess excitation energy as heat. Similarly, the
production of ATP and NADPH in ratios tailored to meet demand is finely tuned by a sophisticated series of controls
that prevents the accumulation of high NAD(P)H/NAD(P) ratios and ATP/ADP ratios that would lead to potentially
harmful over-reduction and inactivation of PET chain components. In recent years, photosynthetic control has also
been extrapolated to the regulation of gene expression because mechanisms that are identical or similar to those
that serve to regulate electron flow through the PET chain also coordinate the regulated expression of genes
encoding photosynthetic proteins. This requires coordinated gene expression in the chloroplasts, mitochondria, and
nuclei, involving complex networks of forward and retrograde signalling pathways. Photosynthetic control operates
to control photosynthetic gene expression in response to environmental and metabolic changes. Mining literature
data on transcriptome profiles of C3 and C4 leaves from plants grown under high atmospheric carbon dioxide (CO2)
levels compared with those grown with ambient CO2 reveals that the transition to higher photorespiratory conditions
in C3 plants enhances the expression of genes associated with cyclic electron flow pathways in Arabidopsis
thaliana, consistent with the higher ATP requirement (relative to NADPH) of photorespiration.