In this sentence, the word "fashion" denotes a process, a mode of action. It is the way plants such as tomatoes have been genetically modified.
A word's denotative meaning is what is signified or referred to by the word according to the context. The denotative meaning of the word fashion in this sentence is making use of the term's second definition; the first one being, as you know, a prevailing usage, style, or practice in a specific period.
