Mycorrhizal fungi and N-Fixation

ecoli at ecoli at
Tue Jun 6 18:21:27 EST 2000

> ==========
> bionet/mycology #4473, from permacltur at, 1850 chars, 17 May 
2000 18:34:18 +0
> ----------
> Article: 7438 of bionet.mycology
> Xref: bionet.mycology:7438
>Dan Hemenway said:
> Greetings:
>  We have been having a discussion on the North American Fruit 
Explorers list
> about mycorrhizal fungi.  One contributor claims that the fungi 
> transfer of Rhizobia from her bean plants to nearby tomatoes where 
> noduated the roots.  She has had the tomatoes checked at a local 
> which also said they were n-fixing nodules and was told, more or less:
>  "Happens all the time."  
>  Yet no one in the know I've been able to contact believes this is 
> While I am n ot quite that absolute, I am skeptical.  Does anyone on 
> newsgroup have info that will dispell my skepticism?
> Dan Hemenway
> Barking Frogs Permaculture Center, Sparr Florida USA

Hello Dan,

I have pondered this for a bit and think there a few things that can 
(and maybe should) be said.I should say right off, I am very sceptical 

(1) Arbuscular mycorhhizal fungi (AMF) are pretty promiscuous so going 
from bean to tomato would not be a problem.
(2) They are non-septate fungi so there are not quite as many barriers 
to tranfer as would be true for a septate fungus.
(3) There are quite a few reports of bacteria within fungal hyphae (just 
as there are of bacteria in apparantly healthy intact plant root cells.

So, potentially there is a mechanism there.

Further there are reports of Rhizobia infecting a non-legume 
(Parasponia) and fixing nitrogen. No nodules are produced and the 
bacterium does not change into the bacteroid form found in fixing 
nodules. The Rhizobium in Parasponia remains within a network of 
"infection threads" but (a big but) these "infection threads" could just 
be fungal hyphae.

Problems seem to be that the Rhizobium Legume association is a pretty 
sophisticated one, just getting a Rhizobium into the plant is very 
unlikely to be enough. The University which said "it happens all the 
time" is being extremely coy about its findings - they should be making 
a fortune out of this discovery.

A few other thoughts. Producing bumps on roots is not quite enough. 
Agrobacterium (a first cousin to Rhizobium) is renowned for producing 
very nasty bumps.
How certain was "the University" about its recognition of Rhizobium ? A 
colony on a plate is not much use.
How certain was "the University" about its measurements of Nitrogen 
fixation ? What method did they use ? Acetylene-ethylene ? Isotopic 
Nitrogen ? All of this takes some time and not a little effort.

On balance I remain unconvinced I'm afraid - but that is not the same as 

Peter Harris

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