AUTHOR=Tay Ignacius Y. Y. , Odang Kristoforus Bryant , Cheung C. Y. Maurice 

TITLE=Metabolic Modeling of the C3-CAM Continuum Revealed the Establishment of a Starch/Sugar-Malate Cycle in CAM Evolution

JOURNAL=Frontiers in Plant Science

VOLUME=11

YEAR=2021

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2020.573197

DOI=10.3389/fpls.2020.573197

ISSN=1664-462X

ABSTRACT=<p>The evolution of Crassulacean acid metabolism (CAM) is thought to be along a C<sub>3</sub>-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<sub>3</sub>, 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<sub>2</sub> exchange during the light period, as a proxy of stomatal conductance, there exists a C<sub>3</sub>-CAM continuum with gradual metabolic changes, supporting the notion that evolution of CAM from C<sub>3</sub> could occur solely through incremental changes in metabolic fluxes. Along the C<sub>3</sub>-CAM continuum, our model predicted changes in metabolic fluxes not only through the starch/sugar-malate cycle that is involved in CAM photosynthetic CO<sub>2</sub> 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<sub>3</sub> crops for improved water use efficiency.</p>