Specificity in degradation of isoflavonoid phytoalexins

Since the first observations by De Wit-Elshove (1969), that pea pathogens, contrary to non-pathogens, are able to degrade, and at least partially detoxify the pea phytoalexin pisatin, similar observations have been made for almost all isoflavonoid phytoalexins. This led to the idea, that breakdown of phytoalexins to products with a lower fungitoxicity is a prerequisite for pathogenic fungi to successfully colonize their host. There are, indeed, striking examples of closely related fungal species (e.g. Ascochyta pisi versus A. fabae; Van’t Land et al. 1975a), or even formae speciales within one species (e.g. Fusarium oxysporum f. sp. pisi versus F. oxysporum f. sp. lycopersici; Fuchs et al. 1976, 1979), in which one, the pea pathogen, readily converts pisatin to less or even non-fungitoxic compounds, whereas the other, the noli-pea pathogen, is not able to do so. In the case of A. pisi this led Van ’t Land et al. (1975b) to attach a significant importance to the ability of pisatin degradation in eliminating feed back inhibition of ascochitine biosynthesis by pisatin. Race specificity within A. pisi (Cf. Van ’t Land & Fuchs 1973), on the other hand, could not be explained in terms of differential elicitation or degradation of phytoalexin (unpublished results). Many examples have been found now which contradict the above-mentioned view, that all – and only – pathogens should be capable of metabolizing host phytoalexins. For instance, Fusarium oxysporum f. sp.phaseoli and Thielaviopsis basicola, both pathogenic to bean, did not metabolize phaseollin in shake cultures (Van den Heuvel & Glazener 1975). Further, Aphanomyces euteiches, although being a notorious pea pathogen, was found to be unable to degrade pisatin (Pueppke & Van Etten 1976). On the other hand, metabolism of phytoalexins appeared not to be restricted to legume pathogens: for instance, Cladosporium herbarum detoxifies phaseollin to la-hydroxyphaseollone (Van den Heuvel & Glazener 1975), Colletotrichum coffeanum hydroxylates medicarpin to 6a,7-dihydroxymedicarpin (Ingham 1976), Septoria nodorum converts phaseollin to 12,13-dihydrodihydroxyphaseollin (Bailey et al. 1977), and Fusarium anguioides and F. avenaceum metabolize pisatin to 6a-hydroxyinermin (= 3,6adihydroxy-8,9-methylenedioxypterocarpan; Lappe & Barz 1978).