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  • Pergamon 0031-9422(94)EO287-3 Phytockmistry, Vol 37, No 1, pp. 19-42, 1994 Eheviet Science Ltd

    Printed in Gnat Britain. oml-9422/94 sz4.cm+om

    REVIEW ARTICLE NO. 92

    A SURVEY OF ANTIFUNGAL COMPOUNDS FROM HIGHER PLANTS, 1982-1993

    RENBE J. GRAYER and JEFFREY B. HARBORNE

    Department of Botany, University of Reading, Whiteknights, Reading RG6 2AS, U.K.

    (Received 30 March 1994)

    IN HONOUR OF PROFESSOR ROBERT HEGNAUERS SEVENTY-FIFTH BIRTHDAY

    Key Word Index-Flowering plants; antifungal agents; constitutive compounds; phytoalexins; second- ary metabolites.

    Abstract-Recent work on the characterization of antifungal metabolites in higher plants is reviewed. Interesting new structures are discussed and the distribution of those substances in different plant families is outlined. Distinction is made between constitutive antifungal agents and phytoalexins, which are specifically formed in response to fungal inoculation. The literature survey covers the 12 years since 1982.

    INTRODUCTION

    A fungal spore landing on the leaf surface of a plant has to combat a complex series of defensive barriers set up by the plant before it can germinate, grow into the plant tissues and survive. The arsenal of weapons against the fungus includes physical barriers (e.g. a thick cuticle) and chemical ones, i.e. the presence or accumulation of anti- fungal metabolites. These can be preformed in the plant, the so called constitutive antifungal substances, or they are induced after infection involving de novo enzyme synthesis, the induced antifungal constituents or phyto- alexins. Since the latter compounds can also be induced in plants by means of abiotic factors, e.g. UV irradiation, Ingham [l] defines phytoalexins as antibiotics formed in plants via a metabolic sequence induced either biotically or in response to chemical or environmental factors.

    Constitutive antifungal substances were called pro- hibitins by Schmidt [2], but Ingham [1] restricts this term to those pre-infectional plant metabolites which are normally present in concentrations high enough to in- hibit most fungi. In other plant species, the concentration of an antifungal substance may normally be low, but may increase enormously after infection in order to combat attack by micro-organisms; Ingham called this type of constituent an inhibitin. A third type of constitutive compound which he called a post-inhibitin, is defined as an antimicrobial metabolite produced by plants in re- sponse to infection, but whose formation does not involve the elaboration of a biosynthetic pathway within the tissues of the host. Post-inhibitins are normally present in the plant in an inactive, bound form, but are converted

    into the active antifungal substance after infection by means of a short and simple biochemical reaction, such as enzymic hydrolysis, e.g. cyanogenic glycosides which release toxic HCN after infection or leaf damage. This process of activation only takes a short time, since the enzyme(s) needed for the reaction are already present in the uninfected plant, though stored in a different com- partment. Damage or infection of the plant brings to- gether the enzyme and inactive form of the compound to produce the active post-inhibitin. In contrast, phytoal- exin production may take two or three days, as by definition it first requires the synthesis of the enzyme systems needed for their biosynthesis.

    For some compounds it is difficult to determine whether they are phytoalexins or constitutive antifungal compounds (especially inhibitins and post-inhibitins) as the distinction between them is not always clear. More- over, the same compound may be a preformed antifungal substance in one species and a phytoalexin in another. For instance, the flavanone sakuranetin (1) is constitutive in blackcurrant leaves (Ribes nigrum, Grossulariaceae) [3], but induced in rice leaves (Oryza sativa, Gramineae) [43. Additionally, some compounds may be phytoalexins in one organ and constitutive in another of the same plant species, e.g. momilactone A which is induced in rice leaves as a phytoalexin [S], but occurs constitutively in rice seeds [6]. In the literature review below we have dis- tinguished between preformed antifungal compounds and phytoalexins, but the former are not further sub- divided into prohibitins, inhibitins and post-inhibitins, since it is not always possible to infer from the data given

    19

  • 20 R. J. GRAYER and J. B. HARBORNE

    2

    COCMS OH Ii

    *

    3 R=a-OH 4 R-f%OIi

    &4 / P R \ m k2

    5 R-iPr,R-R*-0 6 R-iFr,R=OH,R*=H 7 R=OE,R=R2=H

    in papers to which of these subclasses a given pre- infectional substance belongs. Furthermore, the survey is restricted to antifungal metabolites of low molecular weight, although it has recently become apparent that the production of antifungal macromolecules such as pro- teins may also play an important role in the defence systems of higher plants against pathogens.

    CONSTITUTIVE ANTIFUNGAL SUBSTANCES

    Several useful short reviews on the occurrence of preformed antifungal compounds in relation to their role in plant resistance have appeared in the last two decades, notably those by Ingham Cl], Mansfield [7] and Heg- nauer [8]. The present review covers the literature on the subject over the last 12 years. Antifungal compounds described during this period are classified here in two

    ways, first according to their taxonomic distribution, and second according to their chemical structures. This was done to see whether certain plant families or genera specialize in the accumulation of certain types of constitu- tive compounds as they are known to do for induced constituents (e.g. sesquiterpenoid phytoalexins in Solana- ceae; isoflavonoid phytoalexins in Leguminosae [9]), and to see whether any structure-activity relationships are apparent.

    Taxonomic distribution

    Table 1 gives a listing of pre-infectional substances found since 1982 arranged according to their occurrence in plant families and species. The chemical class to which each compound belongs is also given, and additionally the plant organ from which it was isolated and the pathogenic fungus on which the antifungal tests were based. The format of this table is similar to that used by Mansfield [7].

    From Table 1 it is apparent that the antifungal com- pounds found in the taxa surveyed in the last decade belong to a very wide range of chemical classes, and that even closely related species produce their own specific antifungal substances. Thus, although the antifungal compounds newly isolated from Compositae are all phenolic, they belong to different chemical subclasses. Even in the two species of Helichrysum investigated two types of phenolic were recorded: phloroglucinol derivat- ives from H. decumbens [lo] and methylated flavonoids from H. nitens [ll]. In the Gramineae the range of constitutive antifungal substances reported is even wider, e.g. saponins in oats [12], an alkaloid in barley [13], fatty acids in rice [14,15], phenolics in Sorghum [16] and alkadienals in wheat [17]. In the Leguminosae there is also substantial variation, ranging from chalcones, flav- ans and a diphenylpropene in Bauhiniu mama [lS] to isoflavones in Lupinus albus [19,20] and saponins in Dolichos kilimandscharicus [21]. But most species listed in Table 1 in the Compositae, Gramineae and Leguminosae belong to different genera or tribes. On the other hand, species belonging to the same genus in Table 1 generally contain related antifungal substances, the two Heli- chrysum species mentioned above being an exception. Thus, Scutellaria uiolacea and S. woronowii (Labiatae) contain closely related antifungal neo-clerodane diterpen- oids [22], Glycosmis cyanocarpa and G. mauritiana (Ruta- ceae) both produce antifungal sulphur-containing amides [23,24], and the epicuticular leaf wax of both Nicotiana tabacum and N. glutinosa (Solanaceae) contains antifun- gal diterpenoids [25,26]. Finally, cultivated rice (Oryza s&vu, Gramineae) produces a range of fatty acids as constitutive antifungal substances [15], whereas its wild relative, 0. o&in&, produces a biogenetically related compound, jasmonic acid (2) [27]. However, relatively little chemosystematic work on preformed antifungal compounds seems to have appeared in the last 12 years. An exception is perhaps the work by Picman [28] on the antifungal activity of sesquiterpene lactones found in Compositae, but the aim of this research was to investig-

  • Table

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