Why are plants not sentient beings?

Why in Buddhism plants are not recognized as sentient beings? Can it be immoral to kill a plant? Can a plant be reincarnated as an animal? Is a plant alive? **Edit:** I read somewhere a theory according to which Buddhism did not consider plants as sentient beings, part of saṃsāra, because although they are alive they do not have the 5 khandhas and especially they do not have consciousness (*viññāṇa*) although they have perception (*saññā*). A sunflower perceives the sun and reacts by turning towards it but it is not conscious of its perception, it has no metacognition. I found this idea interesting but I don't know how true it is since Buddhism seems to define consciousness more as what allows discrimination than as what allows metacognition.