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Topic 5.
Mechanisms of Pathogenesis and Host responses to
Infection, Including Altered Host Nutrition
I. Introduction
The basic mechanisms of pathogenesis of most nematodes are not well
understood. Since much of the actual damage by nematodes is due to nematodes
interacting with other organisms (such as fungi) many difficulties involved in
investigating these mechanisms.
This lecture will concentrate on specific host responses to infection, namely
gall development because most of the physiological studies have concerned these
gall-forming nematodes.
II- Gall Formation:
A- Meloidogyne spp.
1- Morphological development:
There are two theories (concepts) that describe the processes by which giant
cells (transfer cells or syncytia) are formed:
a) The theory of cell-wall dissolution and coalescence (Christie’s theory)
Christie (1963), as a result of his excellent description of these processes, is
often given credit for this theory (concept), although Kostoff and Kendal (1930) were
the first to suggest this concept.
These processes are as follow: (Owens & Specht 1964)
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The second stage larvae (J2) move intra- and intercellularly in root cells.
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The early multinucleat condition (many nuclei) results from incorporation
of nuclei from pre-existing (neighboring) cells by dissolution of cell walls
and coalescence of cytoplasm from these cells. The essential features of
syncytia (giant cells) are induced by J2.
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During the third (J3), fourth (J4) and fifth (adult females) stages, the
syncytia characteristics are only extensions of processes and changes
already in progress when the nematodes are in J2.
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Formation of syncytia (always) precedes hypertrophy of cortical and stelar
cells.
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The triggering mechanisms for these two phenomena (formation of
syncytia, and hypertrophy of cortical and stelar cells) apparently are
different (the cells of cortex and stele remain mononucleate).
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During J3, expansion of syncytia and further hypertrophy of stelar and
cortical cells continues.
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Giant cell development and maintenance depend on a continuous stimulus
from the nematode, because killing or removal of the nematode results in
breakdown of the giant cells.
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These giant cells have striking cell-wall ingrowths which are apparently
related to their high metabolic activity.
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So, these giant cells are sometimes labeled as transfer cells.
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Factors that inducing syncytia apparently are unable to pass from cell to
cell in significant amount. Thus, large molecules such as proteins or
nucleic acids may be involved.
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The origin of these materials (proteins or nucleic acids) must be with the
feeding of the nematodes.
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Hypertrophy of cortical and stelar cells, which results in gall formation, is
connected with the growth of the nematode body and, possibly, with
excretory products; whereas:
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Hypertrophy of nuclei (up to 200 X normal) results from swelling and,
possibly, by fusion of nuclei.
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Nuclei undergo synchronous mitosis during the 3rd stage of nematode
development. This synchronous mitosis may be a vital part of the syncytial
development.
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In late or final stages of nematode development, many nuclear aberrations
and other changes take place in the syncytia, such as marked thickenings
of their walls as well as prominent protuberances.
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Toward the end of the life cycle, feeding apparently is limited primarily to
one syncytium, the other syncytia show signs of degeneration of nuclei and
cytoplasm.
b) The theory of repeated mitosis without subsequent cytokinesis (Huang
& Maggenti’s theory):
Huang and Maggenti (1969) found that the giant cell nuclei in the roots (Vicia
faba) infected with M. javanica are derived from repeated mitosis of the original
diploid (2n) cells without subsequent cytokinesis. Chromosome numbers (in a giant
cells) of 4n, 8n, 16n, 32n and 64n were established by chromosome counts in the giant
cell. The chromosome number in a giant cell could be predicted by the equation:
N= 12 × 2d
, where:
N= total number of chromosomes (at metaphase).
d= number of mitotic cycles.
12= diploid chromosomes of the host (Vicia faba).
The mitosis in a given giant cell is generally synchronized, except
where prophase and metaphase occur together.
They concluded that if the multinucleated conditions occur through fusion of
neighboring cells, as indicated the first theory, the expected number of chromosomes
(at metaphase) would be as follow:
N= 12 × f
; where f= the number of neighboring cells involved in the fusion.
These two authors found that the chromosome numbers agreed with the first
equation. They concluded, therefore, that giant cells are formed by repeated mitosis
without subsequent cytokinesis.
One weakness of this excellent work of Huang and Maggenti is that the
possibility of all fusion at the 4n stage could not be excluded. If this happens, the
chromosomes of a 4n giant cell could be derived from either equation. However, no
evidence of cell wall dissolution was obtained in their study. For giant cells with
higher ploidy (more than 4n) conditions, the possibility of obtaining this sort of data
by the second equation drastically diminishes.
Mitotic aberrations induced by Meloidogyne are very similar to those induced
by certain chemicals or drugs. For example, in onion, alkylated oxypurines give
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polyploid cells without cytokinesis; also auxines and kinetin in culture media may
result in polyploid mitosis.
In a second paper, Huang and Maggenti (1969) described the process of
secondary wall thickening in developing giant cells (in broad bean and cucumber).
Mechanical breakdown of numerous cell walls is caused by the penetration
and subsequent migration of larvae. The broken cells are collapsed by: the growth of
giant cells, and by the hypertrophy of the tissues around the giant cells. These two
authors concluded, again, that wall breakdown plays no part in giant cell formation as
they saw no breakdown in walls of giant cells themselves. Furthermore, they detected
no fusion of any two neighboring protoplasts. Whenever cell-wall breakdown, the
cells eventually collapsed.
However, despite of this excellent work, Endo (1971) and Gibson et al.,
(1971) have not accepted the theory of Huang and Maggenti. Instead, they have
presented additional data that support the first theory of coalescence, as suggested by
Christie. The results of various studies with Heterodera spp. (especially ultrastructural
studies) also tend to support Christie’s theory. However, the theory of Huang and
Maggenti seems to be more logical. Also, the work by Jones and Payne (1978) with
M. javanica (on Impatiens balsamine) strongly supports the conclusions of Huang and
Maggenti.
2- Physiology of gall development
a- Carbohydrates: Galls have decreased cellulose, free sugars, starch and
phosphorylated intermediates. Giant cells reported to have greater glucose,
G-6-P and ATP, than cells in healthy tissues.
However, contents of carbohydrates differ according to plant-nematode
association, or time in disease cycle.
b- Amino acids: Accumulate primarily in giant cells.
c- Protiens: Accumulate in giant cells and nematode bodies.
d- Nucleic acids: DNA and RNA synthesis increased in developing syncytia.
DNA synthesis is dependent on close association of a feeding nematode.
RNA synthesis once initiated was apparently independent of the nematode.
Gommers and Dropkin (1977) concluded that both the pathogen and host
influence the altered metabolism in giant cells. McClure’s application of
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the term “metabolic sink” to M. incognita-infected roots is an excellent
summation.
e- Growth regulators:
1- Auxins: Goodey (1948) suggested that IAA might be involved in
root-knot disease development. Other workers, later, detected
growth-promoting materials in root galls and concluded they were
IAA or related compounds. Yu and Viglierchio (1964, 1966) found
that the following indole compounds were increased in tissues
infected with Meloidogyne hapla, M. javanica and M. incognita.
IAA: Indole acetic acid
IAN: Indole acetonnitrite
IAE: Indole acetic acid ethyl ester
IBA: Indole butyric acid
but these inole compounds differ according to species (larvae, egg
masses, and host extracts)
2- Cytokinins and gibberellins: Very little information. Extracts
from galled tobacco roots had higher cytokinin activity than did
healthy roots. However, the opposite was found in galled tomato
roots. With nematode infection, a decrease in neutral gebberellins
and an increase production of slightly acid gibberellins.
Dropkin et al. (1969) found that various kinins would prevent
the usual resistant reaction to Meloidogyne in tomato. Resistant
plants treated with these cytokinins supported normal nematode
development (become susceptible).
f- Other growth control mechanisms:
Bird (1967, 1971) and others indicated that materials secreted by
the esophageal glands of Meloidogyne spp. (2 subventrals & 1 dorsal)
apparently are responsible for syncytia development. These glands of
M. javanica apparently produce different substances according to the
development stage and environment. Shortly before hatching and
penetration of the host, the sub-ventral glands synthesized granules
which were mainly protein (may involve cellulases, etc.). Within 1-3
days after entering the host, the subventral ducts are filled mainly with
carbohydrates (granules). The three glands then enlarged and
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nonglanular neutral mucopolysaccharide secretions were present in the
sub-ventral ducts, and giant cell development began. The dorsal gland
secreted glanular, histone-like basic proteins of which most were
produced by adult females (with enlarged dorsal glands). Bird has
suggested that the secretions by the dorsal gland accelerates the
development of the giant cells and thus facilitate nematode feeding. He
further concluded that the nematodes inject these histone-like proteins
into the cytoplasm of the giant cells, and these substances may be
responsible for the control of protein and nucleic acid synthesis in
these areas.
g- Oxidative and certain enzymes:
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