Expression of cyanobacterial ictB in higher plants enhanced photosynthesis and growth

Under many environmental conditions plant photosynthesis and growth are limited by the availability of CO₂ at the site of ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO). We expressed ictB, a gene involved in HCO3 − accumulation in Synechococcus sp. PCC7942, in higher plants. The transgenic Arabidopsis thaliana and Nicotiana tabacum plants exhibited significantly faster photosynthetic rates than the wild types under limiting, but not under saturating CO2 concentrations. Similar results were obtained in Arabidopsis plants bearing ictB from Anabaena sp. PCC7120. Growth of transgenic A. thaliana plants maintained under low humidity was considerably faster than that of the wild type. There was no difference in the amount of RubisCO or the activity of the enzyme activated in vitro in the wild types and the transgenic plants. In contrast, the in vivo RubisCO activity, without prior activation of the enzyme, in plants grown under low humidity was considerably higher in ictB-expressing plants than in their wild types. The CO2 compensation point in the transgenic plants was lower than in the wild types suggesting a higher CO₂ concentration in close proximity to RubisCO. This may explain the higher activation level of RubisCO and enhanced photosynthesis and growth in the transgenic plants. These data indicated a potential use of ictB for the stimulation of crop yield.