Here's an interesting article from Fox News on research that will turn ordinary plants into security devices. The plants turn white when certain elements enter their environment.
Now think about this for writers. Take this to the next level. If technology exists to make plants change in the presence of certain chemicals, what about people? Setup a biological defense against specific aliens. Or even more specific. Say a law enforcement agency has a simple DNA sample. What about plants that could detect that person's DNA?
Think of how this could be used. Rather than a large team of tech experts with sophisticated and complicated equipment, maintenance of the system could be done with a biologist. A story could highlight how this approach could be used at facilities with fewer financial resources. Create plants that, once "fed" a sample of DNA or certain chemicals, it reacts in their presence.
So keep researching and