Tylenchida Tylenchina Tylenchoidea Heteroderidae (Filip'ev & Schuurmans Stekhoven, 1941) Synonyms: Meloidogynidae (Skarbilovich, 1959) Meloidoderidae (Golden, 1971) Ataloderidae (Wouts, 1973)
|Feeding site||Multinucleate syncytium||Multinucleate giant cell|
|Reproductive strategies||Sexual||Mainly parthenogenic|
|Eggs||Mainly retained in female body||Deposited in egg mass|
|Female body||Becomes hardened cyst||Does not form cyst|
|Hatching factors||From host root exudates||Favorable environmental conditions.|
|Root penetration||Inter- and intracellular migration directly across cortex to zone of cell differentiation.||Longitudinally, mainly intercellularly, toward root tip and then turning into zone of differentiating cells.|
Large sectors of the developing root, including areas that would have become vascular tissue are transfomed into syncytia (Heteroderinae) or giant cells (Meloidogyninae). Syncytia and giant cells have many plastids, mitochondria, ribosomes, increased rough endoplasmic reticulum and enlarged lobed nuclei.
Cell wall protruberances increase the surface area of the cell membrane for flow of solutes from the xylem to the syncytium - the transfer cell configuration (Endo, 1975).
More than 50 genes are upregulated to some extent in the development of giant cells (Meloidogyne) and syncytia (Heterodera/Globodera) (Gheysen and Fenoll, 2002). Both types of feeding cells have the genome amplified as a result of multiple shortened cell cycles; but the processes differ. Giant-cells go through repeated (acytokinetic) mitosis. Syncytia undergo repeated S-phase endoreduplication without mitosis or nuclear division.
The eukaryotic cell cycle has four stages.:
1. Nuclear DNA is replicated during synthesis phase (S-phase).
2. DNA synthesis is followed by an interval called the G2 phase (G=gap).
3. Mitosis occurs, the nucleus divides (M-phase).
4. The interval between the completion of mitosis and the beginning of DNA synthesis is the G1-phase,
In normal cell division, the cell divides (cytokinesis) after the mitosis phase.
In the root-knot nematode (Meloidogyne) feeding site there is repeated nuclear division (S and M phases of the cell cycle) but no cell division; this is called acytokinetic mitosis or karyokinesis without cytokinesis.
In the cyst nematode (Heterodera, Globodera) feeding site, the S phase of the cell cycle is activated but not the M phase. Instead, the cells repeatedly go through the S-phase (endoreduplication) and probably through parts of the G1 and G2 phases, but bypass mitosis.
|The Cell Cycle: modified from Gheysen and Fenell, 2002.|
Since nematodes in the Heteroderidae become sedentary from the late second stage onwards (except for the metamorphosis to males), the feeding site in the plant must be maintained in a condition favorable for perhaps five or six weeks to allow the nematode to fulfill its reproductive potential. Besides stimulation of the cell cycle events, pathogen-triggered immunity (PTI) responses, including activation of the salicylic acid pathway, must be suppressed. The salicylic acid pathway leads to production of active oxygen molecules and hypersensitive cell death. In the Meloidogyninae, a possible candidate for effector-triggered suppression of PTI is chorismate mutase, produced in the nematode esophageal glands. In PTI responses, chorismate is converted to salicylic acid to iniate the defense events. Chorismate mutase from the nematode reduces chorismate, and thus salicylic acid (Smant and Jones, 2011). In Heteroderinae, the Hg30C02 effector protein of Heterodera glycines may be involved in active suppression of host defenses. The same gene occurs in H. schachtii but not in Meloidogyne spp. (Hamamouch et al., 2012).
Gheyson, G. and C. Fenoll. 2002. Gene expression in nematode feeding sites. Ann. Rev. Phytopathol. 40: 191-219.
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