Populus simonii × Populus nigra WRKY70 is involved in salt stress and leaf blight disease responses

WRKY transcription factors (TFs) are important regulators in the complex stress response signaling networks in plants, but the detailed mechanisms underlying these regulatory networks have not been fully characterized. In the present study, we identified a Group III WRKY gene (PsnWRKY70, Potri.016G137900) from Populussimonii × Populusnigra and explored its function under salt and pathogen stresses. The promoter sequence that is located 2471-bp upstream from the start codon (SC) of PsnWRKY70 contained many stress-responsive cis-elements. Yeast one-hybrid assay suggested the upstream regulators, PsnWRKY70, PsnNAM (Potri.009G141600), PsnMYB (Potri.006G000800) and PsnGT1 (Potri.010G055000), probably modulate the expression of the PsnWRKY70 gene by specifically binding to the W-box or GT1GMSCAM4 (GT1) element. Yeast two-hybrid assay and transcriptome analysis revealed that HP1 (Potri.004G092100), RRM (Potri.008G146700), Ulp1 (Potri.002G105700) and some mitogen-activated protein kinase cascade members probably interact with PsnWRKY70 TF to response to salt stress. Compared with non-transgenic (NT) plants, PsnWRKY70-overexpressing (OEX) plants exhibited improved leaf blight disease resistance, while PsnWRKY70-repressing (REX) plants displayed enhanced salt stress tolerance. PsnWRKY70, PsnNAM, PsnMYB and PsnGT1 exhibited similar expression patterns in NT under salt and leaf blight disease stresses. The differentially expressed genes (DEGs) from NT vs OEX1 and the DEGs from NT vs REX1 exhibited considerable diversification. Most of the DEGs between NT and OEX1 were involved in aromatic amino acid biosynthesis, secondary metabolism, programmed cell death, peroxisomes and disease resistance. Most of the DEGs between NT and REX1 were related to desiccation response, urea transmembrane transport, abscisic acid response, calcium ion transport and hydrogen peroxide transmembrane transport. Our findings not only revealed the salt stress response signal transduction pathway of PsnWRKY70, but also provided direct evidence for the opposite biological functions of PsnWRKY70 TF in response to salt stress and leaf blight disease in P. simonii × P. nigra.