Key message <p>GhPMEI58 is a functional pectin methylesterase inhibitor, negatively regulates ray petal elongation by inhibiting cell expansion.</p> Abstract <p>Petal size plays a crucial role in the quality of ornamental flowers, which is largely determined by cell expansion. However, the molecular mechanisms controlling this trait are still unknown. In this study, we reported <i>GhPMEI58</i> (<i>pectin methylesterase inhibitor 58</i>), a functional pectin methylesterase inhibitor, negatively regulates ray petal elongation by inhibiting cell expansion. The expression of <i>GhPMEI58</i> was found to be highest in ray florets and increased with ray petal elongation. Subcellular localization analysis showed that the GhPMEI58 protein is localized on the cell membrane. By stably overexpressing <i>GhPMEI58</i> in Arabidopsis, we detected a decrease in pectin methylesterase activity in roots and flowers, and an increase in the degree of pectin methylation, indicating that GhPMEI58 functions as a pectin methylesterase inhibitor. Further transient overexpression and virus-induced gene silencing experiments in gerbera ray florets confirmed that GhPMEI58 is a negative regulator of ray petal elongation. Additionally, the activity assay of pectin methylesterase showed it has a trend of first rising and then falling during ray floret development. This study explores the regulatory mechanism of ray petal elongation in gerbera from the perspective of cell wall methylesterification modification, providing valuable information for improving gerbera varieties and breeding superior cultivars.</p> Graphical abstract <p></p>