Ubiquitin (Ub)-conjugating enzymes (UBCs) are key enzymes involved in ubiquitination. Although UBCs have been shown to play important roles in regulating various aspects of plant growth and development, the role of plant UBCs in abiotic stress response needs to be examined further. Here we report the characterization of a ubiquitin-conjugating enzyme gene AhUBC2 from dehydrated peanut plants. The expression of AhUBC2 gene in peanut plants is responsive to physiological water-stress induced by polyethylene glycol (PEG6000), high salinity, abscisic acid (ABA) or low temperature. The constitutive expression of AhUBC2 gene in wild-type Arabidopsis confers improved tolerance to water-stress induced by sorbitol or soil drought in 35S::AhUBC2 transgenic plants. Constitutive expression of AhUBC2 results in significantly increased expressions of three stress-responsive genes P5CS1, RD29A and KIN1 in 35S::AhUBC2 Arabidopsis grown under normal conditions, whereas the expressions of other four stress-responsive genes NCED3, ABF3, RD29B and RD22 are not affected. The proline level in 35S::AhUBC2 Arabidopsis is significantly higher than that in wild-type Arabidopsis under both soil-drought stressed and control conditions. In contrast, there is no significant difference in the levels of NCED3 transcript and endogenous ABA between wild-type and 35S::AhUBC2 Arabidopsis. These results suggest that constitutive expression of AhUBC2 in Arabidopsis confers improved water-stress tolerance likely through activating an ABA-independent signaling pathway, including regulating the expression of ABA-independent stress-responsive genes and promoting the synthesis of osmolyte proline to protect plants from water deficit. Copyright (C) 2010 The Society for Biotechnology, Japan. Published by Elsevier B.V. All rights reserved.