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

Abstract

The evolution of Crassulacean acid metabolism (CAM) is thought to be along a C3-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 C3, 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 CO2 exchange during the light period, as a proxy of stomatal conductance, there exists a C3-CAM continuum with gradual metabolic changes, supporting the notion that evolution of CAM from C3 could occur solely through incremental changes in metabolic fluxes. Along the C3-CAM continuum, our model predicted changes in metabolic fluxes not only through the starch/sugar-malate cycle that is involved in CAM photosynthetic CO2 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 C3 crops for improved water use efficiency.

CAM Models: Lessons and Implications for CAM Evolution

“…A reduction in atmospheric CO 2 has also been proposed to trigger CAM evolution, as CAM radiation events coincide with declining CO 2 in the geological records ( Keeley and Rundel, 2003 ; Silvera et al, 2009 ). Tay et al (2021) tested this hypothesis by gradually decreasing the simulated concentration of CO 2 and observed CAM flux distributions as well. A sustained condition of limited CO 2 could have selected for more efficient metabolic fluxes over time that resulted in what we know as CAM.…”

“…These results are highly valuable as a long-sought goal of biotechnologists has been the implementation of CAM into C3 crops by means of genetic engineering. Shameer et al (2018) model was the basis for two further works (Töpfer et al, 2020;Tay et al, 2021). Töpfer et al (2020) extended the model by allowing for differences in vacuolar storage capacity, making gas exchange dependent on temperature and relative humidity, and considering the hourly progression of metabolism during a day.…”

“…A similar simulation under a CAM-like vacuole size restriction increased CO 2 uptake at the end of the night together with carboxylic acid accumulation. Tay et al (2021) used the Shameer et al (2018) model to study the C3-to-CAM continuum. They simulated CAM idling and CAM cycling-both considered as weak CAM-by impeding CO 2 uptake during the night and during the whole cycle, respectively.…”

“… Tay et al (2021) used the Shameer et al (2018) model to study the C3-to-CAM continuum. They simulated CAM idling and CAM cycling—both considered as weak CAM—by impeding CO 2 uptake during the night and during the whole cycle, respectively.…”

Dissecting Succulence: Crassulacean Acid Metabolism and Hydraulic Capacitance are Independent Adaptations in Clusia Leaves

“…Under well-watered conditions, Clusia leaves contain a diversity of photosynthetic phenotypes, ranging from obligate C 3 , to strong CAM (Barrera-Zambrano et al , 2014). This range includes several species exhibiting C 3 -CAM intermediate phenotypes, where flux through the CAM cycle represents only a small to moderate fraction of total carbon assimilation (Winter, 2019; Tay, et al , 2021). In addition, many Clusia species exhibit facultative CAM phenotypes, meaning they can increase metabolic flux through the CAM cycle in response to drought (Leverett et al , 2021; Winter & Holtum, 2014).…”