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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.