Plants, or at least the overwhelming majority of them do not have a central nervous system which is what animals require to sense pain and emotions. Plants actually have their own evolved system that allows them to detect and attempt to repair structural compromises. Some plants already do react to tactile stimuli. Whatever it is that they may 'feel', it is compelling enough for them to initiate a programmed system of responses. Plants do not need to be conscious (at the level of animal brains) in order to do this. Microbes also have the ability to detect structural compromises. Our own eucaryotic cells which compose our body also have the ability to detect and repair. You have to understand that these are genetically programmed responses that are there simply because they help avoid the destruction of the organism.
But if it turns out they can feel any pain (Which is extremely unlikely), one can hope that in the far future, we could develop some nanobot that can go around "reprogramming" plants and most animals to not feel pain. And for animals that actually require this to survive? Reprogram an alternative defensive method that does not cause pain.
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