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

LARA, MARÍA VALERIA; ANDREO, CARLOS SANTIAGO

Handbook of Photosynthesis: Third Edition?

CRC press, Taylor & Francis Group

Lugar: Boca Ratón; Año: 2016; p. 307 - 326

All plants and kinds of eukaryotic photoautotrophs use thesame basic pathway for photosynthetic CO2 fixation: the C3cycle (alternatively called photosynthetic carbon reductioncycle or Calvin and Benson cycle). In this pathway, ribulosebisphosphate carboxylase?oxygenase (RuBisCO) catalyzesthe entry of CO2 into the cycle. At ambient CO2 and O2 conditions,the enzyme also acts as an oxygenase incorporatingO2 into the photorespiratory carbon oxidation cycle with theresultant loss of the fixed carbon [1]. To overcome the effect ofO2 on RuBisCO, some plants have developed ways to increasethe 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 oxidationcycle. Among the different photosynthetic modes are theC4 cycle and the crassulacean acid metabolism (CAM), whichare evolutionary derived from C3 photosynthesis [2]. The C4photosynthesis requires the coordination of biochemical functionsbetween two types of cells and the cell type?specificexpression of the enzymes involved [1,3?5]. In these plants,atmospheric CO2 is first incorporated into C4 acids in themesophyll cells by phosphoenolpyruvate carboxylase. TheseC4 acids are then transported to bundle sheath cells, wherethey are decarboxylated, and the released CO2 is incorporatedinto the C3 cycle. The C4 system is more efficient under some environmental conditions as it increases the concentration ofCO2 in bundle sheath cells, suppressing the oxygenase activityof RuBisCO and, thus, photorespiration. On the other hand,CAM is a metabolic adaptation to arid environments: stomataare closed during much of the day and opened at night. Malicacid is accumulated in the vacuoles of mesophyll cells at nightas 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]. Comparedwith C4 plants, leaves of CAM plants have a simple innerstructure [7].Another mechanism found among photosynthetic organismsthat eliminates the O2 inhibition of photosynthesis is theone present in unicellular and multicellular algae [8], and cyanobacteria.In this case, the concentration of inorganic carbonin the site of RuBisCO is the result of different transporterslocated at the plasma membrane or at the chloroplast envelopeand carbonic anhydrase [9?14].