Facultative crassulacean acid metabolism (CAM) plants: Powerful tools for unravelling the functional elements of CAM photosynthesis

Abstract

Facultative crassulacean acid metabolism (CAM) describes the optional use of CAM photosynthesis, typically under conditions of drought stress, in plants that otherwise employ C 3 or C 4 photosynthesis. In its cleanest form, the upregulation of CAM is fully reversible upon removal of stress. Reversibility distinguishes facultative CAM from ontogenetically programmed unidirectional C 3 -to-CAM shifts inherent in constitutive CAM plants. Using mainly measurements of 24 h CO 2 exchange, defining features of facultative CAM are highlighted in five terrestrial species, Clusia pratensis, Calandrinia polyandra, Mesembryanthemum crystallinum, Portulaca oleracea and Talinum triangulare. For these, we provide detailed chronologies of the shifts between photosynthetic modes and comment on their usefulness as experimental systems. Photosynthetic flexibility is also reviewed in an aquatic CAM plant, Isoetes howellii. Through comparisons of C 3 and CAM states in facultative CAM species, many fundamental biochemical principles of the CAM pathway have been uncovered. Facultative CAM species will be of even greater relevance now that new sequencing technologies facilitate the mapping of genomes and tracking of the expression patterns of multiple genes. These technologies and facultative CAM systems, when joined, are expected to contribute in a major way towards our goal of understanding the essence of CAM. © 2014 The Author.