Nematodes are microscopic roundworm that can be found abundantly on earth. Most of the nematodes are free-living organism that inhabits marine, freshwater, and terrestrial environment (covers). Majority of nematodes feeds on bacteria and other microbes, while a small part of nematodes lives as parasites on plants. The annual worldwide losses caused by nematode attack on crops are estimated around 11% (Agrios). We can divide plant-parasitic nematode into four group according to how it feeds on plants: migratory ectoparasite, migratory endoparasite, sedentary ectoparasite and sedentary endoparasites.
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Migratory nematodes will move during its feeding time from one feeding site to another feeding site. On the other hands, sedentary nematodes will form a permanent feeding site. Ectoparasite means that nematode feeds on plant cell from outside the host, while endoparasite feed from within the host cell.
Most of parasitic plant nematode feeds on the root tissue except for migratory endoparasite, some of nematodes that belong to this group can be found on the leaves, stem and even seed. Plants have developed sophisticated defence mechanisms to wander off another organism that attacked it. However, there are still a few pathogens that successfully silenced plant immunity.
Zig-zag model is a well-known model for describing plant immunity. This model describes plant recognised pathogen-associated molecular pattern (PAMP) through it surface localized receptor (PRR). This phase will lead to a broad resistance against pathogen called PAMP-triggered immunity (PTI). Pathogen then produces molecules, called effector, that enable pathogen to overcome PTI resulting a phase called Effector triggered susceptibility (ETS). Subsequently, plant used intracellular receptor (R protein) to recognize effector, resulting Effector triggered immunity (ETI). Again, pathogen will try to overcome ETI either by developing new effector or modify the recognized effector.
These whole models indicate that the battle between plant and pathogen is a never-ending process, plant always tries to develop immunity against pathogens while pathogen will develop new strategies to evade plant immune system.