Root-knot nematodes are plant-
parasitic
Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson ha ...
nematodes from the genus ''Meloidogyne''. They exist in
soil
Soil, also commonly referred to as earth or dirt
Dirt is an unclean matter, especially when in contact with a person's clothes, skin, or possessions. In such cases, they are said to become dirty.
Common types of dirt include:
* Debri ...
in areas with hot climates or short winters. About 2000 plants worldwide are susceptible to infection by root-knot nematodes and they cause approximately 5% of global
crop loss. Root-knot nematode
larva
A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle.
...
e infect plant
root
In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the su ...
s, 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.
Economic impact
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
monocotyledon
Monocotyledons (), commonly referred to as monocots, (Lilianae ''sensu'' Chase & Reveal) are grass and grass-like flowering plants (angiosperms), the seeds of which typically contain only one embryonic leaf, or cotyledon. They constitute one of ...
ous and
dicotyledonous, herbaceous and woody plants. The genus includes more than 90 species, with some species having several races. Four ''Meloidogyne'' species (''
M. javanica,
M. arenaria,
M. incognita, and
M. hapla'') are major pests worldwide, with another seven being important on a local basis.
[Eisenback, J. D. & Triantaphyllou, H. H. 1991 Root-knot Nematodes: ''Meloidogyne'' species and races. In: Manual of Agricultural Nematology, W. R. Nickle. (Ed). Marcel Dekker, New York. pp 281–286.] ''Meloidogyne'' occurs 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. If root-knot nematodes become established in deep-rooted,
perennial
A perennial plant or simply perennial is a plant that lives more than two years. The term ('' per-'' + '' -ennial'', "through the years") is often used to differentiate a plant from shorter-lived annuals and biennials. The term is also wid ...
crops, control is difficult and options are limited.
''Meloidogyne'' spp. were first reported in
cassava
''Manihot esculenta'', commonly called cassava (), manioc, or yuca (among numerous regional names), is a woody shrub of the spurge family, Euphorbiaceae, native to South America. Although a perennial plant, cassava is extensively cultivated ...
by Neal in 1889. Damage on
cassava
''Manihot esculenta'', commonly called cassava (), manioc, or yuca (among numerous regional names), is a woody shrub of the spurge family, Euphorbiaceae, native to South America. Although a perennial plant, cassava is extensively cultivated ...
is variable depending on cultivar planted, and can range from negligible to serious. Early-season infection leads to worse damage. In most crops, nematode damage reduces plant health and growth; in cassava, though, nematode damage sometimes leads to increased aerial growth as the plants try to compensate. This possibly enables the plant to maintain a reasonable level of production. Therefore, aerial correlations to nematode density can be positive, negative or not at all.
[Gapasin, R.M. 1980. Reaction of golden yellow cassava to '']Meloidogyne
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 worldwide are susceptible to infection by root-knot nematodes and they cause appr ...
'' spp. Inoculation. Annals of Tropical Research 2:49–53. Vegetable
Vegetables are parts of plants that are consumed by humans or other animals as food. The original meaning is still commonly used and is applied to plants collectively to refer to all edible plant matter, including the flowers, fruits, stems, ...
crops grown in warm climates can experience severe losses from root-knot nematodes, and are often routinely treated with a chemical
nematicide
A nematicide is a type of chemical pesticide used to kill plant-parasitic nematodes. Nematicides have tended to be broad-spectrum toxicants possessing high volatility or other properties promoting migration through the soil. Aldicarb (Temik), a ca ...
. 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
A drought is defined as drier than normal conditions.Douville, H., K. Raghavan, J. Renwick, R.P. Allan, P.A. Arias, M. Barlow, R. Cerezo-Mota, A. Cherchi, T.Y. Gan, J. Gergis, D. Jiang, A. Khan, W. Pokam Mba, D. Rosenfeld, J. Tierney, an ...
, other diseases). A high level of damage can lead to total crop loss. Nematode-damaged roots do not use water and fertilisers as effectively, leading to additional losses for the grower. In cassava, it has been suggested that levels of ''Meloidogyne'' spp. that are sufficient to cause injury rarely occur naturally.
However, with changing farming systems, in a disease complex or weakened by other factors, nematode damage is likely to be associated with other problems.
Control
Root-knot nematodes can be controlled with biocontrol agents ''
Paecilomyces lilacinus
''Purpureocillium lilacinum'' is a species of filamentous fungus in the family Ophiocordycipitaceae. It has been isolated from a wide range of habitats, including cultivated and uncultivated soils, forests, grassland, deserts, estuarine sedime ...
'', ''
Pasteuria penetrans
''Pasteuria'' is a genus of mycelial and endospore-forming, nonmotile gram-positive bacteria that are obligate parasites of some nematodes and crustaceans. The genus of ''Pasteuria'' was previously classified within the family Alicyclobacilla ...
'' and
Juglone
Juglone, also called 5-hydroxy-1,4-naphthalenedione (IUPAC) is an organic compound with the molecular formula C10H6O3. In the food industry, juglone is also known as C.I. Natural Brown 7 and C.I. 75500. It is insoluble in benzene but soluble i ...
.
Life cycle
All nematodes pass through an
embryonic stage, four
juvenile stages (J1–J4) and an adult stage. Juvenile ''Meloidogynes'' parasites hatch from
egg
An egg is an organic vessel grown by an animal to carry a possibly fertilized egg cell (a zygote) and to incubate from it an embryo within the egg until the embryo has become an animal fetus that can survive on its own, at which point the a ...
s as vermiform, second-stage juveniles (J2), the first
moult
In biology, moulting (British English), or molting (American English), also known as sloughing, shedding, or in many invertebrates, ecdysis, is the manner in which an animal routinely casts off a part of its body (often, but not always, an outer ...
having occurred within the egg. Newly hatched juveniles have a short free-living stage in the soil, in the
rhizosphere
The rhizosphere is the narrow region of soil or substrate that is directly influenced by root secretions and associated soil microorganisms known as the root microbiome. Soil pores in the rhizosphere can contain many bacteria and other microo ...
of the host plants. They may reinvade the host plants of their parent or migrate through the soil to find a new host root. J2 larvae do not feed during the free-living stage, but use
lipid
Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids includ ...
s stored in the gut.
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. 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''. 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
multinucleate
Multinucleate cells (also known as multinucleated or polynuclear cells) are eukaryotic cells that have more than one nucleus per cell, i.e., multiple nuclei share one common cytoplasm. Mitosis in multinucleate cells can occur either in a coordina ...
to form feeding cells, generally known as giant cells, from which the J2 and later the adults feed. Concomitant with giant cell formation, the surrounding root tissue gives rise to a gall in which the developing juvenile is embedded. Juveniles first feed from the giant cells about 24 hours after becoming sedentary.
After further feeding, the J2s undergo morphological changes and become saccate. Without further feeding, they moult three times and eventually become adults. In females, which are close to spherical, feeding resumes and the reproductive system develops.
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 and the survival stage between crops is generally within the egg.
The length of the life cycle is temperature-dependent. The relationship between rate of development and temperature is linear over much of the root-knot nematode life cycle, though it is possible the component stages of the life cycle, e.g. egg development, host
root invasion or growth, have slightly different optima. Species within the genus ''Meloidogyne'' also have different temperature optima. In ''
M. javanica'', development occurs between 13 and 34 °C, with optimal development at about 29 °C.
Gelatinous matrix
Root-knot nematode females lay eggs into a gelatinous matrix produced by six rectal glands and secreted before and during egg laying. 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. As the gelatinous matrix ages, it becomes tanned, turning from a sticky, colourless jelly to an orange-brown substance which appears layered.
Egg formation and development
Egg formation in ''M. javanica'' has been studied in detail, and is similar to egg formation in the well studied, free-living nematode ''Caenorhabditis elegans''.
[Wood, W. B. 1988 Introduction to C.elegans. In::The Nematode ''Caenorhabditis elegans'', W. B. Wood (Ed), Cold Spring Harbour Laboratory, New York. pp 1–16.] 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 stage of ''C. elegans'' has 558 cells, and the J1 of ''M. javanica'' likely has a similar number, since all nematodes are morphologically and anatomically similar.
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.
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
A cyst is a closed sac, having a distinct envelope and division compared with the nearby tissue. Hence, it is a cluster of cells that have grouped together to form a sac (like the manner in which water molecules group together to form a bubble) ...
, not all eggs will hatch when the conditions are optimal for their particular species, leaving some eggs to hatch at a later date.
Ammonium ion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...
s have been shown to inhibit hatching and to reduce the plant-penetration ability of ''M. incognita'' juveniles that do hatch.
Reproduction
Root-knot nematodes exhibit a range of reproductive modes, including sexuality (
amphimixis),
facultative sexuality, meiotic parthenogenesis (
automixis
''Automixis'' is the fusion of (typically haploid) nuclei or gametes derived from the same individual. The term covers several reproductive mechanisms, some of which are parthenogenetic.
Diploidy might be restored by the doubling of the chromoso ...
) and mitotic parthenogenesis (
apomixis).
Species
* ''
Meloidogyne acronea''
* ''
Meloidogyne ardenensis'' Santos, 1968
* ''
Meloidogyne arenaria
''Meloidogyne arenaria'' is a species of plant pathogenic nematodes. This nematode is also known as the ''peanut root knot nematode''. The word "''Meloidogyne''" is derived from two Greek words that mean "apple-shaped" and "female".Howard Ferris ...
''
* ''
Meloidogyne artiellia''
* ''
Meloidogyne brevicauda''
* ''
Meloidogyne chitwoodi''
* ''
Meloidogyne coffeicola''
* ''
Meloidogine exigua''
* ''
Meloidogyne fruglia''
* ''
Meloidogyne gajuscus''
* ''
Meloidogyne hapla
Northern root-knot nematode (''Meloidogyne hapla'') is a species of vegetable pathogens which produces tiny galls on around 550 crop and weed species. They invade root tissue after birth. Females are able to lay up to 1,000 eggs at a time in a l ...
''
* ''
Meloidogyne incognita
''Meloidogyne incognita'' (root-knot nematode - RKN), also known as the "southern root-nematode" or "cotton root-knot nematode" is a plant-parasitic roundworm in the family Heteroderidae. This nematode is one of the four most common species world ...
''
* ''
Meloidogyne javanica
''Meloidogyne javanica'' is a species of plant-pathogenic nematodes. It is one of the tropical root-knot nematodes and a major agricultural pest in many countries. It has many hosts. ''Meloidogyne javanica'' reproduces by obligatory mitotic part ...
''
* ''
Meloidogyne luci''
* ''
Meloidogyne enterolobii
''Meloidogyne enterolobii'' was originally described from a population collected from the Enterolobium contortisiliquum, pacara earpod tree (''Enterolobium contortisiliquum'' (Vell.) Morong) in China in 1983. In 2001 it was reported for the first ...
'' (= ''Meloidogyne mayaguensis'')
* ''
Meloidogyne naasi''
* ''
Meloidogyne partityla''
* ''
Meloidogyne thamesi''
References
External links
APS ReviewPlant Nematology Lab, University of Leeds
{{Taxonbar, from=Q2215256
Tylenchida
Nematode genera
Plant pathogenic nematodes
Taxa described in 1889
Taxa named by Émil Goeldi