Membrane sterols plant cells

Grunwald (1968, 1971) found that the ethanol- and methanol-induced leakage of plant tissues can be prevented by sterols. While free cholesterol was effective even at 0.1 ftM, concentrations of cholesteryl palmitate and cho-lesteryl glucoside at 0.1-100 ftM had no effect on the leakage. The actual penetration of these substances into the plasma membranes of plant cells would be worthy of study. 

Lipid Metabolism. Next we explored changes in lipid metabolism in leaves exposed to ozone. Sterols and sterol derivatives were particularly interesting to us because they have been associated with membrane-containing fractions of leaves ( ). Changes produced in these compounds may be early events in the toxicity of ozone to plant cells. 

Unesterified sterols modulate the function of eukaryotic membranes. In human cells, sterol is esterified to a storage form by acyl-co-enz)me A (CoA) cholesterol acyltransferase (SGTase). In plants, free sterols are associated mainly with microsomal membranes, whereas the steryl esters are stored in lipid granules. The esterification process may, thus, allow regulation of the... 

C. The plant sterols are always present as part of the plant cell membranes and are not free in solution like cholesterol. 

The fungus Phytophthora cinnamomi causes a root and stem-base disease of a wide-range of coniferous and broad-leaved trees. For example, it is associated with the decline of several species such as the beech and the Spanish oak in particular environments. It secretes an ehcitor protein, p-cinnamomin, which ehcits plant defence mechanisms and in sensitive species produces plant cell necrosis. This protein acts as a sterol carrier protein and so it alfects cellular membranes. A related organism, P. ramonan, is the causal agent of a disease known as Sudden Oak Death . It has been found in this country, along with... 

There is ample evidence that the sterols of plant cells are localized in the membranes of the intracellular organelles. Evidence for their function in the membranes is still only indirect. The polyene antibiotic filipin increases the cellular... 

Some T. have major physiological significance. Thus, lanosterol is converted biosynthetically to cholesterol, the precursor of all steroid hormones, bile acids, and vitamin D3. In fungi, lanosterol is converted to er-gosterol (see sterols), an essential component of the fungal cell membrane. Plant cell membranes also incorporate steroids (phytosterols). In prokaryotes, the hopanoids take over the functions of steroids in the cell membranes. As a component of animal and plant waxes T. strengthen the structures. They protect the plant surface from desiccation and attack by microorganisms (e.g., betulin, lupeol, oleanolic acid, and ursolic acid). 

Paclobutrazol is a triazole plant growth retardant which also has fungicidal properties [1]. It is an inhibitor of cytochrome P-450 dependant enzymes such as ent-kaurene oxidase and sterol 14a-methyl-demethylase [1]. The fungicidal activity of azole compounds is believed to result from the inhibition of sterol 14a-demethylation. This causes an accumulation of 14a-methylsterols and loss of ergosterol which may have adverse effects on membrane properties [2]. However, there is evidence for other essential requirements for sterol In cell proliferation in addition to a membrane structural role [3-6].For example, in yeast mutants traces of ergosterol stimulate phospholipid formation [7] and protein kinase activity [8]. This report describes the effects of paclobutrazol on the sterol and phospholipid compositions of membranes from plant and yeast cultures. 

In most higher plants, sterols are present as a mixture of A -sterols. Sitosterol, stigmasterol and 24-methylcholesterol are usually cited as the typical plant sterols (Fig.l). These sterols are concentrated mainly in the plasma membrane (PM) (1). Using sterol biosynthesis inhibitors, we have shown that it is possible to obtain plant cell suspensions or whole plants with a completely modified sterol profile (2,3). Such plants constitute most suitable material for studying structural and functional roles of sterols, which are still largely unknown. 

The free sterols consisted of campesterol (24 %), stigmasterol (4 %) and 8-sitosterol (72 X). This composition changed very little with one or three stress periods. PMEF from plant cells are usually characterized by a high ratio free sterols/total phospholipids. In our system the molar ratio free sterols/total phospholipids of the PMEF was high in the control membranes, 0.58, decreased somewhat after one stress treatment, 0.51, but increased after three stress periods to 0.70. 

Lipids also form an integral part of biological membranes. All living cells are surrounded by a membrane that provides a barrier between the cell and its environment. They also occur within the cell, providing a structure in which many metabolic reactions take place. In mammals, the lipids involved in membrane structures are mainly the glycerophospholipids and unesterified (free) cholesterol, while in plants, the glycosylglycerides are predominant, especially in the chloroplasts and -sitosterol is the most abundant sterol rather than cholesterol. The chemistry of these structural lipids and their role in membrane architecture are described in Chapter 6, while their biochemistry forms the basis for Chapter 7. 

Cholesterol is found almost exclusively in eukaryotic cells. Animal membranes contain substantially more cholesterol than plant membranes, in which cholesterol is usually replaced by other sterols. There is no cholesterol at all in prokaryotes (with a few exceptions). The inner mitochondrial membrane of eukaryotes is also low in cholesterol, while it is the only membrane that contains large amounts of cardiolipin. These facts both support the endosymbiotic theory of the development of mitochondria (see p. 210). 

---