Computational analysis of the productivity potential of CAM

Abstract

There is considerable interest in transferring crassulacean acid metabolism (CAM) to C crops to improve their water-use efficiency. However, because the CAM biochemical cycle is energetically costly, it is unclear what impact this would have on yield. Using diel flux balance analysis of the CAM and C leaf metabolic networks, we show that energy consumption is three-fold higher in CAM at night. However, this additional cost of CAM can be entirely offset by the carbon-concentrating effect of malate decarboxylation behind closed stomata during the day. Depending on the resultant rates of the carboxylase and oxygenase activities of rubisco, the productivity of the PEPCK-CAM subtype is 74-100% of the C network. We conclude that CAM does not impose a significant productivity penalty and that engineering CAM into C crops is likely to lead to a major increase in water-use efficiency without substantially affecting yield.

Model approaches to advance crassulacean acid metabolism system integration

“…A bottom-up approach builds on existing mechanistic models (Owen and Griffiths, 2013a;Hartzell et al, 2018) and metabolic network models (Maurice Cheung et al, 2014;Shameer et al, 2018). Such models posit a null state, in which the only a minimal circadian and/or metabolic regulation is imposed on selective model components.…”

Metabolic modelling of the C3-CAM continuum revealed the establishment of a starch/sugar-malate cycle in CAM evolution

“…However, as a 42 carbon-concentrating mechanism, CAM is thought to be more metabolically expensive than C 3 43 (Winter and Smith, 1996b), which suggests that transferring a CAM pathway into C 3 crops 44 would incur a crop yield penalty. To investigate the energetics of C 3 and CAM, large-scale 45 metabolic models were applied which showed that engineering CAM into C 3 plants does not 46 impose a significant energetic penalty given the reduction in photorespiration from the carbon-47 concentrating mechanism (Cheung et al, 2014;Shameer et al, 2018). 48…”

“…Firstly, a reaction for the accumulation of oxygen, which is produced from 72 water splitting in the photosynthetic light reactions, was added to the model such that we could 73 run the simulations for CAM and CAM idling as oxygen exchange was constrained to zero 74 during the day in these two scenarios. Another modification from Shameer et al (2018) was how 75 the acidification of the vacuole was modelled. Instead of directly setting different fixed pH for 76 the vacuole in C 3 and CAM, protons were allowed to accumulate in the vacuole in our model.…”

“…Predicted metabolic fluxes of C 3 , CAM, CAM cycling and CAM idling 100 In this study, we simulated the metabolism of leaves undergoing C 3 , CAM, CAM cycling 101 and CAM idling using a recently published core metabolic model of Arabidopsis which was used 102 to model C 3 and CAM plants (Shameer et al, 2018). Minor modifications of the model were 103 outlined in the Materials and Methods section.…”

“…The constraints for simulating the core metabolic 104 functions of mature leaves, namely export of sucrose and amino acids into the phloem and 105 cellular maintenance, were set based on the values in Shameer et al (2018). All simulations, 106 except CAM idling, were constrained to have a phloem export rate of 0.259 μ mol m −2 s −1 based 107 on the value of C 3 plants in Shameer et al (2018). The set of constraints for modelling the four 108 different modes of photosynthesis are summarised in Table 1.…”