Caroline,
I re-read a couple of pages in a microbiology textbook, but it's going to take a few more tries to sink in. And probably that text is already out of date. It addressed many kinds of root-fungi symbioses, and vegetables would have been only one or two of those different kinds.
The impression that I got was that it was one-to-many: one genus or species of MR would be acceptable to (or best for) several groups of vegetables. That's just an impression from one general text, so take it with a grain of salt.
But it supports the practice of several vendors who sell various mixes of "generic" MR spores for various applications. The plants will accept the ones they work best with, or the fungi will invade those roots that they work best with. Then the "winners" will multiply and the "losers" will revert to spores (or maybe just die off).
Even back when that text was written, some 3-way and 4-way symbioses were known and documented -
1. plant root
2. first MR
3. first bacterium
4. another MR or bacterium or fungi
Perhaps by now they have learned that it isn't so much that certain 3-way and 4-way combinations "work". Perhaps now the belief is that soil life is more like a very busy orgy with thousands or millions of POSSIBLE interactions, plus not-yet-fully-understood mechanisms that favor BENEFICIAL interactions.
The co-evolution of roots and MR goes WAY back. I've always been impressed and amazed that soil and roots are smarter than I am - provided with minimal water and minerals and organics, the living components always interact to increase the fertility and support more plant growth.
And they do it without scientific journals and planning meetings and budgets. I think soil is smarter than everybody!
There was a suggestion in the textbook that plant roots alone would have had a hard time colonizing sterile soil as life started to move from sea to land. The author suggested that root-MR symbioses were probably as old as vascular plants on plant. I forget how many millions or billions of years old the oldest known root-MR micro-fossil was.
However, that argument ignored things like lichens. Lichens could have covered the earth an inch deep before any vascular plant moved inland, for all I know. MR might then have evolved over hundreds of millions of years.
P.S. I'm no longer as sure that most MR can live in soil independent of plant roots. Maybe it is only the spores that persist. And there are some MR that biologists were having trouble culturing alone in sterile Petri dishes without plants or additional 'helper' microbes.
P.P.S. I think it's cool that there are something like ten times as many microbial species living in soil than microbiologists can get to live in lab conditions (which I have now learned to call " gnotobiotic ").
If the lab environment is so simple that every species present is known, 90% of soil life won't live there. They are THAT interdependent!
