Puente, Beatriz OrosaLoake, GaryAlMutairi, Ahmed2026-07-292028https://hdl.handle.net/20.500.14154/79672This thesis is under an embargo and must not be published or made publicly accessible before 1 July 2028, in accordance with the author's request and the applicable regulations. لا يُسمح بنشر هذه الرسالة أو إتاحتها للاطلاع العام قبل تاريخ 1 يوليو 2028، وذلك بناءً على طلب الباحث ووفقًا للوائح المعمول بهاPlants defend against pathogens by activating complex signalling networks, including a rapid nitric oxide (NO) accumulation at the infection sites that alters redox status. This shift promotes post-translational modifications such as S-nitrosylation, a covalent attachment between NO groups and cysteine residues of proteins. Various immune-related proteins are regulated through S-nitrosylation, demonstrating its significant role in plant immunity. S-nitrosylation is tightly controlled by S-nitrosoglutathione Reductase (GSNOR), which regulates the reduction of the natural NO reservoir S-nitrosoglutathione (GSNO), and Thioredoxin h5 (TRXh5), which directly catalyses the removal of NO moieties from S-nitrosylated proteins, in a process called denitrosylation. Recent studies also link NO signalling with epigenetic regulation, since NO through S-nitrosylation inhibits total histone deacetylase (HDAC) activity, thereby leading to chromatin remodelling and transcriptional activation of immune-responsive genes. However, the direct functional roles of specific HDACs in this pathway remain poorly understood. In this study, we identified histone deacetylase 5 (HDA5) as a novel interactor of GSNOR using yeast two-hybrid (Y2H), in vitro pull-down, and bimolecular fluorescence complementation (BiFC) assays. HDA5 was demonstrated to undergo S-nitrosylation, which inhibits its deacetylase activity while enhancing its protein stability upon NO treatment. Furthermore, we showed that TRXh5 mediates the denitrosylation of HDA5 via direct binding. Finally, functional analysis of hda5 loss-of-function and HDA5 overexpression lines revealed that HDA5 negatively regulates plant immunity by supressing the expression of the defence genes PR1 and PR2. Taken together, these findings uncover a dynamic redox-dependent mechanism for the regulation of HDA5 during plant immunity.153enS-nitrosylationDeacetylationacetylationplant immunityHDA5GSNORTRXh5RedoxThesis