Figure 9.

Regulation of photosynthetic CO₂-fixation and abiotic stress tolerance in the ‘turbocharging’ plants with the AaPEPC1. Overexpression of the AaPEPC1, a highly abundant enzyme catalyzing the primary fixation of CO₂ in CAM plants, increases photosynthetic CO₂-fixation, and rewires the diel accumulation-depletion pattern of malate and glucose. The re-programed malate-dependent carboxylation leads to the feedback up-regulation of the orthologs of key CAM pathway genes, i.e., nocturnal carboxylation and diurnal decarboxylation modules. The increased malate content up-regulates pyrroline-5-carboxylate synthase (P5CS) and pyrroline-5-carboxylate reductase (P5CR), which results in higher proline accumulation. Proline accumulation enhances the salt and drought tolerance of the transgenic plants expressing AaPEPC1. Additionally, a higher glucose content produced from the photosynthetic source is transported as sucrose or glucose to sink tissues and organs to promote cell proliferation, elongation, and expansion, as well as to maintain energy and metabolic homeostasis, resulting in improved biomass production [64,65]. The up arrows indicate up-regulation of expression of genes coding these enzymes or content. CA: carbonic anhydrase; OAA: oxaloacetate; MDH: malate dehydrogenase; ALMT: aluminum-activated malate transporter; TDT: tonoplast dicarboxylate transporter; ME: malic enzyme; PEP: phosphoenolpyruvate; and CBB: Calvin–Benson–Bassham.