Meloidogyne incognita (Kofoid and White) is a major pest of soybean in South Africa and due to its
high level of pathogenicity to the crop it is quintessential that research in this regard should receive
priority. Root-knot nematode control has in the past mostly included the use of nematicides, while
crop rotation and inclusion of cultivars with genetic host plant resistance (henceforth referred to as
resistance only) to these pests were also used. Since no synthetically-derived and/or biological
agents are registered locally as nematicides on soybean, the use of resistant cultivars represents one
of the most viable and environmentally-friendly strategies to protect local soybean crops against
damage resulting from parasitism by M. incognita.
Although numerous exotic soybean cultivars have been identified with resistance to M. incognita,
only a few locally adapted ones have proved to exhibit resistance to the latter species. Moreover, at
present Egret is the only cultivar still available for commercial use in South Africa. Little and
fragmented information is, however, available on the use of plant enzymes, that are interrelated in
biochemical pathways that are expressed in root-knot nematode resistant cultivars, for its use as an
additional parameter to exploit such a trait. Therefore, the present study was undertaken to identify
M. incognita resistance in selected, locally adapted soybean cultivars by quantifying and exploiting
the latter trait by using enzyme activities as an additional parameter. In addition, resistance to M.
incognita in selected resistant soybean cultivars was also verified by means of histopathological
studies to identify cellular changes associated with the trait.
In the first part of the present study, 31 locally adapted soybean cultivars of which 23 were
commercially available in the 2012 growing season were evaluated for resistance to M. incognita.
The latter was done by means of traditional screening protocols for which M. incognita-gall rating,
egg and second-stage juvenile as well as the reproductive factor data per root system for each
cultivar screened were recorded. Two greenhouse experiments were subsequently conducted
concurrently, one of which the abovementioned nematode parameters were recorded 30 and the
other 56 days after inoculation. Reproduction factor values were used as the main criterium to
identify M. incognita resistance in local soybean cultivars since it is considered as a more reliable
parameter for this specific type of evaluations. Reproduction factor values equal to and lower than
one, indicating resistance to the M. incognita population used in this study, were recorded only for cultivar LS5995, as well as seven pre-released GCI cultivars. These eight cultivars also had very
low egg, as well as egg and second-stage juvenile counts per root system, all of which differed
significantly from the susceptible control, as well as a number of other cultivars. Root gall indices,
on the other hand, did not show consistent results in terms of the identification of the host status of
the 31 cultivar screened during this study. Using reproduction factor values, local farmers can thus
be supplied with information on the resistance of commercially-available soybean cultivars.
Eventually, such M. incognita-resistant cultivars can be used to reduce population levels of this
nematode pest in fields of producers and also as valuable germplasm sources in breeding programs
to introgress/stack this trait in newly-developed soybean cultivars.
The second part of the study aimed to verify and exploit M. incognita-resistance in soybean either
identified as resistant or susceptible during the screenings experiments, using enzymatic activity as
biochemical markers. Cultivar LS5995 was included as the resistant and Dundee as the susceptible
standard. The activity of three enzymes, namely guaiacol peroxidase, lipoxygenase and catalase
were recorded at different time intervals in roots and leaf samples of the latter cultivars, of both
nematode-inoculated and nematode-free plants of each cultivar. Significant (P ≤ 0.05) increases in
guaiacol peroxidase activity in leaf and root samples of the M. incognita-resistant cultivars GCI7
and LS5995 (inoculated with J2) were recorded 24 hours (h) after onset of the experiment. Use of
this enzyme thus emanated as a useful parameter to identify soybean cultivars that exhibit resistance
against M. incognita, especially in leaves, which could substantially reduce the time needed to
screen cultivars. In terms of lipoxygenase activity recorded, substantial variation existed between
the cultivars tested. The M. incognita-susceptible cultivar Egret was the only cultivar for which a
significant (P ≤ 0.05) increase in lipoxygenase activity in the roots was evident 24 h after
inoculation. However, during the 48 h sampling time, significant (P ≤ 0.05) differences in
lipoxygenase activity were also recorded for the two M. incognita resistant cultivars GCI7 and
LS5995. Although the increase in lipoxygenase activity for the susceptible cultivar Egret was
unexpected, it may indicate that some level of resistance is present in the latter cultivar, which has
in previous studies been identified as resistant to M. incognita. Other factors such as a different M.
incognita populations used and temperature differences in greenhouse conditions that applied in this
study compared to that for an earlier study may, however, serve as explanations for the latter
differences in host status identification of cultivar Egret. In terms of catalase activity recorded in
leaf samples of the M. incognita-resistant cultivar LS5995, substantial reductions of as much as 35.6 % were recorded for J2-inoculated plants compared to those of the J2-free control plants. In
leaf samples of the susceptible cultivars, Egret and Dundee, catalase was also reduced, but to a
lesser extent and ranged from 6 to 26 %. Conversely, catalase activity in the leaves of J2-inoculated
plants of the highly susceptible cultivar LS6248R was substantially increased by as much as 29.3
%. Enzyme data obtained as a result of the current study thus generally complemented those of
traditional screening assays in which resistance in locally adapted cultivars were identified to a
certain degree. It is, however, recommended that enzyme activity, to be used as bio-markers, still
needs further refinement and more investigation to optimise their use in identification, verification
and exploitation of M. incognita resistance in soybean cultivars.
The third and final part of the study encompassed a comparison of cellular changes induced by M.
incognita in resistant and susceptible soybean cultivars to verify the resistant reactions expressed in
the enzyme data. According to light- and transmission electron microscope observations, distinct
differences in the appearance and development of giant cells in roots of the M. incognita-resistant
cultivars LS5995 and GCI7 existed when compared to those in roots of the susceptible cultivars
Dundee and LS6248R. In the latter cultivars, giant cells that formed were characteristically large
and contained a dense cytoplasm, with thick irregularly surfaced cell walls. Cell walls also
displayed thick aggregations that appeared to be cell-wall ingrowths. These giant cells are optimal
to facilitate M. incognita development and reproduction. In contrast, giant cells that were associated
with the resistant cultivars LS5995 and GCI7 were small, irregularly shaped and contained
increased amounts of deposited cell-wall material in the cytoplasm known as cell wall inclusions.
Necrosis was also present in M. incognita-infected root cells of both cultivars. Such giant cells have
been associated with retarded feeding, development and reproduction of the latter root-knot
nematode species. However, it was evident that neither GCI7 nor LS5995 are immune to M.
incognita since J2 survived and developed to third- and fourth and ultimately mature females that
reproduced in their roots. Optimal giant cells that were formed in the roots of the M. incognitasusceptible
cultivars Dundee and LS6248R thus supported the nutritional needs of the developing
M. incognita individuals and led to significant increases in M. incognita populations 56 days after
inoculation as was evident from the high reproduction factor values that were obtained for such
cultivars during host status assessments that represented the first part of this study. The opposite
was recorded the M. incognita-resistant cultivars LS5995 and GCI7 since sub-optimal giant cells in
their roots could not sustain high offspring from such mature females. The presence of necrotic root tissue adjacent to giant cells, furthermore, indicated that hypersensitive reactions occurred in the
latter resistant cultivars. Enzyme data obtained in the second part of this study supported the
presence of hypersensitive reactions in root cells of the latter resistant cultivars. Guaiacol
peroxidase and lipoxygenase inductions in particular in plant tissues have been reported to play
integral roles in hypersensitive reactions that are exhibited by cultivars that are resistant to pests and
diseases.
Finally, results obtained from the different parts of this study complemented each other. It resulted
in the resistance that was identified in the GCI7 pre-released cultivar being verified and exploited
against that of the resistant standard LS5995. Research that was done during this study also
represented the first investigations into the use of enzymes as biochemical markers of resistance
against M. incognita in soybean in South Africa. / MSc (Environmental Sciences), North-West University, Potchefstroom Campus, 2014
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:nwu/oai:dspace.nwu.ac.za:10394/12088 |
| Date | January 2013 |
| Creators | Venter, Chanté |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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