Abstract
BackgroundA common feature of plant defense responses is the transcriptional regulation of a large number of genes upon pathogen infection or treatment with pathogen elicitors. A large body of evidence suggests that plant WRKY transcription factors are involved in plant defense including transcriptional regulation of plant host genes in response to pathogen infection. However, there is only limited information about the roles of specific WRKY DNA-binding transcription factors in plant defense.ResultsWe analyzed the role of the WRKY25 transcription factor from Arabidopsis in plant defense against the bacterial pathogen Pseudomonas syringae. WRKY25 protein recognizes the TTGACC W-box sequences and its translational fusion with green fluorescent protein is localized to the nucleus. WRKY25 expression is responsive to general environmental stress. Analysis of stress-induced WRKY25 in the defense signaling mutants npr1, sid2, ein2 and coi1 further indicated that this gene is positively regulated by the salicylic acid (SA) signaling pathway and negatively regulated by the jasmonic acid signaling pathway. Two independent T-DNA insertion mutants for WRKY25 supported normal growth of a virulent strain of P. syringae but developed reduced disease symptoms after infection. By contrast, Arabidopsis constitutively overexpressing WRKY25 supported enhanced growth of P. syringae and displayed increased disease symptom severity as compared to wild-type plants. These WRKY25-overexpressing plants also displayed reduced expression of the SA-regulated PR1 gene after the pathogen infection, despite normal levels of free SA.ConclusionThe nuclear localization and sequence-specific DNA-binding activity support that WRKY25 functions as a transcription factor. Based on analysis of both T-DNA insertion mutants and transgenic overexpression lines, stress-induced WRKY25 functions as a negative regulator of SA-mediated defense responses to P. syringae. This proposed role is consistent with the recent finding that WRKY25 is a substrate of Arabidopsis MAP kinase 4, a repressor of SA-dependent defense responses.
Highlights
A common feature of plant defense responses is the transcriptional regulation of a large number of genes upon pathogen infection or treatment with pathogen elicitors
WRKY25 and WRKY33 were phosphorylated by MAP kinase 4 (MPK4) in vitro and a wrky33 knockout mutant expressed elevated levels of PR1 under a short-day growth condition [31]. These results suggest that WRKY25 and WRKY33 may function as downstream components of the MPK4-mediated SArepressing and jasmonic acid (JA)/ET-activating signaling pathways
To examine the DNA-binding activity of WRKY25, we expressed the gene in E. coli, purified the recombinant protein, and assayed its binding to an oligonucleotide that contains two direct TTGACC repeats (Pchn5; Figure 1B) using electrophoretic mobility shifting assay (EMSA)
Summary
A common feature of plant defense responses is the transcriptional regulation of a large number of genes upon pathogen infection or treatment with pathogen elicitors. Some of the pathogen-induced genes encode proteins with direct anti-microbial activities (e.g. the hydrolytic glucanases and chitinases that degrade cell walls of microbial organisms) or enzymes involved in biosynthesis of anti-microbial compounds (e.g. phytoalexins), whereas others encode proteins with regulatory functions in the defense signaling pathways. Many of these defense genes appear to be induced at the transcriptional level through the specific recognition of cis-acting promoter elements and trans-acting sequence-specific DNA-binding transcription factors. WRKY genes have greatly proliferated and form large superfamilies in angiosperms, with more than 70 members in Arabidopsis [6,7]
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