Allelic mutant series reveal distinct functions for <i>Arabidopsis</i> cycloartenol synthase 1 in cell viability and plastid biogenesis

<jats:p> Sterols have multiple functions in all eukaryotes. In plants, sterol biosynthesis is initiated by the enzymatic conversion of 2,3-oxidosqualene to cycloartenol. This reaction is catalyzed by cycloartenol synthase 1 (CAS1), which belongs to a family of 13 2,3-oxidosqualene cyclases in <jats:italic>Arabidopsis thaliana</jats:italic> . To understand the full scope of sterol biological functions in plants, we characterized allelic series of <jats:italic>cas1</jats:italic> mutations. Plants carrying the weak mutant allele <jats:italic>cas1–1</jats:italic> were viable but developed albino inflorescence shoots because of photooxidation of plastids in stems that contained low amounts of carotenoids and chlorophylls. Consistent with the CAS1 catalyzed reaction, mutant tissues accumulated 2,3-oxidosqualene. This triterpenoid precursor did not increase at the expense of the pathway end products. Two strong mutations, <jats:italic>cas1–2</jats:italic> and <jats:italic>cas1–3</jats:italic> , were not transmissible through the male gametes, suggesting a role for CAS1 in male gametophyte function. To validate these findings, we analyzed a conditional <jats:italic>CRE/loxP</jats:italic> recombination-dependent <jats:italic>cas1–2</jats:italic> mutant allele. The albino phenotype of growing leaf tissues was a typical defect observed shortly after the <jats:italic>CRE/loxP</jats:italic> -induced onset of <jats:italic>CAS1</jats:italic> loss of function. In the induced <jats:italic>cas1–2</jats:italic> seedlings, terminal phenotypes included arrest of meristematic activity, followed by necrotic death. Mutant tissues accumulated 2,3-oxidosqualene and contained low amounts of sterols. The vital role of sterols in membrane functioning most probably explains the requirement of CAS1 for plant cell viability. The observed impact of <jats:italic>cas1</jats:italic> mutations on a chloroplastic function implies a previously unrecognized role of sterols or triterpenoid metabolites in plastid biogenesis. </jats:p>