Crassulacean acid metabolism: Molecular genetics

Crassulacean acid metabolism (CAM) is an adaptation of photosynthesis to li mited availability of water or CO2. CAM is characterized by nocturnal CO2 f ixation via the cytosolic enzyme PEP carboxylase (PEPC), formation of PEP b y glycolysis, malic acid accumulation in the vacuole, daytime decarboxylati on of malate and CO2 re-assimilation via ribulose-1,5-bisphosphate carboxyl ase (RUBISCO), and regeneration of storage carbohydrates from pyruvate and/ or PEP by gluconeogenesis. Within this basic framework, the pathway exhibit s an extraordinary range of metabolic plasticity governed by environmental, developmental, tissue-specific, hormonal, and circadian cues. Characteriza tion of genes encoding key CAM enzymes has shown that a combination of tran scriptional, posttranscriptional, translational, and posttranslational regu latory events govern the expression of the pathway. Recently, this informat ion has improved our ability to dissect the regulatory and signaling events that mediate the expression and operation of the pathway. Molecular analys is and sequence information have also provided new ways of assessing the ev olutionary origins of CAM. Genetic and physiological analysis of transgenic plants currently under development will improve our further understanding of the molecular genetics of CAM.