Chapter 2: Literature review
2.2. Morphology and life cycle of root-knot nematodes
Root-knot nematodes are described as cylindrical unsegmented worms that taper towards the head and tail. Depending on the species, stage of development, and sexuality, they vary in size and morphology. The body length of adult males ranges between 791 to 1660 µm, females usually between 419 to 950 µm while infective juveniles have body lengths ranging between 200 to 460 µm (Coetzee, 1956; Whitehead, 1969; Kaur and Attri, 2013; Archidona-Yuste et al., 2018). The mature female nematode of the genus Meloidogyne is characterized by a loss of its worm-like shape and becomes pear-, kidney- or lemon-shaped (Coyne et al., 2007). In comparison to higher animals, root-knot nematodes contain all the critical organ systems except the circulatory and respiratory organs (https://mrec.ifas.ufl.edu/lso/SCOUT/Nematodes.htm).
Their body consists of a transparent covering or exoskeleton known as the cuticle, making it easy to observe their inner parts for species identification (Bernard et al., 2017; Shah and Mahamood, 2017). The body of root-knot nematodes is a typical “tube within a tube” structure in which the inner alimentary canal consists of a digestive tract and gonad, which are surrounded by the outer body wall, which bears a series of dorsally and ventrally placed longitudinal muscles
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that are attached to the hypodermis (Bernard et al., 2017). Dorsal and ventral nerves control the contractions of these muscles for locomotion. A cavity derived from the blastula known as pseudocoelom is filled with fluid (Decraemer and Hunt, 2006; Bernard et al., 2017), a fluid that shelters the reproductive system and other organs. This fluid provides turgor pressure, which sustains the body shape, and serves in the circulation of nutrients and gaseous exchange (Bernard et al., 2017).
Root parasitic nematodes possess a hollow mouth spear (a hypodermic needle-like) called a stylet at the anterior end/head. The stylet is the primary infection structure, and it is adapted for piercing and sucking (Singh and Phulera, 2015; Bernard et al., 2017). When the stylet is injected into the plant host, it punches through the plant cell using a parasitism mechanism and withdraws food and useful metabolites from its host (Bernard et al., 2017). The stylet is linked to the pharynx (esophageal region), atypical tylenchoid esophagus, metacorpus, a narrow isthmus and a posterior glandular basal bulb (Kaur and Attri, 2013; Bernard et al., 2017). The metacorpus serves to transfer enzymes required for primary infection and facilitates the movement of plant nutrients into the intestine. The esophagus is connected to the intestine, while the intestine ends at the rectum and cloacae in the female and male nematode, respectively (Singh and Phulera, 2015). Reproductive structures in females include ovaries, seminal receptacles, a uterus, an ovijector, and a vulva. In contrast, in males, the reproductive system consists of a testis, a seminal vesicle, and a vas deferens opening into a cloaca (Bernard et al., 2017).
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Nematodes are endoparasites; their life cycle is completed inside their host plant, which serves mainly as a source of nourishment and provides a safe haven for reproduction (Caillaudet al., 2008). The life cycle of Meloidogyne spp. consists of six stages that include the egg stage, four juvenile stages (J1, J2, J3, and J4), and an adult stage (Lambert and Bekal, 2002; Osunlola and Fawole, 2014) (Figure 2.1). The eggs are enclosed in a protective jelly-like egg mass outside the surface of galled roots. Briefly, embryogenesis occurs, and the first-stage juvenile (J1) molt takes place within the egg. Therefore, when the egg hatches, the emerging individual is a second-stage juvenile (J2). It is migratory (Bernard et al., 2017) and the only infective stage considered vermiform. In order to survive, J2s are well equipped for parasitism (Perry, 1977). Infective J2 individuals move freely in the soil and are attracted to root tips by root exudates of host plants (Reynolds et al., 2011; Teillet et al., 2013; Bernard et al., 2017). They invade the elongation zone of roots using cell wall hydrolytic enzymes such as cellulases, endoglucanases, endoxylanases, and pectate lyases, secreted into the apoplast from their esophageal or subventral glands (Abad et al., 2008; Perry and Moens, 2011; Bernard et al., 2017).
After root invasion, an intercellular system is formed throughout the aerenchymatous tissues of the cortex as the establishment of a feeding site, and giant cells occur concurrently (Lambert and Bekal, 2002; Massawe, 2014). The galls in the vascular system are formed as the plant responds to nematode parasitism (Bridge et al., 1982). In the absence of host roots, Meloidogyne spp. undergo several biochemical and physiological adaptations by delaying the embryogenesis phase, quiescence, and diapauses, through feeding on its lipid reserves; this prolongs its viability and ensures its survival until host roots are available (Moens et al., 2009). Thus, after the eggs hatch, juveniles emerge and invade the host roots (Perry et al., 2009). After establishing a
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permanent feeding site and becoming sedentary, generally in the vascular cylinder (Palomares- Rius et al., 2017), J2s undergo multiple morphological alterations and molt into third-stage juveniles (J3), then fourth-stage juveniles (J4), before advancing into either female or male adult nematodes (Abad et al., 2003;Thapa, 2018).
The RKNs males are vermiform and, in most cases, exhibit a short life span because Meloidogyne spp. are primarily parthenogenetic. Thus, the males are not necessary for fertilization and reproduction. However, in other cases, males leave the root to copulate with females for breeding, while the female remains intact in the roots (Lambert and Bekal, 2002).
Sexual dimorphism is therefore associated with lifestyle, with the females being sedentary while males are vermiform and motile (Castagnone-Sereno et al., 2013). The female nematode becomes more substantial in size as it matures and begins producing eggs outside the roots (Lambert and Bekal, 2002). Adequate temperature, moisture, and food availability are the major factors that enhance egg hatching (Morris et al., 2011). A female nematode may lay 30 – 50 eggs per day during its life cycle, which generally takes 21 – 28 days to be completed, with a possibility of eight generations per year at 30ºC (Massawe, 2014).
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Figure 2.1.The life cycle of root-knot nematode (Meloidogyne spp.) (Singh and Phulera, 2015).