When every tree, flower and shrub on Earth uproots itself, ascends into space, and forms a gigantic forest-like sphere, an arrogant but gifted space captain and his crew must fly an introverted boy who speaks “plant” through its dangerous caverns to its centre, to plead for the return of all plant-life before humanity starves to death.

This is a great idea I hope you write it.

Thank you sharkeatingman, I agree. I've revised it:

When all plant-life on Earth ascends into space, forming a gigantic spherical
forest, a gifted space captain and his crew must journey into its labyrinthine caverns before humanity starves.

Great concept, but the logline is a bit busy and wordy. Try to reduce it to bare minimums, if possible (25-30 words max). Some things do not need mentioning in logline, like the boy speaking "plant". Your hook is the uprooting and the "Space Jungle" concept. Very unique.