The Influence of Gibberellic Acid on the Permeability of Model Membrane Systems

Various sterols and related steroids were tested for their ability to influence ethanol-induced electrolyte leakage fron Hordeunm vulgare roots. Cholesterol had the greatest influence and, depending on concentration, it stimulated or inhibited the loss of electrolyte. Cholesterol, however, was ineffective if the roots were pretreated with ethanol. These data suggest that sterols protect rather than restore membrane structure. First, modifications in the cholesterol perhydrocyclopentanophenanthrene ring system suggest that at least one double bond is required for membrane activity. Second, increasing the bulkiness of the C1 side chain of cholesterol, as shown with campesterol, stigmasterol, and sitosterol, decreased its activity. Apparently for maximum effectiveness the sterol molecule should have a relatively flat configuration. Third, the C3-hydroxyl group is required for membrane activity since cholesteryl methyl ether, cholest-5-ene-3g-thiol and cholesteryl halogens were without activity. Exception to the foregoing rule was cholestane which was slightly active but which has neither a C3-hydroxyl group nor a double bond in the ring system. In previous publications it was reported that exogenously applied cholesterol, and certain other higher plant sterols, modified the permeability of plant cells (9,11,15). To explain the action of sterols on plant membranes, a hypothesis which incorporated the results of animal and phospholipid film experiments (5-7, 13, 16, 21, 26, 27) was proposed. Briefly, it was suggested that cholesterol, or other sterols, insert themselves into the phospholipid layer of the membrane, with the C.-hydroxyl group interacting by weak ion-dipole-and hydrogen-bonding with the polar nitrogenous base moiety of the phospholipid. In this model, the C, side chain of cholesterol would align itself along the nonpolar terminal of the phospholipid, filling the phospholipid cavity and in turn stabilizing the lipid configuration. According to this hypothesis, the Chydroxyl group of the sterol molecule is an important functional group. This hypothesis would explain why only the free sterols were active and not the steryl esters and steryl glycosides (9,11), all of which are common higher plant constituents. However, sterol analogs with a small polar group in the C. posi- tion, other than a hydroxyl, have never been tested. The foregoing hypothesis of sterol action also assumes that for effectiveness, the sterol molecule must have a flat configuration similar to that of cholesterol (7,26). This requirement was confirmed partially with sterols that differed in branching in the C17 side chain (9, 1 1), but in this respect sterols with modifications in the perhydrocyclopentanophenanthrene ring system have not been studied.For purposes of studying the mechanism of sterol action on plant membranes, cholesterol can be taken as the basic molecule, and this molecule can be divided into three sites of possible physiological importance (Fig. 1). The cholesterol nucleus consisting of ring A, B, C, and D with...

Section: Resultssupporting

confidence: 79%

Sterol Molecular Modifications Influencing Membrane Permeability

Grünwald¹

1974

“…Clearly, the simplest interpretation of the extremely close relationships between phospholipid composition and hormone sensitivity implicates and underlines the possibility that the, GA3 receptor sites are membrane based (31,32). Different phospholipid compositions may be expected to react with GA3 to greater or lesser extents, and even in different ways (22,33), and the structure and function ofsuch membranes will be modulated by GA3, producing effects on the physical parameters of the lipid membrane itself and on the membranebased proteins (19,20).…”

Section: Discussionmentioning

confidence: 99%

Low Temperature-Induced GA₃ Sensitivity of Wheat

Singh

Paleg²

1984

Exposure of isolated aleurone tissue from the wheat (Triticam aetirum) variety Kite which contains the Rht2 allele, to low temperature (5°C) for 20 h prior to addition of exogenous GA3, resulted in significant changes in the content of lipids, especially phospholipids. Sigificat low temperature-induced changes in both the head group and acyl contents of two phospholipids, phosphatidylcholine and phosphatidylethanolamine, were detected. More importantly, these changes displayed a very close temporal relationship with the low temperature-induced increase in GA3 sensitivity. Further, this relationship was paralleled by a highly significant correlation between the changes in the phospholipids and the changes in a-amylase production. These results underline the possibility that the GA3 receptor sites are membranbased lipids.Aleurone tissue of cereal seeds, when exposed to GA3, synthesizes and secretes hydrolytic enzymes, including a-amylase, into the starchy endosperm (5). Deembryonated seed or isolated aleurone layers thus provide convenient experimental systems for the study of GA3-induced a-amylase production. In the preceding paper, low temperature was shown to increase the GA3 sensitivity of otherwise GA3-insensitive aleurone tissue of several wheat varieties. More cubation of the isolated aleurone layers were as described in the preceding paper. Where dry seeds were involved, the aleurone layers were dissected from deembryonated seeds after boiling them for 5 min.Extraction and Analysis of Phospholipids. At the end of the preincubation period, the aleurone layers were immediately frozen in liquid N2 and freeze-dried and lipids were extracted using hot, water-saturated butan-l-ol (28). The total lipid extract was subjected to partition chromatography on Sephadex G-25 to yield a purified total lipid fraction, which was further fractionated by adsorption chromatography on silicic acid column. This fractionated the total lipids into neutral lipids, glycolipids, and phospholipids. These methods have been described in detail previously (1, 28).Phospholipids were separated into individual classes by TLC as described earlier (13) and quantified by GLC according to Douglas and Paleg (3). All experiments were replicated three times, and data were subjected to analysis of variance as well as multivariate analysis of variance.

“…by interaction with a membrane phospholipid component. Evidence suggests that in model membrane systems lipid composition markedly affects hormonemediated responses (11,15), and hormone-phospholipid interaction was influenced by fatty acid composition (9). This hypothesis could be further tested by examining the GA3-response of Avena stem segments with different lipid compositions before hormone treatment.…”

mentioning

confidence: 99%

Correlation between the Lipid Composition and the Responsiveness of Avena sativa Stem Segments to Gibberellic Acid

Jusaitis

Paleg²,

Aspinall³

1982

The lipid compositon of Avena sativa stem segments was malated using BASF 13-338 (formerly Sandoz 9785) and growth temperature, in order to establish whether there were correlations between responsiveness of the tissue to gibberelic acid (GA3) and the presence, before hormone treatment, of specific lipid components. High correlations were obtained between GA3-induced growth and total phospholipid, individual phospholipids, and fatty acids (except for lnolenic acid), total saturated fatty acids, stigmasterol content, and the unsaturated/saturated fatty acid ratio. It was concluded that, although the Upid composition, and particularly the total saturated fatty acid content, seem to be important contributory determinants of the GA,-induced growth response in this system, they may not be obligatory prerequisites, nor the only endogenous factors capable of influencing the response. However, the results are consistent with the hypothesis that membranes are involved in the hormonal mechanism and/or very early stages of the mode of GA3 action in this tissue.Previous studies using Avena sativa stem segments showed that short-term GA3 treatment resulted in no marked changes in lipid components, although significant growth response to the hormone was achieved (6, 7). Moreover, it was suggested (8) that GA3 in this system may be acting in a similar way to GA3 in the liposomal system of Wood and Paleg (16), ie. by interaction with a membrane phospholipid component. Evidence suggests that in model membrane systems lipid composition markedly affects hormonemediated responses (11,15), and hormone-phospholipid interaction was influenced by fatty acid composition (9). This hypothesis could be further tested by examining the GA3-response of Avena stem segments with different lipid compositions before hormone treatment. One of the most obvious ways by which this could be achieved is by alteration of growth temperature. The degree of unsaturation of fatty acids in the membrane systems of many plants and ectothermic organisms is strongly influenced by the environmental temperature (4, 14). Exposure to low temperature seems invariably associated with an increase in unsaturation of the fatty acid chains of phospholipids in a wide variety of species at all levels of biological organization. Similar changes in membrane protein have not been found (5, 12), apart from those occurring as a consequence of changes in the lipid moiety of the membrane in response to the low temperature (17).Another method of altering lipid composition is through the use of compounds which are thought to selectively inhibit synthesis of a membrane component. One such component is BASF 13-338 (4-chloro-5-[dimethylaminoJ-2-phenyl-3[2H]-pyridazinone) (formerly known as Sandoz 9785), a pyridazinone herbicide which reportedly selectively blocks the synthesis of linolenic acid (18:3), thus producing a decrease in 18:3 accompanied by an increase in linoleic acid (18:2), without a shift in the relative proportion of unsaturated/saturated fatty acids of the tissu...