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Pathogen Biology

Genus: Meloidogyne

Root-knot nematodes were first reported in 1855 by Berkeley, who observed them causing damage on cucumbers. Until Chitwood's work in 1949, which defined 4 species and one subspecies (M. incognita acrita) within the genus Meloidogyne, the root-knot nematodes were all considered the same species, Heterodera radicola. In an 1887 paper (reprinted in 1892) Goeldi described Meloidogyne exigua, the type species of the genus. From this description, Chitwood obtained the name we currently use for the root-knot nematodes. The name Meloidogyne is of Greek origin, meaning "apple-shaped female." Approximately 100 species of Meloidogyne have been described. The most widespread and economically important species are M. incognita, M. javanica, M. arenaria, M. hapla, M. chitwoodi and M. graminicola. Root-knot nematodes are primarily tropical to sub-tropical organisms, however M. hapla and M. chitwoodi are well adapted to temperate climates.

Like all plant-parasitic nematodes, root-knot nematodes possess a stylet for injecting secretions as well as ingesting nutrients from host plant cells (Figures 9, 10). Nematodes have no internal skeletal framework, and their "skin" or cuticle acts against internal turgor pressure to maintain body shape and aid locomotion.


Figure 9	Figure 10

Unlike most other plant-parasitic nematodes, root-knot nematode females are globose and sedentary at maturity. They range in length from 400 to 1000 µm. Once they establish a feeding site, they permanently remain at that location within the plant root. The root-knot nematode feeding site is actually a group of cells known as "giant-cells" (Figure 11). When a nematode initially penetrates a plant cell with its stylet, it injects secretory proteins that stimulate changes within the parasitized cells. Parasitized cells rapidly become multinucleate (contain many nuclei) as nuclear division occurs in the absence of cell wall formation. This process is considered to be "uncoupled" from cell division. Cells never actually divide into new cells; they just get bigger and contain more nuclear material. This allows the giant-cell to produce large amounts of proteins which the nematode will then ingest. Giant-cells also act as nutrient sinks, funneling plant nutrients to the feeding nematode. The root-knot nematode does not feed from the cells directly. It forms a feeding tube (from the esophageal gland cell secretions), secreted from the stylet into the plant cell cytoplasm, which acts as a sieve to filter the cytosol that the nematode ingests. As the name implies, giant-cells can grow very large in size. Triggered by nematode esophageal gland cell secretions, an increase in the production of plant growth regulators has been demonstrated to play a role in this increase in cell size and division. Root cells neighboring the giant-cells also enlarge and divide rapidly, presumably as a result of plant growth regulator diffusion, resulting in gall formation.

Figure 11

As the female nematode enlarges, its posterior region may break the epidermis of the root, and the eggs are deposited into a gelatinous egg mass (Figures 12, 13 14, 15, 16). Mature root-knot females (pearly white in color) can be observed without magnification. Second-stage juveniles (J2) and males can only be observed with the aid of a microscope (Figures 17, 18). Generally, females of root-knot nematodes have a short "neck," containing their stylet, metacorpus and esophageal gland cells, with a globose body.


Figure 12	Figure 13

Figure 14	Figure 15

Figure 16	Figure 17

Figure 18

The J2s of the root-knot nematode are most commonly encountered in soils and are vermiform (worm-shaped) (Figure 17). They are usually no larger than 500 µm in length and 15 µm in width. This is the only infective stage.

Root-knot nematode males also are vermiform and range from 1100 to 2000 µm in length (Figure 18). They have distinct lips and strongly developed stylets. In addition, they often have visible spicules, for mating, and a blunt, rounded tail. Many Meloidogyne species are parthenogenic or facultatively parthenogenic. This means that males are not necessary to complete the nematode life cycle and viable eggs can be produced by female nematodes in the absence of fertilization. Because of this, males can be rare in a number of species and are only encountered when the nematode population is subjected to an environmental stress.

Root-knot nematodes can be identified to species using a number of techniques, but one common method is perineal pattern analysis (Figure 19). The perineum (the region surrounding the vulva and anus) of female nematodes displays a pattern of ridges and annulations for each species. While some variation does exist among individuals, these patterns are quite consistent within a species. Enzyme profiles and DNA analyses have also been used to identify different species of root-knot nematodes, however, these techniques can take hours or days and are used primarily as research tools and not diagnostic techniques.

Figure 19