ROS signaling as common element in low oxygen and heat stresses

The activation of the oxidative metabolism in plants under low oxygen conditions has prompted controversial views. The presence of a ROS component in the transcriptome in response to low oxygen has been observed and an overlap with heat stress has been proved. It has been also demonstrated that ROS are produced during both anoxia and heat, but the site of their production remain contentious. Membrane NADPH oxidase and mitochondrial electron transport flow have been indicated as possible ROS generation systems. Both anoxia and heat have been shown to induce the transcription of Heat Shock Factors (HSFs) and Heat Shock Proteins (HSPs), among which HSFA2 and some of its targets. HSFA2 over-expressing plant has been shown to be more tolerant to anoxia, while the knockout hsfa2 lose the capability of wild type plants to cross-acclimate to anoxia through mild heat pre-treatment. The production of ROS seems to be an integral part of the anoxia and heat response, where HSFs likely play a central role in activating the HSP pathway. This mechanism is suggested to result in enhanced plant tolerance to both anoxia and heat

Reactive oxygen species-driven transcription in Arabidopsis under oxygen deprivation

Reactive oxygen species (ROS) play an important role as triggers of gene expression during biotic and abiotic stresses, among which is low oxygen (O2). Previous studies have shown that ROS regulation under low O2 is driven by a RHO-like GTPase that allows tight control of hydrogen peroxide (H2O2) production. H2O2 is thought to regulate the expression of heat shock proteins, in a mechanism that is common to both O2 deprivation and to heat stress. In this work, we used publicly available Arabidopsis (Arabidopsis thaliana) microarray datasets related to ROS and O2 deprivation to define transcriptome convergence pattern. Our results show that although Arabidopsis response to anoxic and hypoxic treatments share a common core of genes related to the anaerobic metabolism, they differ in terms of ROS-related gene response. We propose that H2O2 production under O2 deprivation is a trait present in a very early phase of anoxia, and that ROS are needed for the regulation of a set of genes belonging to the heat shock protein and ROS-mediated groups. This mechanism, likely not regulated via the N-end rule pathway for O2 sensing, is probably mediated by a NADPH oxidase and it is involved in plant tolerance to the stress

The Role of heat shock and heat shock- genes in the acquisition of tolerance to anoxia in Arabidopsis thaliana

Abstract- Abiotic stresses can strongly affect growth and development of plants. Although active research is going on in order to understand the mechanisms involved in plant stress-response, many questions about this issue have still to be answered in model plant system with the goal to transfer the knowledge to crop species. Transcriptome studies of Arabidopsis thaliana under anaerobic condition (lack of oxygen) has allowed to suppose that genes coding for a particular and conservatively recognized group of proteins, -known as heat shock proteins (HSPs) and normally involved in the acquisition of thermotolerance under heat shock-, might play an important role also in survival to anaerobiosis. Anoxia is charadcterized by a decrease in the energy and redox state of the cell, and last but not least, in the production of ROS (Reactive Oxygen Species). Here we investigated the possible role played by HSPs during anoxia response and their possible involvement during post anoxia recovery, likely resulting in oxidative stress. Anoxia induces the expression of genes coding for Heat Shock Proteins (HSPs) in Arabidopsis, and sucrose plays a synergistic role in this process. Sucrose specifically acts as a signalling molecule that triggers the induction of HSPs under anoxia, but is unable to enhance the HSP expression induced by heat. Treatments inducing the expression of heat-shock proteins could enhance Arabidopsis tolerance to anoxia, suggesting that HSPs may play an important role in cells under anaerobic stress. A heat pretreatment can enhance anoxia tolerance in Arabidopsis seedlings, whereas an anoxia pretreatment does not confer tolerance to heat stress. Interestingly, the positive effect exerted by sucrose on the expression of HSPs correlates with enhanced anoxia tolerance. The effects of sucrose appear to be independent of the metabolic role of sucrose, since the inductive effects of sucrose are also observed in the alcohol dehydrogenase mutant (adh), which is unable to carry out the fermentative pathway under anoxia. Plants pretreated with moderately high illumination before anoxia express higher levels of HSPs and display a higher anoxia tolerance when compared to dark-adapted plants. Overall, the results indicate that treatments which pre-induce the expression of HSPs (heat, sucrose, light, hypoxia) also result in enhanced survival to anoxia To deepen the possible role played by HSPs under anoxia, transcriptomic profilings have been analyzed and reveal a significant overlapping between anoxic and heat shock response. In particular a Heat Shock Factor (HsfA2), involved in different abiotic stress responses in Arabidopsis, is likely a “master” upstream regulator of several heat-shock genes induced by anoxia. In this respect, knock-out mutant for this Hsf results affected in the acquisition of anoxia-tolerance via heat-pretreatment , whereas 35S::HsfA2 overexpressing lines can better tolerate anoxia. In this work, we conclude that a possible important involvement of HSPs is present also during plant response to anaerobiosis and we cannot exclude that these proteins might be required during post-anoxic recovery, when re-oxygenation can lead to the ROS formation

The Heat-Inducible Transcription Factor HsfA2 Enhances Anoxia Tolerance in Arabidopsis[W]

Anoxia induces several heat shock proteins, and a mild heat pretreatment can acclimatize Arabidopsis (Arabidopsis thaliana) seedlings to subsequent anoxic treatment. In this study, we analyzed the response of Arabidopsis seedlings to anoxia, heat, and combined heat + anoxia stress. A significant overlap between the anoxic and the heat responses was observed by whole-genome microarray analysis. Among the transcription factors induced by both heat and anoxia, the heat shock factor A2 (HsfA2), known to be involved in Arabidopsis acclimation to heat and to other abiotic stresses, was strongly induced by anoxia. Heat-dependent acclimation to anoxia is lost in an HsfA2 knockout mutant (hsfa2) as well as in a double mutant for the constitutively expressed HsfA1a/HsfA1b (hsfA1a/1b), indicating that these three heat shock factors cooperate to confer anoxia tolerance. Arabidopsis seedlings that overexpress HsfA2 showed an increased expression of several known targets of this transcription factor and were markedly more tolerant to anoxia as well as to submergence. Anoxia failed to induce HsfA2 target proteins in wild-type seedlings, while overexpression of HsfA2 resulted in the production of HsfA2 targets under anoxia, correlating well with the low anoxia tolerance experiments. These results indicate that there is a considerable overlap between the molecular mechanisms of heat and anoxia tolerance and that HsfA2 is a player in these mechanisms

Universal stress protein HRU1 mediates ROS homeostasis under anoxia

Plant survival is greatly impaired when oxygen levels are limiting, such as during flooding or when anatomical constraints limit oxygen diffusion. Oxygen sensing in Arabidopsis thaliana is mediated by Ethylene Responsive Factor (ERF)-VII transcription factors, which control a core set of hypoxia-and anoxia-responsive genes responsible for metabolic acclimation to low-oxygen conditions. Anoxic conditions also induce genes related to reactive oxygen species (ROS). Whether the oxygen-sensing machinery coordinates ROS production under anoxia has remained unclear. Here we show that a low-oxygen-responsive universal stress protein (USP), Hypoxia Responsive Universal Stress Protein 1 (HRU1), is induced by RAP2.12 (Related to Apetala 2.12), an ERF-VII protein, and modulates ROS production in Arabidopsis. We found that HRU1 is strongly induced by submergence, but that this induction is abolished in plants lacking RAP2.12. Mutation of HRU1 through transfer DNA (T-DNA) insertion alters hydrogen peroxide production, and reduces tolerance to submergence and anoxia. Yeast two-hybrid and bimolecular fluorescence complementation (BiFC) analyses reveal that HRU1 interacts with proteins that induce ROS production, the GTPase ROP2 and the NADPH oxidase RbohD, pointing to the existence of a low-oxygen-specific mechanism for the modulation of ROS levels. We propose that HRU1 coordinates oxygen sensing with ROS signalling under anoxic conditions

Universal stress protein HRU1 mediates ROS homeostasis under anoxia

Plant survival is greatly impaired when oxygen levels are limiting, such as during flooding or when anatomical constraints limit oxygen diffusion. Oxygen sensing in Arabidopsis thaliana is mediated by Ethylene Responsive Factor (ERF)-VII transcription factors, which control a core set of hypoxia- and anoxia-responsive genes responsible for metabolic acclimation to low-oxygen conditions. Anoxic conditions also induce genes related to reactive oxygen species (ROS). Whether the oxygen-sensing machinery coordinates ROS production under anoxia has remained unclear. Here we show that a low-oxygen-responsive universal stress protein (USP), Hypoxia Responsive Universal Stress Protein 1 (HRU1), is induced by RAP2.12 (Related to Apetala 2.12), an ERF-VII protein, and modulates ROS production in Arabidopsis. We found that HRU1 is strongly induced by submergence, but that this induction is abolished in plants lacking RAP2.12. Mutation of HRU1 through transfer DNA (T-DNA) insertion alters hydrogen peroxide production, and reduces tolerance to submergence and anoxia. Yeast two-hybrid and bimolecular fluorescence complementation (BiFC) analyses reveal that HRU1 interacts with proteins that induce ROS production, the GTPase ROP2 and the NADPH oxidase RbohD, pointing to the existence of a low-oxygen-specific mechanism for the modulation of ROS levels. We propose that HRU1 coordinates oxygen sensing with ROS signalling under anoxic conditions