Plants produce structurally diverse triterpenoids, which are important for their life and survival. Most triterpenoids and sterols share a common biosynthetic intermediate, 2,3-oxidosqualene (OS), which is cyclized by 2,3-oxidosqualene cyclase (OSC). To investigate the role of an OSC, marneral synthase 1 (MRN1),in planta, we characterized a Arabidopsismrn1knock-out mutant displaying round-shaped leaves, late flowering, and delayed embryogenesis. Reduced growth ofmrn1was caused by inhibition of cell expansion and elongation. Marnerol, a reduced form of marneral, was detected in Arabidopsis overexpressingMRN1, but not in the wild type ormrn1. Alterations in the levels of sterols and triterpenols and defects in membrane integrity and permeability were observed in themrn1. In addition, GUS expression, under the control of theMRN1gene promoter, was specifically detected in shoot and root apical meristems, which are responsible for primary growth, and the mRNA expression of Arabidopsis clade II OSCs was preferentially observed in roots and siliques containing developing seeds. The eGFP:MRN1 was localized to the endoplasmic reticulum in tobacco protoplasts. Taken together, this report provides evidence that the unusual triterpenoid pathway via marneral synthase is important for the growth and development of Arabidopsis.