Crassulacean acid metabolism (CAM)

The biochemical reactions of crassulacean acid metabolism (CAM) were first elucidated at Newcastle in the 1950's by Ransom and Thomas. The physiological consequences of CAM are improved photosynthetic performance in water and/or CO₂-limited environments. CAM is present in more than 20,000 species of plants including desert cacti, many orchids and bromeliads of the tropical rainforest and in some aquatic angiosperms.

CAM has evolved from C₃ photosynthesis many times, and probably under contrasting selective pressures, over the past 100 million years. It is likely that the ecological and agricultural importance of CAM species will increase in the face of global warming and expansion of desertification in semi-arid regions around the world. Thus, there is a critical need for integrated physiological and molecular approaches to the study of CAM.

Temporal separation of metabolism

The day/night separation of carboxylation processes shown below distinguishes CAM plants from C₃ and C₄ species.

Biochemistry of CAM

CAM plants fix CO₂ at night via the enzyme phosphoenolpyruvate carboxylase (PEPC) and release concentrated amounts of CO₂ around the C₃ carboxylase, Rubisco, during the day. The elevated levels of internal CO₂ result in stomatal closure and reduced transpirational water loss while photorespiration is also suppressed. The regulation of PEPC by a circadian rhythm of protein phosphorylation plays a central role in controlling carbon flux through the CAM pathway. Recent studies in our lab also indicate a role for metabolites in entraining PEPC phosphorylation to changes in CO₂ availability over the diel cycle.

The plasticity of CAM

All CAM species show considerable plasticity in adjusting the amount of CO₂ taken up over the day and night in response to changes in environmental conditions. The 4-phase model of CAM shown below indicates the major fluxes of carbon over a day/night cycle. The magnitude and duration of each phase is highly dependent on environmental conditions.

The most remarkable examples of CAM plasticity are shown by inducible species which use the C₃ pathway to maximize growth at times of sufficient water supply, but switch to CAM as a means of reducing water loss while maintaining photosynthetic integrity during periods of limited water supply. Understanding the mechanisms of CAM induction will help to develop engineering strategies for enhancing the growth and productivity of plants in both intermittently and seasonally dry habitats.