Abstract
The plant immune system comprises multiple layers, with receptors on the cell surface or within the cytoplasm recognizing common microbial patterns or specific virulence factors secreted by pathogens, respectively. Such recognition triggers complex cascade of downstream immune signaling, eventually leading to plant resistance. A comprehensive understanding of the plant immune responses is fundamental for innovative solutions to combat plant pathogens and enhance disease resistance. Conversely, a detailed comprehension of how pathogen virulence effectors operate during infection is crucial not only for studying the mechanisms of pathogenesis but also for discerning the functions of host proteins targeted by these effectors. In this study, I employed two approaches to identify and characterize proteins involved in plant-pathogen interactions and unravel the mechanism they are involved in. The first approach used the highly conserved family of AvrE effectors present in various phytopathogenic bacteria, including DspA/E from Erwinia amylovora, AvrE from Pseudomonas syringae and WtsE from Pantoea stewartii to study potential plant targets of those effectors. The second approach investigated potential novel substrates of the key immune signaling receptor-like cytoplasmic kinase (RLCK) BOTRYITIS-INDUCED KINASE 1 (BIK1), whose substrates have been previously shown to execute various immune responses. The first approach led me to the identification of so far undescribed, orthologous groups of the leucine-rich repeat (LRR)- receptor kinase (RK) subfamily III (LRR-RK III) as targets of the conserved AvrE effector family in Arabidopsis, apple and maize (Chapter 2). In this chapter, I could successfully demonstrate the interaction between AvrE effectors and the members of LRR-RK III. Guided by AvrE in planta phenotypes, I aimed to unravel the molecular function of the LRR-RK IIIs, particularly in the context of plant immunity, ABA signaling and water movement (Chapter 3). Mutation in the AvrE-interacting LRR-RK IIIs in Arabidopsis, however, did not result in any plant immune or ABA-phenotype to wild type plants. However, mutation of DIPM3 in apple resulted in increased resistance against E. amylovora highlighting the significance for these proteins in plant-bacterial interactions. The second approach led me to identify RKs from various subfamilies as potential BIK1 substrates. I could confirm RKs from the LRR-RK subfamily I and the family of wall-associated kinases (WAKs) as BIK1 substrates in vitro (Chapter 4). Of those, I focused on WAKs that have previously been described to act as receptors for the pectin-derived cell wall break down products oligogalacturonides (OGs). I characterized the role of WAKs in and beyond OG-perception (Chapter 5). Using a combination of OG-induced immune assays for early and late pattern-triggered immunity (PTI), I could not detect a requirement for WAKs in OG-induced signaling and immunity indicating that they are not bona fide OG receptors. Instead, WAKs seem to be common components of immune receptor complexes suggesting that they rather act as accessory RKs. Additionally, I demonstrated that OG-induced ROS production is dependent on the common LRR-RK co-receptor BRASSINOSTEROID INSENSITIVE 1 (BRI1)-ASSOCIATED KINASE 1 (BAK1) as previously published suggesting the role of an LRR-RK in OG perception. Collectively, I identified and characterized groups of RKs that are potentially involved in different aspects of plant immune signaling and/or establishing compatible pathogen-host interaction.
Herold, Laura