Penetration and Feeding

As mentioned in Sect. 1.1, plant-parasitic nematodes exhibit several feeding strategies. However, for all species, feeding is dependent on the use of the hollow, needlelike mouth spear, or stylet, which is inserted into a cell in order to extract its contents. A framework of definitions for biological function is rarely exact; however, some general groupings can be made that reflect the different feeding strategies.

Sedentary endoparasitic nematodes enter host roots, set up a feeding site within the root tissue and feed internally. As with other aspects of plant-parasitic nema-tode biology, the focus on feeding by endoparasitic species now and in the recent past has been on cyst and root-knot nematodes. However, other nematodes, such as Nacobbus, set up a feeding site as a nutrient sink (Manzanilla-Lopez et al. 2002) and may become important research subjects, especially as comparative genomics progresses.

The convergent evolution of cyst and root-knot nematodes has resulted in the same outcome, feeding on nematode induced nutrient sinks, but the method of achieving the end result and the feeding sites themselves show interesting differences. The preferred invasion site for both groups is behind the root tip in the zone of elongation. J2 of cyst nematodes, with their more robust stylets, cut cortical cell walls and migrate through cells until they reach the differentiating vascular cylinder. This intracellular migration causes considerable damage to host tissue, resulting in necrosis from the invasion point to the feeding site. By contrast, root-knot nematodes migrate intercellularly. After invasion, J2 of root-knot nematodes move towards the root tip until they reach the root apex where they turn around, avoiding the barrier of the endodermis, and migrate back up the root until they reach a site near the vascular cylinder.

Cell wall-degrading enzymes from the subventral glands are secreted through the stylets of both cyst and root-knot nematodes to facilitate migration by weakening or breaking down cell walls. Among the enzymes identified in both nematode groups are cellulases and pectate lyases, and in root-knot nematodes xylanase and polygalacturonase (Davis et al. 2000; Gheysen and Jones 2006). Before the discovery of these enzymes in nematodes, they had been reported only from plants and pathogenic bacteria and fungi. They are not present in non-parasitic nematodes or other invertebrates and it is likely that they were acquired by horizontal gene transfer from bacteria to plant-parasitic nematodes (see Sect. 1.2).

Detailed information of the induction and maintenance of the feeding sites for cyst and root-knot nematodes are given in Chaps. 4 and 5 respectively, this volume. Briefly, a cyst nematode selects a cell, becomes sessile, and a multinucleate feeding site, termed a syncytium, is formed by gradual incorporation of hundreds of adjacent cells as the intervening cell walls disintegrate. Root-knot nematodes become sessile and induce the formation of several binucleate cells followed by mitosis that does not cause cell division. These nuclei further divide and several large multinucleate giant cells result and cells surrounding them also enlarge to form a gall or root-knot. In both cyst and root-knot nematodes, secretions from the dorsal and ventral pharyngeal glands play a central role in the induction and maintenance of the feeding sites.

Only the J2 and adult males of root-knot and cyst nematodes are migratory, whereas in Nacobbus, for example, all juvenile stages, the male and the immature vermiform female are migratory, only the mature female being sedentary. All mobile stages of migratory endoparasitic nematodes, such as Pratylenchus and Radopholus, invade plant hosts but do not become sessile and have no fixed feeding site within the plant, moving around and feeding off numerous cells and causing considerable damage to plant tissue.

Semi-endoparasitic nematodes, such as Rotylenchulus and Tylenchulus, become sessile after penetrating the root but only the anterior part of the nematode penetrates and remains embedded in the root tissue, the rest of the body remains outside the root in the soil. Some genera, such as Hoplolaimus or Helicotylenchus, may be either semi-endoparasitic or migratory ecto-endoparasitic, depending on the host. Ectoparasitic nematodes remain outside the roots to feed externally, puncturing individual cells with their stylet, extracting food and then withdrawing the stylet to move on to another cell to repeat the process. Some ectoparasitic species, e.g. Ca-copaurus, become permanently attached to the root by the deeply embedded stylet.

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