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The photosynthetic assimilation in C3 plants is limited due to the capacity of rubisco to non-specifically bind O2 and CO2 at the same active site. Oxygenation of ribulose-1,5-bisphosphate (RuBP) by rubisco leads to the production of a toxic by product, 2-phosphoglycolate (2-PG). 2-PG is recycled by the photorespiratory pathway, leading to wastage of fixed CO2 and some of the energy generated during photosynthesis. Under ambient conditions, one-third of the rubisco is oxygenated rather than carboxylated. Nature has devised various CO2 concentrating mechanisms (CCM) which could prevent the oxygenation of rubisco under ambient or low CO2 conditions. One such system is present in cyanobacteria that circumvents the oxygenation of rubisco. The cyanobacterial CCM takes place in specialized proteinaceous icosahedral compartments known as carboxysomes that contains the enzymes, rubisco and carbonic anhydrase. The bicarbonate ions diffuse inside these microcompartments which are then converted to CO2 by carbonic anhydrase. This leads to an increase in CO2 concentrations in the vicinity of rubisco, inhibiting its oxygenation.
Inspired by the efficient CCM of the cyanobacteria, we incorporated its components in rice (a C3 plant) to enhance the photosynthetic efficiency. In this study, the carboxysome components of the model cyanobacterium, Synechococcus elongatus PCC 7942 (hereafter, Syn7942) were transformed in rice nuclear genome to create transgenic plants. The cyanobacterial genes were tagged with the rice transit peptide for their targeting to chloroplasts. We developed two types of transgenic rice plants. First type of transgenic lines had the carboxysomal structural protein coding genes (ccmL, ccmO, ccmK) and a chimeric gene (ccmC). The second type of transgenic plants expressed the structural genes (ccmL, ccmO, ccmK and chimeric, ccmC) and the cyanobacterial rubisco along with the RNAi, inhibiting the rice rubisco and carbonic anhydrase genes. It was found that the first type of transgenic lines had growth and yield similar to wild type (WT) rice plants. Whereas, the second type had reduced biomass, yield and photosynthesis compared to WT plants. Both type of plants depicted the presence of protein aggregates and carboxysome-like structure assembly in rice chloroplasts.
We have followed a bottom up approach to assemble the components of CCM of Syn7942 in rice which would help further in stacking up the other essential components to achieve a fully functional CCM in plants. This study represents a significant step towards enhancing photosynthetic efficiency in rice in the future.
Key Words of the Research: Synechococcus elongatus PCC 7942, CO2 concentrating mechanism, CCM, transit peptides, photosynthetic efficiency, rice.
