Fwd: Re: Mycorrhiza

[Julia Frugoli]

>It is my understanding that the endomycorrhizae, known as Vessicular 
>Arbuscular Mycorhhizae (VAM), have very little host specificity.  In 
>other words, they will form symbiotic associations with many species 
>of plants, across family order and even class divisions.  It is 
>beleived that VAM are very ancient symbiotic relationships as they 
>are found in bryophytes, and pteridophytes.  This is exactly the 
>opposite of the rhizobium root nodules, which in some cases is 
>extremely species specific and with a few exceptions is found only 
>in the legume family (Fabaceae).
>
A clarification--most VAM symbionts are from the fungal order 
Glomales (Zygomycota), and only 200 species have been described, due 
to the difficulty of obtaining a culture with a sexual stage apart 
from their hosts (or even in contact with their hosts).  According to 
a recent review (Barker, Yagu and Delp, 1998, Plant Phys. 
116:1201-1207) there is very little host specificity in colonization, 
but not all colonization leads to mutualistic nutrient exchange.  The 
fact that the number of fungal species present has a measurable 
effect on biodiversity and plant competition (van der Heijden, et 
al.,1998, Nature, 396:69-72) suggests to me that there is both some 
specificity and some positive effect, but as Mark pointed out, 
measuring it is no walk in the park.

Plus, since it's possible to obtain plant mutants that no longer set 
up AM symbioses (Harrison, 1997, Trends in Plant Sci, 2:54-56) there 
is some specific signaling going on.  Interestingly, many of the 
legume mutants that are unable to set up symbioses with rhizobia are 
also unable to interact with AM fungi, suggesting the two processes 
share at least part of a common pathway.

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Julia Frugoli
Asst. Professor
Biological Sciences
Clemson University
132 Long Hall
Clemson, SC 29634

PHONE (864) 656-1859
FAX (864) 656-0435
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