Biological function [toxins] in fungi ?

Richard Winder rwinder at PFC.Forestry.CA
Sat Jan 27 19:34:18 EST 1996


In article <4ec0vk$glg at strauss.udel.edu>, 
heytler at strauss.udel.edu (Peter Heytler) writes:
>
>  This "garbage-dump" concept of toxins has been around for a long, long 
>time.  It makes some degree of physiological sense, particularly with 
>regard to non-specific resinous irritants such as occur in some 
>Lactarius species.  However, with the highly selective agents such as 
>the halucinogens and, even more so, the complex deadly amanitoxins, this 
>"dump" concept is biochemically and genetically untenable.
>  Biochemically, these compounds are not a polyterpene gunk that might 
>self-assemble.  They are particular isomers of specialized  compounds 
>which must require a sequence of enzymes for their assembly, and 
>considerable metabolic energy. The amanitins are particularly exotic 
>macrocyclic polypeptides of highly specific and genetically conserved 
>amino acid sequences.  Such things, as we know from studies of 
>polypeptide hormones and of bacterial antibiotics, do NOT just drop out 
>of a pot of amino acids.

Actually, recent detox. work has stressed the role of
mycorrhizal fruiting bodies in concentrating inorganic elements like heavy
metals, copper, etc. away from plant roots.  This kind of soil ammelioration is
beneficial to both symbionts.  Work in Japan has focused on a
detox. role for nitrogen deposited in animal wastes and corpses for a whole
succession of mycorrhizal fungi- amorphous polymers would not 
have to come into play for sequestering N.  With toxic secondary compounds, I
think there is a danger of looking at things too narrowly.  What is toxic to 
one creature may in fact be inert or attractive for another, either through 
coevolution or plain chance or a little of both.  The mode of action for 
amanitoxins on fungi vs. mammals, if it exists, would surely be completely 
different, since we would no longer be talking about creatures with livers, 
etc.  Many of these compounds probably serve dual roles.

[Text about conservation/energy cost deleted]

>  For these reasons I am convinced that such toxins are a competitive 
>mechanism for fungi and almost certainly at the mycelial level.  
>Inhibiting bacterial competition and insect larval predation seem 
>reasonable.  Low tissue levels present in (harvested) mycelium have been 
>cited against this. Still, toxin production by mycelium has been well 
>documented. Since most of these toxins are water-soluble, leeching from 
>the mycelium into surrounding medium appears not only likely but 
>functionally vital. Since such leeching cannot occur in the carpophore, 
>its higher toxin content may be expected.

While I agree that some toxic secondary compounds are produced throughout the
life cycle, and that compounds like hydrophobins could prevent leeching in
masses of tissue, I'm sure that others are probably induced or brought to
greater concentrations in the fruiting bodies, to deter some predators while
attracting vectors.  I think all of these compounds require a lot more 
case-by-case analysis before we know which stories are the prevalent ones.
  -RSW


  RICHARD WINDER                    Title: Research Scientist
  Canadian Forest Service           Phone: (604) 363-0773
  Victoria, B.C.                    Internet: RWINDER at A1.PFC.Forestry.CA



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