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FEEDING
Feeding by
nematodes is a process involving attraction, orientation and contact to
host root system, probing and thursting of stylet in root cells,
ingestion of cell contents and retraction of stylet in the case of
ectoparasite. The morphological structures of the nematodes, involved in
the feeding processes, are the stylet, parts of the oesophagus,
secretory sub-dorsal and ventral oesophageal glands as also valves at
matacorpus and oesophageal-intestinal junction which regulate the flow
of injested food as well as the salivary juices.
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Nematodes differ in their specificity for hosts and for parts of the
host they feed on. In number of cases it has been positively
demonstrated that nematodes are attracted to host roots, presumably
in response to some stimuli. However, there are cases also that
nematodes may be attracted towards non-host roots or organisms
associated with the roots. The parts of the roots most attractive to
nematodes are the tip, zone of elongation and in some cases zone of
root hairs. On reaching the roots, most nematodes tend to explore
the surface, making occasional stylet probes till a suitable site is
located. Once this is achieved, the stylet probes increase in
frequency and vigour and correspondingly the nematode movement is
slowed down.
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The nematode
processes its lip region against the root surface, and probably after it
receives some stimulus (most probably chemical), the stylet thursts
increase in vigour for penetration and feeding. The type and duraiion
of stylet thursts may vary with species. In Criconematids, a firm grip
is maintained between lip of the nematode and host cell by
mucopolysaccharide adhesive plug originating from nematodes. In some of
the dorylaims, especially specrcs of Xiphinema and Longidona,
the stylet is attenuated and the anterior part of the oesophagus is
quite slender. To regulate the flow of the injested food material, two
musculature systems have been recorded in X. index, consisting of
a sheath composed of muscles and basement membrane surrounding the
oesophageal bulb3. This may be visualized as additional
pressure plate, besides the pseudocoelomic pressure, to regulate flow of
sap from host cells to nematode.
One of the
important aspects in feeding is the breaking down of the host cell
barrier before food can be injested. It has been demonstrated in large
number of cases that the nematodes are
equipped with
enzymes capable of hydrolyzing components of plant cell wall. High level
of cellulolytic activity has been recorded from several style bearing
nematodes. The plant parasitic nematodes-have a higher activity of the
enzyme than the fungal or bacterial feeders. One of the hydrolytic
enzymes, which have received considerable attention, is pectinase and
which has been reported from many-plant parasitic nematodes. The
successful feeding on host cells by Ditylenchus dipsaci depends
largely on the nematode ability (through pectinase) to macerate host
tissue through dissolution of middle lamellae without causing death to
the cells. Since pectic compounds are important components of middle
lamellae, pectolytic enzymes facilitate the feeding process by the
nematode.
The secretions
from the nematode glands are either surged forward, at the time of
injection in host tissue, by body contraction movement (as in
ectoparasites) or released slowly (as with endoparasites). The
secretions help in digestion of food. At feeding, the forepart of the
oesophageal lumen gets dilated for sucking of fluid1 juices
followed by posterior dilation and simultaneous anterior closure. The
nematode stylet which injects secretions in host tissues-has also the
capability of preventing viscous or granular materials from blocking the
stylet. With some nematodes, the food being injested is predigested with
the help of the secretions, while in other cases; the juice may be taken
as such with digestive process taking, place inside the lumen of the
oesophagus.
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