Metabolic Modeling of the C(3)-CAM Continuum Revealed the Establishment of a Starch/Sugar-Malate Cycle in CAM Evolution

The evolution of Crassulacean acid metabolism (CAM) is thought to be along a C(3)-CAM continuum including multiple variations of CAM such as CAM cycling and CAM idling. Here, we applied large-scale constraint-based modeling to investigate the metabolism and energetics of plants operating in C(3), CAM, CAM cycling, and CAM idling. Our modeling results suggested that CAM cycling and CAM idling could be potential evolutionary intermediates in CAM evolution by establishing a starch/sugar-malate cycle. Our model analysis showed that by varying CO(2) exchange during the light period, as a proxy of stomatal conductance, there exists a C(3)-CAM continuum with gradual metabolic changes, supporting the notion that evolution of CAM from C(3) could occur solely through incremental changes in metabolic fluxes. Along the C(3)-CAM continuum, our model predicted changes in metabolic fluxes not only through the starch/sugar-malate cycle that is involved in CAM photosynthetic CO(2) fixation but also other metabolic processes including the mitochondrial electron transport chain and the tricarboxylate acid cycle at night. These predictions could guide engineering efforts in introducing CAM into C(3) crops for improved water use efficiency.