Biology subjects

Mhamdi, A.

Publications and source records attributed to Mhamdi, A..

6 recordsLinked to original sources

bioRxiv2026

Cytosolic GLUCOSE-6-PHOSPHATE DEHYDROGENASE 5 is a key player in redox homeostasis during oxidative stress and in oxidative stress-triggered activation of the salicylic acid pathway

Glucose-6-phosphate dehydrogenase (G6PDH) catalyzes the first step of the oxidative pentose phosphate pathway, generating NADPH to sustain redox metabolism and signaling. However, whether individual G6PDH isoforms directly regulate oxidative stress signaling remains unclear. To determine the contribution of the different Arabidopsis G6PDH isoforms to oxidative stress signaling, we introduced single T-DNA mutants into the catalase-deficient cat2 background, a genetic system in which intracellular H2O2 production activates salicylic acid (SA)-dependent cell death and defense pathways. Interestingly, impairment of cytosolic, but not chloroplastic G6PDH activity suppressed cat2-triggered phenotypes, with loss of G6PD5 function fully abolishing lesion formation. The cat2 g6pd5 double mutant phenocopied the SA biosynthesis-deficient mutant cat2 sid2 and showed reversion of defense responses as well as metabolomic and transcriptomic profiles to the wild-type state. Strikingly, despite the suppression of SA-dependent lesions, loss of G6PD5 activity does not appear to reduce stress intensity. On the contrary, cat2 g6pd5 plants exhibit increased glutathione synthesis and oxidation, elevated expression of oxidative stress marker genes, and enhanced accumulation of reactive nitrogen species relative to cat2. Protein-protein interaction analyses revealed that G6PD5 associates with several redox and defense-related proteins. In particular, we confirmed a physical interaction between G6PD5 and thioredoxin h5, a key component of redox-dependent SA signaling. However, analysis of cat2 trxh5 and cat2 npr1 lines indicated that this interaction alone cannot explain the G6PD5-dependent control of SA responses. Our work reveals that cytosolic G6PD5 integrates redox metabolism with immune signaling to control plant responses to oxidative stress.

plant biology↗

bioRxiv2026

Antagonistic functions of the two oxidative pentose phosphate pathway dehydrogenases in shaping oxidative stress responses

The oxidative pentose phosphate pathway (OPPP) is a source of cellular NADPH, generated through the sequential activities of glucose-6-phosphate dehydrogenase (G6PDH) and 6-phosphogluconate dehydrogenase (6PGDH). Using the catalase-deficient cat2 background as a model for H2O2-triggered salicylic acid (SA) signaling we identified the cytosolic G6PDH isoform G6PD5 as a key determinant of redox homeostasis and SA-dependent defense activation (Tremulot et al., companion manuscript). However, the mechanisms underlying this function remain enigmatic. In this work, genetic and transcriptomic analyses show that the role of G6PD5 cannot be explained solely by altered NADPH generation for either NADPH oxidases or the ascorbate-glutathione pathway, suggesting other possible links. To identify such links, a forward genetic screen was employed. We searched for mutations that modulate the suppressed lesion phenotype in cat2 g6pd5 in a photorespiration-dependent manner. This screen identified a mutation in PGD2, encoding the cytosolic 6PGDH. Strikingly, functional analyses of mutants and overexpression lines revealed that PGD2 exerts effects opposite to those of G6PD5 in SA signaling. Our observations uncover an unexpected antagonism between the two cytosolic NADPH-producing steps within the OPPP. Pharmacological analyses support a signaling role for the metabolic intermediate 6-phosphogluconolactone in linking the OPPP to SA signaling. These findings indicate that the OPPP is not solely a source of reducing power during oxidative stress but also acts as a signaling module in which metabolic intermediates contribute to the control of stress-induced immune responses.

plant biology↗

bioRxiv2026

A redox-regulated RCC1-like protein controls catalase activity in Arabidopsis

Reactive oxygen species (ROS) are central regulators of plant growth and stress responses. Cellular ROS levels are tightly controlled by antioxidant systems, including the evolutionarily conserved catalases that detoxify hydrogen peroxide (H2O2) predominantly within peroxisomes. Despite their importance, substantial gaps remain in our understanding of catalase biogenesis, regulation, subcellular targeting, and potential extra-peroxisomal functions. Using affinity purification of the UV-B photoreceptor UVR8 coupled with mass spectrometry, we identified a REGULATOR OF CHROMATIN CONDENSATION 1-like protein in Arabidopsis, which we named CATALASE-INTERACTING RCC1-LIKE 1 (CAIR1). CAIR1 interacts with all three catalase isoforms (CAT1-CAT3) as well as their chaperone NO CATALASE ACTIVITY 1 (NCA1). Loss-of-function cair1 mutants partially phenocopy cat2 and nca1, with reduced catalase activity, enhanced sensitivity to oxidative stress and alkaline growth conditions, and impaired primary root elongation. Mechanistically, cytosolic interaction between CAIR1 and CAT2 enhances total cellular catalase activity by facilitating peroxisomal import and proper subcellular localization of CAT2. In the absence of CAIR1, CAT2 forms aggregates, likely accounting for the observed loss of catalase activity. Notably, CAIR1 undergoes reversible, redox-dependent oligomerization that enhances its interaction with catalases. Mutation of CAIR1 at Cys-356 and Cys-545 compromises this interaction under elevated ROS conditions and fails to rescue the oxidative stress sensitivity of cair1 mutants. Moreover, UV-B exposure suppresses catalase activity by weakening the interaction between CAIR1 and catalases, thus linking environmental light signalling to cellular redox regulation. Together, our findings reveal CAIR1 as a dynamic redox-responsive regulator of catalase activity that maintains cellular redox homeostasis by coordinating catalase localization and function through reversible oligomerization.

plant biology↗

bioRxiv2024

The nuclear sulfenome of Arabidopsis: spotlight on histone acetyltransferase GCN5 regulation through functional thiols

Partial reduction of oxygen during energy generating metabolic processes in aerobic life forms results in the production of reactive oxygen species (ROS). In plants, ROS production is heightened during periods of both abiotic and biotic stress, which imposes a significant overload on the antioxidant systems. Hydrogen peroxide (H2O2) holds a central position in cellular redox homeostasis and signalling, playing an important role by oxidising crucial cysteines to sulfenic acid (-SOH), considered as a biologically relevant post-translational modification (PTM). Until now, the role of the nucleus in the cellular redox homeostasis has been relatively underexplored. The regulation of histone-modifying enzymes by oxidative PTMs on redox-active cysteines or tyrosine residues is particularly intriguing as it allows the integration of redox signalling mechanisms with chromatin control of transcriptional activity. One of the most extensively studied histone acetyltransferases is the conserved GENERAL CONTROL NONDEPRESSIBLE 5 (GCN5) complex. This study investigated the nuclear sulfenome in Arabidopsis thaliana by expressing a nuclear variant of the Yeast Activation Protein-1 (YAP) probe, identifying 225 potential redox-active nuclear proteins subject to sulfenylation. Mass spectrometry analysis further confirmed the sulfenylation of GCN5 at specific cysteine residues, with their functional significance and impact on the protein-protein interaction network assessed through cysteine-to-serine mutagenesis. HighlightProtein cysteine thiols are post-translationally modified under oxidative stress. Through the in vivo capturing of nuclear proteins undergoing sulfenylation in Arabidopsis, we highlight the functionality of particular cysteines in the histone acetyltransferase GCN5.

plant biology↗

bioRxiv2024

MEDIATOR SUBUNIT 25 modulates ERFVII-controlled hypoxia responses in Arabidopsis

Flooding impairs plant growth through oxygen deprivation, which activates plant survival and acclimation responses. Low-oxygen responses are generally associated with activation of group VII ETHYLENE-RESPONSE FACTOR (ERFVII) transcription factors. However, mechanism and molecular components by which ERFVII factors initiate gene expression are not fully elucidated. Here, we show that the Mediator complex subunit AtMED25 is recruited by RELATED TO APETALA 2.2 (RAP2.2) and RAP2.12 to coordinate gene expression during hypoxia in Arabidopsis thaliana.. The med25 mutants display reduced low-oxygen stress tolerance. AtMED25 associates with several ERFVII-controlled hypoxia core genes and its loss impairs transcription under hypoxia due to decreasing RNA polymerase II recruitment. Protein complex pulldown assays demonstrate that the Mediator complex built around AtMED25 is adjusted under low-oxygen conditions. Moreover, during hypoxia, no functional cooperation between AtMED25 and the two subunits AtMED8 and AtMED16 occurs, contrasting previous observations made for other conditions. In addition, AtMED25 function under hypoxia is independent from ethylene signalling. Finally, a functional conservation at the molecular level was found for the MED25-ERFVII module between Arabidopsis thaliana and the monocot Oryza sativa, pointing to a potentially universal role of MED25 in enabling ERFVII-dependent transcript responses to hypoxia in plants.

plant biology↗

bioRxiv2019

On the move: Redox-dependent protein relocation

Compartmentation of proteins and processes is a defining feature of eukaryotic cells. The growth and development of organisms is critically dependent on the accurate sorting of proteins within cells. The mechanisms by which cytosol-synthesized proteins are delivered to the membranes and membrane compartments have been extensively characterised. However, the protein complement of any given compartment is not precisely fixed and some proteins can move between compartments in response to metabolic or environmental triggers. The mechanisms and processes that mediate such relocation events are largely uncharacterized. Many proteins can in addition perform multiple functions, catalyzing alternative reactions or performing structural, non-enzymatic functions. These alternative functions can be equally important functions in each cellular compartment. Such proteins are generally not dual targeted proteins in the classic sense of having targeting sequences that direct de novo synthesised proteins to specific cellular locations. Accumulating evidence suggests that redox post-translational modifications (PTMs) can control the compartmentation of many such proteins, including antioxidant and/or redox associated enzymes.

cell biology↗