How to update a plain-text email by using REST API?

Is there any way to update a plain-text email by using REST API?
HTML email portion can be updated via "POST /rest/asset/v1/email/{id}/content/{htmlId}.json", but it seems there is no endpoint to update plain-text portion.

We need to update more than 400 emails, so we need REST API solution.

I don't want to use "POST /rest/asset/v1/email/{id}/fullContent.json", since this will break a relationship between HTML portion and it's template.