clipped from: en.wikipedia.org Acacia trees produce tannin to defend themselves when they are grazed upon by animals. The airborne scent of the tannin is picked up by other acacia trees, which then start to produce tannin themselves as a protetction from the nearby animals. When attacked by caterpillars, some plants can release chemical signals to attract parasitic wasps that attack the caterpillars.
A similar phenomenon can be found not only between plants and animals, but also between fungus and animals. There exists some sort of communication between a fungus garden and workers of the leaf-cutting ant Atta sexdens rubropilosa. If the garden is fed with plants that are poisonous for the fungus, it signals this to the ants, which then will avoid fertilizing the fungus garden with any more of the poisonous plant.
In theory even an organism with a hormonal system instead of a nervous system could be intelligent in some degree, but it would be an extremely slow brain, to say the least.