The Plant Cell, Vol. 8, 1821-1831, October 1996 0 1996 American Society of Plant Physiologists Preformed Antimicrobial Compounds and Plant Defense against Funga1 Attack Anne E. Osbourn The Sainsbury Laboratory, John lnnes Centre, Colney, Norwich NR4 7UH, United Kingdom INTRODUCTION Plants produce a diverse array of secondary metabolites, many tance to particular pathogens, but often these attempts have of which have antifungal activity. Some of these compounds failed to reveal any positive correlation. However, whereas are constitutive, existing in healthy plants in their biologically preformed inhibitors may be effective against a broad spec- active forms. Others, such as cyanogenic glycosides and trum of potential pathogens, successful pathogens are likely glucosinolates, occur as inactive precursorsand are activated to be able to circumvent the effects of these antibiotics either in response to tissue damage or pathogen attack. This activa- by avoiding them altogether or by tolerating or detoxifying them tion often involves plant enzymes, which are released as a (Schonbeck and Schlosser, 1976; Fry and Myers, 1981; Bennett result of breakdown in cell integrity. Compounds belonging and Wallsgrove, 1994; VanEtten et al., 1995; Osbourn, 1996). to the latter category are still regarded as constitutive because The isolation of plant mutants defective in the biosynthesis of they are immediately derived from preexisting constituents preformed inhibitors would allow a direct genetic test of the (Mansfield, 1983). VanEtten et al. (i994) have proposed the importance of these compounds in plant defense. However, term “phytoanticipin” to distinguish these preformed an- in most cases this approach is technically difficult because timicrobial compounds from phytoalexins, which are synthesized of the problems associated with screening for lossof the com- from remote precursors in response to pathogen attack, prob- pounds. In the absence of plant mutants, a complementary ably as a result of de novo synthesis of enzymes. In recent approach involving the study of fungal mechanisms of resis- years, studies of plant disease resistance mechanisms have tance to preformed inhibitors, and of the contribution of this tended to focus on phytoalexin biosynthesis and other active resistance to fungal pathogenicity to the relevant host plants, responsestriggered after pathogen attack (Hammond-Kosack offers another route toward investigating the importance of and Jones, 1996, in this issue). In contrast, preformed inhibi- these inhibitors in plant defense. tory compounds have received relatively little attention, despite A large number of constitutive plant compounds have been the fact that these plant antibiotics are likely to represent one reported to have antifungal activity. Well-known examples in- of the first chemical barriers to potential pathogens. clude phenols and phenolic glycosides, unsaturated lactones, The distribution of preformed inhibitors within plants is of- sulphur compounds, saponins, cyanogenic glycosides, and ten tissue specific (e.g., Price et ai., 1987; Poulton, 1988; Davis, glucosinolates (reviewed in Ingham, 1973; Schonbeck and 1991; Fenwick et ai., 1992; Bennett and Wallsgrove, 1994), and Schlosser, 1976; Fry and Myers, 1981; Mansfield, 1983; Ku6, r there is a tendency for these compounds to be concentrated 1992; Bennett and Wallsgrove, 1994; Grayer and Harborne, in the outer cell layers of plant organs, suggesting that they 1994; Osbourn, 1996). More recently, 5-alkylated resorcinols may indeed act as deterrents to pathogens and pests. Some and dienes have been associated with disease resistance, in diffusible preformed inhibitors, such as catechol and pro- this case, resistance of subtropical fruits to infection by Col- tocatechuic acid (which are found in onion scales), may lefotrichum gloeosporioides (Prusky and Keen, 1993). However, influence fungal growth at the plant surface. In general, how- only a few classes of preformed inhibitor have been studied ever, preformed antifungal compounds are commonly in detail to determine their possible roles in plant defense sequestered in vacuoles or organelles in healthy plants. There- against fungal pathogens. This review focuses on three of fore, the concentrations that are encountered by an invading these classes-saponins, cyanogenic glycosides, and glu- fungus will depend on the extent to which that fungus causes cosinolates-and summarizes our current knowledge of the tissue damage. Biotrophs may avoid the release of preformed role of these preformed inhibitors in determining the outcome inhibitors by minimizing damage to the host, whereas necro- of encounters between plants and phytopathogenic fungi. trophs are likely to cause substantial release of these compounds. The nature and leve1 of preformed inhibitors to which a potential pathogen is exposed will also vary, depend- SAPONINS ing on factors such as host genotype, age, and environmental conditions (Price et ai., 1987; Davis, 1991). There have been numerous attempts to associate natural Because many saponins exhibit potent antifungal activity and variation in levels of preformed inhibitors in plants with resis- are often present in relatively high levels in healthy plants, these 1822 The Plant Cell molecules have been implicated as determinants of a plant's A resistance to funga1 attack (Osbourn, 1996). A number of other properties are also associated with these compounds, includ- ing piscicidal, insecticidal, and molluscicidal activity; allelopathic action; and antinutritional effects (Price et al., 1987; Fenwick et al., 1992; Hostettmann and Marston, 1995). I Saponins are glycosylated compounds that are widely B-D-gluIl+ 4) Avenacin A-1 distributed in the plant kingdom and can be divided into three major groups, depending on the structure of the agly- cone, which may be a triterpenoid, a steroid, or a steroidal glycoalkaloid. Triterpenoid saponins are found primarily in di- B cotyledonous plants but also in some monocots, whereas steroid saponins occur mainly in monocots, such as the Lilia- ceae, Dioscoraceae, and Agavaceae, and in certain dicots, such as foxglove, which contains the saponin digitonin (Hostettmann and Marston, 1995). Oats (genus Avena) are unusual because Avenacoside A they contain both triterpenoid and steroid saponins (Price et al., 1987). Steroidal glycoalkaloids are found primarily in mem- bers of the family Solanaceae, which includes potato and tomato, but also in the Liliaceae (Hostettmann and Marston, 1995). The saponins produced by oats and tomato have been studied in the greatest detail in relation to their potential role in the defense of plants against phytopathogenic fungi (Osbourn, 1996). These saponins are considered in depth below. a-Tomatine Avenacins and Avenacosides Figure 1. Structures of Oat and Tomato Saponins. Oats contain two different families of saponins, the triterpe- (A) The molecular structure of the major oat root saponin avenacin noid avenacins (A-1, 6-1, A-2, and 6-2) and the steroidal A-I (a monodesmosidictriterpenoid). The structures of the other avena- avenacosides (avenacosides A and B; Price et al., 1987). The cins (not shown) are very similar: avenacin 8-1, like avenacin A-1, is chemical structures of an example of each family (avenacin esterified with N-methyl anthranilic acid, whereas avenacins A-2 and A-1 and avenacoside A) are illustrated in Figure 1. The occur- 8-2 are esterified with benzoic acid. The A-I and A-2 compounds each rence of avenacins and avenacosides is restricted primarily contain one extra oxygen atom, relative to their 8-1 and 8-2 counter- to the genus Avena, although avenacins also occur in the parts. All four avenacin compounds have the same trisaccharide group closely related species Arrhenatherum elafius (Turner, 1953; attached to C-3. (B) The molecular structure of the bisdesmosidic foliar oat saponin Crombie and Crombie, 1986). The distribution of the two types avenacoside A. Avenacoside B (not shown) differs from avenacoside of saponin within the plant is mutually exclusive: the avena- A only in that it has an additional P,1-3-linked o-glucose molecule at- cins are located in the root (Goodwin and Pollock, 1954; Maizel tached to the C-3 sugar chain. et al., 1964), and the avenacosides are located in the leaves (C) The monodesmosidic tomato steroidal glycoalkaloid, a-tomatine. and shoots (Tschesche et al., 1969; Tschesche and Lauven, These saponins all have a sugar chain attached to C-3via oxygen. 1971). Within these tissues, the major avenacin, avenacin A-1, Avenacosides A and B each have an additionalsugar moiety (D-glucose) is restricted to the root epidermal cells (Osbourn et al., 1994), attached via oxygen to C-26. Removal of this C-26 o-glucose by a whereas the avenacosides are present at higher concentra- specific oat glucosyl hydrolase (avenacosidase) converts the avenaco- tions in the epidermis than in the rest of the leaf (Kesselmeier sides to their biologically active monodesmosidic forms (26-desgluco- avenacosides A and 0). and Urban, 1983). The avenacins are referred to as monodesmosidic because they have a single sugar chain attached at the C-3 position (Figure 1A). They exist in the plant in an active form. The avenacosides are bisdesmosidic and have an additional D-glucose molecule at C-26 (Lüning and Schlosser, 1975; o-glucose molecule attached to C-26 (Figure 1B). Avenaco- Nisius, 1988; Gus-Mayer et al., 1994a, 1994b). sides are biologically inactive but are converted into the There has been considerable interest