Involvement of abscisic acid, ABI5, and PPC2 in plant acclimation to low CO(2)

Phosphoenolpyruvate carboxylase (PEPC) plays a pivotal role in the photosynthetic CO(2) fixation of C(4) plants. However, the functions of PEPCs in C(3) plants are less well characterized, particularly in relation to low atmospheric CO(2) levels. Of the four genes encoding PEPC in Arabidopsis, PPC2 is considered as the major leaf PEPC gene. Here we show that the ppc2 mutants suffered a growth arrest when transferred to low atmospheric CO(2) conditions, together with decreases in the maximum efficiency of PSII (F(v)/F(m)) and lower levels of leaf abscisic acid (ABA) and carbohydrates. The application of sucrose, malate, or ABA greatly rescued the growth of ppc2 lines under low CO(2) conditions. Metabolite profiling analysis revealed that the levels of glycine and serine were increased in 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, abi5-1 mutants had lower photosynthetic rates and F(v)/F(m) compared with the wild type under photorespiratory conditions (i.e. low CO(2) availability). However, the growth of these mutants was similar to that of the wild type under non-photorespiratory (low O(2)) conditions. The constitutive expression of ABI5 prevented the growth arrest of ppc2 lines under low CO(2) conditions. These findings demonstrate that PPC2 plays an important role in the acclimation of Arabidopsis plants to low CO(2) availability by linking photorespiratory metabolism to primary metabolism, and that this is mediated, at least in part, through ABA- and ABI5-dependent processes.