The traditional interpretation* is that the plant represents the OT worship according to the letter, which offered some temporary respite from sin to the Jewish people, represented by Jonah, but became obsolete and was destroyed by Jesus Christ, the worm according to the Psalmist and the bait that hooked the Devil as we read in Job. He, our Lord Jesus Christ, is also the sun that arose after His resurrection and ascension and Who scorched the Jews with the trials that He send them because they did not repent of their sins and unbelief. Ninevah of course represents the Church of the Nations.
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* There's another interpretation when mapping the story to our spiritual condition... a topic for another thread.
