Not all Calvin's are equal: Differential control of the Calvin cycle in C₃ versus C₄ plants

Photosynthesis is indispensable for almost all life on our planet as it is a unique process that harnesses light energy to drive the conversion of water and carbon dioxide (CO₂) into oxygen (O₂) and carbohydrates. The CO₂ assimilated by the photosynthetic machinery is the basis for crop production and, therefore, for animal and human food. Thus, the optimization of photosynthetic efficiency is recognized as an auspicious strategy to boost crop yield (Hitchcock et al., 2022). Recent progress in this direction, such as directed bioengineering of chloroplast protein homeostasis and metabolism through the constitutive production of the D1 subunit of photosystem II (Chen et al., 2020), accelerated recovery of plants from nonphotochemical quenching-mediated photoprotection (De Souza et al., 2022) and engineering of plant photorespiration (South et al., 2019), show great potential to improve photosynthetic efficiency and plant yield.

Plants have evolved three types of photosynthetic pathways in response to distinct environmental conditions: C₃, C₄, and crassulacean acid metabolism (CAM). Approximately 85% of plant species use the C₃ photosynthetic pathway whereby CO₂ is fixed by Ribulose bisphosphate carboxylase/oxygenase (Rubisco) in the Calvin cycle, generating the three-carbon compound glycerate-3-phosphate. However, Rubisco can react with O₂ rather than CO₂ especially in circumstances with high [O₂], thereby competing for carbon fixation. This reaction initiates the energy-consuming photorespiration pathway that is essential for C₃ species to purge toxic byproducts such as glycolate. As such, it is not surprising that photorespiration can reduce overall photosynthetic productivity by 20%–50% (South et al., 2019). In contrast to C₃ plants, species with C₄ photosynthesis overcome this limitation by compartmentalization of carbon fixation and the Calvin cycle in mesophyll and bundle sheath (BS) cells, respectively (Figure 1). This allows C₄ photosynthesis to use a biochemical CO₂-concentrating mechanism that enhances the operating efficiency of Rubisco and inhibits ribulose-1,5-bisphosphate (RuBP) oxygenation and associated photorespiratory losses.

Figure 1.

A model for optimization of C₄ photosynthesis. Current results support that increased steady-state levels of Rubisco and Rieske FeS protein of cytochrome b₆f complex result in higher photosynthetic rate in C4 plants, whereas overaccumulation of SBPase fails. OAA, oxaloacetate; PEP, phosphoenolpyruvate. Adapted from Ermakova et al. (2022).

The Calvin cycle is common to all photosynthetic pathways and plays a central role in plant metabolism by providing metabolic intermediates required for starch, sucrose, and shikimic acid biosynthesis (Geiger and Servaites, 1994). The Calvin cycle includes three major steps: carboxylation of RuBP, reduction of 3-phosphoglyceric acid (3-PGA), and regeneration of RuBP. The enzyme sedoheptulose-1,7-bisphosphatase (SBPase) functions between the latter two steps, where assimilated carbon may either enter the regenerative phase or be exported from the cycle for sucrose or transitory starch biosynthesis. The reaction catalyzed by SBPase represents one of the rate-limiting steps of the Calvin cycle. In C₃ plants like tomato (Solanum lycopersicum) and wheat (Triticum aestivum), increasing the abundance of SBPase increases photosynthetic rates and plant biomass and yield (Ding et al., 2017; Driever et al., 2017). However, the effect of the altered steady-state level of SBPase on C₄ plants remained elusive.

In this issue of Plant Physiology, Ermakova et al. (2022) addressed the role of SBPase in C₄ photosynthesis using the C₄ model plant green foxtail (Setaria viridis). The authors generated transgenic lines containing 1.5–3.2 times higher SBPase levels with transcripts predominantly located within the BSs, suggesting correct localization of the protein. Then, they systematically examined the impact of increased steady-state levels of SBPase on macroscopic phenotypes and photosynthetic rate in S. viridis. They found that the abundance of the large Rubisco subunit, relative chlorophyll content, and photosynthetic electron transport chain components were not affected in the transgenic lines, indicating changes to SBPase only (Ermakova et al., 2022). Unlike previous experiments in C₃ plants, Ermakova et al. (2022) found no association between SBPase content and saturating rates of CO₂ assimilation in S. viridis under a range of environmental conditions, including different irradiances, [CO₂], and temperatures. Altogether, their findings indicate increasing SBPase content alone does not improve photosynthesis in C₄ plants.

The authors suggested several potential reasons for these observations. Firstly, the distinct separation of carbon concentration and the Calvin cycle between two different cell types may result in a reduced capacity to regulate carbon flux by the abundance of SBPase relative to C₃ plants where both steps occur within the same cell. Secondly, leaf SBPase content shows a progressive decrease during the evolutionary transition of C₃ to C₄ in Flavaria species (Borghi et al., 2022), suggesting a potentially high SBPase activity whereby further increases in SBPase content would have a minimal impact. Finally, SBPase is stimulated by the plastid ferredoxin/thioredoxin system in response to light. The increased SBPase in the transgenic lines of S. viridis used in this study is more than likely inactive owing to the limited reducing potentials in plastids. In contrast, overaccumulation of components in the photosynthetic electron transfer chain (PETC), such as the Rieske FeS protein of the cytochrome b₆f complex, can substantially enhance C₄ photosynthetic rate (Ermakova et al., 2019). Thus, reducing power provided by the photosynthetic electron transfer chain could be a bottleneck of current C₄ photosynthesis (Figure 1).

The results of Ermakova et al. (2022) have numerous implications for future attempts to improve photosynthetic productivity in plants (Figure 1). A number of key food and energy crops use the C₄ photosynthetic pathway, including maize (Zea mays), Miscanthus sp., sugarcane (Saccharum officinarum), and Sorghum sp. The agricultural potential of engineering C₄ photosynthesis cannot be underestimated. The unique carbon metabolism of these plants means they likely exert different control mechanisms over the regeneration of RuBP and the coordination of the Calvin cycle versus sucrose and starch biosynthesis. As such, any improvements to photosynthetic pathways in C₃ plants do not necessarily indicate corresponding targets for improvement in C₄ plants. Therefore, further studies are required on the control of carbon assimilation in C₄, and likely CAM, species.

Funding

This work was supported by a Leverhulme Trust Early Career Fellowship (A.J.B.), the Deutsche Forschungsgemeinschaft (WA 4599/2-2), the Seed Funding Program for Basic Research (66620211115918), and the Hong Kong Research Grants Council Early Career Scheme (27118022). Any opinions, findings, conclusions, or recommendations expressed in this publication do not reflect the views of the Government of the Hong Kong Special Administrative Region or the Innovation and Technology Commission.