Apoplastic cysteine proteases as inducers of plant defense and their inhibition by microbial effectors

• Applicant: Professor Dr. Gunther Döhlemann
• Subject Area: Organismic Interactions, Chemical Ecology and Microbiomes of Plant Systems
• Term: from 2013 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 244021783

The outcome of plant-microbe interactions is determined by the interplay of the microbial virulence repertoire with the plant immune system. In maize, the pathogenic fungus Ustilago maydis modulates host immunity to establish a biotrophic interaction and cause smut disease. Important hubs for the coordination of plant defence are papain-like cysteine proteases (PLCPs). Our previous work showed that these enzmes are crucial for the activation of salicylic acid (SA)-dependent defenses in maize leaves. During U. maydis infection, the activity of maize PLCPs is modulated by both the host plant via the cystatin CC9, as well as by the fungal effector protein Pit2. During the first phase of this project, we identified a novel, PLCP-release peptide signal (Zip1) which activates SA-mediated immunity. We showed that a fungal effector protein (Pit2) blocks the PLCP-mediated defense mechanism by acting as a molecular mimicry molecule, which releases a cross-kingdom conserved inhibitor peptide (cMIP). We observed an organ-specific, SA-mediated activation of PLCPs in roots, and found that root endophytic bacteria secrete inhibitors of root PLCPs. Based on these findings, we will focus on two main aspects:1) Zip1 signaling and its evolutionary conservation.This part of the project aims to gain mechanistic understanding on the activation of SA-signaling by Zip1. We will address the following questions: How is PROZIP1 localized and how is it processed to release apoplastic Zip1? What is the receptor for Zip1? Is Zip1 signaling specific to maize, or is this mechanismconserved in other plant species? 2) Organ specificity in PLCP-induced SA defense and its suppression in microbial root interactions. This part of the project aims to understand the molecular basis of organ-specific, PLCP-activated SA signaling, as well as to characterize the role of PLCP-inhibitors in the root microbiome. Our major goals are i) the biochemical and functional characterization of root-specific apoplastic PLCPs and their inhibitors; ii) to investigate root-specific SA signaling networks with an emphasis on the role of organ-specific PLCPs, and iii) to study the role of bacterial PLCP inhibitors for the establishment of the root-microbe interactions.

• DFG Programme: Research Grants