The Arabidopsis thaliana onset of leaf death 12 mutation in the lectin receptor kinase P2K2 results in an autoimmune phenotype.

BACKGROUND: Plant immunity relies on the perception of immunogenic signals by cell-surface and intracellular receptors and subsequent activation of defense responses like programmed cell death. Under certain circumstances, the fine-tuned innate immune system of plants results in the activation of autoimmune responses that cause constitutive defense responses and spontaneous cell death in the absence of pathogens. RESULTS: Here, we characterized the onset of leaf death 12 (old12) mutant that was identified in the Arabidopsis accession Landsberg erecta. The old12 mutant is characterized by a growth defect, spontaneous cell death, plant-defense gene activation, and early senescence. In addition, the old12 phenotype is temperature reversible, thereby exhibiting all characteristics of an autoimmune mutant. Mapping the mutated locus revealed that the old12 phenotype is caused by a mutation in the Lectin Receptor Kinase P2-TYPE PURINERGIC RECEPTOR 2 (P2K2) gene. Interestingly, the P2K2 allele from Landsberg erecta is conserved among Brassicaceae. P2K2 has been implicated in pathogen tolerance and sensing extracellular ATP. The constitutive activation of defense responses in old12 results in improved resistance against Pseudomonas syringae pv. tomato DC3000. CONCLUSION: We demonstrate that old12 is an auto-immune mutant and that allelic variation of P2K2 contributes to diversity in Arabidopsis immune responses.

Endoplasmic reticulum oxidoreductin provides resilience against reductive stress and hypoxic conditions by mediating luminal redox dynamics.

Oxidative protein folding in the endoplasmic reticulum (ER) depends on the coordinated action of protein disulfide isomerases and ER oxidoreductins (EROs). Strict dependence of ERO activity on molecular oxygen as the final electron acceptor implies that oxidative protein folding and other ER processes are severely compromised under hypoxia. Here, we isolated viable Arabidopsis thaliana ero1 ero2 double mutants that are highly sensitive to reductive stress and hypoxia. To elucidate the specific redox dynamics in the ER in vivo, we expressed the glutathione redox potential (EGSH) sensor Grx1-roGFP2iL-HDEL with a midpoint potential of -240 mV in the ER of Arabidopsis plants. We found EGSH values of -241 mV in wild-type plants, which is less oxidizing than previously estimated. In the ero1 ero2 mutants, luminal EGSH was reduced further to -253 mV. Recovery to reductive ER stress induced by dithiothreitol was delayed in ero1 ero2. The characteristic signature of EGSH dynamics in the ER lumen triggered by hypoxia was affected in ero1 ero2 reflecting a disrupted balance of reductive and oxidizing inputs, including nascent polypeptides and glutathione entry. The ER redox dynamics can now be dissected in vivo, revealing a central role of EROs as major redox integrators to promote luminal redox homeostasis.

Try or Die: Dynamics of Plant Respiration and How to Survive Low Oxygen Conditions.

Fluctuations in oxygen (O2) availability occur as a result of flooding, which is periodically encountered by terrestrial plants. Plant respiration and mitochondrial energy generation rely on O2 availability. Therefore, decreased O2 concentrations severely affect mitochondrial function. Low O2 concentrations (hypoxia) induce cellular stress due to decreased ATP production, depletion of energy reserves and accumulation of metabolic intermediates. In addition, the transition from low to high O2 in combination with light changes-as experienced during re-oxygenation-leads to the excess formation of reactive oxygen species (ROS). In this review, we will update our current knowledge about the mechanisms enabling plants to adapt to low-O2 environments, and how to survive re-oxygenation. New insights into the role of mitochondrial retrograde signaling, chromatin modification, as well as moonlighting proteins and mitochondrial alternative electron transport pathways (and their contribution to low O2 tolerance and survival of re-oxygenation), are presented.

An antisense noncoding RNA enhances translation via localized structural rearrangements of its cognate mRNA.

A large portion of eukaryotic genes are associated with noncoding, natural antisense transcripts (NATs). Despite sharing extensive sequence complementarity with their sense mRNAs, mRNA-NAT pairs elusively often evade dsRNA-cleavage and siRNA-triggered silencing. More surprisingly, some NATs enhance translation of their sense mRNAs by yet unknown mechanism(s). Here, we show that translation enhancement of the rice (Oryza sativa) PHOSPHATE1.2 (PHO1.2) mRNA is enabled by specific structural rearrangements guided by its noncoding antisense RNA (cis-NATpho1.2). Their interaction in vitro revealed no evidence of widespread intermolecular dsRNA formation, but rather specific local changes in nucleotide base pairing, leading to higher flexibility of PHO1.2 mRNA at a key high guanine-cytosine�(GC) regulatory region inhibiting translation, ∼350-nt downstream of the start codon. Sense-antisense RNA interaction increased formation of the 80S complex in PHO1.2, possibly by inducing structural rearrangement within this inhibitory region, thus making this mRNA more accessible to 60S. This work presents a framework for nucleotide resolution studies of functional mRNA-antisense pairs.

Redox and low-oxygen stress: signal integration and interplay.

Plants are aerobic organisms relying on oxygen to serve their energy needs. The amount of oxygen available to sustain plant growth can vary significantly due to environmental constraints or developmental programs. In particular, flooding stress, which negatively impacts crop productivity, is characterized by a decline in oxygen availability. Oxygen fluctuations result in an altered redox balance and the formation of reactive oxygen/nitrogen species (ROS/RNS) during the onset of hypoxia and upon re-oxygenation. In this update, we provide an overview of the current understanding of the impact of redox and ROS/RNS on low-oxygen signaling and adaptation. We first focus on the formation of ROS and RNS during low-oxygen conditions. Following this, we examine the impact of hypoxia on cellular and organellar redox systems. Finally, we describe how redox and ROS/RNS participate in signaling events during hypoxia through potential post-translational modifications (PTMs) of hypoxia-relevant proteins. The aim of this update is to define our current understanding of the field and to provide avenues for future research directions.

Remodeling of Root Growth Under Combined Arsenic and Hypoxia Stress Is Linked to Nutrient Deprivation.

Root architecture responds to environmental stress. Stress-induced metabolic and nutritional changes affect the endogenous root development program. Transcriptional and translational changes realize the switch between stem cell proliferation and cell differentiation, lateral root or root hair formation and root functionality for stress acclimation. The current work explores the effects of stress combination of arsenic toxicity (As) and hypoxia (Hpx) on root development in Arabidopsis thaliana. As revealed previously, combined As and Hpx treatment leads to severe nutritional disorder evident from deregulation of root transcriptome and plant mineral contents. Both As and Hpx were identified to pose stress-specific constraints on root development that lead to unique root growth phenotype under their combination. Besides inhibition of root apical meristem (RAM) activity under all stresses, As induced lateral root growth while root hair density and lengths were strongly increased by Hpx and HpxAs-treatments. A dual stimulation of phosphate (Pi)-starvation response was observed for HpxAs-treated plant roots; however, the response under HpxAs aligned more with Hpx than As. Transcriptional evidence along with biochemical data suggests involvement of PHOSPHATE STARVATION RESPONSE 1; PHR1-dependent systemic signaling. Pi metabolism-related transcripts in close association with cellular iron homeostasis modulate root development under HpxAs. Early redox potential changes in meristematic cells, differential ROS accumulation in root hair zone cell layers and strong deregulation of NADPH oxidases, NADPH-dependent oxidoreductases and peroxidases signify a role of redox and ROS signaling in root architecture remodeling under HpxAs. Differential aquaporin expression suggests transmembrane ROS transport to regulate root hair induction and growth. Reorganization of energy metabolism through NO-dependent alternate oxidase, lactate fermentation, and phosphofructokinase seems crucial under HpxAs. TOR and SnRK-signaling network components were potentially involved in control of sustainable utilization of available energy reserves for root hair growth under combined stress as well as recovery on reaeration. Findings are discussed in context of combined stress-induced signaling in regulation of root development in contrast to As and Hpx alone.

Multiparametric real-time sensing of cytosolic physiology links hypoxia responses to mitochondrial electron transport.

Hypoxia regularly occurs during plant development and can be induced by the environment through, for example, flooding. To understand how plant tissue physiology responds to progressing oxygen restriction, we aimed to monitor subcellular physiology in real time and in vivo. We establish a fluorescent protein sensor-based system for multiparametric monitoring of dynamic changes in subcellular physiology of living Arabidopsis thaliana leaves and exemplify its applicability for hypoxia stress. By monitoring cytosolic dynamics of magnesium adenosine 5'-triphosphate, free calcium ion concentration, pH, NAD redox status, and glutathione redox status in parallel, linked to transcriptional and metabolic responses, we generate an integrated picture of the physiological response to progressing hypoxia. We show that the physiological changes are surprisingly robust, even when plant carbon status is modified, as achieved by sucrose feeding or extended night. Inhibition of the mitochondrial respiratory chain causes dynamics of cytosolic physiology that are remarkably similar to those under oxygen depletion, highlighting mitochondrial electron transport as a key determinant of the cellular consequences of hypoxia beyond the organelle. A broadly applicable system for parallel in vivo sensing of plant stress physiology is established to map out the physiological context under which both mitochondrial retrograde signalling and low oxygen signalling occur, indicating shared upstream stimuli.

HBI1 Mediates the Trade-off between Growth and Immunity through Its Impact on Apoplastic ROS Homeostasis.

Plants continuously need to adapt to their environment and prioritize either growth or defense responses to secure survival and reproduction. Trade-offs between growth and defense are often attributed to the allocation of energy for growth to adaptation responses. Still, the exact mechanisms underlying growth and defense trade-offs are poorly understood. Here, we demonstrate that the growth-related transcription factor HOMOLOG OF BEE2 INTERACTING WITH IBH 1 (HBI1) regulates apoplastic reactive oxygen species (ROS) homeostasis by differentially controlling the expression of NADPH oxidases (NOXs) and peroxidases (POXs). The HBI1 target genes RESPIRATORY BURST OXIDASE HOMOLOG A (RbohA) and RbohC have contrasting effects on the regulation of cell size. In addition, the HBI1-controlled NOXs and POXs oppositely regulate susceptibility toward Pseudomonas syringae. Our findings reveal that the incompatibility between growth and defense programs can be attributed to the way apoplastic ROS homeostasis is modulated during both processes.

The ACBP1-RAP2.12 signalling hub: A new perspective on integrative signalling during hypoxia in plants.

During their lifetime, plants are frequently exposed to a variety of stresses which negatively impact on growth and vitality. In order to respond specifically to a given stress situation, integration of multiple signal inputs is of utmost importance. Recently, we demonstrated that recognition and adaptation to low-oxygen stress requires integration of signals from energy metabolism, lipid metabolism and oxygen availability. Low oxygen which results in an energy crisis causes a shift in lipid intermediate ratios. Binding of C18:1-CoA by ACYL-COA BINDING PROTEIN 1 (ACBP1) at the plasma membrane concomitantly leads to release and nuclear accumulation of the ERFVII transcription factor RELATED TO APETALA 2.12 (RAP2.12) which is central to the activation of anaerobic metabolism during stress. Moreover, RAP2.12 protein stability is oxygen-dependently regulated and its oxidation results in degradation by the N-end rule pathway. Here, we illuminate the concept of multiple-signal integration under hypoxia and discuss signal inputs merging at the ACBP1-ERFVII signaling hub.

Low-oxygen response is triggered by an ATP-dependent shift in oleoyl-CoA in Arabidopsis.

Plant response to environmental stimuli involves integration of multiple signals. Upon low-oxygen stress, plants initiate a set of adaptive responses to circumvent an energy crisis. Here, we reveal how these stress responses are induced by combining (i) energy-dependent changes in the composition of the acyl-CoA pool and (ii) the cellular oxygen concentration. A hypoxia-induced decline of cellular ATP levels reduces LONG-CHAIN ACYL-COA SYNTHETASE activity, which leads to a shift in the composition of the acyl-CoA pool. Subsequently, we show that different acyl-CoAs induce unique molecular responses. Altogether, our data disclose a role for acyl-CoAs acting in a cellular signaling pathway in plants. Upon hypoxia, high oleoyl-CoA levels provide the initial trigger to release the transcription factor RAP2.12 from its interaction partner ACYL-COA BINDING PROTEIN at the plasma membrane. Subsequently, according to the N-end rule for proteasomal degradation, oxygen concentration-dependent stabilization of the subgroup VII ETHYLENE-RESPONSE FACTOR transcription factor RAP2.12 determines the level of hypoxia-specific gene expression. This research unveils a specific mechanism activating low-oxygen stress responses only when a decrease in the oxygen concentration coincides with a drop in energy.