Is phosphoenolpyruvate carboxylase kinase the bridge between C4 photosynthesis and the crassulacean acid metabolism in Portulaca oleracea?

• Applicant: Dr. Renata Callegari Ferrari
• Subject Area: Plant Physiology
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 542954642

Photosynthesis as the generation of energy using light is fundamental not only for plant metabolism, but for all life on Earth. It is an intricate process that has evolved into three different types across plant lineages: C3, C4 and the crassulacean acid metabolism (CAM). C3 photosynthesis involves CO2 assimilation during the day and the activity of key enzyme Rubisco as the first carboxylation reaction. On the other hand, C4 photosynthesis and CAM are adaptations to concentrate CO2 around Rubisco, and the first carboxylation is actually performed by phosphoenolpyruvate carboxylase (PEPC) on both. Even though possessing some biochemical similarities, C4 and CAM evolved independently and each coordinate with the environment according to specific regulatory steps. For this reason, it is rare for a plant lineage to display both C4 and CAM in the same organism, being Portulaca oleracea one of the few species able to transition from C4 to CAM during drought in a totally reversible manner. The presence of two PEPC genes is central to alternating between C4 and CAM in P. oleracea, since each gene is induced according to water availability conditions. PEPC is activated by a regulatory enzyme, PEPC kinase (PPCK), but instead of also having two PPCKs, P. oleracea only presents one gene copy. Therefore, this project aim is to understand the dynamic of PPCK in regulating the photosynthetic alternation in P. oleracea. I will test the hypothesis that PPCK is the main trigger determining the C4-CAM alternation via a change in activation time of PEPC. For this aim, I will monitor gene expression using transcriptomics, enzyme activity patterns of PEPC and PPCK, and the phosphorylation status of PEPC in leaves of P. oleracea under both well-watered and drought conditions. This will also allow correlating the photosynthetic changes to the circadian clock mechanism, which will be monitored at transcript level. This project seeks to integrate different levels of biological information and will contribute to improving the field of the circadian regulation of photosynthesis. Understanding the extent of the influence of PPCK over the C4-CAM alternation is essential for next steps on unravelling the mechanism allowing a C4-CAM organism to have evolved. Furthermore, in a context of new crop breeding programs and developing strategies for stress resilience in species of economic interest, exploring C4-CAM plants will bring new targets in a context of climate change.

• DFG Programme: WBP Position