Engineering C4 Photosynthesis Pathways into C3 Crops to Improve Nutrient Use Efficiency: A Review

Abstract

Photosynthesis is the cornerstone of plant productivity and, by extension, global food security. C4 photosynthesis, characterized by its biochemical and anatomical adaptations, offers superior nutrient and water use efficiency compared to the more common C3 pathway. With the growing demand for food due to population growth and climate change, engineering C4 photosynthetic traits into C3 crops has emerged as a promising strategy to enhance crop yields and resource use efficiency. This paper provides a comprehensive review of the molecular, biochemical, and anatomical differences between C3 and C4 photosynthesis, followed by an exploration of recent advances in synthetic biology and genetic engineering aimed at introducing C4 pathways into C3 crops. In C3 plants, the carbon fixation process is inefficient under high temperatures and low CO2 concentrations due to photorespiration. On the other hand, C4 plants employ a two-cell system to concentrate CO2 around RuBisCO, minimizing photorespiration. In mesophyll cells, phosphoenolpyruvate carboxylase (PEPC) fixes CO2 into oxaloacetate (OAA), which is then converted into malate or aspartate. These compounds are transported to bundle-sheath cells, where CO2 is released for the Calvin cycle. Several studies have successfully introduced C4 genes into rice, resulting in improved photosynthetic efficiency and nitrogen use efficiency. Moreover, the introduction of C4 enzymes into soybean has led to modest improvements in water use efficiency, particularly under drought conditions. International initiatives, such as the C4 Rice Project, highlight the importance of collaborative efforts in advancing this field. A meta-analysis of recent case studies highlights the progress, challenges, and future prospects of this transformative approach. Looking ahead, interdisciplinary research combining plant physiology, genetic engineering, and computational modeling will be key to unlocking the full potential of C4 engineering in C3 crops.