Abstract text: Pectins are complex polysaccharides characterized by the presence of α-D galacturonic acids (GalA) in their backbones. They are major components of the primary cell wall that can be categorized into four distinct domains: homogalacturonan (HG), rhamnogalacturonan-I, rhamnogalacturonan-II, and xylogalacturonan. GalA residues can be acetylated (O-2 and/ or O-3 position) and methylesterified (C-6). Here, we identified two Arabidopsis PAEs, AtPAE7 and AtPAE11 (EC 3.1.1.6; CE13) controlling the degree of acetylation (DAc) in pectin. While bioinformatic analysis shows high sequence identity (86 %), heterologously expressed enzymes showed distinct differences in biochemical activity. Both enzymes are selective for HG, and AtPAE11 is active at a wide pH range (pH 3.5-7.5), while AtPAE7 requires more neutral pH and the presence of divalent ions. 1H NMR analyses found that AtPAE11 specifically removes O-3 acetylated groups. We also investigated the relationship between methyl-esterification and and PAE activity, and mass spectrometry analyses highlighted increased PAE activity on long pectin polymers previously treated with pectin methylesterase compared to short pectin. This newly identified mechanism suggests a new model where HG degradation proceeds in a sequential manner. Finally, the distinct modes of action AtPAE7 and AtPAE11 provides new insights in fine tuning of pectin acetylation during plant development.