CRASSULACEAN ACID METABOLISM: Molecular Genetics.

Crassulacean acid metabolism (CAM) is an adaptation of photosynthesis to limited availability of water or CO2. CAM is characterized by nocturnal CO2 fixation via the cytosolic enzyme PEP carboxylase (PEPC), formation of PEP by glycolysis, malic acid accumulation in the vacuole, daytime decarboxylation of malate and CO2 re-assimilation via ribulose-1,5-bisphosphate carboxylase (RUBISCO), and regeneration of storage carbohydrates from pyruvate and/or PEP by gluconeogenesis. Within this basic framework, the pathway exhibits an extraordinary range of metabolic plasticity governed by environmental, developmental, tissue-specific, hormonal, and circadian cues. Characterization of genes encoding key CAM enzymes has shown that a combination of transcriptional, posttranscriptional, translational, and posttranslational regulatory events govern the expression of the pathway. Recently, this information has improved our ability to dissect the regulatory and signaling events that mediate the expression and operation of the pathway. Molecular analysis and sequence information have also provided new ways of assessing the evolutionary origins of CAM. Genetic and physiological analysis of transgenic plants currently under development will improve our further understanding of the molecular genetics of CAM.