The last one in the row but decisive: PPDK as a potential key regulator of diurnal deacidification in CAM leaves across varying PPFD and photoperiod conditions

·Crassulacean acid metabolism (CAM) plants primarily fix atmospheric CO ₂ at night and store it as malic acid in their vacuoles. During the light period, vacuolar malate is remobilised and decarboxylated to supply CO ₂ for Rubisco assimilation. Light intensity and photoperiod are believed to play crucial roles in regulating this process, but their influences on the underlying molecular and biochemical mechanisms remain unclear. ·In this study, we integrated physiological, biochemical, and molecular approaches to uncover the temporal patterns and light responsiveness of gene transcript and protein abundances, and the activities of enzymes involved in diurnal malate remobilisation in the obligate CAM model species Kalanchoë fedtschenkoi . ·Vacuolar malate transport was primarily influenced by the endogenous clock and photoperiod, with Kf ALMT4 being a more plausible transporter candidate than Kf tDT. Decarboxylation of the released malate was mainly dictated by photoperiod, with light intensity playing a supplementary role. Both photoperiod and light intensity greatly affected the final processes of CAM photosynthesis i.e. CO ₂ refixation and pyruvate recycling, with PPDK—the last in line—being the most strictly light-regulated player at the mRNA, protein abundance and activity levels, closely matching malate dynamics. ·Collectively, this study revealed the recycling enzyme PPDK as a potential key regulator of light-dependent diurnal deacidification in CAM leaves, rather than the vacuolar malate transport or decarboxylation processes.