What would happen to the plants if the sun died?

In about 4 billion years from now, the sun will have turned all its hydrogen fuel into helium. As helium turns into other heavier elements, the sun will swell up and totally engulf Mercury first, then Venus, and probably the Earth. Of course, as the sun swelled, the seas would have boiled and the rocks melted, so I don't think there's much chance for the plants. Assuming the unlikely event that humankind has not destroyed itself, we had better be living somewhere else in 3 billion years Phil