Expression profiling and functional analysis of Populus WRKY23 reveals a regulatory role in defense

WRKY transcription factors are key regulators that activate and fine-tune stress responses, including defense responses against pathogens. We isolated a poplar (Populus tremulaxPopulus alba) cDNA sequence, PtWRKY23, that encodes the ortholog of Arabidopsis WRKY23 and present the functional analysis of WRKY23, with emphasis on its potential role in resistance to rust infection. To investigate the function of PtWRKY23, we examined PtWRKY23 expression after stress treatments by qRT-PCR and generated PtWRKY23-misexpressing plants. Transgenic plants were assessed for resistance to Melampsora rust and were analyzed using the poplar Affymetrix GeneChip and histological techniques to study the consequences of PtWRKY23 misexpression. PtWRKY23 is rapidly induced by Melampsora infection and elicitor treatments and poplars overexpressing and underexpressing PtWRKY23 were both more susceptible to Melampsora infection than wild type. Transcriptome analysis of PtWRKY23 overexpressors revealed a significant overlap with the Melampsora-infection response. Transcriptome analysis also suggests that PtWRKY23 affects redox homeostasis and cell wall-related metabolism, which was confirmed by analyses that showed that PtWRKY23-misexpressing plants have altered peroxidase activity, apparent H(2)O(2) accumulation and lignin deposition. Our results show that PtWRKY23 affects resistance to Melampsora infection and that this may be caused by deregulation of genes that disrupt redox homeostasis and cell wall metabolism.