Biochemistry and molecular biology of CAM

Summary

Introduction

Crassulacean Acid Metabolism (CAM) has been called a curiosity (Osmond, 1978), the importance of which has been long overlooked in the context of environmental adaptations of plants to cope with arid, hot environments. CAM has since been studied more intensely and our knowledge has been extended by focusing on physiological studies (Ting & Gibbs, 1982; Ting, 1985; Cockburn, 1985), on the biochemical characterisation of important enzymes (O'Leary, 1982; Nimmo et al., 1986), and on ecological aspects (Kluge & Ting, 1978; Lüttge, 1987).

As a result of the diurnal separation of night CO₂ fixation by PEPCase, storage of the acidic product, malate, in the vacuole, and final carbon assimilation by Rubisco, CAM plants conserve water and hence can occupy ecological niches that have limited water or CO₂ availability. To mention only a few examples (see Kluge & Ting, 1978), CAM is expressed in flowering plants that are continually exposed to sea water, in cacti that inhabit true deserts, in climbing vines in the rainforest (Ting et al., 1985), and in freshwater plants where CO₂ is limiting (Keeley & Busch, 1984). CAM may be constitutive in a species, or the pathway may appear during ageing either throughout the plant, or specifically in the older leaves (Guralnick et al., 1984; Ting, 1985). In some plants the pathway may be induced by environmental factors, such as drought, high salinity or low temperature, which affect the availability of water (Winter, 1982).