Effects of simultaneous Rubisco, glyceraldehyde-3-phosphate dehydrogenase, and triosephosphate isomerase overexpression on photosynthesis in rice

Rubisco overexpression does not lead to substantial photosynthesis improvement. One of the possible causes is the over-accumulation of 3-phosphoglycerate in the Calvin-Benson cycle. We have previously suggested that glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and triosephosphate isomerase (TPI), involved in 3-phosphoglycerate metabolism, have control over the Calvin-Benson cycle metabolism and photosynthesis. In the present study, Rubisco, GADPH, and TPI were simultaneously overexpressed in rice plants through crossing to improve photosynthesis and its N use efficiency. The percentage of N allocated to Rubisco and GAPDH and TPI levels in the transgenic plant increased 1.1-, 4.9-, and 2.1-fold, respectively, compared to those in wild-type plants, showing overexpression of these enzymes. However, there were no marked increases in CO2 assimilation rates at different CO2 levels and capacities of Rubisco carboxylation, regeneration of Rubisco substrate, and triose phosphate use calculated using the CO2 assimilation rates in the transgenic plants. The ratios of these photosynthetic capacities to total leaf-N level tended to be slightly lower in the transgenic plants than those in the wild-type plants, implying a marginal decrease in photosynthetic N use efficiency. In the transgenic rice plants substantially overexpressing GAPDH and TPI, photosynthesis at elevated CO2 levels, which is assumed to be limited factors other than Rubisco, was only slightly improved. Therefore, it is suggested that simultaneous Rubisco, GAPDH, and TPI overexpression do not improve photosynthesis and its N use efficiency, as GAPDH and TPI overexpression do not improve the 3-phosphoglycerate metabolism sufficiently to alleviate its over-accumulation caused by Rubisco overexpression.