PHOSPHATIDYLCHOLINE BIOSYNTHESIS AND CELL-GROWTH IN APIUM-GRAVEOLENS

Xenobiotics directed against sterol biosynthesis have proved to be use ful tools in the determination of which sterol molecules are necessary for successful plant cell growth. However, the exact mode of action b y which sterols are able to trigger cell growth remains to be elucidat ed. Previous studies using the triazole paclobutrazol, demonstrated th at in Apium graveolens (cv. New Dwarf White) suspension cultures, ster ol and phosphatidylcholine biosynthesis are co-ordinately regulated (C . E. Rolph and L. J. Goad 1991, Physiol. Plant. 83: 605-610). The stud ies presented herein, were designed to investigate the possible role o f phosphatidylcholine in the stimulation of plant cell growth. Sterol biosynthesis, and hence cell growth, was inhibited by the use of the a zole xenobiotic miconazole. Treatment of the cultures with miconazole lead to compositional changes in the free sterol content of the cells. For example, 30 mu M miconazole treatment led to a reduction in the s tigmasterol/sitosterol ratios from 1.53 to 1.24. In contrast, the phos pholipid content of the cells remained relatively unchanged with phosp hatidylcholine accounting for approximately 25% of the total phospholi pids present in both control and miconazole-treated cells. The cytosta tic effect of miconazole could be partially counteracted by supplement ation of the growth medium with the phytosterol stigmasterol and/or th e unsaturated fatty acids oleate and linoleate. The activity of CTP:ch olinephosphate cytidylyltransferase (EC 2.7.7.15), a rate-limiting enz yme in phosphatidylcholine biosynthesis, was significantly reduced in cells whose growth had been arrested by miconazole treatment. In micon azole-treated cultures whose growth had been partially restored by sup plementation with either specific sterols or unesterified fatty acids, the activity of this key enzyme was increased. In the case of stigmas terol, oleate and linoleate supplementation, the microsomal activity w as found to be similar to that exhibited by control cultures. From the se studies, it may be concluded that certain phytosterols and unsatura ted fatty acids play key roles with respect to phosphatidylcholine bio synthesis and that phosphatidylcholine biosynthesis via the CDP-base p athway is an important pre- and/or co-requisite for successful culture growth.