Engineering of Crassulacean Acid Metabolism
Katharina Schiller, Andrea Bräutigam
Abstract
Crassulacean acid metabolism (CAM) has evolved from a C 3 ground state to increase water use efficiency of photosynthesis. During CAM evolution, selective pressures altered the abundance and expression patterns of C 3 genes and their regulators to enable the trait. The circadian pattern of CO 2 fixation and the stomatal opening pattern observed in CAM can be explained largely with a regulatory architecture already present in C 3 plants. The metabolic CAM cycle relies on enzymes and transporters that exist in C 3 plants and requires tight regulatory control to avoid futile cycles between carboxylation and decarboxylation. Ecological observations and modeling point to mesophyll conductance as a major factor during CAM evolution. The present state of knowledge enables suggestions for genes for a minimal CAM cycle for proof-of-concept engineering, assuming altered regulation of starch synthesis and degradation are not critical elements of CAM photosynthesis and sufficient malic acid export from the vacuole is possible.