Abstract:In this study,a Populus tomentosa cytosolic APX gene,named as PcAPX,was identified and characterized.The recombinant PcAPX protein,encoding a 249-residue protein with a calculated mass of 33.01 kD,showed high activity towards ascorbic acid (AsA) and H2O2.At fixed H2O2 concentration,the Km and Vmax values were (0.71±0.03) mmol·L-1 and (0.41±0.02) mmol·L-1·min-1·mg-1 for AsA.And at fixed AsA concentration,the Km and Vmax values were (0.60±0.21) mmol·L-1 and (0.35±0.12) mmol·L-1·min-1·mg-1 for H2O2.These data showed that the protein was active with AsA and H2O2 and had high catalytic efficiency.Real-time PCR analysis showed that the expression levels of PcAPX was higher in the mesophyll of aged leaves than that of other tissues.This research provided the theoretical support to investigate roles of APX gene family in plant growth regulation.