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High salinity and drought are major abiotic factors limiting productivity of alfalfa. GsWRKY20 encodes a WRKY-type transcription factor from Glycine soja. In our previous study, heterologous expression of GsWRKY20 in Arabidopsis significantly increased the drought tolerance of Arabidopsis. In order to assess whether GsWRKY20 is also involved in salt stress and breed transgenic forage legume with high drought and salt tolerance, GsWRKY20 was overexpressed in alfalfa. We found that transgenic alfalfa overexpressing GsWRKY20 showed increased drought and salt tolerance. Transgenic alfalfa grew well under high-salinity and water-deficit conditions, while wild-type plants exhibited leaf chlorosis, growth retardation and even death. Lower relative membrane permeability and lower malondialdehyde content were observed in transgenic alfalfa, and more free proline and soluble sugars were accumulated in transgenic alfalfa compared with wild-type plants under high-salinity and water-deficit conditions. Transgenic alfalfa accumulated less Na+ and more K+ in both leaves and roots under high-salinity treatment. In addition, transgenic alfalfa had much thicker cuticle, which could decrease the water loss of plants under water deficit. Taken together, these results reveal an important role for GsWRKY20 in salt and drought stress, and the transgenic alfalfa could be used for farming in salt-affected as well as arid and semi-arid areas.