Redox and low-oxygen stress: signal integration and interplay

Abstract

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.

Figure 1

ROS and NO signaling pathways during hypoxia. Hypoxia-induced production of ROS and NO occurs at several sites within the cell. Mitochondrial dysfunction during hypoxia generates ROS and can be pharmacologically mimicked by AA. The mitochondrial MnSOD mediates ${\mathrm{O}}_2^{-}$ conversion to H₂O₂. Apoplastic ROS generation can be attributed to membrane-bound NADPH oxidases (RBOHD). RBOHD-mediated ROS production is modulated by HRU. HRU proteins exist as dimers in the cytosol and regulate ROS production via association with RBOHD in a protein complex including ROP-GTP. ROS production via RBOHD is tightly regulated via several PTMs. This includes: ROS-mediated PTM; NO-mediated S-nitrosylation; Phosphorylation by CDPKs. CDPKs are activated by hypoxia-mediated Ca²⁺ influxes. RBOHD can be synergistically activated via Ca²⁺ binding and CDPK-dependent phosphorylation. Cellular NO production during hypoxia can occur via reductive pathways. This includes cytosolic NRs, which generate NO from ${\text{NO}}_2^{-}$, and the Mt-NiR. Cellular NO levels are regulated primarily via removal by PGBs in a series of reactions known as the PGB–NO cycle. NO and ROS can affect hypoxia responses by influencing the stability of the oxygen labile group VII-ERF TFs. Under normoxic conditions, ERF-VIIs are either degraded via the N-degron pathway, or sequestered via association with Acyl-CoA-binding proteins. Hypoxia triggers ERFVII release and nuclear translocation to activate target gene (RBOHD, HRU, and PGB1) expression. NO and ROS could also influence hypoxia responses by regulating MPK activity. In Arabidopsis, MPK3/6 mediates hypoxia survival by an unknown mechanism. Their activation is triggered by mitochondrial stress signals, potentially either H₂O₂ or peroxynitrate (ONOO⁻) H₂O₂ could also regulate MPKs indirectly via Sucrose nonfermenting1 related Kinase (SnRK1) and a PTP. SnRK1 phosphorylates and activates MPK6, but SnRK1 activity is impaired by oxidative stress. PTP1 inhibits MPK3/6 but is itself inactivated by SnRK1 and also H₂O₂. The reaction of NO with GSH results in GSNO representing an important cellular storage form of NO. GSNO levels in turn are dependent on GSNOR activity. Figure created with BioRender.com.

Figure 2

Major cellular redox systems and the direction of redox shifts during hypoxia. Initial NAD biosynthesis steps occur in plastids, whereas the final step takes place in the cytosol. During hypoxia, a shift toward NADH results in inhibition of the TCA cycle. GSH is synthesized in plastids and GSH is readily transported throughout all cellular compartments. GSH powers enzymes, like GRXs or peroxidases, and acts as a ROS scavenger. Oxidized glutathione (GSSG) is readily converted back to GSH by GR. GRX proteins control oxidative modifications on target proteins at cysteine residues. In a similar fashion, TRXs also modulate redox modification on target proteins in a NAD(P)H-dependent manner. ASC synthesis is performed in the mitochondria and relies on respiratory activity. ASC reacts directly with ROS to form the corresponding oxidized product DHA. DHA is reduced in a GSH-dependent manner through DHA reductase back to ASC. Upon oxygen deprivation, the ASC pool is readily oxidized, which affects multiple enzymes and the cellular capacity for detoxifying ROS. The ER represents a rather oxidative environment due to oxidative protein folding by PDI. Upon oxygen limitation, it is expected that the reduced form of PDI accumulates as its oxidation requires the activity of the oxygen-dependent ERO proteins. Figure created with BioRender.com.