Open questions on facultative C4-CAM photosynthesis in Portulaca

Plants have evolved carbon concentrating mechanisms (CCMs) to optimize carbon fixation under environmental conditions that increase photorespiratory and transpiration rates. C4 and CAM photosynthesis are the two most widespread CCM examples across land plants and were typically considered to be incompatible due to biochemical, anatomical, evolutionary, and regulatory characteristics. However, Portulaca oleracea (purslane) was the first reported case to challenge this assumption more than 40 years ago. Although it remains as one of the most characterized C4-CAM species, many aspects of this dual system are still unresolved. This expert review summarizes recent advances in understanding C4-CAM integration in Portulaca, highlighting key genomic, transcriptomic, biochemical and physiological insights, and outlines the most pressing questions about the functioning and evolution of this combined CCM. Future directions for explaining how these convergent pathways emerged in the same lineage include functional studies on key regulatory elements, spatially resolved analyses for protein distribution and metabolic fluxes across leaves, an updated phylogeny of the family, and the integration of molecular and physiological data. So far, it is evident that expressing a weak CAM cycle in a C4 crop might be possible and could entail increased survival, possibly contributing to higher productivity in a future with climate change.