Isolation and characterization of a subgroup IIa WRKY transcription factor PtrWRKY40 from Populus trichocarpa

Salicylic acid (SA) is a defense-related key signaling molecule involved in plant immunity. In this study, a subgroup IIa WRKY gene PtrWRKY40 was isolated from Populus trichocarpa, which displayed amino acid sequence similar to Arabidopsis AtWRKY40, AtWRKY18 and AtWRKY60. PtrWRKY40 transcripts accumulated significantly in response to SA, methyl jasmonate and hemibiotrophic fungus Dothiorella gregaria Sacc. Overexpression of PtrWRKY40 in transgenic poplar conferred higher susceptibility to D. gregaria infection. This susceptibility was coupled with reduced expression of SA-associated genes (PR1.1, PR2.1, PR5.9, CPR5 and SID2) and jasmonic acid (JA)-related gene JAZ8. Decreased accumulation of endogenous SA was observed in transgenic lines overexpressing PtrWRKY40 when compared with wild-type plants. However, constitutive expression of PtrWRKY40 in Arabidopsis thaliana displayed resistance to necrotrophic fungus Botrytis cinerea, and the expression of JA-defense-related genes such as PDF1.2, VSP2 and PR3 was remarkably increased in transgenic plants upon infection with fugal pathogens. Together, our findings indicate that PtrWRKY40 plays a negative role in resistance to hemibiotrophic fungi in poplar but functions as a positive regulator of resistance toward the necrotrophic fungi in Arabidopsis.