LAUSR

After drought, flooding is the gravest natural disaster that endangers agriculture and food security worldwide. Losses due to flooding reached USD 21 billion between 2008 and 2018, representing 19% of all total agricultural losses in low and lower middle-income countries (FAO, 2021). Upon flooding, plants encounter restricted gas diffusion, leading to oxygen (O2) deprivation, passing from 21% (normoxia) to 51% oxygen availability when submerged (hypoxia). To keep cells alive under hypoxic conditions, plants must metabolically switch from mitochondrial respiration and carbon fixation to glycolysis and ethanolic fermentation for energy production (Sasidharan et al., 2018). During submergence, plants use dynamic variations in the levels of O2, reactive oxygen species (ROS), nitric oxide (NO), and the gaseous phytohormone ethylene as intracellular cues to adjust to flooding conditions. Impeded gas diffusion in water promotes entrapment of ethylene in the plant, leading to increases of up to 20-fold in concentration within the first hour of submergence compared to nonsubmerged tissues (Sasidharan et al., 2018). In the model plant Arabidopsis (Arabidopsis thaliana), pretreatments with ethylene enhanced hypoxia tolerance, increasing rosette sizes and the capacity of the root tip to re-grow during recovery, henceforth root tip survival (Hartman et al., 2019). However, the mechanisms behind ethylene-mediated hypoxia survival are still unclear. In this issue of Plant Physiology, Liu et al. (2022) studied the mechanisms by which ethylene pretreatments help Arabidopsis root tip survival during hypoxia and reoxygenation. Because re-oxygenation after hypoxia triggers a ROS burst that could damage the cell, the authors evaluated cell viability at different recovery timepoints after a 4-h hypoxia treatment. Cell death was observed as soon as 1 h into recovery, and cell death was alleviated in plants pretreated with ethylene, leading to enhanced cell survival. The authors then investigated which processes are associated with ethylene-induced tolerance to hypoxia and reoxygenation. To address this question, they sampled the root tips of Arabidopsis seedlings pretreated with ethylene or air before and after hypoxia treatments and 1 h after reoxygenation. They performed a comprehensive analysis of gene expression and proteome variations in the root tip in all these conditions. Ethylene stabilizes group VII ethylene response factors (ERFVIIs) by inducing expression of PHYTOGLOBIN1 (PGB1), a NO-scavenger (Hartman et al., 2019). ERFVIIs are a component of the O2-/NO-sensing mechanism, and continuous proteasome degradation via PROTEOLYSIS6 and the N-degron pathway limits ERFVII accumulation (Licausi et al., 2011; Gibbs et al., 2014). Hence, submergence-induced ethylene accumulation in the cell promotes nuclear translocation of stabilized ERFVIIs transcription factors, upregulating the expression of hypoxia-response genes. Most of the genes differentially expressed during hypoxia were already differentially expressed immediately after the ethylene pretreatment, indicating that ethylene signaling triggers a transcriptome reconfiguration maintained during hypoxia. Enriched gene ontology (GO) terms of genes upregulated by ethylene treatments were linked to hypoxia response and abscisic acid (ABA), among others. In comparison, GO terms enriched in the downregulated genes were related to decreased cellular maintenance and root growth, such as PLETHORA (PLT) 1 and 2, SCARECROW (SCR), and SHORTROOT (SHR). Moreover, ethylene limited N ew s an d V ie w s