title: Interactions between sterol biosynthesis genes in embryonic development of Arabidopsis.
abstract: The sterol biosynthesis pathway of Arabidopsis produces a large set of structurally related phytosterols including sitosterol and campesterol, the latter being the precursor of the brassinosteroids (BRs). While BRs are implicated as phytohormones in post-embryonic growth, the functions of other types of steroid molecules are not clear. Characterization of the fackel (fk) mutants provided the first hint that sterols play a role in plant embryogenesis. FK encodes a sterol C-14 reductase that acts upstream of all known enzymatic steps corresponding to BR biosynthesis mutants. Here we report that genetic screens for fk-like seedling and embryonic phenotypes have identified two additional genes coding for sterol biosynthesis enzymes: CEPHALOPOD (CPH), a C-24 sterol methyl transferase, and HYDRA1 (HYD1), a sterol C-8,7 isomerase. We describe genetic interactions between cph, hyd1 and fk, and studies with 15-azasterol, an inhibitor of sterol C-14 reductase. Our experiments reveal that FK and HYD1 act sequentially, whereas CPH acts independently of these genes to produce essential sterols. Similar experiments indicate that the BR biosynthesis gene DWF1 acts independently of FK, whereas BR receptor gene BRI1 acts downstream of FK to promote post-embryonic growth. We found embryonic patterning defects in cph mutants and describe a GC-MS analysis of cph tissues which suggests that steroid molecules in addition to BRs play critical roles during plant embryogenesis. Taken together, our results imply that the sterol biosynthesis pathway is not a simple linear pathway but a complex network of enzymes that produce essential steroid molecules for plant growth and development.
authors.affiliation: ZMBP (Zentrum für Molekular Biologie der Pflanzen), Universität Tübingen, Auf der Morgenstelle 3, D-72076 Tübingen, Germany. kathrin.schrick@uni-tuebingen.de
authors.forename: Kathrin
authors.initials: K
authors.lastname: Schrick
authors.forename: Ulrike
authors.initials: U
authors.lastname: Mayer
authors.forename: Gottfried
authors.initials: G
authors.lastname: Martin
authors.forename: Catherine
authors.initials: C
authors.lastname: Bellini
authors.forename: Christine
authors.initials: C
authors.lastname: Kuhnt
authors.forename: Jürgen
authors.initials: J
authors.lastname: Schmidt
authors.forename: Gerd
authors.initials: G
authors.lastname: Jürgens
mesh_terms: D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000187:drug effects / Q000196:embryology / Q000235:genetics* / Q000378:metabolism*; D001483:Base Sequence; D002775:Cholestadienols / Q000494:pharmacology; D018744:DNA, Plant / Q000235:genetics; D004791:Enzyme Inhibitors / Q000494:pharmacology; D008401:Gas Chromatography-Mass Spectrometry; D017343:Genes, Plant; D041743:Herb-Drug Interactions; D008780:Methyltransferases / Q000235:genetics; D008954:Models, Biological; D009154:Mutation; D010088:Oxidoreductases / Q000037:antagonists & inhibitors / Q000235:genetics; D010641:Phenotype; D010840:Phytosterols / Q000096:biosynthesis*; D013251:Steroid Isomerases / Q000235:genetics
chemical_list: D002775:Cholestadienols; D018744:DNA, Plant; D004791:Enzyme Inhibitors; D010840:Phytosterols; C007331:A25822B; D010088:Oxidoreductases; C022672:delta(14)-sterol reductase; D008780:Methyltransferases; C022670:delta 24-sterol methyltransferase; D013251:Steroid Isomerases; C022669:delta(8)-delta(7)-sterol isomerase