Growth, Differentiation and Sexuality

224 C. Schimek and J. Wöstemeyer
towards dehydo-oogoniol exhibit regulatory functions;
7-deoxo-dehydro-oogoniol was described as
competitive inhibitor of the pheromone (McMorris
et al. 1993).
In Oomycota as well as in the Zygomcota,
a common basis seems to exist for sexual
signalling.Alloomycetespeciesanalysedsofar
require sterols to perform sexual development. The
signal system is, nevertheless, more restricted than
in the Zygomycetes, the specific compounds being
active only amongst closest neighbours, possibly
within each genus. Analyses of the induction of
sexual processes in other Oomycetes indicate that
steroids other than the antheridiol structure are
required (Knights and Elliott 1976; Musgrave et al.
1978; Kerwin and Washino 1983). The pronounced
specificity of the reactions becomes apparent from
the non-interchangeable character of oogoniol
and antheridiol in Achlya species (Horgen 1977;
Musgrave et al. 1978).
Self-sterile plant pathogenic species of the genera
Phytophthora and Pythium are unable to synthesize
sterols, and are thus sexually inactive if no
external sterols are available (Kerwin and Duddles
1989). Nevertheless, these are taken up from
the surroundings, presumably the host plants, and
are supposedly converted internally into the necessary
sexual hormones (Langcake 1974; Elliott and
Knights 1981). The actual structure of these active
hormones has not yet been determined. Many
steroid compounds have been tested for their ability
to induce sexual reactions in the genera Phytophthora
and Pythium, andtheresultsofthese
analyses indicate that the usefulness of external
steroids is also defined largely by their configuration
and stereochemistry, particularly that of the
side chain (Elliott 1979).
Substances active in Phytophthora were found
to fulfil the following criteria: they carry a hydroxyl
function at C3 of the sterol nucleus, have one C=C
double bond in ring B, and contain a hydrocarbon
side chain longer than five carbon atoms. Increasing
length of the side chain leads to increasing
activity, the exact position of C29 being the most
important feature. All compounds with the position
of C29 fixedbythepresenceofaC24−28 double
bond were of high activity (Elliott et al. 1966; Elliott
1972). Methylation at C24 also enhances activity; activity
indeed increases concomitantly with the size
of the substituent at C24 (Elliott 1979). Compounds
lacking the double bond in the steroid nucleus and
exhibiting other undesirable structural features are
completely inactive (Knights and Elliott 1976; El-
liott and Sansome 1977). Despite several of the
structural requirements indicating a structure of
the sexual pheromone similar to those of Achlya
sp., and thus raising the possibility of a general signal
mechanism, antheridiol was found to be completely
inactive in P. cactorum (Nes et al. 1980).
This possibly reflects the outcome of evolutionary
adaptation to the parasitic life style. Typical plant
steroids are generally far more effective than fungal
or animal compounds in the plant parasitic species
(Elliott et al. 1966; Nes et al. 1980).
Asecondsignalsystemactiveinsexualreproduction
of Phytophthora was proposed by Ko (1980,
1983, 1985, 1988) who observed intra- and interspecific
sexual reactions leading to self-fertilisation
in heterothallic Phytophthora spp. strains. The response,
accordingly termed hormonal heterothallism
and constituting a compatibility system, occurs
only in the presence of members of both matingtypes,A1andA2.Itisevidentlymediatedby
small molecules, as the resulting cross-induction
of oospore formation also occurred when the two
mating partners were separated by a polycarbonate
membrane (Ko 1980). The putative signal compounds
named hormone α1 andα2 respectively
could be enriched to a limited extent, and were
tentatively identified as lipid-like substances with
a molecular mass between 500 and 100Dalton, α2
being more polar than α1 (Ko 1983; Chern et al.
1996). In a follow-up study, Chern et al. (1999)
demonstrated that the two substances are neither
phospholipids, glycolipids, glycerides nor steroids,
but most probably neutral lipids with hydroxyl
functional group(s).
2. Biosynthesis
Analysis and comparison of steroid content in
Oomycetes, together with feeding experiments
using both natural and synthetic compounds, leads
toapossiblesequenceofeventsinpheromone
synthesis. Common precursor for both antheridiol
and the oogoniols is fucosterol, the most abundant
sterol in Oomycetes (Popplestone and Unrau 1973,
1974; McMorris and White 1977). Fucosterol is also
the sterol with the highest rate of uptake and the
most efficient conversion in Phytophthora (Elliott
et al. 1966; McMorris and White 1977). Saprolegniales
are able to synthesize fucosterol, probably
via 7-dehydrofucosterol (Lenton et al. 1971). In the
following synthesis steps leading to antheridiol,
modification of the steroid ring system and of the
side chain occur independently. For the latter, the