Chapter 1

Introduction

This compilation of electron micrographs is from a series of previously published studies of various anatomical regions of juvenile stages of the soybean cyst nematode, Heterodera glycines Ichinohe, 1952. These previous histological studies of host-parasite interactions provided insight on the extent and nature of root damage caused by endoparasitic nematodes such as the soybean cyst nematode. The penetration of roots by infective juveniles and the subsequent stimulation of a food reservoir in the form of a syncytium readily show how the tissues of roots were modified to the detriment of the host and to the benefit of the parasite.

As the nematode feeds, syncytia develop from stepwise dissolution of contiguous cell walls, which results in a multinucleate cytoplasmic food reservoir. Although subsequent damage to the root system was observed with light microscopy, the details of cell-to-cell interaction in plants and nematodes were revealed only by transmission electron microscopy. Considerable effort was placed on the host reactions to infection as well as the ultrastructure of infective juvenile nematodes.

Various regions and systems of the infective stage of Heterodera glycines have been studied and illustrated in several journals in recent years. This atlas consolidates information on the ultrastructure of early juvenile stages of the soybean cyst nematode. The atlas is an educational tool for those interested in nematode morphology and a reference for researchers interested in applying physiological studies to functional units of the nematode. In bringing together information on the ultrastructure of H. glycines and on its early development, the ultrastructure of the modified host tissues is also included. A list of more detailed work on host responses and nematode anatomy is provided in the Additional Reading list.

Because this atlas is limited to second- and third-stage juvenile nematode anatomy and initial stages of host-parasite relations, many illustrations available in the literature are omitted. Other papers related to the soybean cyst nematode are listed under Additional Reading; many of these are outstanding works by other researchers.

Overview of Host-Parasite Relationship

Figure 1 is an example of field symptoms of soybeans infected by the soybean cyst nematode, Heterodera glycines. A segment of infected roots (fig. 2) shows white-to-brown spherical bodies of adult females, indicating the reproductive potential of the nematode. Histological sections near the nematode feeding sites of infected rosots show cortical penetration by the nematode (fig. 3) and extensive regions of the vascular tissues that are replaced by large multinucleate food reservoirs called syncytia. The syncytia (figs. 4–8) induced by the developing nematode provide nourishment for the nematode at the expense of xylem and phloem development. The destruction or replacement of normal vascular tissues leads to the stunting of soybean plants, which is characteristic of field infestations throughout the soybean-growing regions of the United States and other countries. The planting of resistant cultivars in fields infested with the nematode is considered one of the most effective and economical means of controlling the soybean cyst nematode. Figure 9 shows one of those resistant cultivars.

Figure 1, Figure 2, Figure 3, Figure 4, Figure 5, Figure 6, Figure 7, Figure 8, and Figure 9.

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