Overexpression of ca1pase decreases Rubisco abundance and grain yield in wheat

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) catalyses the fixation of CO2 into organic compounds that are used for plant growth and production of agricultural products. Specific sugar-phosphate derivatives bind tightly to the active sites of Rubisco, locking the enzyme in a catalytically inactive conformation. 2-carboxy-D-arabinitol-1-phosphate phosphatase (CA1Pase) dephosphorylates such tight-binding inhibitors, contributing towards maintaining Rubisco activity. The hypothesis that overexpressing ca1pase would decrease the abundance of inhibitors, thereby increasing the activity of Rubisco and enhancing photosynthetic performance and productivity was investigated in wheat. Plants of four independent wheat transgenic lines showed up to 30-fold increases in ca1pase expression compared to wild-type (WT). Plants overexpressing ca1pase had lower quantities of Rubisco tight-binding inhibitors and higher Rubisco activation states than WT, however the amount of Rubisco active sites decreased by 17-60% in the four transgenic lines compared to WT. The lower Rubisco content in plants overexpressing ca1pase resulted in lower initial and total carboxylating activities measured in flag leaves at the end of the vegetative stage, and lower aboveground biomass and grain yield measured in fully mature plants. Hence, contrary to what would be expected from our theory, ca1pase overexpression caused decreased Rubisco content and compromised wheat grain yields. These results support a possible role for Rubisco inhibitors in protecting the enzyme and maintaining an adequate content of Rubisco active sites available to support carboxylation rates in planta.