A new study led by the Wuhan Botanical Garden of the Chinese Academy of Sciences (WBG, CAS) has identified a unique trait in the aquatic plant Ottelia alismoides that it can simultaneously employ three distinct CO₂-concentrating mechanisms (CCMs). This finding provides new insights into how these pathways, long considered incompatible within the same plant tissue, can operate concurrently.
Many plants maximize photosynthetic efficiency through CCMs, with C4 photosynthesis and Crassulacean acid metabolism (CAM) representing two classic biochemical CCM pathways. These two pathways are generally regarded as incompatible in the same plant tissue, due to significant differences in carbon fixation timing, spatial organization, and metabolite transport.
Ottelia alismoides, however, stands out as an exception. It uniquely possesses all three major CCMs: constitutive C4 photosynthesis (NAD-ME subtype), inducible CAM, and bicarbonate (HCO₃⁻) utilization. The molecular and physiological mechanisms that enable these three CCMs to coordinate without interference at the cellular level have remained unclear.
To address this issue, the research team cultivated O. alismoides under both high and low CO₂ concentrations. They combined diel time-series sampling with a suite of advanced techniques, including key enzyme activity assays, subcellular localization analysis, transcriptomics, proteomics, and ¹³C isotopic labeling. This systematic approach allowed them to unravel the cooperative integration mechanism of C4 photosynthesis, CAM, and HCO₃⁻ utilization within single cells of the plant.
The study revealed that O. alismoides, unlike typical NAD-ME subtype C4 plants-where aspartate (produced by aspartate aminotransferase) serves as the first stable carbon compound-O. alismoides uses malate, generated by a cytosolic malate dehydrogenase. Critically, the plant achieves efficient coordination of its three CCMs through the temporal regulation of key enzyme isoforms, each with distinct expression patterns.
During the night, the phosphorylated isoform 3 of phosphoenolpyruvate carboxylase (PEPC) fixes carbon. This carbon is then converted into malate via catalysis by isoform 2 of malate dehydrogenase 1 (MDH1). The resulting malate is transported into vacuoles by an upregulated tonoplast dicarboxylate transporter (TDT), completing the CAM cycle.
In daylight, HCO₃⁻-supplied by carbonic anhydrase 1 (αCA1) and a solute carrier protein (SLC4)-is fixed by phosphorylated PEPC isoform 2. This initiates C4 metabolism, with malate synthesized under the catalysis of MDH1 isoform 1. Malate from both the CAM (nighttime) and C4 (daytime) pathways is then transported into mitochondria via a dicarboxylate transporter (DTC). There, it is decarboxylated by NAD-ME, releasing CO₂ to fuel the Calvin cycle in chloroplasts.
The findings were recently published in New Phytologist. This work was supported by the National Natural Science Foundation of China, and the CAS Youth Innovation Promotion Association, among other sources.
Coordinated metabolic model of HCO₃⁻ utilization, C₄ and CAM pathways in Ottelia alismoides. (Image by WBG)
The Ottelia alismoides. (Image by FU Wenlong)
