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

Authored by: María Valeria Lara , Carlos Santiago Andreo

Handbook of Photosynthesis

Print publication date:  March  2016
Online publication date:  April  2016

Print ISBN: 9781482230734
eBook ISBN: 9781482230758
Adobe ISBN:

10.1201/b19498-24

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Abstract

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 [1,35]. 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].

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