Regulation of Crassulacean acid metabolism at the protein level in <i>Kalanchoë laxiflora</i>

Crassulacean acid metabolism (CAM) is an adaptation to environments where water availability is seasonal or extremely low. It serves to ensure plant survival and/or maintain productivity in these adverse environments. CAM has repeatedly evolved in many plant lineages, although it requires a large and complex set of enzymes, transporters, and regulatory processes to control metabolite flux and pools. To test the potential levels at which CAM is regulated, we analyzed the CAM plant Kalanchoë laxiflora and compared with the genomes and transcriptomes of other CAM plants across a wide phylogenetic range. We show that CAM-associated transcripts and proteins did not exhibit a binary on/off pattern in abundance between day and night in K. laxiflora. Instead, K. laxiflora and many CAM plants displayed shared amino acid changes among proteins compared to C3 plants, especially in starch metabolism. Phosphoproteomics identified differential phosphorylation in K. laxiflora proteins between day and night. Taken together, our results demonstrate that CAM photosynthesis is regulated at both the transcript and protein levels.