Reactive oxygen species act as signaling molecules to control root hair initiation and tip growth

Abstract

Root hairs (RHs) increase the surface area of roots, facilitating nutrient and water uptake and plant anchorage. RHs form from root epidermal cells and elongate by polar tip growth. Reactive oxygen species (ROS) have recently been implicated as important signals that drive RH formation and elongation using both genetic and imaging approaches. Localized changes in ROS levels in the RH tip are facilitated by hormone-mediated changes in the synthesis and activity of respiratory burst oxidase homologs and class III peroxidases. These findings broaden our understanding of the mechanisms controlling polar tip growth in plants that drive RH formation, which can inform the breeding and engineering of plants that thrive under drought and nutrient stress.

Keywords: class III peroxidase; gene regulatory network; hormone signaling; reactive oxygen species; respiratory burst oxidase homolog; root hair.

Fig. 1

ROS accumulate in trichoblasts in distinct subcellular compartments to promote RH growth and development. (a) Schematic of an Arabidopsis thaliana seedling with the inset showing hair cells (H; trichoblasts) in purple and non‐hair cells (N; atrichoblasts) in dark gray in cross‐ and trans‐sectional views. This figure was created in BioRender (BioRender.com/t2plfps) (Muday, G., 2025). (b) The ROS‐generating protein RBOHC/RHD2 is localized to the growing root hair tip as shown using RHD2::GFP‐RHD2 (reprinted from Ovečka et al., 2022). (c) The H₂O₂‐selective dye PO1 reveals localized increases in ROS in trichoblasts in response to ACC treatment (reprinted from Martin et al., 2022a). (d) RH cells stained with H₂DCFDA and Amplex™ Red reveal subapical and apoplastic ROS, respectively (reprinted from Kuběnová et al., 2023). ACC, 1‐aminocyclopropane‐1‐carboxylic acid; GFP, green fluorescent protein; H₂O₂, hydrogen peroxide; H₂DCFDA, 6‐chloromethyl‐2′,7′‐dichlorodihydrofluorescein diacetate; PO1, Peroxy Orange 1; RBOHC, respiratory burst oxidase homolog C; RHD2, root hair defective 2; RH, root hairs; ROS, reactive oxygen species.

Fig. 2

Model illustrating how hormone‐mediated ROS controls RH tip growth in Arabidopsis thaliana. Auxin, ethylene, and cytokinin regulate the gene encoding RSL4, which transcriptionally activates several ROS‐producing enzymes including respiratory burst oxidase homologs (RBOHs) and class III peroxidases (PERs). Brassinosteroids have been shown to increase ROS and root hair proliferation, but the mechanism is still unknown. RBOHs produce superoxide (O₂ ^ᐧ−) in the apoplast by reducing cytosolic NADPH. Superoxide can be converted into H₂O₂ spontaneously or enzymatically by SUPEROXIDE DISMUTASE (SOD). H₂O₂ is metabolized by PERs to promote cell wall stiffening. Alternatively, PERs convert H₂O₂ to hydroxyl radicals (ᐧOH) to facilitate cell wall loosening. PERs can also oxidize molecular oxygen to create superoxide. RBOHs can be activated by cytosolic calcium. RBOHC is activated by auxin signaling in the apoplast via the FERONIA (FER) receptor‐like kinase. FER activates RBOHC by multiple trafficking and/or signaling pathways (indicated by arrows). FER also controls calcium entry into RHs through MILDEW RESISTANCE LOCUS‐O 15 (MLO15) and other calcium channels (white), but the mechanism for FER regulation of MLO15 is still unknown. H₂O₂ enters the cytoplasm through aquaporins (blue) where it may activate calcium channels (white) or induce oxPTMs on target proteins, which activate secondary signaling pathways. Created in BioRender. Muday, G. (2025) https://BioRender.com/z1edjoq. Arrows through enzymes indicate enzyme‐catalyzed reactions. Dotted arrows indicate unconfirmed in root hairs or unknown. AFB2, Auxin Signaling F‐Box 2; H₂O₂, hydrogen peroxide; ⁻OH, hydroxide anion; RH, root hairs; ROS, reactive oxygen species; RSL4, RHD6‐LIKE 4; TF, transcription factor; TIR1, transport inhibitor response 1.