Abstract:Carboxylesterase (CXE) is widely distributed in plants, and can hydrolyze esters of short-chain fatty acids. CXE belong to α/β hydrolase fold superfamily of proteins, and play key roles in plant growth and development and responses to various stresses. In this study, 52 CXE genes were identified from the Populus trichocarpa genome. These 52 CXE genes were divided into three types, containing 39, 4 and 9 CXE genes, respectively. There are seven CXE clusters on Populus six chromosomes. These seven clusters included 28 CXE genes, and the remaining 24 CXE genes were scattered on Populus 16 chromosomes. Among all the 52 CXEs, expression of eight CXE genes were not detectable in the xylem, phloem, leaf, shooting and root of Populus trichocarpa, while the other 44 CXE genes were differentially expressed in the five tissues. Five CXE proteins (PtCXE4, 5, 6, 41, and 43) were expressed in E. coli, and purified to detect activity. PtCXE5 was found to catalyze the largest number of natural substrates. All the five purified proteins can catalyze the hydrolysis of 4-MU acetate and 4-MU butyrate, but none of them can catalyze the hydrolysis of 4-MU laurate and 4-MU palmitate. The CXE showed lower catalytic activity towards substrates with longer carbon chains.The pH-dependent analysis of enzyme activity showed that the catalytic activity of the five proteins increased with the increase of pH in the range of pH 6.0-9.0. This study provides new insights into the functions of carboxylesterase gene family.