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SUMMARY: Flooding impairs plant growth through oxygen deprivation, which activates plant survival and acclimation responses. Transcriptional responses to low oxygen are generally associated with the activation of group VII ETHYLENE‐RESPONSE FACTOR (ERFVII) transcription factors. However, the exact mechanisms and molecular components by which ERFVII factors initiate gene expression are not fully elucidated. Here, we show that the ERFVII factors RELATED TO APETALA 2.2 (RAP2.2) and RAP2.12 cooperate with the Mediator complex subunit AtMED25 to coordinate gene expression under hypoxia in Arabidopsis thaliana. Respective med25 knock‐out mutants display reduced low‐oxygen stress tolerance. AtMED25 physically associates with a distinct set of hypoxia core genes and its loss partially impairs transcription under hypoxia due to decreased RNA polymerase II recruitment. Association of AtMED25 with target genes requires the presence of ERFVII transcription factors. Next to ERFVII protein stabilisation, also the composition of the Mediator complex including AtMED25 is potentially affected by hypoxia stress as shown by protein‐complex pulldown assays. The dynamic response of the Mediator complex to hypoxia is furthermore supported by the fact that two subunits, AtMED8 and AtMED16, are not involved in the establishment of hypoxia tolerance, whilst both act in coordination with AtMED25 under other environmental conditions. We furthermore show that AtMED25 function under hypoxia is independent of ethylene signalling. Finally, functional conservation at the molecular level was found for the MED25‐ERFVII module between A. thaliana and the monocot species Oryza sativa, pointing to a potentially universal role of MED25 in coordinating ERFVII‐dependent transcript responses to hypoxia in plants. [ABSTRACT FROM AUTHOR]

Tolerance mechanisms to single abiotic stress events are being investigated in different plant species, but how plants deal with multiple stress factors occurring simultaneously is still poorly understood. Here, we introduce Salicornia europaea as a species with an extraordinary tolerance level to both flooding and high salt concentrations. Plants exposed to 0.5 M NaCl (mimicking sea water concentrations) grew larger than plants not exposed to salt. Adding more salt reduced growth, but concentrations up to 2.5 M NaCl were not lethal. Regular tidal flooding with salt water (0.5 M NaCl) did not affect growth or chlorophyll fluorescence, whereas continuous flooding stopped growth while plants survived. Quantitative polymerase chain reaction (qPCR) analysis of plants exposed to 1% oxygen in air revealed induction of selected hypoxia responsive genes, but these genes were not induced during tidal flooding, suggesting that S. europaea did not experience hypoxic stress. Indeed, plants were able to transport oxygen into waterlogged soil. Interestingly, sequential exposure to salt and hypoxic air changed the expression of several but not all genes as compared to their expression upon hypoxia only, demonstrating the potential to use S. europaea to investigate signalling-crosstalk between tolerance reactions to multiple environmental perturbations. The extremophile Salicornia europaea is characterised for its tolerance to high salt and flooding conditions. Here, we studied the effect of multiple and simultaneous environmental perturbations of salinity and waterlogging on its growth. Responses to salt and hypoxia reciprocally affected each other when applied simultaneously, demonstrating the significance to investigate signalling-crosstalk between environmental stress reactions in tolerant species. This article belongs to the Collection Flooding Stress and Hypoxic Responses in Plants. [ABSTRACT FROM AUTHOR]

With this study, we aim to relate the substrate quality of different organic materials derived from plant residues to the respiratory activity of soil microorganisms after amendment, the formation of oxygen gradients upon irrigation, and the leaching of macronutrients and metals in soil. Elemental analyses were performed to determine the chemical composition of wheat straw, green compost, and a biochar product, showing that carbon availability, C/N ratio, and metal contents varied markedly. Consequently, after application to well-aerated sandy loam soil at 1% w/w, only straw increased microbial activity substantially, and nitrate was depleted within one week. Upon intense irrigation of soil columns packed with differently amended soils, strong hypoxia formed only in straw–soil, where microbial oxygen demand for straw degradation was high. This was enhanced after the application of mineral fertilizers, and nitrate leaching was mitigated. With the decreasing redox potential in straw–soil, the leaching of Fe, Mn, Al, Ni, Co, and As was increased. However, nitrate from mineral fertilizer mitigated the reduction of redox potential and, thus, the leaching of these metals. Measuring oxygen at different depths revealed near anoxic conditions at −15 cm of straw–soil with NP-fertilizer applied within 12 h after the start of irrigation and remained for at least 60 h, while oxygen showed extensive fluctuations in the upper few centimeters. This study showed that organic soil amendments with high carbon availability induce microbial respiration to the extent that causes strong and long-lasting hypoxia upon irrigation, even in sandy soil, which leads to substantial effects on the mobility of nutrients and toxic metals. In contrast, organic soil amendments with low carbon availability did not cause such effects. [ABSTRACT FROM AUTHOR]

The majority of eukaryotic organisms rely on molecular oxygen for respiratory energy production (1). When the supply of oxygen is compromised, a variety of acclimation responses are activated to reduce the detrimental effects of energy depletion (2-4). Various oxygen-sensing mechanisms have been described that are thought to trigger these responses (5-9), but they each seem to be kingdom specific and no sensing mechanism has been identified in plants until now. Here we show that one branch of the ubiquitin-dependent N-end rule pathway for protein degradation, which is active in both mammals and plants (10,11), functions as an oxygen-sensing mechanism in Arabidopsis thaliana. We identified a conserved amino-terminal amino acid sequence of the ethylene response factor (ERF)-transcription factor RAP2.12 to be dedicated to an oxygen-dependent sequence of post-translational modifications, which ultimately lead to degradation of RAP2.12 under aerobic conditions. When the oxygen concentration is low--as during flooding--RAP2.12 is released from the plasma membrane and accumulates in the nucleus to activate gene expression for hypoxia acclimation. Our discovery of an oxygen-sensing mechanism opens up new possibilities for improving flooding tolerance in crops., Tolerance to submergence and low oxygen availability (hypoxia) have been considered to be influenced by different members of subgroup VII of the ERF transcription factor family in Arabidopsis (RAP2.12 (ref. [...]