Overexpression of sedoheptulose-1,7-bisphosphatase enhances photosynthesis and growth under salt stress in transgenic rice plants

Activity of the Calvin cycle enzyme sedoheptulose-1,7-bisphosphatase (SBPase; EC3.1.3.37) was increased in the transgenic rice cultivar zhonghua11 (Oryza sativa L. ssp. japonica) by overexpressing OsSbp cDNA from the rice cultivar 9311 (Oryza sativa ssp. indica). This genetic engineering enabled the transgenic plants to accumulate SBPase in chloroplasts and resulted in enhanced tolerance of transgenic rice plants to salt stress at the young seedlings stage. Moreover, CO₂ assimilation in transgenic rice plants was significantly more tolerant to salt stress than in wild-type plants. The analysis of chlorophyll fluorescence and the activity of SBPase indicated that the enhancement of photosynthesis in salt stress was not related to the function of PSII but to the activity of SBPase. Western-blot analysis showed that salt stress led to the association of SBPase with the thylakoid membranes from the stroma fractions. However, this association was much more prominent in wild-type plants than in transgenic plants. Results suggested that under salt stress, SBPase maintained the activation of ribulose-1,5-bisphosphate carboxylase-oxygenase by providing more regeneration of the acceptor molecule ribulose-1,5-bisphosphate in the soluble stroma and by preventing the sequestration of Rubisco activase to the thylakoid membrane from the soluble stroma, and, thus, enhanced the tolerance of photosynthesis to salt stress. Results suggested that overexpression of SBPase was an effective method for enhanncing salt tolerance in rice.