ABSTRACT
Healthy plants grow in an atmosphere crowded with fungal spores, bacterial cells, and viruses; in soil, healthy roots predominate despite the high numbers of fungal spores, bacterial cells, and nematodes that thrive in the rhizosphere (soil immediately around the roots of the plant). In the face of this onslaught of potential pathogens, plants defend themselves with an arsenal of weapons, and, as a result, most plants are resistant to most pathogens. Plants have developed defense strategies that successful pathogens must overcome. Although plants defend themselves against potential pathogens in different ways, scientists have categorized strategies into the following two basic categories: passive (present before pathogen recognition) or active (induced after pathogen recognition by the host). Knowledge and exploitation of host defenses can lead to new pathogen control strategies (e.g., fungi-cides that turn on resistance, transgenic plants that can silence viruses, plants that overproduce bioactive natural products, or plants that produce antimicrobial proteins).
