Roots Engage in Underground Chemical Warfare | Science Blog:
In these experiments, however, root exudates did not kill all of the tested strains of bacteria. One particular strain of Pseudomonas syringae, a bacterium that causes disease in both tomatoes and Arabidopsis, has a seemingly fail-safe mechanism to overcome the plant's defenses. The bacterium not only survives exposure to the antimicrobial substances, it also blocks the plant's ability to produce them.
So plants evolve root antibiotics, then bacteria evolve a response. That's evolution.