gamma-sitosterol and brassinolide

Topics: Arabidopsis; Brassinosteroids; Cell Membrane; Cell Wall; Cholestanols; Cholesterol; Mutation; Phytosterols; Signal Transduction; Sitosterols; Steroids, Heterocyclic

The endogenous brassinosteroids in the dwarf mutant lk of pea (Pisum sativum) were quantified by gas chromatography-selected ion monitoring. The levels of castasterone, 6-deoxocastasterone, and 6-deoxotyphasterol in lk shoots were reduced 4-, 70-, and 6-fold, respectively, compared with those of the wild type. The fact that the application of brassinolide restored the growth of the mutant indicated that the dwarf mutant lk is brassinosteroid deficient. Gas chromatography-selected ion monitoring analysis of the endogenous sterols in lk shoots revealed that the levels of campestanol and sitostanol were reduced 160- and 10-fold, respectively, compared with those of wild-type plants. These data, along with metabolic studies, showed that the lk mutant has a defect in the conversion of campest-4-en-3-one to 5alpha-campestan-3-one, which is a key hydrogenation step in the synthesis of campestanol from campesterol. This defect is the same as that found in the Arabidopsis det2 mutant and the Ipomoea nil kbt mutant. The pea gene homologous to the DET2 gene, PsDET2, was cloned, and it was found that the lk mutation would result in a putative truncated PsDET2 protein. Thus it was concluded that the short stature of the lk mutant is due to a defect in the steroidal 5alpha-reductase gene. This defect was also observed in the callus induced from the lk mutant. Biosynthetic pathways involved in the conversion of campesterol to campestanol are discussed in detail.

Topics: 3-Oxo-5-alpha-Steroid 4-Dehydrogenase; Amino Acid Sequence; Brassinosteroids; Cholestanols; Cholesterol; Conserved Sequence; Dwarfism; Gene Expression Regulation, Plant; Molecular Sequence Data; Mutation; Phytosterols; Pisum sativum; Plant Diseases; Sequence Alignment; Sequence Deletion; Sequence Homology, Amino Acid; Sitosterols; Steroids, Heterocyclic

The Arabidopsis genome contains three distinct genes encoding sterol-C24-methyltransferases (SMTs) involved in sterol biosynthesis. The expression of one of them, STEROL METHYLTRANSFERASE 2;1, was modulated in 35S:SMT2;1 Arabidopsis in order to study its physiological function. Plants overexpressing the transgene accumulate sitosterol, a 24-ethylsterol which is thought to be the typical plant membrane reinforcer, at the expense of campesterol. These plants displayed a reduced stature and growth that could be restored by brassinosteroid treatment. Plants showing co-suppression of SMT2;1 were characterized by a predominant 24-methylsterol biosynthetic pathway leading to a high campesterol content and a depletion in sitosterol. Pleiotropic effects on development such as reduced growth, increased branching, and low fertility of high-campesterol plants were not modified by exogenous brassinosteroids, indicating specific sterol requirements to promote normal development. Thus SMT2;1 has a crucial role in balancing the ratio of campesterol to sitosterol in order to fit both growth requirements and membrane integrity.

Topics: Amino Acid Sequence; Arabidopsis; Blotting, Northern; Brassinosteroids; Cholestanols; Cholesterol; Fruit; Gene Expression Regulation, Plant; Methyltransferases; Molecular Sequence Data; Phenotype; Phytosterols; Plant Growth Regulators; Plant Roots; Plant Stems; Plants, Genetically Modified; Sitosterols; Steroids, Heterocyclic; Sterols; Transgenes