An ER-microtubule bridge: Reticulon 17 links microtubules with ER network organisation in plants.

The plant endoplasmic reticulum (ER) forms a highly dynamic tubular network whose architecture depends on ER-shaping proteins and its interaction with the cytoskeleton. While actin is well known to drive ER movement in plants, how the ER associates with microtubules and how this affects ER network architecture remain poorly understood. Here, we identify Arabidopsis thaliana reticulon 17 (RTN17) as an atypical reticulon that links the ER to the microtubule cytoskeleton. RTN17 features extended, intrinsically disordered N- and C-terminal domains enriched in low-complexity regions, consistent with a scaffolding or hub function. Topology analysis using redox-sensitive roGFP2 constructs shows that both termini face the cytosol, yet RTN17 lacks the amphipathic helix typical of ER-shaping reticulons and does not induce membrane constriction. Instead, RTN17 localises to punctate foci on curved ER membranes, recruits the ER fusogen ROOT HAIR DEFECTIVE3 (RHD3), and co-expression alters ER architecture and dynamics. RTN17 puncta preferentially co-localise with microtubules, and its over-expression promotes ER alignment with the microtubule network. We propose that RTN17 acts as a multifunctional scaffold linking curved ER domains with the microtubule cytoskeleton and localising RHD3 to these sites to regulate ER fusion events. By integrating curvature sensing, cytoskeletal attachment and fusion regulation, RTN17 represents a new class of plant reticulons with scaffolding rather than shaping functions. This work highlights an unrecognised mechanism coordinating ER organisation with the cytoskeleton, providing insights into how plants achieve spatial control of endomembrane architecture and potentially adapt membrane dynamics to developmental or stress cues.