Subject: (none)

From: [email-masked] (kelly decker)
Date: 8 Feb 1996 08:23:04 -0800

Yves Bertheau writes

>As far as I remember Mycorrhizae are divided into:
>
>ectomycorrhizae infecting trees
>ectendomycorrhizae
>endomycorrhizae infecting several kinds of  plants
>
>these latter are subdivided into:
>
>Endomycorrhizae infecting ericaceae 
>Endomycorrhizae infecting orchidaceae
>Vesicular arbuscular mycorrhizae
>
>In all cases the fungi induce mutualistic symbiosis and
>not parasitic symbiosis. The endomycorrhizae of the orchidaceae
>are necessary to the full development of the Cormus of the orchidaceae 
>(the seeds of the orchidaceae could germinate only for a few days without
>these fungi). As I remember some of these fungi might induce
>(e.g. Tanatephorus cucumeris) a mutualistic symbiosis with the orchidaceae
>while inducing a parasitic symbiosis (under the name of Rhizoctonia solani)
>on other plants...
>

Actually there is evidence orchids parasitize the symbiotic fungus before
they produce first leaves. For example, Alexander and Hadley (1985) showed
that in a Goodyera repens- Ceratobasidium cornigerum symbiosis carbon was
transported from fungus to plant during the protocorm stage, but there is a
true mutualistic stage after the orchid has developed. This case is strange
and interesting because mycorrhizal associations exist along a gradient -
from semi-parasitic to mutualistic.Usually the member that benefits the most
is the fungi, because often there is true mutualism (i.e., plant benefits
from the fungi & vice-versa) only during stress periods. One theory of
mycorrhizal evolution is that the symbiosis evolved from a fungal root
parasite. 

Kelly