Well, if we are to follow the parent's mention of Peter Singer, you could go check out just about any source of preference utilitarianism. Briefly:

An animal has a conscious will to live on, a preference so to say, and those preferences are the basis of moral consideration.

A plant lacks those kinds of preferences (lacking any brain activity - what we can see are mostly just hormonal cell responses and such; things that all living cells have) and thus the plant in itself is not worthy of moral consideration.