- Summary Transgenic Arabidopsis thaliana and Nicotiana tabacum plants that express ictB, a gene involved in HCO3− accumulation within the cyanobacterium Synechococcus sp. PCC 7942, exhibited significantly faster photosynthetic rates than the wild-types under limiting but not under saturating CO2 concentrations. Under conditions of low relative humidity, growth of the transgenic A. thaliana plants was considerably faster than the wild-type. This enhancement of growth was not observed under humid conditions. There was no difference in the amount of ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO) detected in the wild-types and their respective transgenic plants. Following activation in vitro, the activities of RubisCO from either low- or high-humidity-grown transgenic plants were similar to those observed in the wild-types. In contrast, the in vivo RubisCO activity, i.e. without prior activation, 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 that express ictB was lower than in the wild-types, suggesting that the concentration of CO2 in close proximity to RubisCO was higher. This may explain the higher activation level of RubisCO and enhanced photosynthetic activities and growth in the transgenic plants. These data indicated a potential use of ictB for the stimulation of crop yield.