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WWW.POTATOGROWER.COM 43 HOW DO PCN INFECT PLANTS? Although many animals that feed on plants are simple grazers, or herbivores, which destroy plants as they feed, potato cyst nematodes, like many of the most damaging plant pathogens, are biotrophic and need to keep their host alive while they feed. The life cycle of the potato cyst nematode starts with cysts—the survival stage of the nematode. Each cyst contains several hundred eggs, and each egg contains a dormant juvenile nematode. The eggshell is an extremely tough, resilient structure that protects the dormant nematode from environmental extremes such as cold temperatures and from fungi and bacteria in the soil. It is this structure that allows the nematode to persist in the field. The nematode is activated by the presence of chemicals produced by the growing roots of plants that the nematode can infest. This response is specific; chemicals from plants that the nematode cannot infest do not activate the nematode. Following detection of these root exudates, a series of changes in the eggshell are induced that make it permeable to water and subsequently lead to the nematode becoming active, and it hatches from the egg. The nematode then moves through the soil and, following chemical gradients to the host plants' growing root tips, invades the root. Perhaps the most remarkable aspect of PCN biology is its feeding behavior. Once inside the root, the nematode migrates destructively through the plant tissues until it finds a specific cell type. There, the nematode induces the formation of a large, metabolically active and multinucleate feeding site—the syncytium. This syncytium is formed by controlled breakdown of the plant cell walls between the initially selected cell and its neighbors, followed by fusion of the protoplasts. This process is repeated until up to 300 cells are gradually incorporated into the syncytium. The nematode can only induce one syncytium and relies on this structure for all the food required for the rest of its development to the adult stage. The details of how the nematode is able to induce this feeding structure are still sketchy, although it is known that the nematode is able to produce mimics of key plant peptide hormones which play important roles in controlling cell fate. Once the feeding site is established, the nematode remains there and feeds, developing through a series of molts to the adult female or male. Females remain attached to the feeding site and grow until they burst through the root surface. Males retain the worm-like body structure, leave the root and find females they mate with. After fertilization, the female dies and her body wall forms the cyst, which encloses the next generation of eggs in the soil. Although the ability to induce these feeding sites is a remarkable adaptation, the fact that the nematode can only induce a single feeding structure is a vulnerability that plant breeders seek to exploit. In a resistant plant, the presence of the feeding structure is detected and a localized cell death response, which either kills or isolates the feeding structure, is triggered. Knowledge of how this process is induced and controlled provides information that can be used to develop more durable resistance. Resistant potatoes will provide an effective solution to producing potatoes on infested land and ultimately will restore infested land to being PCN- free. This article first appeared in the Globodera Alliance's November 2017 newsletter. Geneticists Chuck Brown (left) and Roy Navarre examine diverse potato lines. One of several ways to battle the pale cyst nematode that can infest potatoes is to fi nd genes conferring resistance to the pest. Photo by Stephen Ausmus