Introduction to Fungi, Third Edition

GENERAL ASPECTS OF LICHEN BIOLOGY
453
Fig16.5 The main routes of secondary metabolism in lichenized fungi.The shikimic acid pathway (top) gives rise to vulpinic acid
(¼ pulvinic acid methyl ester) and other pulvinic acid-derived metabolites (see also Fig.19.22).The mevalonic acid pathway gives rise
to triterpenes such as sterols (see Fig.13.16) and tetraterpenes such as carotenoids (see Fig. 24.8).The most typical lichen metabolites
are polyketides, especially those synthesized by polymerization of phenolic acids (orsellinic acid) or orcinols derived from them,
giving rise to a wide range of depsides (e.g. lecanoric acid) or depsidones (e.g. psoromic acid).This biosynthetic route, although
taking place in the mycobiont, is thought to be encouraged by the production of an orsellinic acid decarboxylase inhibitor produced
by the photobiont.Usnic acid is also the product of oxidative coupling of two phenolic-type rings, although the biosynthetic route
does not proceed via orsellinic acid and orcinol.Yet other lichen polyketides (e.g. parietin) arise by cyclization of a single long
polyketide chain; this metabolic pattern is also common in non-lichenized fungi (see Fig.12.46). Modified from Hale (1983),
Masuch(1993)andElix(1996).
(Richardson, 1988; Brodo et al., 2001). Not
surprisingly, lichens containing these and other
toxic substances appear to be avoided by lichengrazing
animals (Masuch, 1993). Many secondary
metabolites of lichens have acidic properties
and are therefore sometimes collectively called
lichen acids. Most of them are produced by the
mycobiont only in the intact lichen thallus but
not in isolation, indicating that the photobiont
may exert a subtle influence on the secondary
metabolism of the mycobiont. As an example,
Culberson and Ahmadjian (1980) have proposed
that a putative decarboxylase inhibitor secreted
by lichen algae inhibits the conversion of
orsellinic acid to phenolic substances which is
common in free-living fungi, leading instead to
esterification of orsellinic acid or orcinol, and
the accumulation of depside-type lichen acids
(see Fig. 16.5). The topic of secondary metabolism
in lichens is vast and has been reviewed by
Lawrey (1986), Fahselt (1994) and Elix (1996).
The main biosynthetic routes towards secondary
metabolism in lichens are the shikimic acid,
terpenoid and polyketide pathways, of which
the polyketide route is particularly important
(Fig. 16.5).
The benefit conveyed by the mycobiont may
thus be the provision of a ‘photobiont cultivation
chamber’ (Honegger, 2001) which permits the
growth of photobionts in situations which might