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Environmental, hormonal and circadian regulation of crassulacean acid metabolism expression.

This paper originates from a presentation at the IIIrd International Congress on Crassulacean Acid Metabolism, Cape Tribulation, Queensland, Australia, August 2001. Expression of crassulacean acid metabolism (CAM) is characterized by the extreme plasticity observed within and between species. Switches between C3 photosynthesis and CAM, and subsequent 24-h patterns of day/night CO2 uptake, are tightly controlled by a variety of environmental and metabolic factors that optimize the response of CAM plants to the most challenging environments over seasonal and daily time scales. Regulation of the genes and enzymes involved in CAM and connected metabolic pathways occurs at a number of levels (transcriptional through to post-translational). Such multiple levels of control are considered to be the key to the photosynthetic plasticity of CAM. Here, we review some of the primary environmental and hormonal factors controlling CAM plasticity in different CAM-inducible species, with emphasis on the regulatory signalling circuits responsible for this control. We also examine the inherent circadian regulation of the pathway, mainly in the context of the diel regulation of phosphoenolpyruvate carboxylase and the dedicated kinase that modulates its activity. We then consider the role of secondary signals, with emphasis on changes in cytosolic [Ca2+]i and the downstream signalling pathways, based on studies conducted on Mesembryanthemum crystallinum L. Besides representing an important metabolic adaptation, CAM provides an intriguing paradigm for studying the complex signalling mechanisms that control and coordinate the expression of genes under a variety of short- and long-term environmental perturbations.