Metabolite compartmentation and transport in CAM plants

Summary

Crassulacean acid metabolism (CAM) represents the third major subdivision of photosynthetic carbon assimilation types in green plants alongside the C3 and C4 pathways. As with the C4 pathway, the CAM pathway can be regarded as an ancillary biochemical mechanism, serving to provide CO₂ at elevated concentrations for fixation in the Calvin cycle of C3 photosynthesis. In contrast to the C4 pathway, however, all the key biochemical components of carbon assimilation in CAM plants are to be found within individual mesophyll cells. CAM, in essence, is a cellular phenomenon, and this highlights the importance of regulating carbon flow between organelles at the subcellular level. The comparative biochemistry of CAM plants has been the subject of excellent reviews (Kluge & Ting, 1978; Osmond, 1978; Osmond & Holtum, 1981; Winter, 1985; Luttge, 1987; Griffiths, 1988; Leegood & Osmond, 1990). These sources can be consulted for detailed information on metabolic pathways and enzymic characteristics. We shall focus here on the significance of subcellular compartmentation for the control of carbon flow in the CAM pathway. While describing the metabolic interconversions characteristic of each compartment, our particular aim is to show how the regulation of metabolite flux between organelles is also fundamental to the carbon assimilation process in CAM plants. Our understanding of these transport processes is improving, but we are only just starting to unravel details of their molecular mechanisms.