Phytoalexin resistance

Figure 4 Chemotaxis, nodulation, and phytoalexin resistance, effects of intermediates of phenylpropane-metabolism. (From Ref. 64.)...

This review summarizes a recent study that evaluated the role of stilbene-based phytoalexin resistance in peanuts using six environmental control plots (Domer, J. W. Cole, R. J. Sanders, T. H. Blankenship, P. D. Phytopathology, in press). 

M. Pamiske, B. Ahiborn, and D. Werner, Isoflavonoid-inducible resistance to the phytoalexin glyceollin in. soybean rhizobia. J. Bacterial 173 2222 (1991). 

HAIN R., REIF, H.J., KRAUSE, E., LANGEBARTELS, R., KINDL, H., VORNAM, B WIESE, W., SCHMELZER, E., SCHREIER, P.H., STOCKER, R.H., STENZEL, K., Disease resistance results from foreign phytoalexin expression in a novel plant, Nature, 1993,361,153-156. 

A recent search for general and specific elicitors from L. maculans demonstrated that the phytotoxins sirodesmin PL (1) and deacetylsirodesmin PL (2) are general elicitors since both induced the production of phytoalexins in resistant brown mustard and in susceptible canola [31]. Furthermore, two specific elicitors, a mixture of cerebrosides C (13) and D (14), were reported from mycelia of liquid cultures of L. maculans virulent on canola (Fig. 9.5) [19]. Previously, cerebrosides C (13) and D(14) were reported from a number of phytopathogenic fungi and were reported to induce the production of phytoalexins in rice plants and disease resistance to the rice blast fungus [32]. 

In the Andes, nearly all of the about 160 varieties of wild potatoes, and two of the eight cultivated species are toxic. Some frost-resistant species that grow above 3600 m have high levels of alkaloids, which are bitter and potentially toxic. These are hybrids between Solanum stenotonum and wild potatoes such as Solanum megistacrolohum and Solanum acaulc. Other secondary plant compounds in potatoes are saponins, phytohemagglutinin, proteinase inhibitors, sesquiterpene phytoalexins, and phenols. 

In the family of the Gramineae, which includes some of man s most important crops, active lignification seems to be of special importance for induced resistance mechanisms (19,20). This may be correlated with the nearly complete absence of phytoalexins in this family (21). In spite of an intensive search for such infection-induced fungitoxic substances, no phytoalexins have been found in wheat to date (22). Nevertheless, induced lignification has been shown to play an important role in disease resistance of wheat against a variety of fungal pathogens (4) ... 

Tiburzy (22,31) obtained similar results by application of the PAL inhibitor aminooxyacetic acid (AOA). However, AOA does not specifically inhibit PAL (99), and PAL is not only involved in lignin biosynthesis (100). Thus, AOA and the related inhibitor aminooxyphenyl propionic acid (AOPP) (101,102) inhibit the biosynthesis of lignin (103,104), anthocyanins (105), other flavonoids (106), and conjugates of cinnamic acids (107) via PAL, as well as ethylene (108-110) via a pyridoxal phosphate dependent enzyme (110,111). In view of the possible function of phenolic compounds as phytoalexins (21,112,113) and the well documented role of ethylene in some resistance reactions (114-116), the above cited experiments with AOA (22,... 

The term elicitor, initially defined as a fungal metabolite capable of inducing phytoalexin production when applied to host plants (122, 123), has since been applied to parasite-derived molecules which induce any facet of resistance in appropriate host plants, including lignification (124). 

He, X.Z. and Dixon, R.A., Genetic manipulation of isofiavone 7-0-methyltransferase enhances biosynthesis of 4 -0-methylated isofiavonoid phytoalexins and disease resistance in alfalfa. Plant Cell, 12, 1689, 2000. 

Dillon, V.M. et al., Differences in phytoalexin response among rice cultivars resistance to blast. Phytochemistry, 44, 599, 1997. 

In peanut, small molecular weight phytoalexins, the stilbenes (76, 77) are induced by the presence of invading aflatoxigenic fungi and appear to be correlated with ability to resist fungal attack. 

De Novo Synthesis of Phytoalexins. Phytoalexins have been studied in great depth by plant pathologists. Excellent review papers are available in Hedin s ACS symposium volume. Host Plant Resistance to Plants (42), and more recently in the book edited by Horsfall and Cowling, Plant Disease (43). The antiherbivory effect of phytoalexins, however, is only now beginning to be fully appreciated. It is apparent that pathogen induced phytoalexins do have a definite effect on insect herbivores. There is mounting evidence that herbivore-inflicted injury may also result in the induction of phytoalexin production and accumulation. 

A better understanding of the role of phytoalexins in plant defenses and of the mechanisms of induced resistance may potentially open a powerful new approach to the control of insect pests of cultivated plants. If indeed, in light of the hypothesis of optimal defense strategies (3), a post-attack response is a more efficient line of defense than the attack-independent accumulation of allelochemics, the exploitation of phytoalexin-producing mechanisms may represent a fertile field for future investigations. Several uses of induced resistance may be conceived. Four of these approaches are briefly discussed. 

Inducible systems of defense (phytoalexins) are widespread in plants and are effective against many types of fungi and bacteria (37,38). Similar systems have been demonstrated in a few cases with insects and are probably common in nature (see for example reference 39). Although Inducible systems of insect resistance would seem to be efficient and effective, no system is foolproof. The larvae of Epilachna tredecimnotata cut a circular trench in Cucurbita leaves and prevent mobilization of the deterrent substances to the area which is then consumed (40). 

Many flavonoids are known to be phytoalexins, antiviral agents, and to serve as antiinflammatory and antitumor compounds. Several isoflavones have estrogenic activity in mammals (70). (-)-Vestitol and sativan, isoflavans from Lotus species, are phytoalexins. 3R(-)-Vestitol from the resistant pasture legume Lotus pedunculatus, has been demonstrated to be a feeding deterrent... 

Systemic Acquired Resistance. SAR is the induction of a transient long-distance (translocated) defence response to fungal attack that is distinct from the local production of phytoalexins. The transient nature of SAR is important and has likely been selected in the course of evolution as a corollary to the energy demands that are made on the plant by the permanent mobilisation of resistance mechanisms. However, plants challenged by fungi are sensitised to subsequent attack and the speed of their SAR response is increased. 

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