I'd hazard a guess that that is precisely why it's toxic to plants. It brings to mind things like Lead poisoning for us, because it takes the place of calcium in the body to horrific effect. And it just never really goes away. Meaning it doesn't really react with anything much, since that would reduce it over time. IIUC of course.

That said, I'm sure lead isn't the best example, since I am merely hazarding a guess right now.

For the Eucalyptus plant, sending the gold to the leaves is probably a good way to get the gold out of the way for more important nutrients to get where they need to go. Worst case scenario for the plant, it loses a few leaves. Which is something it already had to deal with before through Koala bears.

Thats not true. Gold won't patina but it can react with things. It can replace other metals in key enzymes which will inhibit their function. You can also use it as a catalyst both heterogeneous (as nanoparticles) and homogeneous (as an organometalic complex).

Roots obviously don't pick up solid gold. Gold salts are toxic to biological systems.