How Extracellular Reactive Oxygen Species Reach Their Intracellular Targets in Plants

Abstract

Reactive oxygen species (ROS) serve as secondary messengers that regulate various developmental and signal transduction processes, with ROS primarily generated by NADPH OXIDASEs (referred to as RESPIRATORY BURST OXIDASE HOMOLOGs [RBOHs] in plants). However, the types and locations of ROS produced by RBOHs are different from those expected to mediate intracellular signaling. RBOHs produce O₂^•- rather than H₂O₂ which is relatively long-lived and able to diffuse through membranes, and this production occurs outside the cell instead of in the cytoplasm, where signaling cascades occur. A widely accepted model explaining this discrepancy proposes that RBOH-produced extracellular O₂^•- is converted to H₂O₂ by superoxide dismutase and then imported by aquaporins to reach its cytoplasmic targets. However, this model does not explain how the specificity of ROS targeting is ensured while minimizing unnecessary damage during the bulk translocation of extracellular ROS (eROS). An increasing number of studies have provided clues about eROS action mechanisms, revealing various mechanisms for eROS perception in the apoplast, crosstalk between eROS and reactive nitrogen species, and the contribution of intracellular organelles to cytoplasmic ROS bursts. In this review, we summarize these recent advances, highlight the mechanisms underlying eROS action, and provide an overview of the routes by which eROS-induced changes reach the intracellular space.

Keywords: NADPH oxidase; peroxidase; reactive oxygen species; receptor-like kinase; superoxide dismutase.

Fig. 1. A diagram showing the multifaceted roles of extracellular reactive oxygen species (eROS).

(1) Superoxide (O₂^•−) produced by RESPIRATORY BURST OXIDASE HOMOLOGs (RBOHs) is converted to hydrogen peroxide (H₂O₂) by superoxide dismutase (SOD) and migrates to the cytoplasm via aquaporins (AQPs). (2) In Arabidopsis, MSD2, an apoplastic SOD, functions in root skotomorphogenesis and floral organ abscission (Chen et al., 2022; Lee et al., 2022). H₂O₂ is broken down into water and oxygen by catalases (CATs) or peroxidases (PRXs). Extracellular CATs remain to be discovered. (3) Chloride Channel-3 (CLC-3) is responsible for O₂^•− transport in animal cells (Fisher, 2009; Hawkins et al., 2007). O₂^•− transporters in plants remain to be discovered. (4) eROS directly affect the activities of receptor like kinases (RLKs) such as HPCA1 or membrane-localized channels such as SKOR to initiate signal transduction cascades in the cytosol. (5) eROS can directly or indirectly affect cell wall organization, which is sensed by THESEUS1 (THE1)/FERONIA (FER) family RLKs. (6) O₂^•− interacts with nitric oxide (NO^•); this interaction not only affects the bioavailability and action of NO^•, but it also generates the more reactive peroxide peroxynitrite (ONOO−), which functions as a signaling molecule through the post-translational modification of proteins via tyrosine nitration (Pacher et al., 2007). (7) Organellar ROS could contribute to RBOH-dependent cytosolic ROS bursts. Chloroplast-derived ROS contribute to ROS bursts that lead to ABA‐induced stomatal closure (Iwai et al., 2019). The transport of ROS through the stromule restricts ROS accumulation to a localized region, achieving target specificity while minimizing unnecessary oxidative damage (7-1). The endoplasmic reticulum (ER)–mitochondria–peroxisome redox triangle model proposed in animals (Yoboue et al., 2018) might also be applicable to plants (7-2). Solid-lined arrows represent mechanisms previously examined in the literature. Dashed arrows indicate potential mediating pathways.