Plants use an intricate defense system against pests and pathogens i.e. they resist pathogens with combinations of constitutive and induced defense system. Of the induced defense, phytoalexin production is one (Hammerschmidt and Dann 1999). Plants respond to infection by producing and accumulating low-molecular-weight secondary metabolites with antimicrobial activity called phytoalexins. The phytoalexins are generally lipophilic substances that are products of a plant’s secondary metabolism and they often accumulate at infection sites in concentrations that can inhibit the development of the pathogen. Multi-component response mechanism associated with disease resistance include synthesis of phytoalexins in plant cell induced by elicitors (Sharp et al., 1984) , fortification of cell wall, accumulation of pathogenesis –related proteins and biosysnthesis of lytic enzymes.
Resistance and susceptibility in plants are not determined by the presence or absence of genetic information for resistance mechanisms or the genetic information encoding for the enzyme involved in the phytoalexin synthesis, but it is rather determined by the speed with which the information is expressed, the activity of the gene products and the magnitude of the resistance response (Kuć and Rush 1985).
More than 200 phytoalexins have been structurally characterized (Huang 2001) few examples are as follows:
- Brassinin, cyclobrassinin, methoxybrassinin, (Takasugi et al., 1986) from cabbage, radish and turnip
- Sinalexin (Pedras and Smith 1997) and Sinalbins (Pedras and Zaharia 2000) from white mustard
- Ipomeamarone from sweet potato (Kubota and Matsuura 1953)
- Pisatin from pea (Perrin and Bottomley 1962)
- Xanthotoxin from parsnip (Johnson et al., 1973)
- Phaseollin, phaseollidin and kievitone from cowpea (Bailey 1973)
- Tsibulins from onion (Dmitriev et al., 1990)
- Camalexin from Arabidopsis thaliana (Tsuji et al., 1992)
- Glyceollins from soybean (Burden and Bailey 1975)
- Medicarpin, and maackiain from chickpea (Ingham, 1976)
Most phytoalexins produced by the Leguminosae belong to six isoflavonoid classes: isoflavones, isoflavanones, pterocarpans, pterocarpenes, isoflavans and coumestan (Jeandet et al., 2013).
A single mode of action of phytoalexins is difficult to assess because of the diversity of the chemical structure. Most of the effect is on the membrane integrity and respiration. The possible site of the action of phytoalexins are:
- Camalexin disrupts membrane of Pseudomonas maculicola (Rogers et al., 1996)
- Glyceollin acts on soybean tonoplast resulting in proton leakage (Giannini et al., 1991)
- Xanthotoxin acts on mitochondrial membranes of onion root cells. Inhibits oxygen uptake (Kupidlowska et al., 1994)
Plants defend themselves against pathogen by means of phytoalexins. The pathogenicity of certain pathogens depends on their ability to detoxify or tolerate these phytoalexin produced by the plants. In addition to detoxification several other method are there to explain the differential sensitivity to phytoalexins among fungi (Huang 2001).
- Fungi that lack sensitivity to particular phytoalexins may contain a plasmalemma or cell wall which is less permeable to these molecules as compared to that of sensitive organisms
- The site of action in the insensitive fungal cell possess weak affinity for the phytoalexin
- The insensitive organism possesses a metabolic bypass for the process which is inhibited by the phytoalexins and therefore circumvents the toxic effect
Plants are continuously in contact with different microorganisms. The interaction between plant and pathogen depend on the ability of the microorganism to metabolize phytoalexins. Sometime the relationships established with microorganisms are beneficial like the rhizobacteria found in close association to the plant root which are able to control plant diseases caused by soil pathogens (Bais et al., 2006), or the mycorrhizal fungi that helps plant in their growth and development (Gayer and Kokubun 2001) or the symbiotic association of nitrogen fixing bacteria with leguminous plants. During the establishment of symbiotic relationship between Bradyrhizobium japonicum and the soybean plant the rhizobia in the initial stage of interaction acts as a plant pathogen to invade the root hairs of the plant and overcome the possible defense responses. Rhizobia neither takes up the phytoalexin glyceollin nor degrades or detoxifies glyceollin. The possibility is that the plasmalemma or cell wall of the rhizobia are modified during the adaptation period and these modifications prevent glyceollin from entering the rhizobial cell (Parniske et al., 1991).
In plant pathogens, the transporters play an essential role in protection against plant defense compounds during pathogenesis (Sorbo et al., 2000). Phytopathogenic fungi evolved mechanisms of resistance by extruding toxic compounds out of the cell through transporters, conferring them protection against plant defense products. Detoxification of phytoalexins by fungi may limit the practical application of these defense compounds.
Bais, H. P., Weir, T. L., Perry, L. G., Gilroy, S. and Vivanco, J. M. 2006 The Role of Root Exudates in Rhizosphere Interactions With Plants and Other Organisms. Annu. Rev. Plant Biol. 57: 233–266
Bailey, J. A. 1973 Production of Antifungal Compounds in Cowpea (Vigna sinensis) and Pea (Pisum sativum) After Virus Infection J. Gen. Microbiol. 75: 119 – 123
Burden, R. S. and Bailey, J. A. 1975 Structure of the Phytoalexin from Soybean. Phytochem. 14: 1389 – 1390
Dmitriev, A. P., Tverskoy, L.A., Kozlovsky, A. G. and Grodzinsky, D. M. 1990 Phytoalexins from Onion and Their Role in Disease Resistance. Physiol. Mol. Plant Pathol. 37: 235 – 244
Giannini, J. L., Halvorson, J. S. and Spessard, G. O. 1991 High Yield Isolation and Effect on Proton Leakage of Glyceollins I and III. Phytochem. 30: 3233 – 3236
Grayer, R. J. and Kokubun, T. 2001 Plant-Fungal Interactions: The Search for Phytoalexins and Other Antifungal Compounds from Higher Plants. Phytochem. 56: 253–263
Hammerschmidt, R. and Dann, E. K. 1999 The Role of Phytoalexins in Plant Protection. Novartis Found Symp. 223: 175-87
Huang, J-S. 2001 Accumulation of Phytoalexins as a Resistance Mechanism in “Plant Pathogenesis and Resistance Biochemistry and Physiology of Plant-Microbe Interaction” Kluwer Academic Publisher Chapter 9: 525 – 621
Ingham, J. L. 1976 Induced and Constitutive Isoflavonoids from the Stem of Chickpeas (Cicer arietinum L.) Inoculated with the Spores of Helminthosporium carbonum Ullstrup. Phytopathol. Z 87: 353 – 367
Jeandet, P., Clément, C., Courot, E. and Cordelier, S. 2013 Modulation of Phytoalexin Biosynthesis In Engineered Plants for Disease Resistance. Int. J. Mol. Sci. 14: 14136–14170
Johnson, C., Brannon, D. R. and Kuc, J. 1973 Xanthotoxin: A Phytoalexin of Pastinaca sativa Root. Phytochem. 12: 2961 – 2962
Kubota, T. and Matsuura, T. 1953 Chemical Studies on the Black Rot Disease of Sweet Potato. V. Chemical Constitution of Ipomeamarone. J. Chem. Soc. Japan 74: 248 – 251
Kuć, J. and Rush, J. S. 1985 Phytoalexins. Arch Biochem. Biophys. 236(2): 455-72
Kupidlowska, E., Dobrzynska, K., Parys, E. and Zobel, A. M. 1994 Effect of Coumarin and Xanthotoxin on Mitochondrial Structure, Oxygen Uptake and Succinate Dehydrogenase Activity in Onion Root Cells. J. Chem. Ecol. 20: 2471 – 2480
Parniske, M., Ahlborn, B. and Werner, D. 1991 Isoflavanoid-Inducible Resistance to the Phytoalexin Glyceollin in Soybean Rhizobia. J. Bacteriol. 173: 3432 – 3439
Pedras, M. S. C. and Smith, K. C. 1997 Sinalexin a Phytoalexin from White Mustard Elicited by Destruxin B and Alternaria brassicae. Phytochem. 46: 833 – 837
Pedras, M. S. C. and Zaharia, I. L. 2000 Sinalbins A and B Phytoalexins from Sinapis alba: Elicitation, Isolation and Synthesis. Phytochem. 55: 213 – 216
Perrin, D. R. and Bottomley, W. 1962 Studies on Phytoalexins. V. The Structure of Pisatin from Pisum sativum L. J. Amer. Chem. Soc. 84: 1919 – 1922
Rogers, E. E., Glazebrook, J. and Ausbel, F. M. 1996 Mode of Action of the Arabidopsis thaliana Phytoalexin Camalexin and its Role in Arabidopsis- Pathogen Interactions. Mol. Plant-Microbe Interact. 9: 748 – 757
Sharp, J. K., Valent, B. and Albersheim, P. 1984 Purification and Partial Characterization of a Beta-Glucan Fragment That Elicits Phytoalexin Accumulation in Soybean. The Journal of Biological Chemistry 259: 11312-11320
Sorbo, G. D., Schoonbeek, H. and De Waard., M. A. 2000 Fungal Transporters Involved in Efflux of Natural Toxic Compounds and Fungicides. Fungal Genetics and Biology 30 (1): 1-15
Takasugi, M., Katsui, N. and Shirata, A. 1986 Isolation of Three Novel Sulphur- Containing Phtoalexins From the Chinese Cabbage Brassica campestris L. ssp. Pekinensis (Cruciferae) J. Chem. Soc. Chem. Commun. 1077 – 1078
Tsuji, J., Jackson, E. P., Gage, D. A., Hammerschmidt, R. and Somerville, S. C. 1992 Phytoalexin Accumulation in Arabidopsis thaliana During the Hypersensitive Reaction to Pseudomonas syringae pv. Syringae. Plant Physiol. 98: 1304 – 1309