Heterologous Expression of Poplar WRKY18/35 Paralogs in Arabidopsis Reveals Their Antagonistic Regulation on Pathogen Resistance and Abiotic Stress Tolerance via Variable Hormonal Pathways

WRKY transcription factors (WRKY TFs) are one of the largest protein families in plants, and most of them play vital roles in response to biotic and abiotic stresses by regulating related signaling pathways. In this study, we isolated two WRKY TF genes PtrWRKY18 and PtrWRKY35 from Populus trichocarpa and overexpressed them in Arabidopsis. Expression pattern analyses showed that PtrWRKY18 and PtrWRKY35 respond to salicylic acid (SA), methyl JA (MeJA), abscisic acid (ABA), B. cinereal, and P. syringae treatment. The transgenic plants conferred higher B. cinerea tolerance than wild-type (WT) plants, and real-time quantitative (qRT)-PCR assays showed that PR3 and PDF1.2 had higher expression levels in transgenic plants, which was consistent with their tolerance to B. cinereal. The transgenic plants showed lower P. syringae tolerance than WT plants, and qRT-PCR analysis (PR1, PR2, and NPR1) also corresponded to this phenotype. Germination rate and root analysis showed that the transgenic plants are less sensitive to ABA, which leads to the reduced tolerance to osmotic stress and the increase of the death ratio and stomatal aperture. Compared with WT plants, a series of ABA-related genes (RD29A, ABO3, ABI4, ABI5, and DREB1A) were significantly down-regulated in PtrWRKY18 and PtrWRKY35 overexpression plants. All of these results demonstrated that the two WRKY TFs are multifunctional transcription factors in plant resistance.