WRKY transcription factors have been shown to play a major role in regulating, both positively and negatively, the plant defense transcriptome. Nearly all studied WRKY factors appear to have a stereotypic binding preference to one DNA element termed the W-box. How specificity for certain promoters is accomplished therefore remains completely unknown. In this study, we tested five distinct Arabidopsis WRKY transcription factor subfamily members for their DNA binding selectivity towards variants of the W-box embedded in neighboring DNA sequences. These studies revealed for the first time differences in their binding site preferences, which are partly dependent on additional adjacent DNA sequences outside of the TTGACY-core motif. A consensus WRKY binding site derived from these studies was used for in silico analysis to identify potential target genes within the Arabidopsis genome. Furthermore, we show that even subtle amino acid substitutions within the DNA binding region of AtWRKY11 strongly impinge on its binding activity. Additionally, all five factors were found localized exclusively to the plant cell nucleus and to be capable of transactivating expression of a reporter gene construct in vivo.

Disease resistance of plants involves two distinct forms of chemical communication with the pathogen: recognition and defense. Both are essential components of a highly complex, multifaceted defense response, which begins with non-self recognition through the perception of pathogen-derived signal molecules and results in the production, inter alia, of antibiotically active compounds (phytoalexins) and cell wall-reinforcing material around the infection site. To elucidate the molecular details and the genomic basis of the underlying chains of events, we used two different experimental systems: suspension-cultured cells of Petroselinum crispum (parsley) and wildtype as well as mutant plants of Arabidopsis thaliana. Particular emphasis was placed on the structural and functional identification of signal and defense molecules, and on the mechanisms of signal perception, intracellular signal transduction and transcriptional reprogramming, including the structural and functional characterization of the responsible cis-acting gene promoter elements and transacting regulatory proteins. Comparing P. crispum and A. thaliana allows us to distinguish species-specific defense mechanisms from more universal responses, and furthermore provides general insights into the nature of the interactions. Despite the complexity of the pathogen defense response, it is experimentally tractable, and knowledge gained so far has opened up a new realm of gene technologyassisted strategies for resistance breeding of crop plants. Most plant͞pathogen interactions are fierce battles of attack and counterattack. These battles are fought with highly sophisticated means for the survival of the individual and, in the end, of the entire population or species. On the plant side, the most immediate defense response includes the reprogramming of cellular metabolism and highly dynamic, structural rearrangements within and around the attacked cells. In the cases of locally invading, fungal or fungus-like pathogens, the counterstroke of the plant commences in a highly localized fashion with the perception of chemical and physical signals from the intruder and ends with the accumulation of soluble, antibiotically active compounds and wall-bound, barrier-forming substances. The initiating event, attempted penetration of a potential pathogen, immediately activates an elaborate safe-guard system of non-self recognition based on specifically adapted plant receptors. These receptors recognize characteristic pathogen-borne surface molecules and transduce that information to numerous genes through a network of intracellular signaling cascades that orchestrate an extensive, defense-oriented transcriptional reprogramming of the affected cell. Among the major changes in cellular metabolism is the rapid accumulation of various secondary metabolites, some of which are likely to be integral to the complex, multicomponent defense response.This network of events, from the initial stage of recognition by the plant to the successful confinement or death of the pathogen, is ...