There is evidence of fungi appearing over 1 billion years ago [1].
Plants first appeared around 400 million years ago or later [2]. In fact, early plants required symbiosis with fungi to grow. Even now, plants grow better if they have symbiosis with fungi and most plants (such as tomatoes) can grow symbiotically with fungi. But it is cheaper to use fertilizers, and those are used instead.
Still, forest ecosystems still depend on fungi and require them as a way to recycle plant material (fallen leaves, dead plants and trees).
Evolutionary, fungi existed well before plants managed to evolve.
Chemoautotrophs are understood to be the earliest form of life on Earth, but photosynthetic bacteria date back 3.4 billion years. Cyanobacteria show up in the fossil record some 700 million years later (oxygen producing photosynthetic bacteria) and 300 million years after that, oxygen existed in sufficient quantities in the atmosphere to appear in the geologic record.
The common ancestor basically tells us who we are related to more, but does not tell you when they branched out. So we are more closely related to fungus than to almonds, and that is what we learn from the tree of life. But the common ancestor of plants was born after the common ancestor of fungus.
In Human analogy you could think of it like that cousin uncle (non native speaker, so don't know what if that is the right term) who was born after you, but in the family tree will be shown before you.
Edit: Googled a bit. I have a first cousin once removed, who was born after me (referred to as cousin uncle above)
I thought: "how bad can it be?" but that really is a rabbithole of almost limitless depth. It really drives home just how many species there are that we have no pictures of. And how far you have to zoom in to find anything you can recognize.
My guess is that simosx is using the term "plant" in the strict/colloquial sense, not the either Plantae kingdom (so not including things like cyanobacteria).
We probably lack a big chunk of the whole picture. Phylogeny is based in fossils, but as most fungi are typically soft, lack of characteristic fruits or wood only a few were fossilized. Even worse, most micelia are indistinguishable from other fungi unless you pick a microscope, thus much less fossil diversity and branching is expected.
Well they work on their own timeslines :) I had to cut down a tree about 6-7 years ago. The leftover stump at the base couldnt be removed without major excavation due to the elaborate root network. That stump has attracted all kinds, sizes, shapes and colors of fungi over the years and they are still busy working on it.
It already is a selling point used by many companies, primarily those targeting marijuana cultivators. "Photosynthesis Plus[1]" is a one such product. I have used it to grow both tomatoes and pot. It does not replace the need to use fertilizer (or compost, etc.) so at least for the home grower, there is no cost savings. At scale it could allow you to use less expensive types of fertilizer or something, I am not sure.
Theoretically it helps to break down organic matter into forms more readily bio-avialible to plants. In my experience I'm not sure I've noticed much difference, although I did see some mushrooms popping out of the side of one of my fabric pots once.
Products like this should be looked at with the same skepticism as "Audiophile grade capacitors" and such. If you want to smell the absolute worst stench -- like a transcendentally rotten smell -- I'd definitely recommend paying $20 for a bottle of germs at your local hydroponics shop.
Marketing BS aside, myorrhizal associations[2] are pretty fascinating, and relatively understudied.I enjoy the analogy sometimes used of it being "the internet for plants[3]."
There already are some product like this. At least for small scale gardening. Not sure about cost difference since I do not buy fertilisers.
I've already used it for about 40 tomato plants. But not sure if it really works (or at least that particular product). Will have to try with some control group next time.
Plants first appeared around 400 million years ago or later [2]. In fact, early plants required symbiosis with fungi to grow. Even now, plants grow better if they have symbiosis with fungi and most plants (such as tomatoes) can grow symbiotically with fungi. But it is cheaper to use fertilizers, and those are used instead. Still, forest ecosystems still depend on fungi and require them as a way to recycle plant material (fallen leaves, dead plants and trees).
Evolutionary, fungi existed well before plants managed to evolve.
1. https://en.wikipedia.org/wiki/Evolution_of_fungi
2. https://en.wikipedia.org/wiki/Evolutionary_history_of_plants