Phytoalexins: Properties and their Role in Nematode Resistance in Plants
Authors: Bharat H. Gawade and Zakaullah Khan
Division of Plant Quarantine
ICAR-National Bureau of Plant Genetic Resources
Pusa Campus, New Delhi-110012


Phytoalexins are low molecular weight secondary metabolites with antimicrobial activity, which are synthesized in plants with response to pest/pathogen attack. These are the natural products secreted by plants, accumulates temporarily and have inhibitory effect against bacteria, fungi, nematodes, insects etc. Among phytoalexins, pisatin was the first chemically characterized phytoalexin from pea plants after which large number of phytoalexins were isolated and reported from various crops such as beans, rice, barley, banana etc. (Braga et al. 1991). It able the plants to respond nematode infection by production of antibiotic compounds to limit further spread and development of nematode (Veech, 1982). The word phytoalexin was derived from two greek words- phyto means plant and alexin means warding off compound. The phytoalexin concept was first given by Muller and Borger in 1941 during observation of potato tubers infected with incompatible race of Phytophthora infestans which was capable of induction of hypersensitive response (HR) in potato. They found that the phtyoalexin produced with response to this race significantly reduced the infection by another compatible race of P. infestans.

General properties of phytoalexins:

• These are heterogeneous group of compounds with biological activity against pathogens
• These are considered as molecular markers of disease resistance and often associated with hypersensitive response (HR)
• Production of phytoalexins is elicited by nematode infection in resistant plants
• They are produced in plants after few hours on infection and continue till few days
• Phytoalexins are reported from plant families like leguminosae/fabaceae, solanaceae, brassicaceae/cruciferae
• Phytoalexins are induced HR in cells surrounding the feeding cells of nematodes

There are more than 300 types of phytoalexins reported, characterized and classified into different types of chemical compounds such as coumarins, diterpenes, flavonoids, alkaloids, phenolic compounds etc. The time and site of production, concentration of phytoalexin accumulation are key factors which decide the reaction to the nematode infection. Few phytoalexins that are studied in plant parasitic nematodes infected plants are listed below.

Sr no.	Name of phytoalexin	Crop/plant	Produced in response to	Reference
1.	Coumestrol Psoralidin	Phaseolus lunatus	Pratylenchus scribneri	Rich et al., 1977
2.	Glyceollin	Glycine max	Meloidogyne incognita	Kaplan et al., 1980
3.	Glyceollin I	Glycine max	Heterodera glycines	Huang and Barker, 1991
3.	Gossypol (Terpenoid Aldehydes)	Gossypium hirsutum	M. incognita	Veech, 1982
4.	Phaseolin	Phaseolus vulgaris	Pratylenchus penetrans	Abawi et al., 1971
5.	Medicarpin	Medicago sativa	P. penetrans	Baldridge et al., 1998
6.	Phenyl-phenalenones: anigorufone	Musa spp.	Radopholus similis	Holscher et al., 2014

Conclusion:

The above reports suggest that phytoalexins are not extensively studied for nematode resistance in plants but these can have potential in nematode management as in other pathogens.

References

• Abawi, G.S., Van Etten, H. D., and Mai. W. F. 1971. Phaseollin production induced by Pratylenchus penetrans inPhaseolus vulgaris. Journal of Nematology, 3:301.
• Baldridge, G. D., O’Neill, N. R. and Samac D. A. 1998. Alfalfa (Medicago sativa L.) resistance to the root-lesion nematode, Pratylenchus penetrans: defense-response gene mRNA and isoflavonoid phytoalexin levels in roots. Plant Molecular Biology, 38: 999-1010.
• Braga, M.R. et al. 1991. Phytoalexins induction in Rubiaceae. Journal of Chemical Ecology, 17:1079-1090.
• Holscher et al. 2014. Phenalenone-type phytoalexins mediate resistance of banana plants (Musa spp.) to the burrowing nematode Radopholus similis. PNAS, 111.1: 105–110.
• Huang, J. and Barker, K.R. 1991. Glyceollin I in soybean-cyst nematode interactions- spatial and temporal distribution in roots of resistant and susceptible soybeans. Plant Physiology, 96: 1302-1307.
• Kaplan, D. T., Keen, N. T. and Thomason, I. J. 1980. Studies on the mode of action of glyceollin in soybean incompatibility to the root-knot nematode, Meloidogyne incognita. Physiological Plant Pathology, 16: 319-325.
• Rich, J. R., Keen, N. T. and Thomason, I. J. 1977. Association of coumestans with the hypersensitivity of lima bean roots toPratylenchus scribneri. Physiological Plant Pathology, 10: 105-I16.
• Veech, J. A. 1982. Phytoalexins and their role in the resistance of plants to nematodes. Journal of Nematology ,14(1): 149-161.

About Author / Additional Info:
Scientist: Nematology,
Division of Plant Quarantine, ICAR-National Bureau of Plant Genetic Resources, Pusa Campus, New Delhi-110012