Introduction to Fungi, Third Edition

ZYGOMYCETES: MUCORALES
175
but is converted to the active hormone trisporic
acid after diffusion into the mycelium of the
complementary strain (Gooday, 1994). Trisporic
acid stimulates further synthesis of b-carotene
and of the two prohormones, leading to amplification
of its own synthesis by a ‘cascade’
mechanism. The 15 20-fold enhanced synthesis
of b-carotene upon mating of two compatible
strains of B. trispora holds potential for commercial
production of this substance (Lampila et al.,
1985; Sandmann & Misawa, 2002).
The role of the trisporic acid in inducing
b-carotene synthesis and zygophore formation is
widespread in the Mucorales, having been
characterized in Blakeslea, Phycomyces, Mucor and
even Mortierella (Schimek et al., 2003). It is also
known that trisporic acid is involved in the
sexual response of some homothallic Mucorales
such as Zygorhynchus moelleri, Mucor genevensis and
Syzygites megalocarpus (Lampila et al., 1985). The
common nature of the hormones of homothallic
and heterothallic species could also be inferred
from earlier observations of attempted matings
between such forms, either at the interspecific or
intergeneric level.
Zygophores show directional growth towards
each other in response to volatile hormones.
Gooday (1994) has suggested that these may be
the mating type-specific prohormones, methyl-
4-dihydrosporate of the (þ) strain and trisporol of
the ( ) strain.
Thigmotropic reactions
When compatible zygophores make contact, they
become firmly attached to each other and
develop into progametangia. In Mucor mucedo
there is evidence that the cell wall chemistry of
the zygophores is distinct from that of the
vegetative mycelium and that the (þ) and ( )
zygophores are bound together by lectins, i.e.
glycoproteins exhibiting specific binding for
polysaccharides (Jones & Gooday, 1977). In
Fig 7.9 Collaborative biosynthesis of trisporic acid
by cross-feeding of intermediates between (þ)and
( ) mating types of Blakeslea trispora. b-Carotene
(I) is metabolizedby both (þ)and( ) mating-types
via retinal (II) to 4-dihydrotrisporol (III).This is
metabolized by (þ)strainsto
4-dehydrosporic acid and its methyl ester (IV)and
by ( ) strains to trisporol (V).These are converted
to trisporic acid (VI) only after diffusing to the ( )
and (þ) strains, respectively. Redrawn from
Gooday (1994), with kind permission of Springer
Science and Business Media.