Allelic mutant series reveal distinct functions for Arabidopsis cycloartenol synthase 1 in cell viability and plastid biogenesis.
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 Arabidopsis thaliana. To understand the full scope of sterol biological functions in plants, we characterized allelic series of cas1 mutations. Plants carrying the weak mutant allele cas1-1 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, cas1-2 and cas1-3, were not transmissible through the male gametes, suggesting a role for CAS1 in male gametophyte function. To validate these findings, we analyzed a conditional CRE/loxP recombination-dependent cas1-2 mutant allele. The albino phenotype of growing leaf tissues was a typical defect observed shortly after the CRE/loxP-induced onset of CAS1 loss of function. In the induced cas1-2 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 cas1 mutations on a chloroplastic function implies a previously unrecognized role of sterols or triterpenoid metabolites in plastid biogenesis.
Babiychuk
Elena
E
Department of Plant Systems Biology, Flanders Institute for Biotechnology, Technologiepark 927, 9052 Ghent, Belgium.
Bouvier-Navé
Pierrette
P
Compagnon
Vincent
V
Suzuki
Masashi
M
Muranaka
Toshiya
T
Van Montagu
Marc
M
Kushnir
Sergei
S
Schaller
Hubert
H
D017360:Arabidopsis / Q000201:enzymology*; D001483:Base Sequence; D002470:Cell Survival / Q000235:genetics / Q000502:physiology; D017931:DNA Primers / Q000235:genetics; D008401:Gas Chromatography-Mass Spectrometry; D019751:Intramolecular Transferases / Q000235:genetics* / Q000502:physiology*; D008854:Microscopy, Electron; D008969:Molecular Sequence Data; D015394:Molecular Structure; D009154:Mutation / Q000235:genetics; D010641:Phenotype; D018547:Plant Stems / Q000648:ultrastructure; D018087:Plastids / Q000235:genetics / Q000502:physiology*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D017422:Sequence Analysis, DNA; D013185:Squalene / Q000031:analogs & derivatives / Q000032:analysis
C002821:2,3-oxidosqualene; D017931:DNA Primers; D013185:Squalene; D019751:Intramolecular Transferases; C021923:cycloartenol synthase