Abstract
In C3 plants, the carbon fixation step catalyzed by ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) represents a major rate‐limiting step due to the competing oxygenation reaction, which leads to the energy‐intensive photorespiration and lowers the overall photosynthetic efficiency. Hence, there is great biotechnological interest in replacing the Rubisco in C3 crops with a more efficient enzyme. The Rubisco enzymes from red algae are among the most attractive choices due to their remarkable preference for carboxylation over oxygenation reaction. However, the biogenesis of Rubisco is extremely complex. The Rubisco enzymes in plants, algae, and cyanobacteria are made up of eight large and eight small subunits. The folding of the large subunits and the assembly of the large subunits with the small subunits to form a functional holoenzyme require specific chaperonin complexes and assembly factors. As a result, previous success in expressing foreign Rubisco in plants has been limited to Rubisco large subunits from closely related plant species and simpler bacterial enzymes. In our previous work, we successfully replaced the Rubisco in tobacco with a cyanobacterial enzyme, which was able to support the phototrophic growth of the transgenic plants. In this work, we used the same approach to express the Rubisco subunits from the red alga Griffithsia monilis in tobacco chloroplasts in the absence of the tobacco Rubisco large subunit. Although the red algal Rubisco genes are being transcribed in tobacco chloroplasts, the transgenic plants lack functional Rubisco and can only grow in a medium containing sucrose. Our results suggest that co‐expression of compatible chaperones will be necessary for successful assembly of red algal Rubisco in plants.
Highlights
A promising concept to increase crop yields is to improve plant productivity through more efficient photosynthesis (Long, Zhu, Naidu, & Ort, 2006; Ort et al, 2015)
One major limitation in photosynthesis that has been challenging to overcome is the carbon fixation step, which is catalyzed by a highly inefficient enzyme known as ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) (Whitney, Houtz, & Alonso, 2011)
Rubisco carries out two competing reactions: carboxylation and oxygenation of ribulose-1,5-bisphosphate (RuBP) with the latter leading to photorespiration and lowering the overall photosynthetic efficiency
Summary
A promising concept to increase crop yields is to improve plant productivity through more efficient photosynthesis (Long, Zhu, Naidu, & Ort, 2006; Ort et al, 2015). We previously replaced the tobacco rbcL gene with genes encoding the large and small subunits from a cyanobacterium Synechococcus elongatus PCC7942, which were able to assemble functional L8S8 complex and support the phototrophic growth of the tobacco transformants (Lin, Occhialini, Andralojc, Parry, & Hanson, 2014; Occhialini, Lin, Andralojc, Hanson, & Parry, 2016). Those findings indicated that the folding and assembly requirements of cyanobacterial Rubisco are not as strict as the Rubisco from higher plants and can be readily met in tobacco. We report our latest attempt and relate our results with recent advances in Rubisco research
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