Kalanchoë PPC1 Is Essential for Crassulacean Acid Metabolism and the Regulation of Core Circadian Clock and Guard Cell Signaling Genes

Unlike C3 plants, Crassulacean acid metabolism (CAM) plants fix CO2 in the dark using phosphoenolpyruvate carboxylase (PPC; EC 4.1.1.31). PPC combines phosphoenolpyruvate with CO2 (as HCO3 -), forming oxaloacetate. The oxaloacetate is converted to malate, leading to malic acid accumulation in the vacuole, which peaks at dawn. During the light period, malate decarboxylation concentrates CO2 around Rubisco for secondary fixation. CAM mutants lacking PPC have not been described. Here, we employed RNA interference to silence the CAM isogene PPC1 in Kalanchoë laxiflora Line rPPC1-B lacked PPC1 transcripts, PPC activity, dark period CO2 fixation, and nocturnal malate accumulation. Light period stomatal closure was also perturbed, and the plants displayed reduced but detectable dark period stomatal conductance and arrhythmia of the CAM CO2 fixation circadian rhythm under constant light and temperature free-running conditions. By contrast, the rhythm of delayed fluorescence was enhanced in plants lacking PPC1 Furthermore, a subset of gene transcripts within the central circadian oscillator was upregulated and oscillated robustly in this line. The regulation of guard cell genes involved in controlling stomatal movements was also perturbed in rPPC1-B These findings provide direct evidence that the regulatory patterns of key guard cell signaling genes are linked with the characteristic inverse pattern of stomatal opening and closing during CAM.

A roadmap for research on crassulacean acid metabolism (CAM) to enhance sustainable food and bioenergy production in a hotter, drier world

Crassulacean acid metabolism (CAM) is a specialized mode of photosynthesis that features nocturnal CO2 uptake, facilitates increased water-use efficiency (WUE), and enables CAM plants to inhabit water-limited environments such as semi-arid deserts or seasonally dry forests. Human population growth and global climate change now present challenges for agricultural production systems to increase food, feed, forage, fiber, and fuel production. One approach to meet these challenges is to increase reliance on CAM crops, such as Agave and Opuntia, for biomass production on semi-arid, abandoned, marginal, or degraded agricultural lands. Major research efforts are now underway to assess the productivity of CAM crop species and to harness the WUE of CAM by engineering this pathway into existing food, feed, and bioenergy crops. An improved understanding of CAM has potential for high returns on research investment. To exploit the potential of CAM crops and CAM bioengineering, it will be necessary to elucidate the evolution, genomic features, and regulatory mechanisms of CAM. Field trials and predictive models will be required to assess the productivity of CAM crops, while new synthetic biology approaches need to be developed for CAM engineering. Infrastructure will be needed for CAM model systems, field trials, mutant collections, and data management.