|Larva of root-knot nematode, Meloidogyne incognita, magnified 500X, shown here penetrating a tomato root|
Larva of root-knot nematode, Meloidogyne incognita, magnified 500X, shown here penetrating a tomato root
Root-knot nematodes are plant-parasitic nematodes from the genus Meloidogyne. They exist in soil in areas with hot climates or short winters. About 2000 plants are susceptible to infection by root-knot nematodes and they cause approximately 5% of global crop loss. Root-knot nematode larva infect plant roots causing the development of root-knot galls that drain the plant's photosynthate and nutrients. Infection of young plants may be lethal, while infection of mature plants causes decreased yield.
Root-knot nematodes (Meloidogyne spp.) are one of the three most economically damaging genera of plant-parasitic nematodes on horticultural and field crops. Root knot nematodes are distributed worldwide and are obligate parasites of the roots of thousands of plant species including monocotyledonous and dicotyledonous herbaceous and woody plants. The genus includes more than 60 species with some species having several races. Four Meloidogyne species (M. javanica, M. arenaria, M. incognita, M. hapla) are major pests worldwide with another seven important on a local basis (Eisenback and Triantaphyllou, 1991). Meloidogyne occur in 23 of 43 crops listed as having plant-parasitic nematodes of major importance, ranging from field crops, through pasture and grasses, to horticultural, ornamental and vegetable crops (Stirling et al, 1992). If root-knot nematodes become established in deep-rooted perennial crops, control is difficult and control options are limited. Vegetable crops grown in warm climates can experience severe losses from root-knot nematodes and are often routinely treated with a chemical nematicide. Root-knot nematode damage results in poor growth, a decline in quality and yield of the crop and reduced resistance to other stresses (e.g. drought, other diseases). A high level of root-knot nematode damage can lead to total crop loss. Nematode damaged roots do not utilise water and fertilisers as effectively, leading to additional losses for the grower.
The root-knot nematode life cycle
All nematodes pass through an embryonic stage, four juvenile stages (J1 - J4) and an adult stage. Juvenile Meloidogyne hatch from eggs as vermiform second stage juveniles (J2), the first moult having passed within the egg. Newly hatched juveniles have a short free-living stage in the soil, in the rhizosphere of the host plant. They may reinvade the host plant of their parent or migrate through the soil to find a new host root (Fig 1.1). J2 do not feed during the free living stage, but use lipids stored in the gut (Eisenback and Triantaphyllou, 1991). An excellent model system for the study of the parasitic behaviour of plant-parasitic nematodes has been developed using Arabidopsis thaliana as a model host (Sijmonds, et al, 1991). The Arabidopsis roots are initially small and transparent, enabling every detail to be seen. Invasion and migration in the root was studied using M. incognita (Wyss et al, 1992). Briefly, second stage juveniles invade in the root elongation region and migrate in the root until they became sedentary. Signals from the J2 promote parenchyma cells near the head of the J2 to become multi-nucleate (Hussey and Grundler, 1998) to form feeding cells, generally known as giant cells, from which the J2 and later the adults feed (Sijmons et al , 1994). Concomitant with giant cell formation, the surrounding root tissue gives rise to a gall in which the developing juvenile is embedded (Fig. 1.2ii). Juveniles first feed from the giant cells about 24 hours after becoming sedentary. After further feeding the J2 undergo morphological changes and become saccate (Fig. 1.2iii). Without further feeding they moult three times and eventually become adults. In females, which are close to spherical (Figs. 1.2i and 2.1i), feeding resumes and the reproductive system develops (Eisenbach and Triantaphyllou, 1991). The life span of an adult female may extend to three months and many hundreds of eggs can be produced. Females can continue egg laying after harvest of aerial parts of the plant (Fig. 1.2i) and the survival stage between crops is generally within the egg. The length of the life cycle is temperature dependent (Madulu and Trudgill, 1994; Trudgill, 1995). The relationship between rate of development and temperature is linear over much of the root-knot nematode life cycle, though it is possible that component stages of the life cycle, e.g. egg development, host root invasion or growth, have slightly different optima. Species within the Meloidogyne genera also have different temperature optima. In M. javanica, development occurs between 13 and 34 C, with optimal development at about 29 C.
Gelatinous matrix of root-knot nematodes Root-knot nematode females lay eggs into a gelatinous matrix (GM) which is produced by six rectal glands and secreted before and during egg laying (Maggenti and Allen, 1960). The matrix initially forms a canal through the outer layers of root tissue and later surrounds the eggs, providing a barrier to water loss by maintaining a high moisture level around the eggs (Wallace, 1968). As the gelatinous matrix ages, it becomes tanned, turning from a sticky colourless jelly to an orange-brown substance which appears layered (Bird, 1958).
Egg formation and development Egg formation in M. javanica has been studied in detail (McClure and Bird, 1976) and is similar to egg formation in the well studied free-living nematode Caenorhabditis elegans (Wood, 1988). Embryogenesis has also been studied and the stages of development are easily identifiable with a phase contrast microscope following preparation of an egg mass squash. The egg is formed as one cell, with two-cell, four cell and eight cell stages recognisable. Further cell division leads to the tadpole stage, with further elongation resulting in the first stage juvenile, which is roughly four times as long as the egg. The J1 of C. elegans have 558 cells and it is likely that J1 of M. javanica have a similar number since all nematodes are morphologically and anatomically similar (Wood, 1988). The egg shell has three layers, with the vitelline layer outermost, then a chitinous layer and a lipid layer innermost.
Egg hatching Preceded by induced changes in eggshell permeability, hatching may involve physical and/or enzymatic processes in plant-parasitic nematodes (Norton and Niblack, 1991). Cyst nematodes such as Globodera rostochiensis may require a specific signal from the root exudates of the host to trigger hatching. Root-knot nematodes are generally unaffected by the presence of a host but hatch freely at the appropriate temperature when water is available. However, in an egg mass or cyst, not all eggs will hatch when the conditions are optimal for their particular species, leaving some eggs to hatch at a later date. Ammonium ions have been shown to inhibit hatching and to reduce plant-penetration ability of M. incognita juveniles that do hatch (Surdiman and Webster, 1995).
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