I think animals can suffer. The fact that the animals can't tell us how much they hurt gives us the idea that they don't suffer alot. I also think that humankind wants to conquer earth and all its inhabitants. Killing animals makes us more dominant and puts us at the top where we want to be. I am not saying that killing animals is unreasonable, I am just saying that animals can suffer.
Plants are the same way.
I don't think that in the future there will be a equality between plants, animals, and humankind because we will always need the feeling of being in control of the elements that surround us.