Susceptible: the relationship between a plant and a nematode species when the plant will support feeding and abundant reproduction of the nematode and may be subject to damage. Example: Juglans regia is susceptible to Pratylenchus vulnus.
Resistant: describes the effect of the plant on nematode reproduction. Usually infers that some (maybe most) varieties or cultivars of that plant species are susceptible. There may be different levels of resistance among cultivars. Example: the Prunus rootstock Nemaguard is resistant to Meloidogyne incognita , however, it is susceptible to Mesocriconema xenoplax.
Vertical resistance may be indicated when a variety differs in susceptibility to races, implying one or a few genes involved. May select for races or biotypes of the nematode that are able to overcome the resistance mechanism. Example: Mi is a single dominant gene that confers resistance to Meloidogyne incognita in tomato, Solanum lycopersicum.
Single-gene resistance against soybean cyst nematode (Heterodera glycines) in soybeans.
Horizontal resistance (multigene) when resistance to races or biotypes of the nematode species is uniform - reduces selection pressure. Example: resistance to Globodera pallida in the potato relative Solanum vernei.
Immune: describes a cultivar or variety of plant that will not support feeding or reproduction of the nematode species. Example: Dog Ridge rootstock (Vitis champinii) of grape is immune to Meloidogyne javanica.
Non-host: usually used when no varieties or cultivars of a plant species support the feeding and reproduction of a nematode species. Example: Citrus spp. are non-hosts to the potato cyst nematode, Globodera rostochiensis.
Tolerant: a host plant that can support reproduction of the nematode while sustaining little damage. Example: the grape (Vitis vinifera) variety "Thompson Seedless" is quite tolerant to Meloidogyne incognita.
Intolerant: a host plant that is heavily damaged at relatively low population levels of a nematode species. Example:
Compatible: interaction between a nematode species and host plant that is favorable to nematode reproduction.
Incompatible: interaction between a nematode species and host plant that is unfavorable to nematode reproduction. An alternative description of the resistant condition.
Hypersensitivity: the condition in which host plant cells die in response to nematode stimulus rather than develop into a feeding site; a mode of resistance that occurs in several plant species. Note, the mechanism underlying the hypersensitive response probably varies among plant species. Example: resistance of potato to Globodera rostochiensis conferred by the H1 gene from Solanum andigena.
Note the effect of vertical resistance on selection for
aggressiveness on potatoes in Europe, tobacco in NC, soybean cyst nematode in
the southeastern US. Will
this happen with the Mi gene in tomato?
Consider the potential for stabilizing selection and management of population gene frequencies since genes for aggressiveness may not confer advantage in absence of selection pressure.
This is an important area in pest and nematode management.
- How is the resistance expressed relative to the nematodes?
a. Resistance to penetration
b. Resistance to development
c. Resistance to productivity (reproduction)
A very complete understanding of resistance mechanisms is being developed through the work of Dr. Valerie Williamson and colleagues. They study the Mi gene which confers resistance to Meloidogyne spp. in tomato. The gene provides resistance to Meloidogyne incognita, M. arenaria, and M. javanica. It also confers resistance to the potato aphid, Macrosiphum euphorbiae, and the white fly Bemisia tabaci.
There are important developments in the understanding of resistance in soybeans to Heterodera glycines.
Many resdistant varieties were developed over several years by USDA scientist Dr. Hartwig. In general, resistance in soybeans is expressed as a hypersensitive response. Syncytia are initiated in both resistant and susceptible cultivars but degenerate in the resistant cultivars. They become necrotic and unable to support nematode development (Huang, 1998). Some of the varieties developed by Hartwig did not yield as well as susceptible varieties in the absence of soybean cyst nematode. Many of the sources of resistance have now been integrated into a high-yielding variety, CystX.
In 2011 and 2012, resesarchers at Southern Illinois University Carbondale and the University of Missouri at Columbia identified and validated the gene at the Rhg4 (for resistance to Heterodera glycines 4) locus. The Rhg4 gene encodes a serine hydroxymethyltransferase (SHMT). The enzyme catalyzes conversions between serine and glycine and is essential for cellular carbon metabolism. Alleles of Rhg4 conferring resistance or susceptibility differ by two genetic polymorphisms that alter a key regulatory property of the enzyme. (Liu et al., 2012).
Roberts, P.A. 1982. Plant resistance in nematode pest management. Journal of Nematology 14:24-33.
Kaplan, D.T. and E.L. Davis 1987. Mechanisms of pant incompatibility with nematodes. Pp 267-276 in: J.A. Veech and D.W. Dickson (eds). Vistas on Nematology. E.O Painter, Florida.
Huang, J.S. 1998. Mechanisms of resistance. Pp353-368 in Sharma, S.B.
(ed). The Cyst Nematodes. Kluwer, Dordrecht.
Huang, J.S. 1998. Mechanisms of resistance. Pp353-368 in Sharma, S.B. (ed). The Cyst Nematodes. Kluwer, Dordrecht.
Liu, S., Kandoth, P.K., Warren, S.D., Yeckel, G., Heinz, R.,Alden, J.,
Yang, C., Jamai, A., El-Mellouki, T.,. Juvale, P.S., Hill, J., Baum, T.J.,
Cianzio, S., Whitham, S.A., Korkin, D., Mitchum, M.G.,Meksem, K. 2012. A soybean
cyst nematode resistance gene points to a new mechanism of plant resistance to
pathogens. Nature doi:10.1038/nature11651.
Liu, S., Kandoth, P.K., Warren, S.D., Yeckel, G., Heinz, R.,Alden, J., Yang, C., Jamai, A., El-Mellouki, T.,. Juvale, P.S., Hill, J., Baum, T.J., Cianzio, S., Whitham, S.A., Korkin, D., Mitchum, M.G.,Meksem, K. 2012. A soybean cyst nematode resistance gene points to a new mechanism of plant resistance to pathogens. Nature doi:10.1038/nature11651.