Plant Cell http://doi.org/7sp (2015)

Fast activation of a central MAP kinase cascade and the production of reactive oxygen species (ROS) by membrane NADPH oxidases (known as RBOHs) are two common responses to multiple biotic and abiotic danger signals. Adachi et al. now show that both are connected in immune responses through so-called ‘WRKY’ transcription factors.

The ROS response to pathogens is biphasic. The fast and transient first stage is specific to pattern-induced immunity, post-translationally regulated by calcium and the kinase BIK1, which directly and functionally connects immune receptors with Arabidopsis RBOHD. The slow and sustained second stage is activated transcriptionally, specific to effector-induced immunity, and important for the hypersensitive response that limits pathogen spread.

The authors show that four WRKYs are substrates of the activated MAPKs in tobacco. Once phosphorylated, these transcription factors will directly bind and activate the promoter of tobacco RBOHB and produce the secondary ROS burst needed for the hypersensitive response. The increased susceptibility to oomycete Phytophthora infestans of a plant with silenced WRKYs confirms the biological significance of this mechanism. These results are a significant step toward unravelling the role of pathways downstream of MAPK cascades in defence, and maybe extending this understanding to further danger signals.