Q:

Can we use "next of kin" for things as a metaphor?

Wheat has been man's next of kin.

Does this sentence make sense to native English speakers??
It's supposed to be a simile, meaning wheat is like family to humans.
It's a translated sentence from a Persian text.

A:

This sounds very strange to me. "Next of kin" describes a close blood relationship, which typically carries legal obligations like inheritance, medical decision making, etc. To say that wheat is man's next of kin seems like a massive overstatement of that relationship - there is no genetic link whatsoever, and it would be unusual to have a very close personal "kinship" bond with a plant that can't really do anything except sit in place and grow.
You might be able to jokingly say "my dog is my next of kin", since that at least carries some form of personal bond that would be reminiscent of kinship. But to say that a species of plant is the next of kin of a species of animal implies a personal, genetic, or societal link between the two, none of which are really the case. Mankind is not rearing wheat to one day take its place, which is what one generally does with a next of kin.
In sum, "next of kin" can be used in a non-literal sense, but it carries the connotation of a close personal bond, as well as the sense of the succession of a lineage. Wheat seems far too dissimilar to a person or a normal "kin" relationhip to aptly describe it as man's "next of kin" (unless you happen to be writing a book about a post-apocalytic world that is dominated by wheat).

