Which Metropolis feature(s) does Plasma require?

I’ve read and seen in some talks that Plasma can be implemented on Ethereum after Metropolis launches. What feature does Metropolis introduce that Plasma is going to require? I figure it is something to do with account abstraction, but I’m not certain the details.

Plasma could have been implemented on Frontier. The only reason it will take until after Metropolis launches is because Plasma implementations will take a while to actually write and test the code for.

Thanks. Got it.

So when Joseph Poon said, “post Metropolis this doesn’t require and changes to ethereum”, that was true, but it was also true to say the same thing subbing “Frontier” for "Metropolis.

Poon quote here