C-like Sesuvium sesuvioides (Aizoaceae) exhibits CAM in cotyledons and putative C-like + CAM metabolism in adult leaves as revealed by transcriptome analysis.

Background: The co-occurrence of C and CAM photosynthesis in a single species seems to be unusual and rare. This is likely due to the difficulty in effectively co-regulating both pathways. Here, we conducted a comparative transcriptomic analysis of leaves and cotyledons of the C-like species Sesuvium sesuvioides (Aizoaceae) using RNA-seq.

Results: When compared to cotyledons, phosphoenolpyruvate carboxylase 4 (PEPC4) and some key C genes were found to be up-regulated in leaves. During the day, the expression of NADP-dependent malic enzyme (NADP-ME) was significantly higher in cotyledons than in leaves. The titratable acidity confirmed higher acidity in the morning than in the previous evening indicating the induction of weak CAM in cotyledons by environmental conditions. Comparison of the leaves of S. sesuvioides (C-like) and S. portulacastrum (C) revealed that PEPC1 was significantly higher in S. sesuvioides, while PEPC3 and PEPC4 were up-regulated in S. portulacastrum. Finally, potential key regulatory elements involved in the C-like and CAM pathways were identified.

Conclusions: These findings provide a new species in which C-like and CAM co-occur and raise the question if this phenomenon is indeed so rare or just hard to detect and probably more common in succulent C lineages.