Isolation And Characterization Of A Transcription Factor PtrWRKY40 From Populus Trichocarpa

WRKY transcription factors (TFs) are one of the largest families of transcriptional regulators in plants. Many WRKY TFs from various species have been described as positive or negative regulators in plant defense against different types of pathogens at multiple levels. WRKY TFs works in a complex way in plant response to various biotic and abiotic stresses at multiple levels. They have a very significant role in plant immunity against different types of pathogens with different strategies and mediating through different hormonal signaling. Therefore, it is important to know the mechanism of function of WRKYs in plant defense responses in poplar so that we can better equip ourselves in understanding the plant immune responses.Populus is an important wood species for timber and other commercial purposes. But many hybrid poplars are susceptible to many bacterial and fungal pathogens, which severely affect wood productivity. At least 100 genes of WRKY family have been found in the poplar genome and increasing evidence has confirmed important roles of WRKY TFs in poplar defense process. Several poplar WRKY genes have been identified to be involved in defense response.In this study, a subgroup Ha WRKY gene PtrWRKY40 was isolated from Populus trichocarpa, which displayed amino acid sequence similarity to Arabidopsis AtWRKY40, AtWRKY18 and AtWRKY60. PtrWRKY40 transcripts accumulated significantly in response to SA, methyl jasmonate (MeJA), abscisic acid (ABA) and hemi-biotrophic fungus Dothiorella gregaria Sacc. in wild type poplar. Overexpression of PtrWRKY40 in transgenic poplar conferred more 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 as compared to 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 these findings indicate that PtrWRKY40 plays a negative role in resistance to hemi-biotrophic fungi in poplar but functions as a positive regulator of resistance toward the necrotrophic fungi in Arabidopsis.On the other hand, WRKYs have been elaborated as key molecules in ABA signaling in Arabidopsis and other species. But no functional account is available about any poplar WRKY transcription factor in ABA signaling. So it is of prior significance to find out the role of Poplar WRKYs in ABA signaling. In this study, we found the involvement of PtrWRKY40 in ABA signal transduction in Arabidopsis as a negative regulator against ABA inhibition of root growth and germination. These findings suggested that PtrWRKY40 is involved in ABA signal transduction.