Aren't there some religions which believe that
plants possess a soul?
Yes. This is particularly true of certain religions which believe that after we die, we are reincarnated as plants or animals. Many of these religions state that a plant's soul spends its entire life in a state similar to what we are in when we are in deep sleep. When we are in deep sleep, we are not aware of our surroundings, nor are we dreaming. Therefore, many people who believe that plants possess a soul do not have a problem accepting that plants are incapable of feeling pain or a desire to live.
Follow up questions:
How do you know that plants can not feel pain?
What would you do if we hypothetically discovered that plants do indeed feel pain?
Don't plants move their leaves to follow the sun?