"Plants react to external stimuli, are capable of communication during predator attacks. Recent research also demonstrates plants can learn and remember"

But so can robots, and can be in ways that aren't simply programmed by the designer. There's the phenomenon of "emergent behaviour", which has been observed in automata from the days of simple relay- operated obstacle- avoiders - behaviour that shows adaptation, learning, and reactions that were not put in by the circuit designer. No one would call them sentient, and in that their behaviour wasn't intended or foreseen, they can't be dismissed as a simple extension of the sentience of the designer. They appear, in fact, to have some sort of free will. It's worth looking up the work of Stafford Beer, who tried to use the phenomenon as a way of organising industrial production. With some success, even though shareholders don't like tghe idea of unpredictability.

Some greedy reductionists would have it that "intelligent" behaviour is "no more than" an extension of this emergent behaviour in multiple interactions of massively complicated networks. No one has ever suggested how I feel it though. Perehaps I don't.