This is how Pythor reveals he planted the Plunderseeds goes in My Little Pony Transformers Prime: Prince Optimus Prime.
[The Autobots and Mane Six come out of the Everfree Forest when Pythor shows up]
Pythor P. Chumsworth: Bravo, ladies and Autobots, bravo! How ever did you save the day this time? Blast the beastie with your magic necklaces, I presume? Where are those little trinkets of yours? You know, the ones you use to send me back to my extremely uncomfortable stone prison?
Applejack: [sigh] Gone.
Pythor P. Chumsworth: Gone? Gone?
Fluttershy: We gave them up.
Ratchet: For the sake of Ponyville and ourselves.
Pythor P. Chumsworth: Well, that seems kind of weird.
Twilight Sparkle: But your friendship with me and Celestia remains. And if you want to remain friends, you'll stop thinking whatever it is you're thinking and help us clean up.
Pythor P. Chumsworth: Fine. [holds up a picture with a window on it] But I don't do windows.
Applejack: One thing I don't get. Why did all this happen now?
Bulkhead: I was kinda thinking the same thing.
Pythor P. Chumsworth: I have no idea. Those seeds I planted should have sprouted up ages ago.
Optimus Prime: What did you say?!
Twilight Sparkle: Optimus!
Pythor P. Chumsworth: Oh, why should I try to explain it when you can see for yourself? [hands Optimus the potion with his tail]
Twilight Sparkle: Just let him show you!
Optimus Prime: [gulping]
[Optimus drinks the potion and his eyes turn white. Flashback to when the Princesses first imprisoned Pythor]
Princess Celestia: Play time is over for you, Pythor!
Pythor P. Chumsworth: Oh, I doubt that. [munching] Hungry? [voice over] Well, obviously things didn't go according to my original plan. My plunderseeds should have stolen the magic from the Tree of Harmony and captured Princess Celestia and Princess Luna thousands of moons ago. Alas, it seems the tree had enough magic to keep the seeds from growing up big and strong. Until now, that is.
Optimus Prime: You realize this is information we could have used hours ago?!
Pythor P. Chumsworth: And rob you of a valuable lesson about being prince? What kind of friend do you think I am?
[Pythor snaps his fingers, snapping Optimus out of his flashback. Later]