Sites in maize (Zea mays L.) coleoptile homogenates that reversibly bind naphthalene-1-acetic acid with high affinity and may represent receptor sites for auxins are located primarily on cellular membranes that show the enzymic and buoyant density characteristics of membranes of the rough endoplasmic reticulum. The sites remain attached to the endoplasmic reticulum (ER) membranes after the ribosomes have been stripped off them. Binding sites for naphthylphthalamic acid, an inhibitor of auxin transport, are located on membranes different from those that carry the naphthalene-1-acetic-acid (NAA)-binding sites, and which are probably plasma membrane. The two kinds of binding sites can be largely separated by appropriate density gradient centrifugation. The results raise the possibility that primary auxin action occurs at ER membranes and could represent facilitation of the transfer of hydrogen ions and nascent secretory protein into the ER lumen followed by secretory transport of these products to the cell exterior via the Golgi system.

Auxin-binding Sites of Maize Coleoptiles Are Localized on Membranes of the Endoplasmic Reticulum.

Publisher Summary This chapter presents detailed analysis of sterols and sterol derivatives of well-characterized (by a rich array of both morphological and biochemical markers) membrane fractions from etiolated maize coleoptiles, with a special focus on a plasma membrane (PM)-rich fraction. Then data about the different cellular sites of plant sterol biosynthesis in the same material are presented. It is presented in the chapter that it is possible to modify the sterol profile of higher plant membranes using sterol biosynthesis inhibitors. Results that have obtained with maize roots treated by fenpropimorph are given. If both free sterols and sterol conjugates of membrane fractions are to be analyzed, then the total lipid must be extracted three times with 6 vol of dichloromethane-methanol (2:1, v/v). To obtain sterols from steryl esters (SE), the corresponding fraction can be hydrolyzed by heating for one hour with 5 ml of 6% (w/v) KOH in methanol and pyrogallol.

[58] Plant membrane sterols: Isolation, identification, and biosynthesis

Sixty-one sterols and pentacyclic triterpenes have been isolated and characterized by chromatographic and spectral methods fromZea mays (corn) Several plant parts were examined; seed, pollen, cultured hypocotyl cells, roots, coleoptiles (sheaths), and blades By studying reaction pathways and mechanisms on plants fed radiotracers ([2-14C]mevalonic acid, [2-14C]acetate, and [2-3H]acetate), and stable isotopes (D2O), we discovered that hydroxymethylglutaryl CoA reductase is not “the” rate-limiting enzyme of sitosterol production Additionally, we observed an ontogenetic shift and kinetic isotope effect in sterol biosynthesis that was associated with the C-24 alkylation of the sterol side chain Blades synthesized mainly 24α-ethyl-sterols, sheaths synthesized mainly 24-methyl-sterols, pollen possessed an interrupted sterol pathway, accumulating 24(28)-methylene-sterols, and germinating seeds were found to lack an activede novo pathway Shoots, normally synthesizing (Z)-24(28)-ethylidine-cholesterol, after incubation with deuterated water, synthesized the rearranged double-bond isomer, stigmasta-5,23-dien-3β-ol Examination of the mass spectrum and1H nuclear magnetic resonance spectrum of the deuterated 24-ethyl-sterol indicated the Bloch-Cornforth route originating with acetyl-CoA and passing through mevalonic acid to sterol was not operative at this stage of development An alternate pathway giving rise to sterols is proposed

Developmental regulation of sterol biosynthesis in Zea mays.

Sterol biosynthetic capability of purified membrane fractions from maize coleoptiles

Lipids are important constituents of all biological membranes and play a central role in the molecular organization of these membranes. Structural lipids in plant biomembranes are essentially phospholipids, galactolipids, and sterols. In this chapter we shall first review the lipid composition of plant biomembranes; then we shall examine where these lipids come from in plant cells, how they are built, and how they integrate the various biomembranes or cell organelles. However, membrane lipids have not only a structural role. The activities of many membrane-bound enzymes are controlled by the extent and the quality of the lipid environment within the membranes. The last part of this chapter will deal with the results of some recent studies concerning the effect of lipids upon the activities of some membrane-bound enzymes.

G. Normand

Papers

Sterol composition of plasma membrane enriched fractions from maize coleoptiles