CEFOBI   05405
CENTRO DE ESTUDIOS FOTOSINTETICOS Y BIOQUIMICOS
Unidad Ejecutora - UE
capítulos de libros
Título:
Photosynthesis in Nontypical C4 Species
Autor/es:
LARA M. V.; ANDREO CS
Libro:
Handbook of Photosynthesis, 2nd edition
Editorial:
PESSARAKLI:
Referencias:
Año: 2015; p. 391 - 421
Resumen:
All plants use the same basic pathway for photosynthetic CO2 fixation: the C3 cycle (alternatively calledphotosynthetic carbon reduction cycle or Calvin andBenson cycle). In this pathway, ribulose bisphosphatecarboxylase?oxygenase (RuBisCO) catalyzes theentry of CO2 into the cycle. At ambient CO2 and O2conditions, the enzyme also acts as an oxygenaseincorporating O2 into the photorespiratory carbonoxidation cycle with the resultant loss of the fixedcarbon [1]. To overcome the effect of O2 onRuBisCO, some plants have developed ways to increase the level of CO2 at the location of RuBisCO inthe plant, decreasing in this way the oxygenationreaction and thus the carbon flux through the photorespiratory carbon oxidation cycle. Among the different photosynthetic modes are the C4 cycle and the392 Marı ´crassulacean acid metabolism (CAM), which are evolutionarily derived from C3 photosynthesis . The C4photosynthesis requires the coordination of biochemical functions between two types of cells and the celltype-specific expression of the enzymes involved [1,3].In these plants, atmospheric CO2 is first incorporatedinto C4 acids in the mesophyll cells by phosphoenolpyruvate carboxylase. These C4 acids are then transported to bundle sheath cells where they aredecarboxylated and the released CO2 is incorporatedinto the C3 cycle. The C4 system is more efficientunder some environmental conditions as it increasesthe 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 areclosed during most 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 thephosphoenolpyruvate carboxylase. During the day,malic acid is decarboxylated and the released CO2is refixed in the C3 cycle . Compared with C4plants, leaves of CAM plants have a simple innerstructure .Another mechanism found among photosyntheticorganisms that eliminates the O2 inhibition of photosynthesis is the one present in unicellular and multicellular algae and cyanobacteria. In this case, theconcentration of inorganic carbon in the site ofRuBisCO is the result of different transporters located at the plasma membrane or at the chloroplastenvelope and carbonic anhydrase.Although most C4 plants present Kranz anatomyand C4 biochemical features in a constitutive manner,many variations as well as transitions to and fromother photosynthetic modes have been described.These nontraditional C4 plants can be grouped asfollows: (a) submersed aquatic species like Egeriadensa, Elodea canadensis, and Hydrilla verticillatathat show induction of a C4-like metabolism withoutKranz anatomy under conditions of high temperatures and light intensities ; (b) amphibious species like the sedge Eleocharis vivipara, which has traitsof C4 plant in the terrestrial form and those of a C3plant in the submerged form ; (c) C4 photosynthetic plants belonging to the Chenopodiaceae whereC4 photosynthesis functions within a single photosynthetic cell though lacking the Kranz anatomy ;and (d) C4 succulent species of Portulaca that exhibit,under water stress, transition to a crassulacean acidlike metabolism (as in Portulaca oleracea) orinduction of a CAM-cycling metabolism compartmentalized in a different cell type while the C4 pathway is also operating (Portulaca grandiflora ).