Gene duplications facilitate C4-CAM compatibility in common purslane.

Common purslane (Portulaca oleracea) integrates both C4 and crassulacean acid metabolism (CAM) photosynthesis pathways, and is a promising model plant to explore C4-CAM plasticity. Here, we report a high-quality chromosome-level genome of NAD-ME subtype common purslane that provides evidence for two rounds of whole-genome duplication (WGD) with an ancient WGD (P-β) in the common ancestor to Portulacaceae and Cactaceae around 66.30 million years ago (Mya) and another (Po-α) specific to common purslane lineage around 7.74 Mya. A larger number of gene copies encoding key enzymes/transporters involved in C4 and CAM pathways were detected in common purslane than in related species. Phylogeny, conserved functional site and collinearity analyses revealed that the Po-α WGD produced the phosphoenolpyruvate carboxylase (PEPC) encoded gene copies used for photosynthesis in common purslane, while the P-β WGD event produced two ancestral genes of functionally differentiated (C4- and CAM-specific) beta carbonic anhydrases (β-CA) involved in the C4+CAM pathways. Additionally, cis-element enrichment analysis in the promoters showed CAM-specific genes have recruited both evening and midnight circadian elements as well as the ABA independent regulatory module mediated by ethylene-response factor (ERF) cis-elements. Overall, this study provides insights into the origin and evolutionary process of C4 and CAM pathways in common purslane, as well as potential targets for engineering crops by integrating C4 or CAM metabolism.