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PLANT PHYSIOLOGY , Vol 113, Issue 3 667-676, Copyright © 1997 by American Society of Plant Biologists
Molecular Genetics of Crassulacean Acid Metabolism
J. C. Cushman and H. J. Bohnert
Department of Biochemistry and Molecular Biology, 350 Noble Research Center, Oklahoma State University, Stillwater, Oklahoma 74078 (J.C.C.)
Most higher plants assimilate atmospheric CO2 through the C3 pathway of
photosynthesis using ribulose-1,5-bisphosphate carboxylase/oxygenase
(Rubisco). However, when CO2 availability is reduced by environmental
stress conditions, the incomplete discrimination of CO2 over O2 by Rubisco
leads to increased photorespiration, a process that reduces the efficiency
of C3 photosynthesis. To overcome the wasteful process of photorespiration,
approximately 10% of higher plant species have evolved two alternate
strategies for photosynthetic CO2 assimilation, C4 photosynthesis and
Crassulacean acid metabolism. Both of these biochemical pathways employ a
"CO2 pump" to elevate intracellular CO2 concentrations in the vicinity of
Rubisco, suppressing photorespiration and therefore improving the
competitiveness of these plants under conditions of high light intensity,
high temperature, or low water availability. This CO2 pump consists of a
primary carboxylating enzyme, phosphoenolpyruvate carboxylase. In C4
plants, this CO2-concentrating mechanism is achieved by the coordination of
two carboxylating reactions that are spatially separated into mesophyll and
bundle-sheath cell types (for review, see R.T. Furbank, W.C. Taylor [1995]
Plant Cell 7: 797-807;M.S.B. Ku, Y. Kano-Murakami, M. Matsuoka [1996] Plant
Physiol 111: 949-957). In contrast, Crassulacean acid metabolism plants
perform both carboxylation reactions within one cell type, but the two
reactions are separated in time. Both pathways involve cell-specific
changes in the expression of many genes that are not present in C3 plants.
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