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.
References:
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