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2.1 Introduction

Worldwide, economic benefits of nematode
resistance in plants are being evaluated as alternative to synthetic chemical
nematicides (Mashela et al., 2016). Three
years prior to withdrawal of methyl bromide in 2005, yield loses due to
nematodes were estimated at US$125 billion (Chitwood, 2003). Three and eight
years after the withdrawal, several studies are underway to develop crops with
resistance genes against various Meloidogyne species (Norshie et al.,


Recent screening
of 12 sweet potato cultivars (Pofu et al.,
2016) demonstrated that most cultivars were hosts to M. incognita races 2 and 4, whereas ‘Bophelo’, ‘Bosbok’ and
‘Mvuvhelo’, which are South African orange and cream-fleshed cultivars respectively,
were non-hosts to all Meloidogyne
species and races. Generally, host-status in plant-parasitic
nematodes is assessed using reproductive factor (RF), which is quotients of
final nematode numbers (Pf) and initial nematode numbers (Pf): RF = Pf/Pi
(Seinhorst, 1965). Host-status and host-sensitivity concepts are both used as
indicators of whether a host is resistant, tolerant or susceptible to
plant-parasitic nematodes (Seinhorst, 1967). Based on nematode-chemical
interaction, nematode resistance mechanisms had been concentrated to two,
concepts, pre-infectional and post-infectional nematode resistance.


The nematode bodies are covered with
sensory organs, which are used to detect chemicals in small quantities,
therefore influencing the direction to which nematode should move.
Chemo-attractants and chemo-repellents attract and repel nematodes,
respectively (Wuyts et al., 2006; Zhao et al., 2000). However, a recent
literature review on mechanisms of nematode resistance in nematode-resistant
transgenic plants suggested that at a molecular level, plants used three
different strategies to prevent nematode infection (Mashela et al., 2016).


2.2 Work done on problem statement

2.2.1 Nematodes-resistance in sweet potato

Assessments of sweet potato cultivars for resistance to M. incognita populations showed that galls
occurred on roots as an indication of host-status, suggesting that the nematode
was able to reproduce within the tested cultivars (Olabiyi, 2007). Osunlola and Fawole (2015) investigated the
pathogenicity of M. incognita on
sweet potato and showed that the nematode caused reduction in growth, yield and
quality reduction in sweet potato. In other studies, yield losses due to Meloidogyne species in sweet potato
production had been estimated at (6%) for South Africa (Kleynhans et al., 1996).


Pofu et al. (2016) screened 12 sweet potato
cultivars against Meloidogyne species
and races and demonstrated that most cultivars were host to M. javanica and M. incognita races 2 and 4, whereas ‘Bophelo’, ‘Bosbok’ and
‘Mvuvhelo’ were non-hosts to all Meloidogyne
species and races. Karuri et al. (2017)
conducted a survey of root-knot nematodes resistance in sweet potato varieties
and observed that 68.0% tested sweet potato varieties were highly resistant to M. incognita, whereas 11.1% were
classified as susceptible. Similar results were observed on the resistance of
sweet potato clones to M. incognita races 1 and 2, were among 63
analysed clones, 78% were resistant to M. incognita race 1 79% to M. incognita race 3 and 67% showed
multiple resistance to all M. incognita races (Gomes et al., 2015).


The Japanese
Sweet Potato Breeding Programme, conducted work with a putative hexaploid I.
trifida (K123) wild relative, where it was crossed with various genotypes
to establish nematode-resistant cultivars. After several generations of
backcrossing, the M. incognita resistant
and high-yielding cultivar called ‘Minamiyutaka’ was developed (Iwanaga, 1988). That was the first successful use of
wild sweet potato germplasm to transfer desirable traits like root-knot
nematode resistance into sweet potato cultivars, with the success having a
major impact on sweet potato breeding programme in Japan (Iwanaga, 1988). Dean and Struble (1953) observed
that root-knot nematode resistance was of high frequency in seedling
populations from selected sweet potato resistant parents. Similar observations
were made by Giamalva et al. (1961)
when they crossed sweet potato lines with different degrees of resistance to M.
incognita, where different ratios in resistance responses among the progeny
were obtained.


Jones and Dukes
(1980) studied the inheritance of resistance in sweet potato to M. incognita
and M. javanica and found that resistance to the two species was not
correlated, suggesting the independent inheritance theory. The two researchers
used three measures of resistance, namely, number of egg masses, gall index,
and necrosis index and reported heritability estimates of 0.69, 0.78 and 0.72,
respectively. Gasapin (1984) suggested that the different degrees of resistance
showed by different sweet potato cultivars to M. incognita and M.
javanica could be attributed to the differences in genes for resistance
possessed by the different cultivars. Although nematode resistant genotypes
hardly existed in the tested orange-fleshed sweet potato cultivars for South
Africa nematode populations, the findings suggested the existence of resistant
genotypes in local cream-fleshed cultivars (Pofu et al., 2016). In the trials, Pofu et al. (2016) used the reproductive potential, which does not
provide an indication of whether the tested plants were resistant or tolerant
to the tested nematode.


2.2.2 Nematode resistance in other crops

In sweet stem
sorghum (Sorghum bicolor) J2 failed
to penetrate roots in S. bicolor cv. ‘Ndendane-X1’ which led to the conclusion that the cultivar
had pre-infectional mechanism of nematode resistance to both M. incognita race 2 and M. javanica (Mashela and Pofu, 2016). Host-status and host-sensitivity of C. africanus
and C. myriocarpus to Meloidogyne species were investigated
and both C. africanus and C. myriocarpus were shown to be
resistant to M. incognita races 2 and 4 and M. javanica, which
are dominant in South Africa (Pofu et al.,
2010). Nematode races are
morphologically identical within the same species, but can be separated using
differential hosts and/or molecular approaches (Mashela et al., 2015).


According to Dayan
et al. (2010), S. bicolor produces chemical compound, sorgoleone (C22H29O4),
which has nematicidal properties that result in inhibition of nematode
mobility. Out of 30 tested cabbage (Brassica
oleracea) cultivars for nematode resistant, seven white head cabbage
cultivars were reported to be highly resistant to Heterodera cruciferae (Aydinli and Mennan, 2012). Cruciferous
plants from Brassicaceae family
contain glucosinolate (C17H32O11NS3)
chemical compounds which release nematode toxic products such as thiocyanate
(SCN-) and isothiocyanate (C4H5NS) when they decompose
(Brown et al., 1991; Matthiesen and
Kirkegaard, 2006; Ntalli and Caboni,
2012), which are toxic to nematodes (Larkin, 2013; Petersen et al., 2001). Dry bean (Phaseolus vulgaris) cultivars ‘Apore’
and ‘Talisma’ were highly resistant to M.
javanica, whereas snap bean cultivars
‘Macarrao atibaia’ and ‘Macarrao preferido’ were moderately resistant to M. javanica (Ferreira et al., 2010). Common bean cv. ‘Polder’
was shown to be resistant to M. chitwoodi
and M. fallax (Wesemael and
Moens, 2012). Resistant Lima bean (P.
lunatus) inoculated with P. penetrans produced the
phytoalexin, coumestrol (C15H8O5) (Veech,


Resistance to M.
arenaria was expressed in soybean (Glycine max) as small, poorly
formed giant cells, with reduced cell number and cell size of cells surrounding
selected feeding cell (Pedrosa et al., 1996). Meloidogyne species produced glyceollin (C20H18O5),
a nematode-toxic chemical compound (Veech, 1982). Carrot (Daucus carota)
cv. ‘Brasilia’ was shown to be resistant to M.
javanica (Huang, 1986; Huang et al., 1986). Generally, when M. chitwoodi J2 penetrated roots of carrot cv. ‘Parmex’ and cv.
‘Berlanda’, fewer egg masses were observed on each cultivar (Sone, 2010), with high male to female ratio and
numerous rootlets. Wesemael and Moens (2008) also reported egg masses of
less than 20% in cv. ‘Parmex’ and cv.
‘Berlanda’ infected with M. chitwoodi. According to Osei et al.
(2010), leguminous plants contain numerous chemicals, some of which were
nematostatic or influence nematode behaviour. The absence of galls on the roots
of cowpea (Vigna unguiculata) varieties led to the conclusion that V.
unguiculata varieties had the ability to inhibit the formation of feeding
sites that are required to support the reproduction of females after
penetration. The gene responsible for resistance to M. incognita in V.
unguiculata appeared to confer resistance to other Meloidogyne
species (Fery, 1980).


Mashela and Pofu (2012) conducted a trial on host
response of Capsicum frutens cultivar ‘Capistrano’ to M. incognita
race 2, the reproductive factor was less than unity, while nematode infection
had no effect on plant growth and concluded that the cultivar was resistant to M.
incognita race 2. The reproductive factor of M. javanica on hemp cultivars were
greater than one, without the cultivars suffering damage from the nematode
infection. Results suggested that the four cultivars were tolerant to M.
javanica (Pofu et al., 2010). Growth of beetroot
cultivar ‘Detroit Dark Red’ was significantly stimulated and inhibited at low
and high nematode infection levels, respectively. In contrast, RF values for M.
javanica on cultivar ‘Crimson Globe’ were below unity, without any significant
effects on plant growth, it was concluded that, ‘Detroit Dark Red’ was tolerant
to M. incognita, whereas ‘Crimson Globe’ was resistant to M. javanica
(Mashela, 2017). Reproductive factors suggested that three open-pollinated varieties,
namely: OBATAMPA, QPM-SR and QS-OBA were non-host to both M. incognita race
2 and M. javanica. Penetration indices suggested that OBATAMPA had
post-infectional non-host status, whereas QPM-SR and QS-OBA had pre-infectional
non-host status (Ngobeni et al., 2012).


2.2.3 Assessment concepts in
nematode-plant resistance

The concepts of host-status and
host-sensitivity was introduced to describe nematode-plant relations
(Seinhorst, 1967), which had since been widely used in plant-parasitic
nematology. Host-status was described using the proportion of the final
nematode population density (Pf) and the initial nematode population density
(Pi), referred to as the reproductive factor (RF = Pf/Pi). Using the RF
concept, when Pf = Pi, the population is at equilibrium (E) point, beyond which
nematodes have intensive competition for resources, while RF is invariably less
than unity (Seinhorst, 1967). Generally, before E point, nematodes are at the
lowest competition for resources and if the plant is a host, RF is invariably
greater than unity. Ferris (1981) and later Duncan and McSorley (1987), explained
the host-status concepts using various mathematical models, which, assist
nematode specialists in better understanding of nematode reproduction and
density-dependent growth patterns (Salisbury and Ross, 1992) and thereby
improving nematode management tactics. Also, E point, assist in selecting
appropriate inoculation levels, since excessively high levels could result in
RF being lower than zero due to competition as opposed to nematode resistance.


Host-sensitivity was described in relation
to damage inflicted by nematodes to plants, with Seinhorst (1965) using a model
to formulate three concepts: (i) susceptible, (ii) tolerance and (iii)
resistance, which had since been widely used in nematode-plant relations (Mashela,
2017). Susceptible hosts are
plants that have the ability to build up nematode populations and suffer
subsequent damage in terms of growth reduction (Trudgill, 1992). Seinhorst
(1967) defined tolerance to nematodes as the capacity of the plant to withstand
nematode damage. Most nematodes can reproduce in tolerant hosts without causing
any significant reduction in growth and yield (Seinhorst, 1967; Trudgill,
1985). However, tolerant hosts are not suitable for use in crop rotation systems
since they invariably increase nematode population densities, which may
eventually produce virulent biological races. Resistant hosts neither allow nematode reproduction nor suffer
nematode damage (Seinhorst, 1967; Taylor and Sasser, 1978). Resistance to
nematodes is usually associated with the inability of the nematode to induce a
normal feeding site or reproduce inside the host (Miller and Guyla, 1987).


2.2.4 Mechanisms of nematode resistance

Active nematode resistance responses occur post-infection,
following penetration into the root or other host tissues by the nematodes
(Mashela et al., 2016). Post-infectional
resistance is expressed by delayed or retarded development of the nematodes after
penetration into the plant, or by non-development of the nematode to maturity
in the plants (Gasapin, 1986). In post-infectional resistance, plant have the ability
to defend itself against nematode parasitism by releasing chemicals present in
low levels to higher levels in the host tissues after penetration of nematodes
(Kaplan and Davis, 1987). Naturally, resistant plants carrying major R genes
for resistance to nematodes are invaded like susceptible plants. Pre-infectional
resistance may be manifested as physical or chemical barriers, or as
nutritional inadequacies. Pre-infectional resistance is mainly due to
pre-formed chemicals, which are fully expressed in root tissues before
infection and do not rise to higher levels in response to attacks by invading
nematodes (Ferraz and Brown, 2002). Failure of J2 to penetrate roots in S. bicolor
cv. ‘Ndendane-X1’ led to the
conclusion that the cultivar had pre-infectional mechanism of nematode
resistance to both M. incognita race
2 and M. javanica (Mashela and Pofu,
2016). Asparagus (Asparagus officinalis)
also possesses pre-infectional nematode resistance (Gommers, 1981). Sunn hemp (Crotalaria juncea)
released chemicals into the rhizosphere that prevented infection by M. incognita
J2 prior to penetration (McSorley and Gallaher, 1991; Roberts, 1992). Tomato
cv. ‘Nemared’ was reported to be pre-infectional resistant to M. incognita and Pratylenchus. penetrans (Hung and Rohde, 1973). The J2 of M. incognita and P. penetrans could not penetrate roots of the ‘Nemared’ cultivar
(Ohri and Pannu, 2010).


In other plants,
resistance is post-infectional. In this mode of resistance, a hypersensitive
response (HR) and accumulation of toxic metabolites, and some resistance
results in the degeneration of nematode feeding sites (Veech, 1981). After
invasion into the plant, the activation of incompatible interactions can result
in nematodes emigrating from the roots or nematode death within resistant roots
(Roberts et al., 1998). This results
in a lower number of egg masses and reduced size of egg laying females in
resistant cultivars as compared to susceptible cultivars (Gasapin, 1986). Dean
and Struble (1953) reported that on resistant and susceptible sweet potato
varieties, juveniles (J2) enter the roots in equal number, but fewer nematodes
develop to egg-laying maturity on resistant varieties. The two workers
suggested that resistance to nematode in sweet potato was related to an
extensive necrosis of root tissues. Histological studies of ‘Porto Rico’ sweet potato
showed that the primary root penetration by J2 occurred at the tips of young roots
in the region of tissue differentiation. Another major nematode penetration
site in sweet potato occurred through the loose ruptured cells of enlarging
roots where lateral roots emerged (Krusberg and Nielsen, 1958). Several types
of host-parasite reactions are related with root-knot nematode resistance in sweet
potato. These are: none to trace amounts of galling on the host, moderate to severe
root tip necrosis, general inability of nematode larvae to reach mature stages,
little or no reproduction by the nematode, and reduced number of eggs where reproduction
does occur (Davide and Struble, 1966). Recent mechanism of resistance study suggested
that post-infectional nematode resistance was in place in the two wild
indigenous Cucumis species. The results confirmed that
the identified nematode resistance in C.
africanus and C. myriocarpus to Meloidogyne species was post-infectional
(Ramatsitsi, 2017).


2.2.5 Molecular approaches in nematode resistance

Molecular approaches suggested that there were three strategies for
nematode resistance namely (i) RNA-resistance strategy, (ii) anti-gene products
strategy and (iii) anti-plant gene strategy which were recently reviewed by Mashela
et al. (2016) in nematode-resistant
transgenic plants.


RNA-resistance strategy: Hewezi and Baum (2015)
reported that the RNA interference (RNAi) disrupts the nematode gene products
through host-induced gene silencing approach. The RNAi genes had revealed
accurate selectivity for the target organisms with slight side effects (McDowell
and Woffenden, 2003). Cathepsin L-like cysteine proteinases, produced by R
genes in nematode resistant transgenic plants, were shown to be an attractive
group of candidate genes for RNAi-induced downregulation due to their high
level of specificity to the target nematode gene products (McDowell and
Woffenden, 2003), resulting in silencing effects on host-induced gene products.
Also, the host-produced RNAi of Mi-cpl-1 gene confers resistance to M. incognita by inducing negative
effects on nematode infection, development and the subsequent reproduction
(McDowell and Woffenden, 2003).


Anti-gene products strategy: Plant-parasitic nematodes
secret chemical compound called gene products through the sub-ventral and
dorsal gland cells during migration and sedentary phases, respectively (Gheysen
and Fenoll, 2002; Tripathi et al.,
2015). The secretion of gene products is important, especially during the
formation of nematode feeding sites, which allow for nematode development to
subsequent stages (Curtis, 2008; Siddique et
al., 2014). During migratory
phases, roots are wounded upon which, chemical compounds referred to as defence
plant genes, comprising peroxidase, chitinase, lipoxygenase, extension and
proteinase inhibitors are activated (Gheysen and Fenoll, 2002; Hewezi and Baum,
2015). The anti-gene products strategy in nematode resistant plants, ranged
from those during both migratory and sedentary phases, in respect to those that
silence the expression of the gene products (Mashela et al., 2016).


Anti-plant gene strategy: The host plant genes that
respond to nematode feeding and secretions to allow for successful partnerships
between gene products and gene plants are silenced in anti-plant gene strategy
(Mashela et al., 2016). Thus, the
phytotoxic chemical compounds that destroy the feeding structures, syncytium
and giant cells, are upregulated (Mashela et
al., 2016). Mostly the plant
releases certain plant genes in order to protect the nematode and such
chemicals could be suppressed, thereby leaving the bodies of nematodes exposed
(Hewezi and Baum, 2015). The anti-plant gene strategy had been successfully
used in certain transgenic plants (Mashela et
al., 2016).


2.3 Work not yet done on problem statement

distinction between screening for host-status and nematode had been
re-emphasised recently (Mashela et al.,
2016). In screening, one inoculum level of nematode is used, with results
expressed using the concept of reproductive potential (RP) (Mashela et al., 2016), whereas in nematode
resistance, a series of nematode levels are used, with findings being expressed
using reproductive factors (RF) (Seinhorst, 1965). Although screening results
suggest that sweet potato cultivars ‘Bophelo’, ‘Bosbok’, and ‘Mvuvhelo’, were
non-host to all tropical Meloidogyne
species in South Africa, it would not be wise to
conclude that the three sweet potato cultivars were
resistant to Meloidogyne species. Hence, empirical-based information on the degree of nematodes
resistance in the three sweet potato cultivars would be necessary.

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