Transgenic poplar expressing Arabidopsis NDPK2 enhances growth as well as oxidative stress tolerance Yun-Hee Kim 1,† , Myoung Duck Kim 1,† , Young Im Choi 2,† , Sung-Chul Park 1 , Dae-Jin Yun 3 , Eun Woon Noh 2 , Haeng-Soon Lee 1 and Sang-Soo Kwak 1, * 1 Environmental Biotechnology Research Center, Korea Research Institute of Bioscience and Biotechnology (KRIBB), Daejeon, Korea 2 Department of Forest Genetic Resources, Korea Forest Research Institute, Suwon, Korea 3 Division of Applied Life Science, and Plant Molecular Biology and Biotechnology Research Center, Gyeongsang National University, Jinju, Korea Received 12 March 2010; revised 22 June 2010; accepted 24 June 2010. *Correspondence (fax: +82 42 860 4608; email sskwak@kribb.re.kr) † These authors contributed equally to this work. Keywords: antioxidant enzyme, auxin, nucleoside diphosphate kinase 2, stress-inducible promoter, transgenic poplar. Summary Nucleoside diphosphate kinase 2 (NDPK2) is known to regulate the expression of antioxidant genes in plants. Previously, we reported that overexpression of Arabidop- sis NDPK2 (AtNDPK2) under the control of an oxidative stress-inducible SWPA2 pro- moter in transgenic potato and sweetpotato plants enhanced tolerance to various abiotic stresses. In this study, transgenic poplar (Populus alba · Poplus glandulosa) expressing the AtNDPK2 gene under the control of a SWPA2 promoter (referred to as SN) was generated to develop plants with enhanced tolerance to oxidative stress. The level of AtNDPK2 expression and NDPK activity in SN plants following methyl viologen (MV) treatment was positively correlated with the plant’s tolerance to MV-mediated oxidative stress. We also observed that antioxidant enzyme activities such as ascorbate peroxidase, catalase and peroxidase were increased in MV-treated leaf discs of SN plants. The growth of SN plants was substantially increased under field conditions including increased branch number and stem diameter. SN plants exhibited higher transcript levels of the auxin-response genes IAA2 and IAA5. These results suggest that enhanced AtNDPK2 expression affects oxidative stress tolerance leading to improved plant growth in transgenic poplar. Introduction Environmental stress is one of the most severe agricultural problems affecting plant growth and crop yield (Toennies- sen et al., 2003). Recent studies have identified a large number of genetic and molecular networks underlying plant adaptations to adverse environmental growth condi- tions (Sreenivasulu et al., 2007). All of these studies empha- size the complexity of various traits and their polygenic inheritance. The current notion is that defence mechanisms of plants against stress conditions are tightly associated with the species-specific growth habits, and hence every claim of tolerance enhancement needs to be tested on a crop-yield basis, coupled with its economic significance from an agricultural point of view (Flowers, 2004; Passioura, 2007). This consideration has only recently become gener- ally accepted, providing the justification for a major focus on crop-specific gene transfer to enhance stress tolerance. Trees have great values as a source of essential ele- ments for human living. They have unique characteristics, such as perennial growth, developmental phase changes, secondary growth and metabolism, and trees also exhibit resistance systems to extreme environmental conditions (Bhalerao et al., 2003; Gallardo et al., 2003). They are generally exposed to recurrent cycles of injury by a variety of biotic and abiotic stresses. This is because of both the technical difficulty of transformation and the extended life cycle. Therefore, the development of techniques leading to avoidance or reduction in injuries imposed by environ- mental stress is important for trees. Because of the slow growth, growth enhancement of trees provides an impor- tant incentive, because growth enhancement coupled to stress tolerance are important factors not only from an environmental but also from an economical point of view. Among the various tree crops, poplar occupies a promi- nent place as a model system for functional genomics ª 2010 The Authors 334 Plant Biotechnology Journal ª 2010 Society for Experimental Biology, Association of Applied Biologists and Blackwell Publishing Ltd Plant Biotechnology Journal (2011) 9, pp. 334–347 doi: 10.1111/j.1467-7652.2010.00551.x