phytosterols and Dwarfism

Auxin and brassinosteroid (BR) play essential roles in diverse aspects of growth and developmental processes in plants mainly through coordinate regulation of cell division, elongation, and differentiation. Consistent with the overlapped roles, accumulating evidence indicates that the two growth hormones act in a synergistic as well as in an interdependent manner in many cases, although the underlying molecular mechanisms are not fully understood. Here, we demonstrate that auxin and BR signaling pathways are interconnected at the transcriptional level via a negative feedback loop. An Arabidopsis activating tagging mutant dlf-1D exhibited dwarfed growth with small, dark-green leaves and reduced fertility. Hormone feeding assays revealed that the mutant phenotype is caused by the reduction of endogenous BR level. Consistent with this, a gene encoding the CYP72C1 enzyme that catabolizes BR was up-regulated. Notably, the transcript level of the ARF8 transcription factor gene, which modulates the expression of auxin-responsive genes, was significantly elevated in the mutant. In addition, the ARF8 gene expression was significantly reduced by BR but induced by brassinazole, a BR biosynthetic inhibitor. On the other hand, two BR catabolic pathway genes, DLF (CYP72C1) and BAS1, were induced by auxin. Our observations indicate that at least part of auxin and BR signaling pathways are unified through a transcriptional feedback control of the DLF and ARF8 genes.

Topics: Arabidopsis; Arabidopsis Proteins; Cell Growth Processes; Cytochrome P-450 Enzyme System; DNA-Binding Proteins; Dwarfism; Feedback, Physiological; Fertility; Gene Expression Regulation, Plant; Indoleacetic Acids; Mutation; Phytosterols; Plant Growth Regulators; Receptor Cross-Talk; Signal Transduction; Triazoles

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