Gibberellic acid increases the permeability of model membranes composed of various plant-source lipids, a sterol, and dicetyl phosphate. As a result of hormone treatment, the flux of uncharged molecules such as glucose or sucrose, or charged ions such as chromate, through the model membranes (liposomes or micelies) is increased. The revelance of this finding to the in vivo effects of the hormone is briefly discussed.Plant hormones are both fewer in number and less variable in structure and function than their animal counterparts. In only one case, the gibberellins, do plant hormones approach the diversity apparent with animal steroids. These two classes of compounds, steroids and gibberellins, also share the properties of isoprenoid structure (and, thus, a common biosynthetic pathway), similarity in structural diversity (through insertion or deletion of double bonds, radicals, subsidiary ring structures), and control of comparable physiological functions. It would seem possible that gibberellins and steroids also share at least one common trigger or hormonal mechanism.One of the simplest manifestations of steroid action is the alteration of permeability of synthetic model membrane systems (3,9,21). The models are based on the fact that phospholipids, one of the major components of natural membranes, when dispersed in aqueous media, produce self-ordered particles which display many of the properties of natural membranes (6). The physiological pertinence of the models was enhanced by the finding that they demonstrated comparable responses to compounds which affect membranes in vivo (3). The permeability of the model structures (micelles or liposomes) is influenced by detergents, some antibiotics, and toxins, which increase permeability, and anti-inflammatory drugs and anaesthetics, which decrease permeability (16). Sterols may either increase or decrease permeability of the liposomes, and in one study, there was a high degree of correlation between the release of acid phosphatase from lysosomes and the increase in permeability of liposomes, with a range of sterols (3). Some sterols which decrease permeability of the model system have also been shown to act in a similar way in higher plant tissue, i.e., they protect red beet tissue from alcohol-induced damage in vivo (8). In addition, the in vivo effect of the polyene antibiotic, filipin, in increasing permeability of pea stem tissue, red beet, and potato discs (12,17), closely parallels its effect on liposomes. Furthermore, the effects of filipin, in both the in vivo and the in vitro systems, can be overcome by cholesterol (12).An explanation of hormone action in terms of alterations in membrane permeability has been a perennially proposed possibility. In general, the concept has met with only very limited success, for several reasons, and most workers continue to concentrate on more "metabolic explanations." In this and succeeding papers the potential role of gibberellin as a regulator of membrane permeability will be explored.
MATERIALS AND METHODSTo pre...
