In photosynthesis, Rubisco catalyses the stable assimilation of atmospheric CO2.In many, but not all plants, 2-Carboxy-D-arabinitol 1-phosphate (CA1P), a naturally occurring inhibitor of Rubisco, limits photosynthetic CO2 assimilation in low light.In bright irradiance, 2-Carboxy-D-arabinitol 1-phosphate phosphatase (CA1Pase) dephosphorylates CA1P released from Rubisco by Rubisco activase.The dephosphorylation produces 2-Carboxy-D-arabinitol which is not inhibitory.Genes for CA1Pase from several species have been identified and cloned.The derived amino acid sequences reveal the existence of two distinct domains, suggesting a multifunctional enzyme.We have recently demonstrated that CA1Pase is promiscuous and dephosphorylates misfire products during Rubisco catalysis and from RuBP degradation.This indicates a central role for CA1Pase in maintaining Rubisco activity and provides a rationale for its presence in species which lack significant amounts of CA1P, such as wheat and Arabidopsis.