mycorrhizae tolerance to fertilizer

brateaver at brateaver at
Fri Jan 3 23:39:32 EST 1997

Another idea for study for Michael Roberts: 
  How does the mycorrhizal fungus "know" to help a plant root? Does it
just normally have a go at any root it finds? Does any root welcome it
with open arms? 

   Since myc. are known to involve themselves with plants growing where at
least certain minerals, such as P, are low, does that mean that every
plant in that environment will welcome the "intrusion" of that kind of

  Is there some kind of rapport, something more or less like the Lorelei
song the root hair sings to the Rhizobium, with lectins involved, and
carbohydrates, in N fixing?  Is the root opening its doors, between
epidermal cells, to facilitate entry? or does the fungus provide the
enzymes that opens the doors thru chemical action? or is it just physical

   Why does the VAM enter and do its job inside cells, while the ECTO
types just get immediately busy wrapping themselves so thoroughly that
they take over the job of root hairs?  (Yes, Mr. Grunden indicated the
role of hormones, but what of the myc. fungus' side of the action?)

   Why does the VAM just protrude via the cell membrane, instead of
breaking thru it and exposing its "naked" self to the actual cytoplasm?
and how does it manage to exert such a pressure that it can push the cell
membrane inward? How does it absorb thru, or give thru, the plasma
membrane, and how does this exchange differ from what would be done if the
vesicle were an actual intrusion into the cytoplasm? Does it not WANT to
get right into actual contact with cytoplasm, or is it prevented from
doing so? Why?

   How is the exchange of nutrients carried out? Is osmotic pressure of
some kind involved?or is that impossible at the site? 

   What happens to the endocytotic surface of the plasma membrane at that
point? or is the "indentation" made by the VA vesicle just another form of
vesicle formation in an unusual endocytotic situation?   (This I should
really like to know.)

   What induces the VA vesicle to intrude itself into the cell? Are
enzymes, hormones or other items involved?

   What effect does the mycor. have on the other normal nutrient exchange
provisions of a cell?

   Is there any way to determine the amount of give and take that VAM
offer or demand? At what point is there more of either? and would that
figure, whatever it is, determine how long the intruding fungus is allowed
to continue this effort?

  How long does an individual invading fungal thread keep up this
exchange? At what point does this fungus have to quit doing the job? Does
the cell say it has had enough, and does it kill off the vesicle or just
quit allowing it to exchange?  Then would the vesicle dry up or  does the
cell make provision for dissolving the fungal tissue at that point? What

   Does the fungus maintain the same number of nuclei  it had to start
with, or does its functioning in the nutrient exchange entrain a
multinucleate condition? If so, what nutrient balance prompts this?

   What triggers formation of spores, and where? Do any form inside the
invasion site? or are all spore forming processes restricted to the
mucigel area?

   Is there a limit to the number of vesicles that may form inside any one
cell? What controls this? is time involved? concentrations? Number of
invading fungi per sq cm of root surface?

   Do the root exudates have any effect on the attraction of the fungi
toward the root epidermis? Does the approaching fungus utilize any of the
exudate molecules?
Would there be a variation in exudation sufficient to affect fungal
infection? Variation in amount? or kind of molecule?

   To what extent does the mucigel affect the growth of the fungus? What
organisms in the mucigel affect the intrusion of the fungal hyphae? Are
they helpful to it? or not? In what way, by what mechanisms?

   In photos, the fungi look like streamers making a beeline for the root
epidermis. It would seem that they know where they are going and go
straight toward the desired site. Again, is there a Lorelei song involved?
and response? Is the response mediated by an enzyme or enzymes? Which
ones? What are the reactions involved?

   Why is it that only when the soil is low in certain nutrients that the
fungus "knows" to approach a root? Or is it just looking around for a job
and somehow finds that this particular root needs help: so to speak.

   What triggers the relationship between beneficial fungi like these, to
make them link up with others, to form a web through the soil sites? At
what point do these fungi find the soil so adequately supplied with
nutrients that roots no longer need their help? Could one make artificial
sites to illustrate such a transition of nutrient balances?

  Would a rhizotron help in explaining any of these points? What kinds of
experiments would indicate some answers that I want to have?   

OK here are some 20 ideas. Can you use any of them, or do they suggest
other avenues of exploration? How long would I have to wait to get
answers? I assume there are others equally anxious to know.



B. Rateaver

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