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Molekulare Analyse der Virulenz- und Avirulenzaktivitäten des Xanthomonas Effektors XopH und verwandter Effektorproteine

Fachliche Zuordnung Organismische Interaktionen, chemische Ökologie und Mikrobiome pflanzlicher Systeme
Förderung Förderung von 2019 bis 2024
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 418274535
 
Erstellungsjahr 2025

Zusammenfassung der Projektergebnisse

Certain microbial pathogens, such as Gram-negative phytopathogenic bacteria, utilize a syringe-like structure to deliver type III effector (T3E) proteins directly into plant cells. These effectors interfere with plant signaling pathways, often suppressing PAMP-triggered immunity (PTI) to benefit the pathogen. In response, plant resistance (R) genes enable the recognition of microbial effectors, triggering effector-triggered immunity (ETI), which often culminates in localized cell death known as the hypersensitive response (HR). Many R genes encode nucleotide-binding leucine-rich-repeat proteins (NLRs), yet the mechanisms by which NLRs detect microbial effectors remain unclear in most cases. In previous work, we demonstrated that the Xanthomonas T3E XopH exhibits 1-phytase activity, dephosphorylating the inositol phosphate InsP6 (phytate), a key phosphate storage molecule considered to also play a role in pathogen defense, into InsP5 [1-OH] (Blüher et al., 2017). Initially, we hypothesized that this activity might be critical for XopH-mediated immunity in plants carrying the Bs7 R gene. However, while XopH triggers plant resistance, the advantage of this effector for the pathogen, and whether XopH dephosphorylates higher phosphorylated inositol pyrophosphates (PP-InsPs)—critical regulators of hormone-mediated defense—remained unresolved. At the beginning of the funding phase, we discovered that, in addition to InsP6, XopH and related proteins can hydrolyze various InsP7 isomers to generate specific PP-InsP4 isomers (i.e., molecules with the same molecular mass as InsP6 but containing one PP moiety). In consequence, the identity of XopH products responsible for triggering immunity in resistant plants was unclear and our findings challenged our initial assumption that the key physiological activity of XopH was InsP6 degradation and that the primary XopH product responsible for ETI was InsP5 [1-OH]. Instead, our results suggested the need for a more comprehensive understanding of both the inventory of InsPs and PP-InsPs and the roles of the associated InsP/PP-InsP kinases and phosphatases with the idea that XopH-type effectors at least in part antagonize these activities. Significant progress was made on multiple fronts during the funding phase of this project. We optimized a novel technology, capillary electrophoresis coupled to electrospray ionization mass spectrometry (CE-ESI-MS), to identify previously unknown plant PP-InsPs, including 4- and 6-InsP7 isomers, which are enantiomers indistinguishable in the absence of chiral selectors. Both enantiomers were readily hydrolyzed in vitro by XopH and related proteins, producing PP-InsP4 isomers (likely 4PP-InsP4 [1-OH] and 6PP-InsP4 [1-OH]) that do not normally occur in plants. Like InsP5 [1- OH], these novel XopH products are strong candidates for triggering Bs7-dependent HR upon bacterial infection. Additionally, we identified ITPK1 and ITPK2 as kinases responsible for synthesizing 5-InsP7, another substrate of XopH. This led to the inclusion of 5PP-InsP4 as another potential candidate eliciting HR in Bs7 plants. Beyond XopH’s immediate functions, we uncovered important roles for PP-InsPs in phosphate starvation responses, auxin signaling, and salicylic acid (SA)-dependent immunity, further motivating experiments to explore why bacteria utilize XopH-like proteins. We also investigated endogenous phosphatases unrelated to XopH, including PFA-DSP-type and NUDT-type PP-InsP pyrophosphatases. These studies aimed to determine whether such endogenous hydrolytic activities could mirror XopH-like functions in different tissues and developmental contexts. Interestingly, even though not a primary focus of our work, we also leveraged the enzymatic activity of the XopH homolog RipBL1 from Ralstonia solanacearum to resolve the isomer identity of InsPs in human kidney stones, demonstrating the broader utility of these enzymes. In addition, we made substation contributions to the lipid arm of inositol phosphate signaling related to work on SEC14 lipid transfer proteins. Overall, the project funded by DFG grant SCHA 1274/5-1 was highly successful, as evidenced by 10 peer-reviewed publications, seven of which were led by our group and three where we made significant contributions. Publications appeared in highly visible journals, including Molecular Plant, PLOS Biology, and Plant Physiology. While a significant portion of our work on XopH is still ongoing, we are making good progress toward elucidating its role in plant defense and bacterial virulence." In terms of personnel development and training of students, Esther Lange (née Riemer), partially financed by this grant, successfully completed her PhD and received two prestigious awards for her PhD thesis. Robin Schneider, also partially funded by this grant, has already submitted his thesis to the faculty and will defend soon.

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