Photosynthesis in Nontypical C4 Species C4 Cycles without Kranz Anatomy and C4 Crassulacean Acid Metabolism Transitions

All plants and kinds of eukaryotic photoautotrophs use the same basic pathway for photosynthetic CO₂ fixation: the C₃ cycle (alternatively called photosynthetic carbon reduction cycle or Calvin and Benson cycle). In this pathway, ribulose bisphosphate carboxylase–oxygenase (RuBisCO) catalyzes the entry of CO₂ into the cycle. At ambient CO₂ and O₂ conditions, the enzyme also acts as an oxygenase incorporating O₂ into the photorespiratory carbon oxidation cycle with the resultant loss of the fixed carbon [ 1]. To overcome the effect of O₂ on RuBisCO, some plants have developed ways to increase the level of CO₂ at the location of RuBisCO in the plant, decreasing in this way the oxygenation reaction and, thus, the carbon flux through the photorespiratory carbon oxidation cycle. Among the different photosynthetic modes are the C₄ cycle and the crassulacean acid metabolism (CAM), which are evolutionary derived from C₃ photosynthesis [ 2]. The C₄ photosynthesis requires the coordination of biochemical functions between two types of cells and the cell type-specific expression of the enzymes involved [ 1, 3– 5]. In these plants, atmospheric CO₂ is first incorporated into C₄ acids in the mesophyll cells by phosphoenolpyruvate carboxylase. These C₄ acids are then transported to bundle sheath cells, where they are decarboxylated, and the released CO₂ is incorporated into the C₃ cycle. The C₄ system is more efficient under some environmental conditions as it increases the concentration of CO₂ in bundle sheath cells, suppressing the oxygenase activity of RuBisCO and, thus, photorespiration. On the other hand, CAM is a metabolic adaptation to arid environments: stomata are closed during much of the day and opened at night. Malic acid is accumulated in the vacuoles of mesophyll cells at night as a result of fixation of CO₂ by the phosphoenolpyruvate carboxylase [ 6]. During the day, malic acid is decarboxylated, and the released CO₂ is refixed in the C₃ cycle [ 4]. Compared with C₄ plants, leaves of CAM plants have a simple inner structure [ 7].