I think the focus on sentience is misplaced. Rather, the value should be rationality, meaning here the ability use one's mind to survive and to live by reason alone. This is valuable to any other rational living creature because rationality allows one to eschew violence and live cooperatively, so it is in the best interest of every rational creature to, at least on a basic level, value and respect the life of any other rational creature. The reason we don't apply and respect rights to animals and plants is because they are incapable of doing the same to us, or even being aware of the concept of 'rights'. You should interact with other forms of life on the highest level it is capable of interacting with you.