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Chapter 7
The Impact of Plant-Parasitic Nematodes on
Agriculture and Methods of Control
Gregory C. Bernard, Marceline Egnin and
Conrad Bonsi
Additional information is available at the end of the chapter
http://dx.doi.org/10.5772/intechopen.68958
Abstract
Plant-parasitic nematodes are costly burdens of crop production. Ubiquitous in nature,
phytoparasitic nematodes are associated with nearly every important agricultural crop
and represent a signicant constraint on global food security. Root-knot nematodes
(Meloidogyne spp.) cyst nematodes (Heterodera and Globodera spp.) and lesion nematodes
(Pratylenchus spp.) rank at the top of list of the most economically and scientically
important species due to their intricate relationship with the host plants, wide host range,
and the level of damage ensued by infection. Limitations on the use of chemical pesti-
cides have brought increasing interest in studies on alternative methods of nematode
control. Among these strategies of nonchemical nematode management is the identi-
cation and implementation of host resistance. In addition, nematode genes involved in
parasitism represent key targets for the development of control through gene silencing
methods such as RNA interference. Recently, transcriptome proling analyses has been
used to distinguish nematode resistant and susceptible genotypes and identify the spe-
cic molecular components and pathways triggered during the plant immune response
to nematode invasion. This summary highlights the importance of plant-parasitic nema-
todes in agriculture and the molecular events involved in plant-nematode interactions.
Keywords: plant-parasitic nematodes, agriculture, crops, genetics, resistance,
Meloidogyne
1. Introduction
Over millions of years, the association of plants and nematodes has resulted in the evolution
of the plant-parasitic nematode. Widely distributed pathogens of vascular plants, enormous
losses in yields have been aributed to the presence of nematodes. The intricate relationship
© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons
Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,
distribution, and reproduction in any medium, provided the original work is properly cited.
between the parasitic nematode and plant has culminated in an “evolutionary arms race”.
Phyto-parasitic nematodes have evolved strategies to suppress host immune responses for
the development of feeding sites. In turn, plants have developed specic molecules to rec-
ognize pathogens signaling the activation of immune responses. Declining use of chemical
pesticides has brought great aention to research in alternative methods of nematode control.
An eective strategy for nematode management involves the utilization and implementation
of nematode-resistant cultivars into crop breeding programs. Currently, genetic sequencing
analyses are widely utilized in the identication of molecular components of nematode para-
sitism and is also used to distinguish nematode-resistant and susceptible plant genotypes.
These detailed analyses have signicantly contributed to our overall understanding of the
dynamic and complex nature of plant-nematode interactions.
2. Nematode morphology
Nematodes are a fascinating, biologically diverse group of organisms. Their ability to adapt
to a wide variety of habitats including; marine, soil and aquatic, provides an evolutionary
advantage for species longevity. Phylum Nematoda is largely distinguished by three major
monophyletic groups including: Enoplia (marine), Dorylaimia (parasitic trichinellids and
mermithids and Chromadoria (nematodes of various environments). Nematodes belong to
the group Ecdysozoa, which comprises animals that can shed their cuticle. Over 30,000 spe-
cies of round worms are found in Nematoda [1] typically ranging in size from 0.2 mm to
over 6 m. Nematode body structure is relatively simple and characterized as limbless, cylin-
drical, and elongated. Essentially the body plan is a “tube within a tube”, the inner tube or
alimentary canal, consists of a digestive tract and gonad which are surrounded by an outer
tube; a body wall containing a series of dorsal and ventral longitudinal muscles aached to
the hypodermis. These muscles are activated by the dorsal and ventral nerves and their con-
tractions allow for locomotion in sinusoidal waves. In plant-parasitic nematodes, a primary
infection structure called a stylet is located at the anterior end of the nematode which is fol-
lowed by an esophageal region that connects the stylet to the intestines. A typical tylenchoid
esophagus consists of an anterior procorpus, a median bulb and the posterior basal bulb. The
median bulb functions in the transfer of enzymes involved in primary infection and facilitates
the movement of plant nutrients into the intestine. Inside of the exterior body wall lies the
pseudocoelom, a unlined, pressurized, uid-lled cavity formed directly from the blastula
surrounding the gut cavity. The pseudocoelom is lled with uid which provides turgor pres-
sure for the entire body containing the internal organs and aides in the transfer of nutrients,
oxygen and metabolic products. The excretory system is composed of four distinctive cells, an
excretory pore cell, a duct cell, one canal cell, and a fused pair of gland cells. Nematodes are
enclosed within an exoskeleton called a cuticle which is secreted by inner hypodermal cells,
and is primarily composed of collagens, insoluble proteins (cuticlins), glycoproteins and lip-
ids. The cuticle plays an important role in movement, environmental protection and growth
and development [2]. The typical male reproductive structures include a testis, a seminal
vesicle and a vas deferens leading to a cloaca, while the female reproductive system is tubular
containing one or two ovaries, seminal receptacles, an uterus, ovijector and a vulva.
Nematology - Concepts, Diagnosis and Control122
3. Evolution of plant-parasitic nematodes
Why do some nematodes become plant parasites? The dynamic association of nematode and
plant host has resulted in plant parasitism which has evolved three times culminating in
substantial benets for nematode survival and development [3, 4]. An existing evolutionary
hypothesis places the origins of these ancient microscopic roundworms around 400 million
years before the explosion of animal phyla (pre-“Cambrian explosion”) [5]. Evidence suggests
the initial presence of plant-parasitic nematodes to have occurred around 235 BC [6] while the
rst described plant parasitic nematodes were reported by Needham who observed symp-
toms of galling in wheat [7]. An agriculturally important species of plant-parasitic nematodes
called root-knot nematodes, were initially identied by Berkeley who observed the presence
of galls on cucumber roots [8].
The plant-nematode association has resulted in the development of specic feeding struc-
tures and secretory products that are involved in host infection and nutrient absorption. Plant
parasitic nematodes are specialized by the stylet and subventral and dorsal esophageal glands
which are considered the most signicant evolutionary adaptations for plant parasitism [4, 9].
Plant-parasitic nematodes utilize a hollow, needle-like, protrusible stylet to probe plant tis-
sue and release an assortment of proteinaceous secretions from the subventral and dorsal
glands which comprises the integrity of the host cell and allow for nematode entry. These
glandular secretions induce cellular remodications that are essential for development of a
metabolically active feeding cell [10]. Among the secretory molecules are a group of carbohy-
drate-active enzymes. Since cellulose is the primary component of plant cell walls, cellulases
(β-1,4-endoglucanases) are secreted to degrade the cell wall which allows nematode entry into
host tissue. Genomic analyses of root-knot nematodes have revealed the presence of a suite
of enzymes called CAZymes (cellulases, xylanases and other glycosyl hydrolase family mem-
bers (GHFs)) [11]. Beta-1,4-endoglucanase genes have been isolated from plant-parasitic cyst
nematodes with catalytic domains belonging to family 5 of the glycosyl hydrolases [12, 13].
Glycosyl hydrolase families G5 and G45 have been identied in plant-parasitic nematodes.
Plant-parasitic nematode GH5 cellulases show closes homologies with bacterial G5s, which
suggests an initial horizontal gene transfer of bacterial G5 cellulases into nematode genomes
during the evolution of the plant-parasitic order Rhabditida (suborder Tylenchina) [14, 15].
G45 cellulases have been found in plant-parasitic nematode Bursaphelenchus xylophilus of the
Aphelenchida order [16]. Phylogenetic analyses have shown similarities in gene structure
between G45 sequences found in these nematodes and ascomycetous fungi which supports
the hypothesis of a horizontal gene transfer event from fungi to nematodes [17].
Plant-parasitic nematodes dier in lifestyles. Some nematodes will invade the plant cells while
others simply obtain nutrition externally. Ectoparasitic nematodes remain outside the host
cells and feed on plant roots while endoparasitic nematodes establish residence within plant
tissue. An example of ectoparasitic nematode is Xiphinema (California dagger nematode)
which transmits the Grapevine fanleaf virus. The resulting viral infection causes tremendous
economic losses in grapes worldwide [18]. Endoparasitic nematodes are further divided into
migratory and sedentary groups. Migratory endoparasitic nematodes move within the root
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and remove cytoplasm killing the host cell while sedentary nematodes become immobile after
the development of a feeding site within the host tissue [19]. Migratory endoparasitic nema-
todes of economic signicance include Pratylenchus spp. (lesion nematode), Radopholus spp.
(burrowing nematodes) and Hirschmanniella (rice root nematode).
4. The impact of plant-parasitic nematodes on crops
Plant-parasitic nematodes are a costly burden in agricultural crop production. Over 4100 spe-
cies of plant-parasitic nematodes have been identied [20]. Collectively, they cause an esti-
mated $80–$118 billion dollars per year in damage to crops [21, 22]. Encompassing 15% of
all identied nematode species, the most economically important species directly target plant
roots of major production crops and prevent water and nutrient uptake resulting in reduced
agronomic performance, overall quality and yields. Nematodes in the order Tylenchida are
pathogens of plants, invertebrates, and fungi and are considered the most important agricul-
tural pests [22].
Of all the important plant-parasitic nematodes, the most successful species are the seden-
tary groups which establish a permanent feeding site within the plant host and obtain nutri-
ents while completing their lifecycles. Sedentary nematodes have a natural advantage over
their migratory relatives due to a fascinating and complex method of host cell transforma-
tion resulting in the development a sustainable feeding structure. Interestingly, with over
4000 described plant-parasitic nematodes, only a small amount produce signicant economic
losses in crops. In a survey conducted on a variety of crops in the U.S, the major genera of phy-
toparasitic nematodes reported to cause crop losses were Heterodera, Hoplolaimus, Meloidogyne,
Pratylenchus, Rotylenchulus, and Xiphinema [23].
4.1. Wheat
Wheat (Triticum aestivum) is the most important cereal crop in the world. A staple food source
for 40% of the world's population, approximately 758 million tons are produced globally [24].
Wheat yields are signicantly decreased by the presence of cereal cyst nematodes (Heterodera
spp.) in the Heterodera avenae group (H. avenae, Heterodera lipjevi, and Heterodera latipons)
which also damage other important cereals including barley (Hordeum vulgare) and oat (Avena
sativa). An estimated 3.4 million in prots are lost each year in U.S. wheat cultivating states
Idaho, Oregon, and Washington [25]. In some wheat elds, the losses caused by H. avenae can
range from 30 to 100% [26, 27]. In addition to cereal cyst nematodes, further losses of wheat
are caused by root-lesion nematodes Pratylenchus neglectus and Pratylenchus thornei, and the
seed gall or ear-cockle nematode, Anguina tritici. An inverse relationship between H. avenae
and P. neglectus was shown on P. neglectus resistant and susceptible wheat cultivars infested
with H. avenae [28] where a reduction in P. neglectus population densities was observed on
both wheat genotypes. Anguina tritici is often a vector for Rathayibacter tritici, a Gram-positive
soil bacterium which associates with Clavibacter tritici causing seed gall [29].
Nematology - Concepts, Diagnosis and Control124
4.2. Rice
Rice (Oryza sativa L.) is a staple food crop for most of the world's population with an estimated
480 million tons currently produced [30]. Plant-parasitic nematodes rank as one of the most
important soil borne pests of rice and may account for annual yield losses of 10–25% world-
wide. Over 100 species of nematodes aect rice production. Meloidogyne spp. is distributed
worldwide and are signicant pathogens of rice and other crops cultivated in temperate and
tropical areas [31]. One of the most important species of Meloidogyne, is M. graminicola, may
reduce rice yields up to 80% [32]. Symptoms of infection manifests as hook shaped galls,
stunting, decreased tiller numbers and poor growth and reproduction [33]. The rice root-nem-
atode Hirschmanniella oryzae, i.e., rice root nematode (RRN), is commonly found in irrigated
rice production systems [34]. Widely distributed, H. oryzae has been reported in Asian coun-
tries such as India, Pakistan, Bangladesh, Sri Lanka, Nepal, Thailand, Vietnam, Indonesia, the
Philippines, China, Korea and Japan [35] and in the U.S states, Louisiana and Texas.
4.3. Maize
Maize (Zea mays) is grown largely throughout the world with three largest production in
North America Asia and Europe [21]. Over 50 species that are known to parasitize corn in the
globally however, the most devastating genera include the root knot nematodes, Meloidogyne
spp., the root lesion nematodes, Pratylenchus spp. and the cyst nematodes, Heterodera spp. [21].
In the U.S., the most economically important species are the lesion nematodes (Pratylenchus
spp.) and the corn cyst nematode (Heterodera zeae). In most cases, symptoms of infection
caused by these nematodes include poor development and leaf chlorosis with minor galling
[36]. The needle nematode Longidorus breviannulatus is associated with stunting in corn and
may cause economic losses in yields up to 60% [37].
4.4. Potato
The potato (Solanum tuberosum) is a member of the Solanaceae family, and is considered
the third most important crop in the world with total world potato production estimated at
over 376 million tonnes in 2013 [38]. Cyst nematodes are prolic pathogens of potato caus-
ing dramatic losses in yields. Globodera rostochiensis and Globodera pallida originate from S.
America and are known pests of other members of the Solanaceae family including toma-
toes and woody nightshade [39]. These nematodes are classied as quarantine pests in a
number of countries including the U.S. and an estimated £ 50m year in prots are lost each
year in the U.K. [40]. Other major plant-parasitic nematodes of potato include root-knot
nematodes (Meloidogyne spp.), and the stem nematode Ditylenchus destructor. Among the
four species of root-knot nematodes that aect potato production in the U.S., the Columbian
root-knot nematode (Meloidogyne chitwoodii) is considered the most important species [41].
In addition to potato, sweetpotato (Ipomoea batatas L. Lam) is a major host for D. destruc-
tor, up to 100% yield losses have occurred in major production regions including the top
producer China [42, 43]
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4.5. Sweetpotato
The sweetpotato [Ipomoea. batatas (L) LAM] has been regarded as a plant of great signicance
throughout human history. Its cultivation dates to the prehistoric era, and it has been grown
continuously as a staple food source. Global productions of sweetpotato is estimated at 105 mil-
lion metric tons [44]. Currently, the sixth most important food crop, sweetpotato produc-
tion has improved the economic status for communities throughout the world particularly in
developing nations where it ranks as the fth most important crop [44]. Approximately 10.2%
of sweetpotato yields are lost each year due to the presence of plant-parasitic nematodes [20].
Root-knot nematodes (RKNs) are signicant pests of sweetpotato causing symptoms of infec-
tion which include: stunted plant growth, yellowing of leaves, abnormal ower production,
and gall production on roots leading to decreased nutrient and water absorption and necrosis
and cracking on eshy storage roots. Due to the economic importance of the storage root,
root cracking is a primary concern for producers. Successful sweetpotato root-knot nematode
resistant breeding programs involve the determination of resistance genes. Nematode resis-
tance is governed by genotype [45] and is primarily quantitative [46]; therefore, the identica-
tion of genetic markers associated with root-knot nematode resistance requires broad scale
molecular studies.
4.6. Root-knot nematodes
In a recent survey, the top 10 most important genera of parasitic nematodes in molecular plant
pathology were ranked based on scientic and economic importance [47]. Ranked at the top
of the list are root-knot nematodes (Meloidogyne spp.). The root-knot nematode (Meloidogyne
spp.) comprises over 100 species, with Meloidogyne javanica, Meloidogyne arenaria, Meloidogyne
hapla, and Meloidogyne incognita representing the most devastating threat to agricultural crop
production [48]. The Meloidogyne spp. are globally distributed, have enormous host range and
develop dynamic disease complexes with fungal species and bacteria which may exacerbate
disease incidences in cultivated plants. The lifecycle of Meloidogyne spp. involves four develop-
mental stages including larval stage 1 (within the egg), larval stage 2 (migratory), larval stage
juvenile 3 (sedentary), larval stage 4 (sedentary) and adult stage (sedentary). Under favorable
environmental conditions, rst stage moulting to J1 larval stage within the egg occurs result-
ing in hatching, with or without the presence of a chemical stimulus. Infective second-stage
juveniles (J2s) are often aracted to root exudates and migrate to root tips where they inl-
trate behind the root cap at the elongation zone. Root knot nematodes aenuate plant cells
by stylet thrusting and secrete cell wall degrading enzymes to separate the middle lamella
during intercellular migration through root cortex cells as they target the undierentiated
procambium cells of the vascular cylinder. During later stages in primary infection, dorsal
gland activity increases to promote shuling of secretory granules to the stylet where pro-
teinaceous secretions are released in the development of the primary feeding site—the giant
cell [49]. The multi-nucleated giant cell is a result of nematode-induced endoreduplication
within the host cell in the absence of cytokinesis. Cellular ingrowths arise to sequester solutes
from the xylem [50] further enhancing nutrient availability. J2 larvae moult into larval stage
3 (J3) during the initial intake of plant nutrients from giant cells. Additional moulting occurs
Nematology - Concepts, Diagnosis and Control126
resulting into the J4 and nal adult stage. Further reproductive development in females results
in the characteristic “apple” shape associated with the Greek nomenclature Meloidogyne. The
lifecycle completes when eggs are released into the soil from the gelatinous egg matrix formed
on epidermal root tissue. Root-knot nematode infection is typically characterized by stunted
growth, wilting, root galling and abnormalities in root formation.
4.7. Cyst nematodes
Cyst forming nematodes, or cyst nematodes, (Heterodera and Globodera spp.) rank second to
root-knot nematode in agricultural and economic importance. The biology of cyst nematodes
is similar to that of root-knot nematodes where J2 larvae infect the host and develop to adult
stages within host tissue. In contrast, to root-knot nematode reproduction where eggs are
deposited into a gelatinous matrix on root systems, eggs produced by cyst nematodes are
preserved within the body of the female and are protected after her death until hatching under
favorable conditions. Cyst nematodes enter root tips and induce specialized feeding structures
in the infected plant roots called syncytia via esophageal gland secretions released by the sty-
let [51]. These secretions promote cell wall degradation and protoplast fusion of numerous
adjacent cells to form the syncytium [52]. In agriculture, the most signicant cyst nematode
species are the potato cyst nematodes Globodera rostochiensis and G. pallida, the soybean cyst
nematode (Heterodera glycines) and cereal cyst nematodes (CCNs) (including Heterodera avenae
and H. lipjevi. In the U.S., losses due to H. glycines is estimated at 1.286 billion [53]. Globodera
pallida originated in South America and is now widely distributed in 55 countries. Yield losses
of potato due to G. pallida range from 50 to 80% in heavily infested soils [54]. Although the beet
cyst nematode, Heterodera schachtii, is a primary pathogen of sugar beets, it can parasitize plant
species in 23 dierent plant families with losses of 30% in the families of Chenopodiaceae [55, 56].
4.8. Lesion nematodes
Ranked third among the most damaging nematodes in agriculture [57], approximately 70 spe-
cies of root-lesion nematodes (Pratylenchus spp.) are distributed worldwide with a host range
of nearly 400 plant species [57]. Among Pratylenchus spp., P. thornei is associated with yield
reductions in wheat by as much as 85% in Australia, 70% in Israel, 50% in Oregon and 37% in
Mexico [58]. Lesion nematodes are migratory, feeding mainly in the root cortex and may
enter vascular tissues obtaining nutrients. Infection typically results in lesion formation and
necrosis on roots with aboveground symptoms of chlorosis as well as reductions in leaf num-
ber and size [58, 59]. Host tissue injury resulting from infection may represent areas for sec-
ondary infection from other pathogens. Recently two new species of root-lesion nematodes
(Pratylenchus. kumamotoensis, Pratylenchus. pseudocoeae) were identied in Korea by morpho-
metric and molecular analyses of internal transcribed spacer (ITS) and ribosomal DNA [60].
4.9. Burrowing nematode
The burrowing nematode, Radopholus similis [(Cobb, 1893) Thorne, 1949] is a migratory plant
parasitic nematode, listed as a quarantine plant pest worldwide [61]. Over 250 plant species
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serve as hosts for R. similis where it causes severe economic losses in yields. R. similis damages
banana, citrus, pepper, coee and other agronomic and horticultural crops and is considered
the most important phytopathogenic nematode in banana-growing areas [62]. Eective con-
trol of R. similis remains problematic worldwide, and eective approaches must be identi-
ed and implemented. Radopholus Calreticulin (CRT) is a Ca2+-binding protein that plays key
roles in parasitism and represents a candidate target for controlling R. similis. R. similis CRT
(Rs-CST) is expressed in the esophageal, reproductive and gastrointestinal regions as well as
the eggs. Using plant-mediated RNA interference, Rs-CRT expression was signicantly inhib-
ited in the nematodes, and enhanced resistance was demonstrated in transgenic tomato plants
[63]. In a bioassay-based study, phenylphenalenones extracted from Musa spp. showed anti-
nematode eects on R. similis which was demonstrated by nematode motility inhibition [64].
5. Nematode parasitism genes
Nematode parasitism is conferred by the actions of a variety of genes that are upregulated
during host infection. In an earlier review, a comprehensive discussion highlighted the struc-
ture, origin and functions of nematode parasitism genes and further supported the acquisi-
tion of parasitism genes through horizontal transfer from bacteria [49]. Since parasitism genes
are usually required for infection, they represent important targets for the development of
control measures. Parasitism genes often encode for eectors which are proteins or chemi-
cals that elicit an immune response and/or trigger changes in the host cell architecture [51].
Recently two eector genes (MhTTL2 and Mh265) were identied in the root-knot nematode
M. hapla and were shown to be upregulated during primary infection [65]. MhTTL2 encodes
for a secreted protein bearing a transthyretin-like protein domain and is expressed in the
amphids, with a potential role in the nervous system while Mh265 is expressed in subventral
glands. Nematode eectors including expansin, β-1,4-endoglucanase and polygalacturonase
are released during primary infection and feeding site development. In plants, expansin pro-
teins are secreted during growth processes to allow for cell enlargement [66]. Nematodes
are believed to cause dierential expressions of plant genes encoding cell wall modifying
proteins including expansins [67] quite possibly to mimic endogenous expansin production
during feeding site development. HaEXPB2, a predicted expansin-like protein found in cereal
cyst nematode Heterodera avenae was associated with cell death in tobacco plants [68]. During
primary infection of tobacco, HaEXPB2 gene expression was localized in subventral glands
of J2 nematodes and was later found in the cell wall. Silencing of HaEXPB2 by RNA interfer-
ence was associated with reduced nematode infectivity. Transcriptome sequencing analyses
of early stage H. avenae juveniles has revealed a variety of potential eectors including plant
cell wall-modifying proteins and homologues of secreted proteins involved in the detoxi-
cation of reactive oxygen species (ROS) including: peroxiredoxin, glutathione peroxidase,
glutathione-S-transferase [69]. ROS release is associated with onset of plant defense signal-
ing. New evidence suggests that root-knot nematodes may utilize plant peroxidase to reduce
ROS levels and parasitize plants bearing the Mi-1 root-knot nematode resistance gene [70].
In plants, pathogens may trigger a hypersensitive response which involves programmed
Nematology - Concepts, Diagnosis and Control128
cell death (a form of apoptosis) in the site of infection to prevent pathogen colonization.
Apoptosis regulator BAX (BCL-2 protein 4) is a member of the Bcl-2 family of proteins found
in plants and animals [71]. Two secretory eector candidate genes (No. 5, No. 100) identied
by transcriptome proling in Meloidogyne enterolobii suppressed BAX-induced programmed
cell death suggesting their roles as plant immune modulators for nematode infection [72].
The SPRY (SPla and the RYanodine Receptor) protein domain is most likely a scaold for
mediating protein-protein interactions [73]. SPRY eectors from Globedera spp. was shown to
suppress the plant defense responses [74].
6. Molecular basis of nematode resistance
The development of a resistance response may encompass a variety of physiological out-
comes including: minor or complete absence of galling, dierences in the degree of necrosis,
the inability of the nematode to establish a permanent feeding site, and a decrease in female
fecundity or egg output. To date, the majority of plant-parasitic nematode resistance genes
bear the characteristic NBS-LRR (Nucleotide binding site—Leucine Rich Repeat) domains.
These include the Mi-1 gene from Solanum peruvianum (formerly Lycopersicon peruvianum)
[75], Hs1pro-1 from sugar beet [76] and Gpa2 and Gro1-4 from potato [77, 78].
Resistance to Meloidogyne in commercial resistant tomato cultivars (Lycopersicon esculentum)
was originally identied in its wild relative L. peruvianum Mill. [79] followed by introgres-
sion of resistance into commercial breeding lines through backcrossing [80]. Several root-
knot resistance gene homologues have been identied in tomato. Mi-1.2 (referred to as
Mi-1) confers resistance to multiple species of root-knot nematodes, [75] the potato aphid,
Macrosiphum euphoribiae [81] and the whitey, Bemisia tabaci [82]. Rme1 is considered a poten-
tial component of the Mi-1-mediated signaling pathway as studies have indicated tomato
Rme1 mutants lack resistance to nematodes and whiteies [66]. Molecular changes in Rme1
protein conformation due to the presence of pathogens, may be recognized by Mi-1.1 which
signals the hypersensitive response in the “guard hypothesis” [83]. In carrots, inherited dom-
inance of two root-knot nematode resistance genes Mj1 [84] and Mj2 [85] conferred resistance
to M. javanica. The RMIa gene located in a subtelomeric position 300 kb physical distance
between AMPP117 and AMPP116 markers and is associated with M. incognita resistance in
peach (Prunus spp.) [86]. The Myrobalam plum (Prunus cerasifera) harbors dominant alleles
(Ma1, Ma2, and Ma3) of a single gene Ma, a TIR-NBS-LRR class resistance gene, which confers
broad spectrum resistance to multiple Meloidogyne spp. [87]. Using polymorphic sequencing
analyses and genetic linkage mapping (RFLP, SSR) the Ma loci was precisely identied in the
Myrobalan plum linkage group 7, while in a Japanese plum variety, a Rjap gene was localized
at the same position in co-segregation with SSR markers previously associated with root-
knot nematode resistance [88]. In sweetpotato, 275 candidate resistance gene analogs have
been identied by degenerate PCR and molecular mining [89]. Plant-parasitic nematodes
have been shown to manipulate host gene expression, therefore the identication of dieren-
tial expression paerns of transcript levels for defense-related genes is a critical component
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in the determination of molecular factors of root-knot nematode resistance. Traditional iden-
tication of root-knot nematode resistance has involved the use of bulk segregant analysis
[90] to map out qualitative traits between pooled plant genomes. Bulk segregant analysis has
been used in tandem with random amplied polymorphic DNA assays to identify molecular
markers at specic loci associated with root knot resistance in sweetpotato where genotypes
are often isogenic [91]. Polymorphic events in resistance genes that confer eector recogni-
tion has been demonstrated in Arabidopsis resulting in a bifurcation that distinguishes resis-
tant and susceptible allele clades [92].
Genome-wide expression proling analyses using next-generation sequencing technologies
are often employed in the analysis of host-nematode interactions. The resultant data from
global transcriptome assays are used to target genetic traits associated with plant immune
responses in response to various pathogens and to distinguish plant genotypes for resistance
or susceptibility to certain diseases. Our general understanding of discreet molecular events
involved in compatible (susceptible) and incompatible (resistant) plant-nematode interac-
tions is limited in comparison to other signicant host-pathogen associations. Recently, RNA-
Sequencing has been frequently used in plant pathological studies to prole gene expression
paerns in host plants and pathogens [93, 94]. Dierential genetic expression proles of many
specic genes involved in plant immune responses has been shown in resistant and suscep-
tible plants challenged by root-knot nematodes [48, 95]. The identication of novel defense-
related transcripts and the elucidation of pathways involved in plant immune responses
to nematodes have been recorded for important economic crops including coon [96], rice
[97], and soybean [98]. Transcriptome proling of resistant and susceptible tobacco varieties
infected with root-knot nematodes has shown dierential expression paerns among genes
involved in cell wall modication, auxin production and oxidative stress [99].
6.1. Plant immune responses
Due to their immobile lifestyle, plants have developed sophisticated molecular strategies
to prevent pathogen invasion [100]. Plant defense has been characterized as a two- prong
approach. In incompatible (resistant) plant-pathogen interactions, the presence of microbial/
pathogen/-associated molecular paerns (M/PAMPs) including: toxins, glycoproteins, carbo-
hydrates, fay acids and proteins can trigger the upregulation of a network of host genes and
corresponding proteins involved in an innate response termed pathogen-triggered immu-
nity (PTI). Plant pathogens have evolved specialized eector molecules to suppress this rst
line of defense leading to eector-trigged susceptibility (ETS). In turn, plants have developed
resistance genes which recognize specic eectors triggering a more robust defense response
characterized as eector-triggered immunity (ETI). A hallmark of ETI is a hypersensitive cell
death response (HR) at the infection site which prevents pathogen colonization [101].
6.2. Reactive oxygen species and antioxidant production
During plant metabolic processes, the accumulation of reactive oxygen species (ROS) by-
products including superoxide anion (O2
−), hydrogen peroxide (H2O2), singlet oxygen (1O2)
Nematology - Concepts, Diagnosis and Control130
and hydroxyl radicals (−OH) is often continuous, as these highly reactive molecules are local-
ized to various cellular compartments. ROS are primarily generated by NADPH oxidases and
superoxide dismutases and production is associated with numerous abiotic and biotic stress
responses. Activation of ROS was shown to be critical during the defense response to root-
knot nematode invasion [102]. ROS accumulation is toxic to nematodes and can often lead to
induced oxidative destruction of infected cells during the hypersensitive response, to prohibit
pathogen colonization. Increased ROS production is often correlated with the activation of
antioxidant gene expression. These oxidative/reduction reactions must be tightly regulated to
eliminate inadvertent plant tissue damage. Antioxidant enzymes including peroxidases are
primarily responsible for the maintenance of a steady-state ROS level however, certain classes
of peroxidases act as producers of ROS depending on the cyclic (catalytic or the hydroxylic)
nature of the enzyme. ROS- producing Class III peroxidase genes were upregulated during
an incompatible reaction in H. avenae-resistant wheat cultivars [103]. Peroxidase reduces H2O2
levels via H2O2-dependent polymerization of hydroxycinnamyl alcohols which promotes
defense responses including lignin synthesis and cell-wall reinforcement by the cross-linkage
of cell wall proteins [104]. Higher induction of peroxidase groups was observed in resistant
plant species during H. avenae and M. incognita infection [105].
6.3. Pathogenesis-related proteins
Presently, 17 families of pathogenesis-related (PR) proteins have been identied based pri-
marily on their enzyme function, activity and amino acid sequence homologies [106]. The PR
family are characterized as plant allergens inclusive of an assortment of proteins such as: b-1,3
glucanases, chitinases, proteinase inhibitors, defensins, ribonucleases and thionins. PR gene
expression is often induced by ethylene, salicylic acid, jasmonic acid, xylanase, and systemin
signaling pathways. The molecular functions of PR proteins are often species specic with
great diversity in the mode of action and structure between protein groups. Most PRs pos-
sess antifungal, antiviral, antibacterial and insecticidal activity and are primarily involved in
plant developmental processes and environmental stress responses. PR proteins were initially
reported in tobacco leaves during a hypersensitive response to the tobacco mosaic virus (TMV)
[107, 108] and have been induced in response a wide variety of pathogens including nema-
todes. During nematode infections, PR transcripts may accumulate in high concentrations and
are associated with the long distance immune response termed systemic acquired resistance
(SAR) [109]. Increased expression of PR-1(P4) transcripts was observed at 3 days' post-infection
in the G. rostochiensis-infected resistant plants compared with the uninoculated controls [110].
6.4. Callose deposition
Cell wall modications often occur during plant-pathogen interactions which are demon-
strated by the deposition of cell wall appositions leading to the development of papillae.
Structural components associated with papillae formation are: callose, phenolics including lig-
nin, phenolic conjugates such as phenolic–polyamines, reactive oxygen species, peroxidases,
cell wall structural proteins (arabinogalactan proteins and hydroxyproline-rich glycopro-
teins) and cell wall polymers (pectin and xyloglucans). Callose (beta-1-3-glucan) deposition,
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lignication and suberization are plant developmental processes further associated with the
restriction of systemic pathogen movement during PTI. Defense-associated cell wall strength-
ening through lignin and callose synthesis is signaled by cell wall degradation in a feedback
mechanism which occurs in response to pathogens [111]. In addition to promoting declines
in localized microbial populations, callose deposition also prevents the translocation of PTI-
suppressive eectors. Interestingly, the cuticular chitin derivatives of plant-parasitic nema-
todes may activate the innate immune response. Although the cuticle is generally believed
to be devoid of chitin, it is possible that chitin derivatives or chitin previously deposited in
the stylet are recognized by the host which activates callose deposition at the site of penetra-
tion. The overexpression of the ethylene response transcription factor RAP2.6 in Arabidopsis
enhanced plant basal resistance to H. schachtii [112]. Increased expression of jasmonic acid-
related genes and callose deposition were observed at nematode infection sites.
7. WRKY transcription factors
WRKY transcription factors are transcriptional regulators of many developmental processes
in plants and are associated with abiotic and biotic stress responses. The WRKY domain is
almost exclusive to plants characterized by a highly-conserved core WRKYGQK motif and a
zinc nger region. The critical role of WRKY transcription factors (WRKY TFs) in plant defense
responses has been well documented [113, 114]. Their ability to bind to pathogen responsive
cis acting W-box promoter elements in PR1 genes is indicative of their role in plant immunity
[113]. Arabidopsis WRKY72 was reported to have a signicant contribution to Mi-1-mediated
defense against RKNs, potato aphids [114] and oomycete pathogen Hyaloperonospora arabidop-
sidis the causal agent of downy mildew [115]. WRKY gene expression is altered during plant-
parasitic nematode interactions. The development of cyst nematode H. schachtii feeding site
(syncytia) involves the up-regulation of WRKY23 [116]. Conversely, endogenous WRKY33
gene expression levels were strongly downregulated in syncytia formed in Arabidopsis roots,
while plants overexpressing WRKY33 showed a 20–30% reduction in the presence of female
nematodes [117] a possible indication of its role in plant defense.
7.1. Calreticulin proteins
In animals, endoplasmic reticulum (ER) localized calreticulin proteins are integral compo-
nents in calcium homeostasis as well as protein folding and are involved in other signi-
cant cellular functions [118]. Ubiquitously expressed in plants, calreticulin performs similar
functions to its animal counterpart despite 50% dierences in amino acid sequence homol-
ogy. Plant calreticulin is described as a molecular calcium-binding chaperone that promotes
protein folding, calcium signaling and homeostasis, and oligomeric assembly in a calreticu-
lin/calnexin cycle. Calreticulin may interact with a majority of monoglucosylated glycopro-
teins synthesized in the ER, while certain isoforms have been associated with the expression
and quality control of the elongation factor Tu receptor-like protein kinase (EFR) [119] an
important event in M/PAMP-triggered immune responses. The signicance of calreticulin
Nematology - Concepts, Diagnosis and Control132
isoform-3 (AtCRT-3) function through gene deletion was identied in Arabidopsis plants [120].
Plant transformants with repressed AtCRT-3 gene activity were impaired in perception of M/
PAMP-associated e-18 and decient in EFR protein expression and anthocyanin content.
Furthermore, they concluded that AtCRT-3 may be involved in the unfolding and activation
of EFR based on its primary molecular function and recognition of EFR N-glycosyl binding
sites. Recently, studies have shown that root-knot nematodes secrete calreticulin, which plays
an important role in infection [121].
7.2. Plant proteinase inhibitors
Plants utilize an arsenal of defensive mechanisms to evade infection from nematodes. One
important strategy involves limiting nematode feeding capabilities. Plant proteinase inhibitors
are involved in many physiological processes including protein turnover and proteolysis dur-
ing metabolism however; other evidence has supported an alternative role in defense against
plant pathogens [122]. Plant proteinase inhibitors degrade nematode proteases preventing
the breakdown of food material which reduces nutrient absorption in the nematode. As early
as 1947, the idea of proteinaceous protease inhibitors was formulated as Mickel and Standish
observed dierences in larval development on soybean cultivars [123]. The applicability of
proteinase-inhibitors in nematode resistance was initially demonstrated in transgenic potato
expressing a serine proteinase-inhibitor cowpea trypsin inhibitor (CpTI) [124]. CpTI expres-
sion directly inuenced the sexual fate of G. pallida toward a higher ratio of smaller males with
reduced damage observed on roots. Out of the four major classes of plant proteinases inhibi-
tors (cysteine, serine, aspartic, metallo-proteinases) cysteine and serine proteinase inhibitors
have gained considerable interest as eective defense molecules nematodes due to their speci-
city in the degradation of the major digestive enzymes (proteases) in plant-parasitic nema-
todes [125].The eectiveness of proteinase inhibitors can be aributed to its small size, which
benets its inclusion with nutrient molecules absorbed by some plant-parasitic nematodes. In
tomato, overexpression of phytocystatin gene, CeCPI isolated from taro (Colocasia esculenta)
showed enhanced resistance to root-knot nematodes demonstrated by reduced galling and
an inuence on sex determination [126]. In sweetpotato, sporamin which is classied as a
Kuni-type trypsin inhibitor, accounts for 60–80% of total soluble protein. Sporamin is consti-
tutively expressed in the tuberous root in comparison to in the stem or leaves and is expressed
systemically in response to wounding and other abiotic stresses [127]. In previous studies,
three forms of sweetpotato sporamin showed strong trypsin inhibitory activity invitro [128].
Additional research has resulted in the identication of sporamin-mediated resistance to cyst
nematodes [129]. Decreased nematode development correlated with trypsin inhibitor activity
of sporamin which was the critical factor for inhibition of growth and development of cyst
nematodes on sugar beet roots. Plant genotypes that produce high sporamin levels may have
a selective advantage in defense to plant-parasitic nematodes.
7.3. Plant hormones
The roles of plant developmental hormones, ethylene, jasmonic acid and salicylic acid have
been well established during plant immunity [130, 131]. Jasmonic acid (JA) and ethylene (ET)
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signaling pathways work synergistically while the salicylic acid (SA) pathway is antagonistic
to JA/ET pathways [132]. In a prior study, exogenous ethyhlene (ethephon) and jasmonic
acid (methyl jasmonate) application triggered the induction of PR proteins and the activa-
tion of systemic defense against root-knot nematodes on rice [133] These ndings suggest a
critical role of an active intact jasmonic acid pathway during the activation of systemically
induced resistance. The combination of exogenous jasmonic acid and biogenic elicitor arachi-
donic acid, decreased galling on tomato roots two-fold in comparison to controls [134]. The
role of salicylic acid has been well documented in the ecacy of host resistance to root-knot
nematodes. Pathogenesis-related protein expression was associated with salicylic acid-depen-
dent systemic required resistance in tomatoes pretreated with salicylic acid under root-knot
nematode challenge [109]. Expression of a NahG which encodes for an enzyme that degrades
salicylic acid to catechol, reduced Mi-1 gene- mediated root-knot nematode resistance in
transgenic tomatoes [135].
8. Disease management of plant-parasitic nematodes
8.1. Cultural control
For many years, crop rotation and cover cropping are often utilized in integrated pest manage-
ment protocols to reduce plant-parasitic nematode incidence and replenish soil nutrient levels.
Soil nematode levels have been eectively decreased by rotational cultivation of non-host cul-
tivars however, the wide host range of Meloidogyne spp. often diminishes the eectiveness of
crop rotation [136]. Planting corn as a rotational crop has been shown to reduce northern root-
knot nematode (M. hapla) incidence however; population densities of other Meloidogyne spp.
may increase with persistent cultivation. Plant species with resistance to mixed Meloidogyne
populations have been identied. Leguminous cover crops Mucuna pruriens L., and Crotalaria
spectabilis showed multiple resistance to three species of root-knot nematodes (Meloidogyne
arenaria, M. incognita, M. javanica) [137]. In certain cases, the very nature of crop production
may suppress the magnitude of infection. Rice is cultivated under ooding conditions which
does not favor the nematode lifestyle. In Taiwan, crop rotations with rice or taro combined
with cultural control methods including ooding and bare fallowing was shown to decrease
nematode soil populations and increase strawberry yields [138].
8.2. Plant extracts
Plant extracts often contain a myriad of compounds which demonstrate nematode suppressive
properties. Ethanolic extracts of Azadirachta indica (neem), Withania somnifera (ashwagandha),
Tagetes erecta (marigold) and Eucalyptus citriodora (eucalyptus) were reported to show nema-
ticidial activity against Meloidogyne incognita, Helicotylenchus multicinctus and Hoplolaimus
which was comparable to chemical nematicide controls [139]. In other reports, increased plant
growth and development were shown in plants propagated with the addition of a variety of
extracts. Root-knot nematode egg hatch and larval development was dramatically reduced
by leaf extracts from Hunteria umbellata and Mallotus oppositifolius which coincided with
Nematology - Concepts, Diagnosis and Control134
increased growth of cashew seedlings [140]. Plant height, fruit production and weights of M.
incognita-infected tomato were signicantly increased by the addition of ethanol extracts from
Azadirachta indica leaves, Capsicum annuum fruits, Zingiber ocinale rhizomes and Parkia biglo-
bosa seeds in comparison to non-treated controls [141].
8.3. Biological control
With increasing demands in organic agriculture and concerns for environmental welfare,
the use of chemical pesticides has decreased. Alternative means of pest management such as
the use of biological controls are of great interest for crop producers. The ecacy of nema-
tophagous bacteria and fungi in the control some nematode pests, including cyst and root-
knot nematodes has been well-documented [142, 143]. Parasitic bacteria of Pasteuria spp. have
been reported to infect 323 nematode species including both plant-parasitic nematodes and
free-living nematodes [144]. Three methods of application for P. penetrans were evaluated
for nematode control including seed, transplant, and post-plant treatments [145]. In green-
house studies involving cucumber, all three Pasteuria treatments were shown to reduce gall-
ing caused by M. incognita as well as soil nematode numbers and nematode reproduction.
In other reports, M. incognita suppression was observed in eld soil treated with P. penetrans
in comparison to untreated soil [146]. Other genera of bacteria including Bacillus spp. have
shown great promise in nematode management. B. cereus strain S2 treatment resulted in a
mortality of 90.96% to M. incognita [147]. B. rmus YBf-10 exhibited nematicidal activity against
M. incognita, which was clearly demonstrated by an inhibition of egg hatch and motility [148].
Nematophagous fungi Pochonia chlamydosporia has potential as a biological control agent for
M. incognita in vegetable crops. Along with crop rotational methods, P. chlamydosporia was
shown to reduce nematode levels in soil previously used for root-knot nematode susceptible
tomato [149]. Nematophagous fungal products including chitinases show great potential for
the development of biopesticides. Certain root-knot nematode species have transparent pro-
tective chitin-containing shells. Puried chitinase LPCHI1 from Lecanicillium psalliotae was
shown to degrade M. incognita eggs [150].
8.4. Host resistance
Chemical nematicides are often used in the management of root-knot nematodes however;
EPA restrictions in some soil fumigants due to increased environmental toxicity coupled
with the expensive costs associated new nematicide development limit their availability. The
very nature of these mammalian pesticides poses a signicant risk to humans. Plant-parasitic
nematodes often reside in plant tissue which makes soil delivery applications of the chemical
challenging. The incorporation of plant varieties that harbor multiple resistance to an array
of plant pathogens is an aractive and practical approach for plant breeders. However, the
conserved use of specic genotypes of disease resistant cultivars may contribute to increased
pathogen aggressiveness resulting in epiphytotic conditions; therefore the identication
of additional resistant varieties becomes increasingly necessary for long term control. For
many years crops have been articially selected for their inherent disease-resistant proper-
ties through phenotypic screenings and genetic analyses. Nematode-resistant genes found
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in gene pools of a variety of plant species have been introgressed into the genomes of eco-
nomically important crops with natural susceptibility through transgenic technologies such
as agrobacterium-mediated transformation [151, 152].
Plants synthesize and release an array of volatile organic compounds in response to damage.
Plant terpenes/terpenoids are secondary metabolites produced by terpene synthases in plants
and are involved plant survival and biotic and abiotic stress responses. Functional character-
ization of one member of the soybean TPS gene family, designated GmAFS suggested an anti-
nematode role [153]. Transgenic hairy roots overexpressing GmAFS were generated in an H.
avenae-susceptible soybean line. Plants showed signicantly higher resistance to H. avenae burden
than controls.
RNA interference (RNAi) is a method of gene silencing observed in a wide range of organ-
isms. This method of gene silencing has become a useful tool for biologists to study biological
processes and has been developed into a novel control strategy for engineering plants with
nematode resistance. First identied in plants [154] the mechanism of action was elucidated
in the nematode model organism C. elegans [155]. RNAi involves the suppression of specic
transcripts to minimum expression levels as a method of post-transcriptional gene silencing
during developmental processes and is believed to be a response to double-stranded viral
entry. RNAi is premised on the cell’s ability to recognize and degrade double-stranded RNA
(dsRNA). The dsRNA is processed into small interfering RNA (siRNA) by the enzyme Dicer,
a ribosome III-like enzyme. Double-stranded siRNA is unwound into two single-stranded
RNAs and one strand serves as a guide which associates with the RNA-induced silencing
complex (RISC). This complex associate with the specic complementary mRNA expressed
in the cell where the RNAse H enzyme Argonaute degrades the mRNA resulting in gene
silencing. Since the discovery of RNA-interference, researchers have developed transgenic
constructs that specically target genes for functional characterizations. More recently, plants
have been engineered to expresses double-stranded RNA that silence important genes in
plant-parasitic nematodes [156, 157]. As nematodes feed on the plant cytoplasm, the uptake
of the siRNA triggers the endogenous RNAi mechanism within the nematode, silencing the
target gene involved in infection [158]. The RNAi approach was applied, using sequence
fragments from M. incognita genes that encode for two heat-shock protein 90 (HSP90) and
isocitrate lyase (ICL). Heterologous expression of RNAi constructs in tobacco plants corre-
lated to a signicant level of resistance against M. incognita. Delayed galling and decreased
egg production was observed in plants expressing HSP90 dsRNA. The 16D10 eector gene
encodes for a secretory peptide synthesized in the subventral esophageal glands of root-knot
nematodes which plays an important role in giant cell formation cells [156]. In planta expres-
sion of 16D10 dsRNA in Arabidopsis conferred in resistance eective against the four major
root-knot nematode species [156]. In transgenic lines of potato expressing a 16D10 RNAi
construct (Mc16D10L), the number of M. chitwoodi egg masses and eggs was signicantly
decreased in comparison to empty vector controls [159]. Mc16D10L expression was reduced
in eggs and juveniles developed on transgenic potato which suggest a stable heritability of
the construct. Decreased egg production was also observed in transgenic grape lines express-
ing 16D10L [160].
Nematology - Concepts, Diagnosis and Control136
The use of site-specic DNA endonucleases including Zinc nger nucleases (ZFNs), [161]
transcription activator-like eector nucleases (TALENs) [162] and now clustered regularly
interspaced short palindromic repeats (CRISPR)/Cas9 [163] have equipped researchers
with the ability to specically inactivate genes and target genetic regions for homologous
recombination of input DNA. In general, double-stranded breaks introduced by nucleases
activates DNA repair mechanisms which generate mutations in the target sequence con-
ferring a loss of expression i.e., gene editing. Homologous recombination of exogenously
supplied sequences can result in genetic modications (knock-ins). CRISPR technology has
important advantages over TALENS and ZFNs including; ease of use [164] target site selec-
tion [165] and overall eciency, although o-target eects remains an important issue of
concern [166]. CRISPR/Cas9 system may be used to alter the expression of resistance genes
for constitutive expression against plant-parasitic nematodes. For example, point muta-
tions in the snc1 (suppressor of npr1-1, constitutive 1) locus in Arabidopsis plants resulted
in constitutive expression of pathogenesis-related proteins and enhanced disease resistance
against two plant pathogens [167]. The mutation was mapped to a single nucleotide change
in 120-kb region on chromosome 4 which contains a cluster of resistance genes. In a recent
sweetpotato study, putative disease resistance gene DRL23 showed elevated expression
in resistant sweetpotato genotypes when compared to susceptible plants at days 14 and
46 post-inoculation with Meloidogyne incognita inoculum [168]. To identify any polymor-
phisms in amino sequences between DRL23 from resistant and susceptible cultivars, protein
alignments using the NCBI BLAST (Basic Local Alignment Search Tool) was performed.
Interestingly, variations in amino acid sequences occurred between resistant (positions 187–
231) and susceptible (positions 57–102) which corresponded to the NBS domain. Mutations
in the NB-ARC domain often abolish R-protein function, indicative of the functional rel-
evance of this domain [169]. Precise targeting by CRISPR may be useful in restoring gene
function by sequence replacement in defense-related genes thereby enhancing resistance to
nematode infection.
Acknowledgements
The authors would like to acknowledge the IBREED program under USDA-NIFA Grant
#2014-38821-22448 and the Tuskegee University George Washington Carver Agricultural
Experiment Station.
Author details
Gregory C. Bernard*, Marceline Egnin and Conrad Bonsi
Address all correspondence to: gbernard4673@mytu.tuskegee.edu
Tuskegee University, Tuskegee, Alabama, United States
The Impact of Plant-Parasitic Nematodes on Agriculture and Methods of Control
http://dx.doi.org/10.5772/intechopen.68958
137
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