TOPLESS promotes plant immunity by repressing auxin signaling and is targeted by the fungal effector Naked1

In plants, the antagonism between growth and defense is hardwired by hormonal signaling. The perception of pathogen-associated molecular patterns (PAMPs) from invading microorganisms inhibits auxin signaling and plant growth. Conversely, pathogens manipulate auxin signaling to promote disease, but how this hormone inhibits immunity is not fully understood. Ustilago maydis is a maize pathogen that induces auxin signaling in its host. We characterized a U. maydis effector protein, Naked1 (Nkd1), that is translocated into the host nucleus. Through its native ethylene-responsive element binding factor-associated amphiphilic repression (EAR) motif, Nkd1 binds to the transcriptional co-repressors TOPLESS/TOPLESS-related (TPL/TPRs) and prevents the recruitment of a transcriptional repressor involved in hormonal signaling, leading to the derepression of auxin and jasmonate signaling and thereby promoting susceptibility to (hemi)biotrophic pathogens. A moderate upregulation of auxin signaling inhibits the PAMP-triggered reactive oxygen species (ROS) burst, an early defense response. Thus, our findings establish a clear mechanism for auxin-induced pathogen susceptibility. Engineered Nkd1 variants with increased expression or increased EAR-mediated TPL/TPR binding trigger typical salicylic-acid-mediated defense reactions, leading to pathogen resistance. This implies that moderate binding of Nkd1 to TPL is a result of a balancing evolutionary selection process to enable TPL manipulation while avoiding host recognition. Navarrete F., Gallei M., Kornienko A.E., Saado I., Khan M., Chia K.-S., Darino M.A., Bindics J., and Djamei A. (2022). TOPLESS promotes plant immunity by repressing auxin signaling and is targeted by the fungal effector

Automated summary

In plants, the interaction between growth and defense is controlled by hormonal signaling. A maize pathogen, Ustilago maydis, manipulates auxin signaling to promote disease, but the mechanism of how auxin inhibits immunity is not fully understood. Researchers have discovered that the effector protein Nkd1 from U. maydis induces auxin signaling in the host plant, leading to increased susceptibility to pathogens.