Directory UMM :Data Elmu:jurnal:P:PlantScience:Plant Science_BioMedNet:281-300:

trends in plant science
reviews

Plant sterols and the membrane
environment Marie-Andrée Hartmann
Sterols are essential for all eukaryotes. In contrast to animal and fungal cells, which contain
only one major sterol, plant cells synthesize a complex array of sterol mixtures in which
sitosterol, stigmasterol and 24-methylcholesterol often predominate. Sitosterol and 24methylcholesterol are able to regulate membrane fluidity and permeability in a similar manner
to cholesterol in mammalian cell membranes. Plant sterols can also modulate the activity of
membrane-bound enzymes. In contrast, stigmasterol might be specifically required for cell
proliferation.

S

terols are part of the vast family of isoprenoids. The conversion of farnesyl diphosphate into squalene marks the
channelling of the isoprenoid pathway into the branch that
produces sterols. These compounds are known to be essential for
all eukaryotes, either synthesized de novo or taken up from the
environment. Since the discovery of cholesterol, they have continued to be the focus of the research activities of many chemists,
biochemists and clinicians, as attested by the 13 Nobel prizes
awarded between 1910 and 1985 that have been associated with

work on sterols. The sterols show a fascinating chemical complexity (over 200 natural 3b-monohydroxysterols so far indexed)
and a remarkable diversity of function in living organisms.
Sterols are membrane components and as such regulate membrane fluidity and permeability. This structural role is often
described as the ‘bulk’ function, because it is played by significant amounts of sterols and can be fulfilled by virtually any of the
compounds. However, sterols can also participate in the control of
membrane-associated metabolic processes, a function for which
only a few specific sterol molecules are required; their involvement in signal transduction events has also been reported in mammalian cells. Moreover, sterols are the precursors of a vast array
of compounds involved in important cellular and developmental
processes in animals (e.g. steroid hormones and bile acids), fungi
(e.g. ecdysteroids, antheridiol and oogoniol) and higher plants

Fig. 1. Basic structure of a sterol with standard carbon numbering
according to the 1989 IUPAC-IUB recommendations4. The carbons C241 and C242 come from S-adenosyl-L-methionine and
were previously designated as C28 and C29. The stereochemistry
of the alkyl group is a. According to the former nomenclature system, the two methyl groups located at C24 and the methyl group
at C14 were numbered C30, C31 and C32, respectively.

170

May 1998, Vol. 3, No. 5

(e.g. brassinosteroids1). Finally, in plants, sterols are substrates for
the synthesis of a wide range of secondary metabolites – such as
cardenolides, glycoalkaloids, pregnane derivatives and saponins –
to which precise physiological functions have not been assigned.
Saponins have been proposed to play a role in resistance to fungal
attack2. Whereas mammalian and fungal cells generally contain
one major sterol, cholesterol and ergosterol, respectively, plants
have a characteristically complex sterol mixture. Thus, as many as
61 sterols and pentacyclic triterpenes have been identified in
maize seedlings3, although some of these compounds are biosynthetic precursors that occur in very low amounts.
Several questions urgently need to be addressed to understand
the function of plant sterols:
• Why do plants require a mixture of sterols instead of one
unique sterol?
• Does such a mixture bestow significant advantages to plants
compared with mammals and fungi?
• Does each individual sterol play a specific role in plant cell
metabolism?
Here, the structural roles of sterols in higher plant cells are examined in an attempt to assess how they relate to functional aspects

of membrane behaviour.
Structure, biosynthesis and cellular localization of sterols
Structural patterns

Sterol was the name originally proposed to describe a 3b-monohydroxy compound based upon the perhydro-1,2-cyclopentano
phenanthrene ring system, with methyl substitution at C10 and
C13 and a side chain with 8–10 carbon atoms (Fig. 1). The nomenclature of sterols is largely based on the 1989 IUPAC-IUB
recommendations4, although many phytochemists continue to use
the old numbering system5 (Fig. 1). In plants, sterols are always
present as a mixture. Structural variations arise from different
substitutions in the side chain and number and position of double
bonds in the tetracyclic skeleton6–8. There have also been many
reports on sterol composition of algae9. Detailed and accurate
sterol analyses of such complex mixtures have been made possible
by the availability of powerful methods of separation (e.g. gas
chromatography and high-performance liquid chromatography)
and identification (high resolution 1H- and 13C-NMR spectroscopy
and gas chromatography/mass spectrometry)7,8. D5-sterols with a
24-ethyl group at C24, such as sitosterol and stigmasterol (Fig. 2),
are by far the most abundant compounds, but sterols with the D7

nucleus are frequently encountered in members of some plant
families (e.g. Caryophyllaceae, Theaceae and Cucurbitaceae)6. The
introduction of an alkyl group at C24 renders this position chiral
and thus two epimers are possible. Concerning the stereochemistry of the sterol side chain, two sets of rules are also in use: the a/b
and R/S nomenclatures5. The a and b notations are independent of

Copyright © 1998 Elsevier Science Ltd. All rights reserved. 1360 - 1385/98/$19.00 PII: S1360-1385(98)01233-3

trends in plant science
reviews
substituents at neighbouring atoms, with a
in front of the plane and b behind the plane
(Fig. 1). According to the R/S nomenclature, the configuration depends on the substituent pattern associated with C24. Thus,
a 24a-alkyl group can be R or S depending
on the absence or presence of a double bond
at C22 (Ref. 5). Whereas 24-ethylsterols
mainly have a side chain with only one type
of configuration, usually 24a, 24-methylsterols consist of a mixture of epimers.
Thus, 24-methylcholesterol is a mixture of
campesterol (24a) and 22-dihydrobrassicasterol (24b). Cholesterol, the major sterol

in mammals, is also found in plants. It generally accounts for a few percent of the
sterol mixture of most plants, but some
families (e.g. Solanaceae) contain higher
amounts. In the early stages of apical development of Brassica campestris10 or in the
epicuticular waxes of rape leaves11, this
sterol represents around 70% of total sterols. Plants also contain a great variety of
minor 4,4-dimethyl- and 4a-methylsterols,
which are biosynthetic precursors of the
main sterols.
In most higher plants, sterols with a free
3b-hydroxyl group, also called free sterols,
are the major end products. However, sterols also occur as conjugates in which the
3-hydroxyl group is either esterified (by a
long-chain fatty acid to give steryl esters) or
b-linked (to the 1-position of a monosaccharide, usually glucose) to form steryl glucosides or, when the 6-position of the sugar
is esterified by a fatty acyl chain, acylated
steryl glucosides12.
Biosynthesis

The occurrence of phylogenetic differences in the sterol biosynthesis routes operating in various organisms is now well

established (Fig. 2). In plants, the sterol
pathway represents a sequence of more than
30 enzyme-catalyzed reactions, all associated with membranes6,13. It is characFig. 2. Different sterol biosynthesis pathways in living organisms. In higher plants, 2,3terized by steps restricted to plants, such
squalene oxide is cyclized to produce cycloartenol, a 9b,19-cyclopropylsterol. The opening
as the cyclization of squalene oxide into
of the cyclopropane ring is catalyzed by the cycloeucalenol obtusifoliol isomerase, a step
cycloartenol; the opening of the 9b,19occurring only in photosynthetic phyla. End-pathway sterols are always produced as a mixcyclopropane ring of cycloeucalenol, catature. Broken arrows indicate multiple steps.
lyzed by the cycloeucalenol-obtusifoliol
isomerase; and the second stage of the sidechain alkylation at C24. In higher plants,
squalene oxide can also be converted into a wide range of pentaSterols appear to be very stable, as attested by the large increase
cyclic triterpenes such a- and b-amyrins. Cholesterol is synthe- in the sterol-to-phospholipid ratio observed in senescent memsized via cycloartenol, but its biosynthetic pathway is not entirely branes16. So far, there are no available data about the mean halfknown. The post-squalene biosynthetic pathway is clearly charac- life of plant sterol molecules or their catabolism.
terized by critical rate-limiting steps (e.g. the methylation of
cycloartenol)14. All plant tissues are able to form their own sterols. Cellular distribution
Most of the enzymes involved in sterol biosynthesis from squa- As in animal and fungal cells, free sterols reside predominantly in
lene formation are associated with the membranes of endoplasmic the plasma membranes of plant cells15. Compared with other cell
reticulum (ER)15, but the participation of the plasma membrane at membrane systems, the plasma membrane contains both the greatfinal steps of the pathway (e.g. in stigmasterol synthesis) cannot est sterol content in proportion to the amount of protein present
be excluded.
and the highest sterol-to-phospholipid molar ratio. Sterols are
May 1998, Vol. 3, No. 5

171

trends in plant science
reviews
present in low amounts in ER (Ref. 15), tonoplast17 and mitochondrial membranes18. Also, a relatively small proportion of sterol
molecules is present in the envelope of chloroplasts, but they are
absent from thylakoid membranes15 (see note added in proof).
It appears that there is no specific association of an individual
sterol molecule with a given membrane compartment, and that all
the membranes contain the same sterols in similar proportions15.
Cholesterol has been found in all the membranes. Cellular concentrations of free sterols and their intracellular distribution are probably tightly regulated, but factors that maintain sterols in the plasma
membrane are unclear. Their partitioning between the two leaflets
of the plasma membrane and their organization in the lateral plane
of the membrane have not been investigated. The synthesis of sterols in the ER and accumulation in the plasma membrane implies
transport between these two membrane compartments. Unfortunately, evidence for sterol domains in the plasma membrane is still
lacking, although this might help to clarify whether they play a role
in exocytosis, endocytosis, or during signal transduction in plants.
Experimental approaches for investigating sterol functions
Biosynthesis inhibitors

The ergosterol biosynthetic pathway is the target of many antifungal drugs developed against animal and plant pathogens. Thus, a
wide range of molecules is now available to inhibit this pathway at
different levels19. These compounds have also been shown to interfere with the phytosterol biosynthetic pathway13,20–22. Two main
groups of sterol biosynthesis inhibitors can be considered depending on the enzyme target. If the target is an enzyme of the general
isoprenoid pathway, such as the 3-hydroxymethyl-3-glutaryl coenzyme A (HMG-CoA) reductase or an enzyme at the beginning
of the sterol pathway (i.e. the squalene synthase or squalene
epoxidase), the effect of the inhibitor is a net decrease in the total
amount of membrane free sterols. In contrast, an inhibitor acting
downstream of cycloartenol (the first sterol with a tetracyclic
skeleton) triggers the replacement of the naturally occurring D5sterols by biosynthetic intermediates as well as by unusual sterols
such as 9b,19-cyclopropylsterols or 14a-methyl D8-sterols13,20–22
(Fig. 3). Sterol biosynthesis inhibitors are therefore useful tools
for manipulating plant sterol synthesis in vivo and thus for obtaining
whole plants or plant cell suspensions with a wide variety of sterol
profiles23. Such plants are very convenient for studying the consequences of unusual sterols on the chemical composition, physical
properties and functions of membranes. More-detailed studies
have been performed with plants treated by two major classes of
compounds, N-alkylmorpholines (e.g. fenpropimorph) and azole
derivatives, which are the most commonly used in agriculture19
and inhibit the cycloeucalenol-obtusifoliol isomerase and the

obtusifoliol 14-demethylase, respectively22 (Fig. 3). In the case of
maize roots treated with fenpropimorph24, it has been shown that:
• The growth rate of seedlings is little affected, even after
replacement of 95% of the typical D5-sterols by 9b,19-cyclopropylsterols.
• These unusual sterols are readily incorporated into membranes.
• The modification of the sterol profile is accompanied by a dramatic modification in the cellular distribution of sterols, the ER
becoming the richest membrane in terms of free sterols instead
of the plasma membrane.
• No change occurs in the partitioning of the main phospholipid
classes, but an increase in the degree of unsaturation of ER
phospholipid acyl chains is observed.
There are limitations to the use of inhibitors. First, there is the
need to apply the inhibitor during the whole experiment to
observe its effects. These compounds are also generally lipophilic
molecules that can bind to membrane lipids and therefore interfere
172

May 1998, Vol. 3, No. 5

with the parameter under study. Finally, the best modification of

the sterol profile accounts for 95% of total sterols, the remaining
5% D5-sterols corresponding to compounds already present in the
seed. This means that some catalytic roles played by minute quantities of sterols might be not detected.
Membrane model systems

To gain more insight into structural and functional roles of each
individual sterol molecule, one approach is the use of simple,
well-defined in vitro membrane models (liposomes). A large body
of work has thus been devoted to interactions of cholesterol with
a vast array of lipid species, and information about the structural
and dynamic properties of cholesterol in biological membranes has
been obtained by a wide range of physical techniques (e.g. fluorescence depolarization of suitable probes, 2H-NMR, electron spin
resonance spectroscopy and differential scanning calorimetry).
Phospholipids from plant membranes are generally characterized
by a higher proportion of unsaturated fatty acyl chains than those
from animal or fungal membranes. It is therefore of crucial importance to investigate the behaviour of plant sterols with lipids of
plant origin. A series of experiments has recently been performed
with large unilamellar vesicles prepared from soybean phosphatidylcholine, a representative plant phospholipid, and various plant
sterols in different molar ratios. Effects of sterols on membrane
fluidity were investigated by diphenyl hexatriene fluorescence

polarization measurements25 and 2H-NMR spectroscopy with perdeuderiated dimyristoylphosphatidylcholine on the sn-2 acyl chain
as a probe26,27. Permeability changes triggered by plant sterols were
monitored by measuring the swelling rates of vesicles following
an osmotic shock in a stopped-flow spectrophotometer26.
Liposomes are also very useful for studying the roles of sterols
on the activity of membrane-bound enzymes in reconstitution
experiments. A recent study has been devoted to effects of plant
sterols on the plasma membrane H+-ATPase (Ref. 28).
Mutants and transgenic plants

Sterol auxotrophs are very convenient tools for investigating the
roles of sterols in membranes. The yeast Saccharomyces cerevisiae has been the most widely used organism. A wide range of
mutants defective in ergosterol biosynthesis has been isolated
using polyene antibiotics as selective agents. In particular, many
studies have been devoted to the mutant GL7, and valuable information about the structural features of the sterol molecule required
for yeast membrane functions is readily available29–31. Various
investigations have established multiple roles for sterol during
yeast growth31. All relevant genes of the ergosterol pathway have
been cloned32, allowing isolation of mutants by gene disruption. In
contrast, most of the genes encoding enzymes involved in the
plant sterol pathway are unknown33. However, major advances in
this field are expected, mainly because the large international
sequencing programmes with Arabidopsis have made available
many expressed sequence tag (EST) clones. A plant sterol mutant,
STE1, has recently been isolated in Arabidopsis34. This mutant is
defective in the D7-sterol C5-desaturase and thus accumulates D7sterols34, but displays no particular phenotype.
Information is also expected from studies using transgenic plants
in which genes encoding enzymes of sterol biosynthesis can be
overexpressed. For example, in tobacco plants overexpressing the
HMG-CoA reductase gene, the free sterol content of membranes
was found to be unchanged, whereas the excess metabolic intermediates are diverted from the main pathway and accumulated as
steryl esters in lipid droplets35. Finally, new avenues will be opened
by the isolation of mutants in which genes encoding enzymes of
the sterol pathway are disrupted by T-DNA.

trends in plant science
reviews

Fig. 3. Use of sterol biosynthesis inhibitors to manipulate the sterol profile of plant membranes in vivo. The main pathway from acetyl-CoA to
end products (sitosterol, stigmasterol and 24-methylcholesterol) via mevalonic acid (MVA) is indicated by open arrows. Downstream of
cycloartenol, the first cyclic intermediate, two enzymes, the cycloeucalenol obtusifoliol isomerase (COI) and the obtusifoliol 14-demethylase
(OBT14DM), are the targets of two classes of fungicide widely used in agriculture, N-alkylmorpholines and triazole derivatives, respectively.
The inhibition of COI leads to an accumulation of 9b,19-cyclopropylsterols (e.g. 24-methylpollinastanol); the inhibition of OBT14DM triggers
an accumulation of 14a-methyl D8-sterols (e.g. 14a,24-dimethylcholest-8-en-3b-ol) at the expense of the naturally occurring D5-sterols. The
boxed sterols are not present in untreated plants. The inhibition of any step located upstream of cycloartenol (e.g. HMG-CoA reductase) results
in a net decrease of the free sterol content of membranes.

Structural roles of sterols

Sterol molecules are incorporated into membranes with the 3bOH facing the water interface and the side chain extending into the
hydrophobic core to interact with fatty acyl chains of phospholipids and proteins. Thus, they modulate the physical state of bilayers by restricting the motion of fatty acyl chains (ordering effect),
which at physiological temperatures are in the liquid-crystalline
phase. Previous investigations led to the identification of several
structural features of the sterol molecule required for a membrane
function: a free 3b-hydroxyl group; a planar tetracyclic skeleton;
and an aliphatic side chain with 8–10 carbon atoms29. The typical
plant sterols (sitosterol, stigmasterol and 24-methylcholesterol) all
satisfy these requirements. Experiments with soybean phosphatidylcholine bilayers indicate that all the plant sterols tested are able
to regulate membrane fluidity, but with different efficiency25–27.
Sitosterol and 24-methylcholesterol are the most efficient sterols for
restricting the mobility of phospholipid fatty acyl chains. The introduction of a trans-oriented double bond at C22 in the side chain of
the sterol molecule significantly reduces its ordering ability, as
attested by the comparison between sitosterol and stigmasterol.
Such a reduced ordering ability was observed only in the presence

of an alkyl group in the side chain, as cholesterol and D22-dehydrocholesterol have similar efficiencies27. The stereochemistry at
C24 was also shown to interfere with the ordering ability of the
sterol36. The ability of two unusual sterols, 24-methylpollinastanol
(a 9b,19-cyclopropylsterol) and 14a,24-dimethylcholest-8-en-3bol (a 14a-methyl D8-sterol), in regulating membrane fluidity was
examined. These two sterols are isomers. As they accumulate in
plants treated with fungicides used in agriculture19, the functional
consequences of their incorporation into the membranes of such
plants were investigated. Whereas the 9b,19-cyclopropylsterol exhibits a high ability to order soybean phosphatidylcholine bilayers,
similar to that of sitosterol, the 14a-methyl D8-sterol appears to be
a far less efficient compound27.
Effects triggered by the same range of sterols on the water
permeability of soybean phosphatidylcholine bilayers have also
been investigated26. Sitosterol and 24-methylcholesterol appeared
to be very active in reducing membrane permeability. In contrast,
stigmasterol was very inefficient. Of two unusual sterols tested
previously, 24-methylpollinastanol was found to be functionally
equivalent to sitosterol, and the 14a-methyl D8-sterol was found
not to be very effective.
May 1998, Vol. 3, No. 5

173

trends in plant science
reviews
In conclusion, with soybean phosphatidylcholine bilayers,
there is an excellent correlation between sterol effects on acyl
chain ordering and membrane permeability to water. Although
these results have been obtained with a model membrane system,
they suggest that the most efficient sterols for regulating both
functions are able to influence the properties of plant membranes.
Stigmasterol might be required for a different function, although
the interaction of this sterol with another class of phospholipid
(e.g. phosphatidylethanolamine) remains to be determined.
Metabolic and regulatory functions of sterols
Cellular proliferation and differentiation

Active sterol synthesis occurs following seed germination to meet
the needs for new membranes, as attested by the several-fold
increase in free sterol concentration3. The rate of sterol synthesis
then gradually decreases with seed maturity.
Evidence that sterols may play an important metabolic role in
the cell proliferation process, in addition to the purely structural
function of controlling the physical state of membranes, has been
emerging recently. For such a function, only a small fraction of
specific sterol molecules might be necessary. A more specific
requirement for stigmasterol has been shown in the case of celery
cells treated by an inhibitor of the obtusifoliol 14-demethylase37.
Cholesterol at 50–100 mM restored growth to 40–50% of that of
the control, but full growth was only achieved in the presence
of stigmasterol (50 mM). Lower concentrations of stigmasterol
(0.05–0.5 mM) were unable to restore growth, but a combination
of a low concentration of stigmasterol together with a high concentration of cholesterol (50 mM) was as effective as stigmasterol
alone (50 mM). However, most plant cells exhibit a very low ability for exogenous sterol uptake.
Sterols as effectors of membrane-bound enzymes

Recent evidence suggests that plant sterols are able to modulate
the activity of the plasma membrane H+-ATPase from maize
roots28. In particular, cholesterol and stigmasterol were found to
stimulate proton pumping, especially at a low concentration (5
mol %), whereas all the other sterols tested (e.g. sitosterol and 24methylcholesterol) behaved as inhibitors at any concentration.
When given at a concentration of 3 mM to intact maize roots, these
two sterols are also able to stimulate H+ secretion38. They might
therefore be considered as effective ligands in much the same way
that cholesterol is for the Na+/K+ ATPase of animal cells39. In the
same context, specific sterol molecules might be required by surface receptors and participate in some signal transduction events
as in mammalian cells.
Conclusions

As is the case for animal and yeast cells, sterols play many roles in
higher plants. One role is as a bulk sterol for new membranes synthesized during plant development, and this function can be satisfied by a variety of sterol compounds. Most of the free sterols of
the cell reside in the plasma membrane, where they contribute to
the maintenance of an intermediate state of fluidity consistent
with optimal functioning of integral enzymes and surface receptors. The plant sterols sitosterol and 24-methylcholesterol probably play a role similar to that of cholesterol in mammalian cells.
Stigmasterol, another major plant sterol, which differs from sitosterol only by an additional double bond at C22 in the side chain,
appears not to be involved in the regulation of membrane properties. The ability of 9b,19-cyclopropylsterols to replace D5-sterols
as effectors of membrane fluidity and permeability is quite
remarkable. Small amounts of specific D5-sterols also appear to be
required to promote cell proliferation and to regulate metabolic
174

May 1998, Vol. 3, No. 5

events. Stigmasterol, but also cholesterol, might be involved in
such processes. The lack of plant mutants auxotrophic to sterols
has been a serious handicap to research in this field, but progress
in cloning genes encoding enzymes of sterol biosynthesis as well
as in gene-targeted disruption techniques will undoudtedly help to
clarify the functions of plant sterols.
References
01 Yokota, T. (1997) The structure, biosynthesis and function of brassinosteroids,
Trends Plant Sci. 2, 137–143
02 Osbourn, A. (1996) Saponins and plant defence – a soap story, Trends Plant
Sci. 1, 4–9
03 Guo, D.A., Venkatramesh, M. and Nes, W.D. (1995) Developmental
regulation of sterol biosynthesis in Zea mays, Lipids 30, 203–219
04 Moss, G.P. (1989) The nomenclature of steroids: recommendations by the
IUPAC–IUB joint commission on biochemical nomenclature, Eur. J.
Biochem. 186, 429–458
05 Parker, S.R. and Nes, W.D. (1992) Regulation of sterol biosynthesis and its
phylogenetic implications, Am. Chem. Soc. Symp. Ser. 497, 110–145
06 Nes, W.R. (1977) The biochemistry of plant sterols, Adv. Lipid Res. 15,
233–324
07 Goad, L.J. (1991) Phytosterols, in Methods in Plant Biochemistry (Vol. 7)
(Charlwood, B.V. and Banthorpe, D.V., eds), pp. 369–434, Academic Press
08 Akihisa, T., Kokke, W.C.M.C. and Tamura, T. (1991) Naturally occurring
sterols and related compounds from plants, in Physiology and Biochemistry of
Sterols (Patterson, G.W. and Nes, W.D., eds), pp. 172–228, American Oil
Chemists’ Society
09 Patterson, G.W. (1991) Sterols of algae, in Physiology and Biochemistry of
Sterols (Patterson, G.W. and Nes, W.D., eds), pp. 118–157, American Oil
Chemists’ Society
10 Douglas, H.H. et al. (1996) Changes in lipid composition during floral
development of Brassica campestris, Phytochemistry 42, 335–339
11 Noda, M. et al. (1988) Occurrence of cholesterol as a major sterol component
in leaf surface lipids, Lipids 23, 439–444
12 Wojciechowski, Z.A. (1991) Biochemistry of phytosterol conjugates, in
Physiology and Biochemistry of Sterols (Patterson, G.W. and Nes, W.D., eds),
pp. 361–395, American Oil Chemists’ Society
13 Benveniste, P. (1986) Sterol biosynthesis, Annu. Rev. Plant Physiol. 37,
275–308
14 Nes, W.D. and Venkatramesh, M. (1997) Enzymology of phytosterol
transformations, in Biochemistry and Function of Sterols (Parish, E.J. and Nes,
W.D., eds), pp. 111–122, CRC Press
15 Hartmann, M-A. and Benveniste, P. (1987) Plant membrane sterols: isolation,
identification and biosynthesis, Methods Enzymol. 148, 632–650
16 Duxbury, C.L. et al. (1991) Alterations in membrane protein conformation in
response to senescence-related changes, Phytochemistry 30, 63–68
17 Yoshida, S. and Uemura, M. (1986) Lipid composition of plasma membranes
and tonoplasts isolated from etiolated seedlings of mung bean (Vigna radiata
L.), Plant Physiol. 82, 807–812
18 Méance, J., Dupéron, P. and Dupéron, R. (1976) Répartition des substances
stéroliques à l’intérieur des mitochondries de l’inflorescence de Chou-fleur,
Physiol. Vég. 59, 745–756
19 Köller, W. (1992) Antifungal agents with target sites in sterol functions and
biosynthesis, in Target Sites in Fungicide Action (Köller, W., ed.), pp.
119–205, CRC Press
20 Burden, R.S., Cooke, D.T. and Carter, G.A. (1989) Inhibitors of sterol
biosynthesis and growth in plants and fungi, Phytochemistry 28, 1791–1804
21 Mercer, E.I. (1993) Inhibitors of sterol biosynthesis and their applications,
Prog. Lipid Res. 32, 357–416
22 Rahier, A. and Taton, M. (1997) Fungicides as tools in studying postsqualene
sterol synthesis in plants, Pestic. Biochem. Physiol. 57, 1–27
23 Schmitt, P., Rahier, A. and Benveniste, P. (1982) Inhibition of sterol
biosynthesis in suspension cultures of bramble cells, Physiol. Vég. 20,
559–571

trends in plant science
reviews
24 Grandmougin, A. et al. (1989) Cyclopropyl sterols and phospholipid
composition of membrane fractions from maize roots treated with
fenpropimorph, Plant Physiol. 90, 591–597
25 Schuler, I. et al. (1990) Soybean phosphatidylcholine vesicles containing
plant sterols: a fluorescence anisotropy study, Biochim. Biophys. Acta 1028,
82–88
26 Schuler, I. et al. (1991) Differential effects of plant sterols on water
permeability and on acyl chain ordering of soybean phosphatidylcholine
bilayers, Proc. Natl. Acad. Sci. U. S. A. 88, 6926–6930
27 Krajewsky-Bertrand, M-A., Milon, A. and Hartmann, M-A. (1992)
Deuterium-NMR investigation of plant sterol effects on soybean
phosphatidylcholine acyl chain ordering, Chem. Phys. Lipids 63, 235–241
28 Grandmougin-Ferjani, A., Schuler-Muller, I. and Hartmann, M-A. (1997)
Sterol modulation of the plasma membrane H+-ATPase activity from corn
roots reconstituted into soybean lipids, Plant Physiol. 113, 163–174
29 Bloch, K.E. (1983) Sterol structure and membrane function, Crit. Rev.
Biochem. 14, 47–82
30 Nes, W.D. et al. (1993) The structural requirement of sterols for membrane
function in Saccharomyces cerevisiae, Arch. Biochem. Biophys. 300, 724–733
31 Rodriguez, R.J. et al. (1985) Multiple functions for sterols in Saccharomyces
cerevisiae, Biochim. Biophys. Acta 837, 336–343
32 Lees, N.D. et al. (1995) Cloning of the late genes in the ergosterol
biosynthetic pathway of Saccharomyces cerevisiae – a review, Lipids 30,
221–226
33 Bach, T.J. and Benveniste, P. (1997) Cloning of cDNAs or genes encoding
enzymes of sterol biosynthesis from plants and other eukaryotes: heterologous
expression and complementation analysis of mutations for functional
characterization, Prog. Lipid Res., 36, 197–226

34 Gachotte, D., Meens, R. and Benveniste, P. (1995) An Arabodopsis mutant
deficient in sterol biosynthesis: heterologous complementation by ERG3
encoding a D7-sterol-C5-desaturase from yeast, Plant J. 8, 407–416
35 Schaller, H. et al. (1995) Expression of the Hevea brasiliensis (H.B.K.) Müll.
Arg. 3-hydroxy-3-methylglutaryl coenzyme A reductase 1 in tobacco results in
sterol overproduction, Plant Physiol. 109, 761–770
36 Marsan, M.P., Muller, I. and Milon, A. (1996) Ability of clionasterol and
poriferasterol (24-epimers of sitosterol and stigmasterol) to regulate membrane
lipid dynamics, Chem. Phys. Lipids 84, 117–121
37 Goad, L.J. (1990) Application of sterol synthesis inhibitors to investigate the
sterol requirements of protozoa and plants, Biochem. Soc. Trans. 18, 63–65
38 Cerana, R. et al. (1984) Regulating effects of brassinosteroids and of sterols
on growth and H+ secretion in maize roots, Z. Pflanzenphysiol. 114, 221–225
39 Cornelius, F. (1995) Cholesterol modulation of molecular activity of
reconstituted shark Na+,K+-ATPase, Biochim. Biophys. Acta 1235, 205–212
Note added in proof
M. Havaux has recently discussed the absence of sterols in thylakoid membranes
(Trends Plant Sci. 3, 147–151). In such membranes the function played by sterols
might be fulfilled by other isoprenoids, such as xanthophyll carotenoids (e.g.
zeaxanthin) and a-tocopherol.

Marie-Andrée Hartmann is at the Institut de Biologie Moléculaire
des Plantes du Centre National de la Recherche Scientifique
(CNRS), 28 rue Goethe, 67083 Strasbourg, France
(tel +33 3 88 35 83 70; fax +33 3 88 35 84 84;
e-mail ma.hartmann@ibmp-ulp.u-strasbg.fr).

The molecular basis of cytoplasmic
male sterility and fertility restoration
Patrick S. Schnable and Roger P. Wise
Cytoplasmic male sterility (CMS) is a maternally inherited condition in which a plant is unable
to produce functional pollen. It occurs in many plant species and is often associated with
chimeric mitochondrial open reading frames. In a number of cases, transcripts originating
from these altered open reading frames are translated into unique proteins that appear to
interfere with mitochondrial function and pollen development. Nuclear restorer (Rf or Fr )
genes function to suppress the deleterious effects of CMS-associated mitochondrial abnormalities by diverse mechanisms. There are now several well-characterized CMS systems, for
which the mitochondrial sequences thought to be responsible have been described. Possible
mechanisms by which nuclear restoration occurs in these systems can now be postulated.

C

ytoplasmic male sterility (CMS) has been observed in over
150 plant species1–3. CMS systems make excellent models
in which to study the interaction between nuclear and cytoplasmic factors, because fertility restoration relies on nuclear
genes that suppress cytoplasmic dysfunction (Fig. 1). Analyses of
the molecular mechanisms by which fertility loss and restoration
occur can also help elucidate pollen development and normal
mitochondrial function.
Nuclear restoration allows the commercial exploitation of CMS
systems in the production of hybrid seed. This is because, in combination with CMS, it eliminates the need for hand emasculation
and yet ensures the production of male-fertile, first-generation (F1)
progeny. For example, prior to the epidemic of southern corn leaf

blight in 1970, male-sterile T (Texas)-cytoplasm maize system
was used to produce approximately 85% of hybrid seed in the
USA. Breeders produce hybrid seed using a CMS system by
developing female lines that carry CMS cytoplasm but lack
restorer genes and by developing male lines that carry the appropriate restorer genes. F1 hybrid seed produced by the female lines
carry the CMS cytoplasm but yield fertile plants because of the
action of the paternally contributed nuclear restorers.
Origin of cytoplasmic male sterility

Because of their value in hybrid seed production, CMS systems
have been identified and characterized in many crop species, including Phaseolus vulgaris, beet, carrot, maize, onion, petunia,

Copyright © 1998 Elsevier Science Ltd. All rights reserved. 1360 - 1385/98/$19.00 PII: S1360-1385(98)01235-7

May 1998, Vol. 3, No. 5

175