J HELDER, G SMANT, A GOVERSE, J BAKKER and A SCHOTS

Department of Nematology, Wageningen Agricultural University, P.O. Box 8123, 6700 ES Wageningen, The Netherlands

Background and objectives
Endoparasitic nematodes exploit their hosts in a sophisticated manner. Best described are the relationships between cyst- and root knot nematodes and their host plants. Both penetrate the root, migrate through the cortical tissue and induce the formation of a feeding site within the central cylinder. The different ways these feeding sites are referred to, syncytia and giant cells, reflect the difference in ontogeny of these sites. Penetration of the root by a preparasitic juvenile and feeding site induction are two separate processes.
Secretions produced by the subventral and the dorsal oesophageal glands and released via the stylet are thought to play a crucial role in both processes. Hatching in potato root diffusate induces J2 to produce stylet secretions in standard pore water (SPW), no such secretions could be collected upon exposure to SPW solely [1]. In this respect the potato cyst nematode distinguishes itself from several other cyst nematodes. Strong artificial agents such as the serotonin analogue DMT are currently used to induce the production of stylet secretions in Heterodera spp. Analyses of naturally induced stylet secretions of the potato cyst nematode Globodera rostochiensis gives new insights in the interaction between the nematode and its host plant.

Results and conclusions
Subventral gland secretions, previously thought to be involved in food digestion within the nematode, were shown to be excreted via the stylet. These secretions include -1,4-endoglucanases (EGase, EC 3.2.1.4), an enzyme that degrades polysaccharides possessing - 1,4-glucan backbones such as cellulose and xyloglucan. EGase is produced endogenously by the potato cyst nematode. This is an unusual ability; previously identified EGase genes, mostly involved in the digestive system of animals, originated from symbiotic microorganisms. A possible function of EGases is cell wall softening. Hence, intracellular migration of the preparasitic J2 towards the vascular cylinder presumably involves both enzymatic and mechanical weakening of the cell wall. Gland secretions are thought to be involved in syncytium induction, too. DNA endoreduplication and the partial breakdown of cell wall between the initial syncytial cell and adjacent cells are among the early events in feeding site induction. Interestingly, a number of similarities can be named between the interactions Rhizobium - leguminous plants and cyst-nematode -host plant. By ballistic microtargeting it was recently shown that lipo-chitin oligosaccharides can change locally the auxin/cytokinin ratio in roots of Vicia sativa. This resulted in reactivation of the cell cycle [2]. We are currently investigating whether similar events take place in the interaction potato cyst nematode - potato.

References
1. Smant G, Goverse A, Stokkermans JPWG, de Boer JM, Pomp H, Zilverentant JL, Overmars HA, Helder J, Schots A, Bakker J. 1997. Phytopathology 87, 839-845
2. Schlaman HRM, Gisel AA, Quaedvlieg NEM, Bloemberg GV, Lugtenberg BJJ, Kijne JW, Potrykus I, Spaink HP, Sautter C. 1997. Development 124, 4887-4895