STEROL ACYL TRANSFERASE AND STERYL ESTER HYDROLASE ACTIVITIES IN A TOBACCO MUTANT WHICH OVERPRODUCES STEROLS

The tobacco mutant LAB 1-4 overproduces sterols, most of which are esterified and stored as lipid droplets in the cytoplasm (P. Maillot-Vernier, L. Gondet, H. Schaller, P. Benveniste and G. Belliard, Mel. Gen. Genet. 231 (1991) 33-40). We studied the enzymes involved in the esterification of sterols and hydrolysis of steryl esters in tobacco calli and leaves. These activities were shown to be bound to microsomal membranes. The microsomes were delipidated by acetone treatment so as to discard endogenous sterols and steryl esters; optimal conditions were set up which allowed the selective determination of either the sterol acyl transferase (SATase) activity or the steryl ester hydrolase (SEHase) activity. The SATase activity of the LAB 1-4 mutant calli was shown to be twice that of the wild type calli whereas the activities were the same in the leaves of both genotypes. The SEHase activity of LAB 1-4 calli and leaves was only slightly higher than that of the wild type tissues. The results strongly suggest that the activity of the SATase is increased when the ratio sterols/enzyme exceeds a certain threshold. Both enzymes might be involved in the control of the free sterol content of the sterol-overproducing mutant and, more generally, in the homeostasis of free sterols in the plant cells.