Download Mechanisms of plant resistance to nematodes

MECHANISMS OF PLANT RESISTACE TO NEMATODES
Different mechanisms may operate in different cultivars.
Resistance mechanisms may be:
"constitutive" (= present before infection) or
"induced" after infection.
1. Preinfection factors for resistance
constitutive factors most important
A. Effects on egg hatching
egg hatching may be affected by plant exudates
may be hatch inhibitors or
lack of "hatching factors" needed to stimulate egg hatch
B. Effects on preinvasion by soil migrating stages
Allelochemics = plant chemicals that may affect other organisms,
= beneficial or deleterious
Those that are deleterious are "allelopathic" and are generally root exudates.
The proposed mechanism(s) may;
"not attract" nematodes,
actively repel nematodes
or be toxic.
Cucurbitacins (cucumber)
= Triterpenoids that accumulate as aglycones or glycosides
other examples of compounds toxic to nematodes at 1 ppm or less.
2. Resistance factors affecting infection (Probably not important mechanism)
Penetrating action of stylet and cell wall degrading enzymes are capable of overcoming
all superficial mechanical barriers.
3. Postinvasion factors for resistance
Constitutive factors are less important after invasion.
Plant must "recognize" parasite, usually by compounds on cuticle or secretions made by
nematode, to induce mechanisms of resistance.
Nematode "tries to disguise itself"
Once it has done so you may have a new race.
A. Effects on transfer cells
In some plants/cultivars, transfer cells fail to develop or to function, nematode dies
Resistant plants that are infected may undergo a "hypersensitive response", affected
cells turn brown and die (or just die) and nematode dies (or moves away?).
Ranges from nonspecific tissue necrosis that may not impair development
to localized necrosis around the nematode feeding sites that affect nutrition so that
males may develop but not females
to a strong hypersensitive response that prevents development of feeding site as
well as nematodes.
There appears to be several biochemical stimuli that induce hypersensitive reactions
and these differ between different nematode-plant relationships.
B. Phytoalexins and phytoalexin elicitors
= Antibiotic chemicals that are:
1) in response to a metabolic interaction between the host and the pathogen and
2) inhibits growth of the pathogen
An "elicitor" stimulates accumulation of phytoalexin either by increasing its
biosynthesis or by inhibiting its degradation.
1. Coumestans, Coumestrol (lima bean vs Pratylenchus scribneri)
This compound is "genus specific".
Meloidogyne javanica does not elicit response, not affected by coumestrol.
2. Glyceollin (soybean vc Meloidogyne incognita)
hypersensitive response and increase in glyceollin occurs in the same
This compound is "species specific"
M. javanica does not induce response, is not affected by glyceollin.
3. Terpenoids
"gossypol" = cotton anti-fungal phytoalexin which affects Verticillium dahliae
and M. incognita.
accumulate in endodermis (and stele?) near site of nematode.