While RKNs move intercellularly, CNs move intracellularly both with destinations toward the vascular cylinder, where they establish specialized feeding sites and become sedentary. Another distinction between RKNs and CNs is their mode of migration through the plant. While CNs penetrate roots at undefined locations, RKN penetrate roots mainly behind the tip, in the root elongation zone. Under proper environmental conditions, their infective stage, the second-stage juvenile (J2), hatches from eggs, is attracted to and penetrates roots of hosts. This group includes the most intensely studied PPNs, the RKNs and cyst nematodes (CNs) of the genera Heterodera and Globodera ( Jones et al. In contrast, sedentary endoparasites, as the name indicates, penetrate the host and form an intimate relationship with their host, feeding on the same site for life and becoming sessile as soon as they initiated feeding. In a wide context, these nematodes can all be referred to as lesion nematodes, since they trigger the formation of lesions while migrating inside plant tissues ( Fosu-Nyarko and Jones 2016). Migratory endoparasites, such as the root lesion nematodes ( Pratylenchus spp.), burrowing nematode ( Radopholus similis), rice root nematode ( Hirschmanniella oryzae), pine wilt nematode ( Bursaphelenchus xylophilus), and red ring nematode ( Bursaphelenchus cocophilus), are known to inflict great damage during their migration inside plant tissues and are known for causing severe necrotic symptoms. The endoparasites are further divided into migratory and sedentary depending on their motility after initiating feeding. PPNs are divided into ectoparasites, endoparasites, and semi-endoparasites depending on their location on the host when they feed. Different modes of adaptation and parasitic behavior allow nematodes to succeed in a variety of different habitats, both above- and belowground. Nematodes use their stylets to penetrate plant roots and inject secreted effectors originating from the esophageal glands to establish parasitism on a variety of plant species ( Vieira and Gleason 2019).
A common feature ubiquitous among PPNs is the specialized mouthpart or the stylet, which resembles a hypodermic needle.
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