Engineering of Crassulacean Acid Metabolism.

Crassulacean acid metabolism (CAM) has evolved from a C ground state to increase water use efficiency of photosynthesis. During CAM evolution, selective pressures altered the abundance and expression patterns of C genes and their regulators to enable the trait. The circadian pattern of CO fixation and the stomatal opening pattern observed in CAM can be explained largely with a regulatory architecture already present in C plants. The metabolic CAM cycle relies on enzymes and transporters that exist in C 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.