PLANT PHYSIOLOGY , Vol 105, Issue 2 509-518, Copyright © 1994 by American Society of Plant Biologists
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METABOLISM AND ENZYMOLOGY |
Regulation of Sterol Content in Membranes by Subcellular Compartmentation of Steryl-Esters Accumulating in a Sterol-Overproducing Tobacco Mutant
L. Gondet, R. Bronner and P. Benveniste
Institut de Biologie Moleculaire des Plantes, Centre National de la Recherche Scientifique, Departement d'Enzymologie Cellulaire et Moleculaire, Institut de Botanique, 28 rue Goethe, 67083 Strasbourg Cedex, France (L.G., P.B.)
The study of sterol overproduction in tissues of LAB 1-4 mutant tobacco
(Nicotiana tabacum L. cv Xanthi) (P. Maillot-Vernier, H. Schaller, P.
Benveniste, G. Belliard [1989] Biochem Biophys Res Commun 165: 125-130)
over several generations showed that the overproduction phenotype is stable
in calli, with a 10-fold stimulation of sterol content when compared with
wild-type calli. However, leaves of LAB 1-4 plants obtained after two steps
of self-fertilization were characterized by a mere 3-fold stimulation,
whereas calli obtained from these plants retained a typical
sterol-overproducing mutant phenotype (i.e. a 10-fold increase of sterol
content). These results suggest that the expression of the LAB 1-4
phenotype is dependent on the differentiation state of cells. Most of the
sterols accumulating in the mutant tissues were present as steryl-esters,
which were minor species in wild-type tissues. Subcellular fractionation
showed that in both mutant and wild-type tissues, free sterols were
associated mainly with microsomal membranes. In contrast, the bulk of
steryl-esters present in mutant tissues was found in the soluble fraction
of cells. Numerous lipid droplets were detected in the hyaloplasm of LAB
1-4 cells by cytochemical and cytological techniques. After isolation,
these lipid granules were shown to contain steryl-esters. These results
show that the overproduced sterols of mutant tissues accumulate as
steryl-esters in hyaloplasmic bodies. The esterification process thus
allows regulation of the amount of free sterols in membranes by subcellular
compartmentation.
