I wonder how,--- or if this works for grafted plants?

Good point! Grafted plants with top half from "Harry" have roots from "George". T he root fungi must communicate with George. Maybe the question is: "Can Harry tell George that aphids are biting Harry?"

One thin g that must help is that (I think, I guess) that the species and variety of root fungi that associate with plants prob ably is pretty constant through the whole species - probably,. maybe. Thus Harry and Gedorge would BOTH be used to communicating with the same varieties of root fungi.

It would be really cool if they could discover different "dialects" of root / root fun gi communication!

May I recommend a book by William Cullina titled "Understanding Perennials". It is outstanding, and covers this topic, pages 45 to 47 "my fungus , my friend".

I found it in half price books for about $7, and feel I should send him a couple of bucks. It should be on your shelf.