Spatial resolution of an integrated C4+CAM photosynthetic metabolism (Sci. Advances)

C₄ and CAM are both photosynthetic strategies that concentrate CO₂ upstream of RuBisCO, somewhat uncoupling photosynthetic carbon fixation from transpirational water loss. Both strategies use the enzyme PEP-carboxylase to produce an organic acid, which can then be decarboxylated to provide CO₂ to RuBisCO. In the C₄ system, PEP-carboxylase operates in mesophyll cells and RuBisCO in bundle sheath  cells, with the organic acid shuttling between them as a CO₂ carrier. Rather than this physical separation, CAM plants separate their carboxylases by time; CO₂ uptake occurs at night when transpiration is lower, it is stored as organic acids in the vacuole, and then released by decarboxylation in daylight when RuBisCO is active.  Moreno-Villena et al. explored the carbon-concentrating mechanisms of Portulaca oleracea, a C₄ species that also carries out CAM under drought conditions. They carried out RNA sequencing on isolated mesophyll and bundle sheath cells from well-watered and droughted plants. Their first surprise was that genes involved in both C₄ and CAM were expressed in the same bundle sheath cells, arguing against a previous assumption that these reactions took place in spatial isolation. The second finding was that the two pathways are actually linked. This finding comes from a metabolic model as well as flux balance analysis. Under drought conditions, CAM-produced malate feeds into the C₄ decarboxylation cycle and final CO₂ assimilation. The authors then speculate on why this efficient coupling of C₄ and CAM is apparently rare, suggesting that Portulaca was most likely an ancestrally C₃+CAM species that evolved C₄. They also observe that as fast-growing, self-compatible plants, Portulaca is amenable to development as a model system, and that further study could lead to the incorporation of CAM into C₄ crops, thereby increasing their drought tolerance without sacrificing productivity. (Summary by Mary Williams @PlantTeaching) Sci. Advances 10.1126/sciadv.abn2349