Significance This study demonstrates, in vivo, that reticulon (RTN) proteins, responsible for the shaping and maintenance of endoplasmic reticulum (ER) membrane tubules, rely on a highly conserved C-terminal amphipathic helix (APH) for their morphogenic function. Previously it was thought that RTN could bend the ER membrane both by assuming a wedge-like topology and by forming oligomeric arcs. We show here that deleting or mutating the APH region abolishes the function of a plant RTN but does not affect its capacity to oligomerize. These findings indicate that proteins of the RTN family use an APH to affect membrane curvature: a mechanism that is shared by several other membrane-shaping protein families.