The Structure of Syncytia

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Infective second stage juveniles of the cyst forming genera of plant parasitic nematodes (Heterodera and Globodera) induce the formation of feeding sites in roots of resistant and susceptible plants. Under special experimental conditions in Arabidopsis thaliana, future males select the initial syncytial cell (ISC) in the pericycle and future females select their ISC in the procambium. After a preparation phase the nematode feeding site is still unicellular. Cytoplasmic streaming and cytoplasm density, amount of endoplasmic reticulum (ER) and volume of nucleus are increased while the volume of central vacuole is decreased and new small cytoplasmic vacuoles are formed. The cell walls are thickened and covered by a layer of callose-like material. The developing syncytia expand along the vascular cylinder. The expansion of the syncytium triggers the proliferation of cambial and peridermal tissues, in a manner similar to secondary growth. Compared with syncytia associated with females, syncytia of males are less hypertrophied and are composed of more cells. Distinctive cell wall openings are mostly found between the few strongly hypertrophied pericyclic syncytial elements. The ultrastructure of both types of syncytia is very similar but shows conspicuous differences in the structure and localisation of cell wall ingrowths.

Cultivars resistant to nematodes share some common reactions upon nematode infection. At the stage of migration and ISC selection there are no essential differences between susceptible and resistant plants. However, at the stage of syncytium induction some resistant plants show a very quick defence based on the hypersensitive response (HR). Many other resistant plants do not exhibit the HR. In these cases resistance develops more gradually during syncytium formation. The resistance can be correlated with changes in symplast and apoplast. In the symplast they usually deal with the ER. Another characteristic change is gradual reduction of cytoplasm density indicating loss of its physiological activity. Changes in apoplast can be responsible for the obstruction of syncytial development. The syncytial cell walls are abnormally thickened and restructured. The cell wall openings are smaller and less numerous. The syncytia are often surrounded by a conspicuous layer of necrotic cells. Syncytia developing in roots of resistant plants necrotise prematurely.