Changes in the structure of Arabidopsis thaliana during female development of the plant-parasitic nematode Heterodera schachtii

The beet cyst nematode Heterodera schachtii is able to establish a feeding structure (syncytium) in the vascular tissue of roots and shoots of Arabidopsis thaliana. Histological and ultrastructural studies were performed to assess plant responses during the development of juvenile females under monoxenic conditions. After destructively invading a root the nematode selects and pierces a single procambial cell with its styler and transforms it into an initial syncytial cell (ISC) by secretory activity. The first most obvious changes in the ISC occur in the vacuolar system and at the wall. Differentiation of a central vacuole is impeded resulting in the formation of numerous small vacuoles. Multivesicular and paramural bodies are formed. An electron translucent material is deposited on the cell wall. Partial dissolution of the cell wall leads to the formation of a syncytium. At the juveniles' last pre-adult developmental stage the syncytium attains its maximum longitudinal and radial extension, occupying a major part of the central cylinder. Its features are indicative of a very high level of metabolic activity. The hypertrophied syncytium is ensheathed by a peridermal cover in which secondary xylem and phloem elements are interspersed. When females die the syncytia degenerate. The ultrastructural and histological features of syncytia described from roots are also found in syncytia induced in aerial parts of the plant.