PYRUVATE ORTHOPHOSPHATE DIKINASE (PPDK) as a putative key regulator of diurnal deacidification in CAM leaves across varying light intensities and photoperiods

Crassulacean acid metabolism (CAM) plants primarily fix atmospheric CO2 at night and store it as malic acid in their vacuoles. During daytime, the vacuolar malate is remobilised and decarboxylated to supply CO2 for Rubisco assimilation. Light intensity and photoperiod play crucial roles in regulating this process, but their influences on the underlying molecular and biochemical mechanisms remain unclear. In this study, physiological, biochemical, and molecular approaches were integrated 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 plant Kalanchoë fedtschenkoi. Vacuolar malate transport was primarily influenced by the endogenous clock and photoperiod, with the ALUMINIUM-ACTIVATED MALATE TRANSPORTER 4 (ALMT4) being a more plausible transporter candidate than the TONOPLAST DICARBOXYLATE TRANSPORTER (tDT). Malate decarboxylation was mainly dictated by photoperiod, with light intensity playing a supplementary role. Both photoperiod and light intensity greatly affected CO2 refixation and pyruvate recycling, with PYRUVATE ORTHOPHOSPHATE DIKINASE (PPDK) being the most strictly light-regulated player at the mRNA, protein abundance and activity levels, closely matching malate dynamics. Overall, PPDK seems to be a key regulator of light-dependent diurnal deacidification in CAM leaves, rather than the vacuolar malate transport or decarboxylation processes.