Transcriptomics of C 3 +CAM Peperomia species reveals potential initial steps toward a CAM pathway
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Background The genus Peperomia (family Piperaceae) is the only known lineage with Crassulacean Acid Metabolism (CAM) in the Magnoliids.. Previous studies have demonstrated C 3 +CAM in multiple Peperomia species, yet there are ~ 1,600 species in the genus and very few have been assessed for CAM. Further, no studies have used genomics to understand CAM evolution in the Magnoliids. This study not only aims to find evidence of CAM in Hawaiian Peperomia , but also seeks to reveal genomic changes associated with C 3 +CAM evolution via transcriptomics and gene regulatory network analysis. Results While carbon isotope ratios suggested the majority of Hawaiian Peperomia species were C 3 , our detailed physiological measurements on a subset of species unveiled two two C 3 +CAM species. Transcriptomic analysis showed typical CAM-like expression of a number of genes in the core CAM pathway in both C 3 and C 3 +CAM species studied, but with increasing levels of expression in the C 3 +CAM species. One core CAM gene, phosphoenolpyruvate carboxylase kinase ( PPCK ), had clear upregulation of expression in the C 3 +CAM species, and also had induced expression under drought. Cluster and network analysis of PPCK expression indicated that circadian rhythm factors are also highly co-expressed with PPCK in C 3 +CAM species. Conclusions Our findings not only reveal the existence of the C 3 +CAM pathway in Hawaiian Peperomia species, but highlights PPCK as a regulatory bridge that possibly connects the evolution between C 3 and C 3 +CAM. With further analysis, understanding functional regulators of PPCK will be a key to understanding how CAM evolved from ancestral C 3 .
