Ultrastructure of feeding plugs and feeding tubes formed by Heterodera schachtii

The development of feeding plugs and feeding tubes formed in syncytia induced by the cyst-forming nematode Heterodera schachtii in roots of Arabidopsis thaliana was examined at the ultrastructural level. The feeding plug was first observed 24 h after selection of the initial syncytial cell (ISC) and was present throughout the entire nematode life cycle. In later stages of nematode development the feeding plug became increasingly robust and infiltrated by fibrillar syncytial wall material while the central part, through which the nematode stylet was inserted, retained an amorphous structure. Neither the feeding plug nor the nematode stylet were observed to penetrate the plasmalemma of the syncytium. After the nematode completed the preparation phase for feeding, the first secretions were released from the stylet orifice and emitted through the plasmalemma into the cytoplasm. They formed uniformly osmiophilic wavy tubes without an electron translucent lumen. The first typical feeding tubes were found 24 h after ISC selection and were composed of an electron dense wall and an electron translucent lumen. The size of a single feeding tube was about 1 x 4 mu m. No difference occurred between feeding tubes formed by male and female juveniles. Frequently, membranes of the endoplasmic reticulum were connected to the wall of feeding tubes. After the nematode completed feeding, the tubes were disassociated from the styler orifice and were dispersed in the syncytial cytoplasm. The feeding tube lumen was filled with cytoplasm and the wall gradually degraded.