Photosynthesis is a vital biochemical process fueling life on earth. In plants, during photosynthetic reactions, atmospheric CO2 is converted into sugar molecules with the help of an enzyme called RuBisCO (ribulose-1,5-bisphosphate carboxylase-oxygenase). RuBisCO is a bifunctional enzyme that performs carboxylation and oxygenation activities depending on cellular CO2 and O2 levels, respectively. Carboxylation activity of RuBisCO is important for fixation of carbon into sugars, while oxygenation activity results in the formation of toxic by-products such as 2-phosphoglycolate, glycolate and ammonia. However, these toxic compounds are recycled back into nontoxic products by photorespiration but at the expense of energy and net loss of fixed carbon. Thus, photorespiration limits photosynthetic efficiency and contributes up to 20-50% productivity losses in C3 crops such as wheat, rice, soybean etc. Global population expansion and climate change are aggravating food demand, which require crop improvement to ensure global food security. Improving photosynthetic efficiency by re-engineering photosynthesis and minimizing losses occurred due to photorespiration pathway has been recognized as a potential biotechnological target. Although few studies have previously demonstrated the potential of reducing photorespiration to enhance growth and production, but was limited to Arabidopsis and controlled environmental conditions. Recent two studies implemented synthetic biology approaches to re-engineer plant photosynthesis by bypassing photorespiration. In the first study, South et al transferred multiple genes from bacteria, Arabidopsis, pumpkin and chlamydomonas to tobacco, and simultaneously silenced the glycolate-glycerate transporter via RNAi. The transgenic lines generated showed up to 40% increase in growth and productivity in replicated field trials. Further physiological and biochemical analysis revealed that these engineered plants possess increased photosynthetic rates, improved quantum efficiency and altered metabolic profile of photorespiration related metabolites. In the second study, Shen et al overexpressed three self-originating genes in rice, which encoded for photorespiration pathway related enzymes such as glycolate oxidase, oxalate oxidase, and catalase. Engineered plants showed significant improvement in growth and productivity under green house and field conditions. This growth enhancement was contributed by improvement in photosynthetic rates and reduced photorespiration as revealed by physiological and metabolite data. Importantly, both these studies highlight that photosynthetic efficiency of crops can be enhanced further by bypassing photorespiration, and that can result in higher crop yield under field conditions.

References:

1)  South et al 2019, Science; DOI: 10.1126/science.aat9077; Weblink: http://science.sciencemag.org/content/363/6422/eaat9077

2)  Shen et al 2019, Molecular Plant; DOI: 10.1016/j.molp.2018.11.013; Weblink:https://www.cell.com/molecular...