Journal of Energy Bioscience 2025, Vol.16, No.4, 205-215 http://bioscipublisher.com/index.php/jeb 206 2 The C4 Photosynthetic Pathway: An Overview 2.1 Biochemical mechanisms The C4 photosynthetic pathway is an efficient way to concentrate carbon. It reduces photorespiration and enhances the efficiency of photosynthesis by concentrating CO2 at the site of Rubisco enzyme through the division of labor between mesophyll cells and vascular sheath cells. There are three main types of C4 pathways, which are classified according to the different decarboxylases used as: NADP-malate type (NADP-ME), NAD-malate type (NAD-ME), and phosphoenolpyruvate carboxykinase type (PCK). They all differ in metabolic processes, enzyme activity regulation, substrate transport and energy requirements, etc. For instance, the decarboxylation reaction of NADP-ME type mainly takes place in chloroplasts, while that of NAD-ME type relies on mitochondria. The degree of dependence of different types on the REDOX state of chloroplasts and mitochondria also varies. Some key enzymes in C4 plants, such as PEPC, MDH, and PPDK, usually have higher catalytic efficiency and special cellular localization. The genes of these enzymes originated from the homologous genes of the C3 ancestor, but through functional changes and adjustment of expression patterns, they acquired the function of C4 (Furbank et al., 2000; Rao and Dixon, 2016; Brautigam et al., 2018; Fan et al., 2021; Alvarez and Maurino, 2023; Chen et al., 2023). 2.2 Anatomical requirements C4 photosynthesis requires special leaf structures, the most typical of which is the Kranz structure. Its characteristic is that mesophyll cells and vascular bundle sheath cells are closely arranged, and Rubisco enzyme is mainly concentrated in vascular bundle sheath cells. CO2 is first fixed in mesophyll cells and then transported to vascular sheath cells for further reactions. C4 plants generally have a high vein density, well-developed vascular sheath tissue, low gas diffusivity, and also have a special chloroplast distribution and morphology. The anatomical details of different C4 lineages are not exactly the same, such as the size, quantity, distribution of vascular bundle sheath cells and the way they connect with mesophyll cells. Some C3 plants also have structures similar to C4, which indicates that the anatomical features of C4 may only require a small amount of genetic changes to form (Westhoff and Gowik, 2010; Ludwig, 2013; Lundgren et al., 2014; Ermakova et al., 2019; Alenazi et al., 2023; Alvarez and Maurino, 2023). 2.3 Natural diversity of C4 crops According to existing research, we can find that the C4 photosynthetic pathway has independently occurred in angiosperms over 60 times, involving 19 families, which is a typical phenomenon of convergent evolution. C4 plants are mainly distributed in tropical and subtropical regions, including food and energy crops such as corn (Zea mays), sorghum (Glycine max) and sugarcane (Saccharum officinarum). Different C4 lineages have distinct biochemical types, anatomical features, gene regulation and adaptability, etc. The C4 gene families in different crops are diverse, and the selective pressures they face during survival also vary. Wild species generally retain more allele resources. Compared with C3 plants, C4 plants have higher utilization rates of water and nitrogen and are more likely to survive in adverse conditions (Sage, 2004; Rao and Dixon, 2016; Tao et al., 2019; Upadhyay et al., 2020; Chen et al., 2023). 3 Rationale for C4 Engineering in C3 Crops 3.1 Expected benefits for wheat Wheat is a globally important staple food crop, but its photosynthetic efficiency is limited by the C3 pathway, and its yield is prone to damage under conditions of high temperature and drought. C4 engineering may bring the following benefits to wheat:
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