Isoflavonoid Phytoalexins and their Biosynthetic Enzymes

Abstract Phytoalexins are low molecular weight antimicrobial compounds which accumulate in plant tissues after exposure to micro-organisms. In the case of isoflavonoid phytoalexins , the rates of synthesis and accumulation of these compounds in leguminous plants can be correlated with the resistance of the host to the pathogen (for recent reviews see references 1-3). The biosynthesis of isoflavonoid phytoalexins (for example medicarpin) proceeds via a multistep pathway which begins with phenylalanine (Figure J). The pathway may also be extended to produce more complex isoflavonoid phytoalexins, such as the prenylated pterocarpans phaseollin and the glyceollins. In this review we will use the biosynthetic route to medicarpin to illustrate the methods used to analyse isoflavonoids and to measure the activities of the enzymes involved in their synthesis. The exact methods of treatment for induction of phytoalexins will depend on the biological system used. Whole plant tissues may be elicited with suspensions of fungi (spores or mycelia) or bacteria either by surface application or by infiltration. Alternatively, abiotic elicitors such as heavy metal salts, may be used at a final concentration of 10-2- 10-3 M. However, biotic elicitors derived from micro-organisms or plants (see Chapter 8) are frequently the inducing agents of choice. When treating suspension-cultured plant cells the sterilized elicitor is added in aqueous solution with sterile water serving as a control treatment. In the case of crude elicitor preparations from fungal cell walls elicitation may be optimized in the range 10-100 mg glucose equivalents per litre of cell culture medium. Elicitor active fractions may be most conveniently obtained from yeast (4).