A thing being alive (collection of cells being "not dead") and sentient are two different things. If we assume sentience is some kind of concentration of information, then I would assume regular plants' synapses are not complex enough to create a relevant feedback loop of consciousness. Mushrooms and their networks (interconnected myceliums) on the other hand may be a different case.
This is actually a really interesting comparison. From what I have studied, mushrooms behave similar to a neural network. This is one of the reasons they are so good at revitalizing forests and are seemingly unaffected by viruses. Not to mention their benefits on immune regulation in primates.
