Intracellular transport and pathways of carbon flow in plants with crassulacean acid metabolism.

The massive daily reciprocal transfer of carbon between acids and carbohydrates that is unique to crassulacean acid metabolism (CAM) involves extensive and regulated transport of metabolites between chloroplasts, vacuoles, the cytosol and mitochondria. In this review of the CAM pathways of carbon flow and intracellular transport, we highlight what is known and what has been postulated. For three of the four CAM pathway variants currently known (malic enzyme- or PEP carboxykinase-type decarboxylase, and starch- or soluble sugar-type carbohydrate storage), the mechanisms of intracellular transport are still hypothetical and have yet to be demonstrated experimentally. Even in malic enzyme starch-storing species such as Kalanchoë daigremontiana Hamet et Perr. and Mesembryanthemum crystallinum L., the best-described variants of plants with the second-most common mode of photosynthetic carbon metabolism known, no tonoplast or mitochondrial transporter has been functionally described at a molecular level.