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.