Phosphorylation of Phosphopyruvate Carboxylase Is Essential for Maximal and Sustained Dark CO Fixation and Core Circadian Clock Operation in the Obligate Crassulacean Acid Metabolism Species .

Phosphopyruvate carboxylase (PPC; EC 4.1.1.31) catalyzes primary nocturnal CO fixation in Crassulacean acid metabolism (CAM) species. CAM PPC is regulated posttranslationally by a circadian clock-controlled protein kinase called phosphopyruvate carboxylase kinase (PPCK). PPCK phosphorylates PPC during the dark period, reducing its sensitivity to feedback inhibition by malate and thus enhancing nocturnal CO fixation to stored malate. Here, we report the generation and characterization of transgenic RNAi lines of the obligate CAM species with reduced levels of transcripts. Plants with reduced or no detectable dark phosphorylation of PPC displayed up to a 66% reduction in total dark period CO fixation. These perturbations paralleled reduced malate accumulation at dawn and decreased nocturnal starch turnover. Loss of oscillations in the transcript abundance of was accompanied by a loss of oscillations in the transcript abundance of many core circadian clock genes, suggesting that perturbing the only known link between CAM and the circadian clock feeds back to perturb the central circadian clock itself. This work shows that clock control of prolongs the activity of PPC throughout the dark period in , optimizing CAM-associated dark CO fixation, malate accumulation, CAM productivity, and core circadian clock robustness.