Involvement of abscisic acid, ABI5, and PPC2 in plant acclimation to low CO2.

Phosphoenolpyruvate carboxylase (PEPC) plays a pivotal role in the photosynthetic CO2 fixation of C4 plants. However, the functions of PEPCs in C3 plants are less well characterized, particularly in relation to low atmospheric CO2 levels. Of the four genes encoding PEPC in Arabidopsis, PPC2 is considered as the major leaf PEPC. Here we show that the ppc2 mutants suffered a growth arrest when transferred to low atmospheric CO2 conditions, together with decreases in the maximum efficiency of photosystem II (Fv/Fm) and lower levels of leaf ABA and carbohydrates. The application of sucrose, malate or ABA greatly rescued the growth of ppc2 under low CO2 conditions. Metabolite profiling analysis revealed that the levels of glycine and serine were increased in the ppc2 leaves, while the abundance of photosynthetic metabolites was decreased under these conditions. The transcript levels of encoding enzymes involved in glycine or serine metabolism was decreased in ppc2 in an ABI5-dependent manner. Like the ppc2 mutants, the abi5-1 mutants had lower photosynthetic rates and Fv/Fm compared to the wild type under photorespiratory conditions i.e. low CO2 availability. However, the growth of these mutants was similar to the wild type under non-photorespiratory (low O2) conditions. The constitutive expression of ABI5 prevented the growth arrest of ppc2 under low CO2 conditions. These findings demonstrate that PPC2 plays an important role in the acclimation of Arabidopsis plants to low CO2 availability by linking photorespiratory metabolism to primary metabolism, and that this is mediated, at least in part, through ABA and ABI5-dependent processes.