plant life vs organism

plant life and organism both are nouns.

Word	Noun	Adjective	Verb	Adverb
plant life	Yes	No	No	No
organism	Yes	No	No	No

As nouns, organism is a hypernym of plant life; that is, organism is a word with a broader meaning than plant life:

plant life: (botany) a living organism lacking the power of locomotion
organism: a living thing that has (or can develop) the ability to act or function independently

Other hypernyms of plant life include being.

plant life (noun)	organism (noun)
(botany) a living organism lacking the power of locomotion	a living thing that has (or can develop) the ability to act or function independently
	a system considered analogous in structure or function to a living body