Beziehungen zwischen Indolessigsäure-induziertem Wachstum und dem Steringehalt der Plasmamembran-Fraktion bei Avena-Koleoptilen

Summary After etiolated Avena coleoptiles have been incubated either for 1 h or 20 h in buffered 10- 4 or 10- 5 MIAA, the plasma membrane fraction was isolated by means of homogenization and centrifugation on a linear sucrose gradient of 15-45 % (w/w). Centrifugation concentrated plasma membrane material in the sucrose gradient fractions of 33-37 %. This material was identified by electron microscopy and with the aid of marker enzymes. After extraction of the plasma membrane fraction, cholesterol, campesterol, stigmasterol, and sitosterol could be detected in the purified extract by means of gc-ms-analysis. Quantitative determinations of the sterols contained in the plasma membrane after IAA-treatment revealed that, in comparison to the untreated control, 10- 5 M IAA applied 1 h before analysis leads to an increased concentration (11-24 %) of the four identified sterols, whereas incubation for 20 h in 10- 4 M IAA shows a weaker promotive effect (2-15 %); in fact, the concentration of sitosterol deereases. As it is well known that sterols can influence microviscosity and permeability in biomembranes, it may be concluded from the above-mentioned results that a relationship exists between the primary action of the tested auxin and the sterol composition of the plasma membrane. However, these alterations in the sterol concentration observed in extracts of complete membranes are rather small; but with regard to this observation, it must be taken into consideration (ORCI et al. 1980) that the distribution of sterols in membranes is not homogeneous. Thus, it might be possible that sterol concentrations underwent strongly localized alterations after IAA treatment of the plasma membrane. These locations could be situated near transport proteins or ATPases responsible for proton pumping. It seems reasonable to believe that many effects observed after steroid treatment of plants may be attributable to the interrelation of auxins and sterols at the membrane level.