Figure 6.

Pathway of Crassulacean acid metabolism (CAM), highlighting underlying genes detected in this study as differentially expressed, with gene family expansion, with signature of adaptive sequence evolution or elevated TE insertion counts. The color of the process symbols indicates the species showing diel or elevated expression, increased gene family size, or increased TE insertion rate. Enzymes are shown in boxes, while pathway products are shown in bold, outside boxes. Enzymatic members of CAM metabolism pathways are shown in orange boxes, while stomatal and circadian regulators are highlighted in yellow. Stomatal regulators are shown on the left outside the cell and circadian regulators on the right. A) CO₂ is absorbed at night and first converted to HCO₃⁻ by carbonic anhydrase (CA). Then, it is converted to malate by carboxylating phosphoenolpyruvate (PEP), a key component of the glycolysis. In a first step, PEP carboxylase (PEPC) converts PEP to oxaloacetate, after being activated by PEPC kinase (PPCK). In a second step, malate dehydrogenase (MDH) converts oxaloacetate to malate. Malate is then transported into the vacuole by 2 possible transporters, either a tonoplast dicarboxylate transporter or an aluminum-activated malate transporter, which are assisted by V-ATPase proton pumps. During the day, the accumulated malate becomes the main source of CO₂ for photosynthesis. This allows the stomata to remain closed, which greatly enhances the water use efficiency of the plant. Malate is again transported out of the vacuole and reconverted to oxaloacetate by MDH, and then decarboxylated to PEP and CO₂ by PEP carboxykinase (PEPCK). The CO₂ will cycle through the Calvin cycle and generate sugars. GLR2.8, glutamate receptor 2.8; ABCC4, ABC transporter C 4; SDH, succinate dehydrogenase; XCT, XAP5 CIRCADIAN TIMEKEEPER; GI, protein GIGANTEA; RVE1, REVEILLE 1. B) Glycolysis, transitory starch, and sugar metabolism are tightly linked with the core CAM pathway as providers of starting materials such as PEP. During the day, CAM plants can store starch in the chloroplast and hexoses in the vacuole. In Bromeliaceae, the relative importance of soluble sugars versus starch as a source for PEP is variable across species (Christopher and Holtum 1998). At night, the stored starch and/or sugars are transported into the cytoplasm, converted to glucose or fructose, and broken down via the glycolysis to PEP. G6P, glucose-6-phosphate; GPT, glucose-6-phosphate/phosphate translocator; F6P, fructose-6-phosphate; PFK, phosphofructokinase; F1,6BP, fructose-1,6-biphosphate; DHAP, dihydroxyacetone phosphate; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; 1,3BPG, 1,3-bisphosphoglyceric acid; PEP, phosphoenolpyruvate; SSI, starch synthase I; SUT2, sucrose transporter 2; ERD6, EARLY RESPONSE TO DEHYDRATION 6. For a detailed description and accompanying per-gene expression profiles, see Supplementary Fig. S9 and Supplementary SI Note S10.