The ultimate plants in space movie (unlike Day of the Triffids which takes place on Earth) is Silent Running, a movie made back in the seventies but with excellent sfx for its day and made with a very pro-plant agenda.

The movie is about a space botonist who has the job of preserving the last surviving trees and plants of Earth on a several space ships/greenhouses far away from the pollution and other problems of Earth.

In a real tribute to capitalism, the guy gets the call from Earth that the cost of maintaining the space craft and the plants is too much so they need to terminate the project.

This Wall-E is a human being but his programming tells him to save the plants anyway-result is a major effort to turn his space craft into a modern day botanical Noahs ark.

Excellent movie and predates Wall-E by quite a bit....