It is true that plants have defenses when attacked or injured, but I'm not convinced that this indicates sentience.
For example, if someone were in a coma, their immune system would still function. The immune response is obviously important for survival, but it doesn't involve any consciousness.
Another thought: suppose a plant could feel and be aware. What good would this do a plant? Unless a plant were able to move away from an animal who was eating it, or maybe fight them off, there would be no benefit to being aware of being munched on. Even animals which have little or no ability to move seem to have less going on, neurologically speaking, than animals who can move.