Pretty rad isn't it?! You've got the core idea but I can't help but ackchually you on a couple technical details
Multi-cellular fungi like the mycorrhiza you mentioned typically have genome sizes in the range of 35 million base pairs or more. Fungi in the range of 3.5 million are small single-celled yeasts. For reference, baker's yeast has about 12 million. The second point is that RNA are functional elements almost always derived from some DNA blueprint that can do all sorts of things depending on internal and external conditions. They're very small molecules whereas the diameters of mycorrhizal filaments are large enough to fit all sorts of stuff. It's really astounding the quantity and size of things they can transport--including viruses and other microbes!