Phototoxic Metabolites of Tropical Plants - ACS Publications

Chapter 25

Phototoxic Metabolites of Tropical Plants 1

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Lee A. Swain and Kelsey R. Downum

Downloaded by UNIV MASSACHUSETTS AMHERST on August 1, 2012 | http://pubs.acs.org Publication Date: January 9, 1991 | doi: 10.1021/bk-1991-0449.ch025

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Department of Biological Sciences, Florida International University, Miami, FL 33199 Fairchild Tropical Gardens, 10901 Old Cutler Road, Miami, FL 33156

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Over 400 species of tropical plants from 76 families were assayed for phototoxic activity. Furanocoumarins, an important class of phototoxins, were identified from three genera of the Moraceae (fig family).Dorstenia,an herbaceous member of this family, was particularly rich in these metabolites. The distribution of furanocoumarins in the Moraceae as well as evolutionary and ecological aspects are discussed.

Phototoxic phytochemicals, or "photosensitizers", exhibit broad spectrum biocidal activity against a range of organisms including viruses, bacteria, fungi, nematodes, insects and other plants (1-12). They are also responsible for causing serious health problems in range animals and man (13-191 After absorbtion of light (usually in the UV-A range, 320-400nm), photosensitizers undergo a transition to an excited state. Some of them (type II phototoxins) can transfer the excitation energy to molecular oxygen to form singlet oxygen, which is capable of oxidizing many biomolecules. Cell membranes and cell wall components are particularly vulnerable to these types of phototoxins (20). Others (photogenotoxins) react directly with DNA and RNA, though reactions with other cellular components are known (21-25). Although most of the information concerning the biological activity of these phototoxins has been demonstrated jn vitro, it is likely that they provide a viable defensive mechanism against nonadapted organisms. Phototoxins in the wild parsnip, for example, are effective in killing or deterring generalist insects such as the armyworm (Spodoptera eridania). while they have little effect on swallowtail butterfly larvae (Papilio polyxenes) which utilize the wild parsnip as a food plant (26-31). Swallowtail caterpillars are able to detoxify the chemicals and excrete them as harmless waste products (32-37).

0097-6156/91/0449-0361$06.00/0 © 1991 American Chemical Society In Naturally Occurring Pest Bioregulators; Hedin, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


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NATURALLY OCCURRING PEST BIOREGULATORS

Distribution of phototoxins Various biosynthetic classes of phototoxic compounds have been isolated from over thirty plant families (5). Some plant families produce more than one type of phototoxin. Three different types of phototoxins have been isolated from the Apiaceae ( celery family), for example, and eight different types have been identified so far from the Rutaceae (citrus family). Among other families that contain phototoxins are the Asteraceae (sunflower family), Euphorbiaceae (spurge family), Fabaceae (pea family) and Moraceae (fig family) (£). It was suggested that photosensitizers would be most effective in plant species that evolved under high-light environments and that the incidence of phototoxin-containing plants might be greater in these areas (2). A recent survey for phototoxins in one high-light environment, the desert southwestern United States, suggested that light-activated phytochemicals were relatively common in families in which phototoxins had previously been described (38). Over 35% of the extracts from members of the Asteraceae exhibited phototoxic activity, with many of these belonging to a limited number of tribes of this family. Almost half of the members of the tribe Heliantheae, for example, tested positive for lightactivated toxins. All of the extracts of the Pectidinae, a subtribe within the Heliantheae, were phototoxic toward the test organisms. Photobiocides from tropical plants We recently surveyed a cross-section of plants from many tropical regions of the world in a search for photosensitizers to further test the above hypothesis. The methods used to test for phototoxic phytochemicals are described in detail elsewhere (22). Briefly, methanolic extracts were spotted onto sterile filter-paper discs and allowed to dry. The dried discs were placed onto replicate nutrient agar plates that had been spread with E . coli B/r (a U V resistant bacterium). The plates were incubated in the dark at 37°C for 30 min. Half of the plates were irradiated for 60 min. with eight Sylvania F40BLB U V A lamps (18W m' ), while the other half were kept in the dark. All plates were incubated overnight in the dark at 37°C, after which the zones of inhibition surrounding the filter paper discs were measured. Over 400 tropical/subtropical species representing 76 families were collected at either Fairchild Tropical Gardens or Chapman Field, USDA Plant Induction Station in Miami, F L Table I lists the number of genera and species of the families that were examined. Only five of the 76 families tested elicited phototoxic responses from E.coli. These families are listed in boldface caps in Table I . The Asteraceae has been studied extensively in regards to its phototoxic components (38). as have members of the Rutaceae (citrus family; 5.7)and the Moraceae (fig family: 40). The compound(s) responsible for phototoxicity in the Sapotaceae (sapodilla family) is not known and is the only instance of lightenhanced toxicity from this family that has been reported to date (41). Elucidation of the structure of this phytochemical is in progress and we are also examining other members of the Sapotaceae for phototoxicity. 2

Biocidal compounds of the Moraceae In contrast to the Asteraceae and the Rutaceae, the Moraceae is almost

In Naturally Occurring Pest Bioregulators; Hedin, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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SWAIN AND DOWNUM


Table I.

Phototoxic Metabolites of Tropical Plants

Tropical plants examined for phototoxic antimicrobial properties (#genera,#species examined).

Acanthaceae (6,9) Amaryllidaceae (2,2) Angiopteridaceae (1,1) Annonaceae (4,5) Apocynaceae (9,11) Aquifoliaceae (1,1) Araceae (1,1) Araucariaceae (1,1) Arecaceae (1,1) Aristolochiaceae (1,3) Asclepiadaceae (1,1) •ASTERACEAE (4,5) Barringtoniaceae (1,1) Bignoniaceae (13,15) Bombacaceae (3,3) Bromeliaceae (2,2) Buddlejacaceae (1,1) Burseraceae (1,3) Cactaceae (1,1) Capparidaceae (1,1) Caryophyllaceae (1,1) Celastraceae (2,2) Chrysobalanaceae (1,1) Cistaceae (1,2) Clusiaceae (2,2) Combretaceae (5,11) Convolvulaceae (2,2) Costaceae (1,1) Cupressaceae (2,3) Cyperaceae (1,1) Ehretiaceae (2,9) Eleagnaceae (1,1) Empetraceae (1,1) Ericaceae (1,1) Euphorbiaceae (4,4) Fabaceae (20,36) Flacourtiaceae (7,9) Heliconiaceae (1,1)

* HYPERICACEAE (1,2) Iridaceae (1,1) Lamiaceae (2,2) Lecythidaceae (1,1) Liliaceae (1,1) Lythraceae (1,1) Malphigiaceae (3,5) Malvaceae (1,1) Meliaceae (5,6) Menispermiaceae (2,2) •MORACEAE (7,86) Myrtaceae (1,1) Onagraceae (1,1) Oxalidaceae (1,2) Phileiaceae (1,1) Piperaceae (1,1) Pittosporaceae (1,4) Poaceae (1,1) Podocarpaceae (1,5) Polygalaceae (1,2) Polygonaceae (4,9) Rhamnaceae (2,3) Rosaceae (1,1) Rubiaceae (6,6) •RUTACEAE (23,57) Sapindaceae (3,4) •SAPOTACEAE (7,21) Selaginellaceae (1,1) Simaroubaceae (3,3) Solanaceae (4,8) Sterculiaceae (1,1) Taxaceae (1,1) Theophrastinaceae (1,3) Ulmaceae (1,1) Urticaceae (2,4) Verbenaceae (4,4) Zamiaceae (2,2) Zygophyllaceae (3,4)


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exclusively a tropical/subtropical family (42). Many members of this family are economically important (42-44). Besides the edible fig (mainly Ficus carica). the jakfruit (Artocarpus heterophyllus) and breadfruit (A. atilis) are important food sources throughout the tropics. In addition to their importance as food plants, many members of this family are widely used in treating diseases and other health problems (43-45). Species of Dorstenia are used for everything from mouthwash and hangover cures to emetics and diuretics. Other members provide paper (Broussonetia papyrifera) and rubber (Castilloa elastica). while still others are popular ornamentals. A summary of phototoxicity in the various genera of the Moraceae is shown in Table II (40). Of the eight genera tested, only nine

Table II.

Genus

Distribution of phototoxic activity in extracts of various members of the Moraceae.

#species assayed

Artocarpus Brosimum Cecropia Cudrania Dorstenia FatQua Ficus Morus

4 3 2 1 5 1 69 2

#species with phototoxic activity 0 0* 0 0 5 1 2"

0

* 1 species listed by Murray, £ i .al, 1982. ** 5 additional species listed by Murray, £ l .al, 1982.

species from three genera elicited phototoxic responses from E.coli. Although 72 species of Ficus were assayed, only two, or about 3%, showed phototoxic activity. In contrast, all five species of Dorstenia were phototoxic. Only one species of Fatoua was assayed, which was the third genus that tested positive for photosensitizers. HPLC analysis was performed on the extracts of the Moraceae, and a number of furanocoumarins were identified including psoralen (I) and 5methoxypsoralen (It) (5-MOP). Furanocoumarins are potent photosensitizers and their presence in a small number of Ficus species has already been reported (4642). Table III lists the distribution of furanocoumarins in Ficus. One important point that should be noted is the limited number of Ficus species from which furanocoumarins have been identified. The actual number of Ficus species tested for these compounds is unknown, but furanocoumarins have been detected in only seven species from a genus with roughly 1000 members. Furanocoumarins were also detected in Dorstenia and Fatoua. the only



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(IX R = H (IIX R = O C H (IIIX R = OOCCH(CH )C^CHCH CH=(CH )

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3

3

Table III.

Species

F. palmata

£. pymila F. religiosa F. salicifolia

£. sywmorus

3

2

Distribution of furanocoumarins in Ficus determined by HPLC.

psoralen

F. asprima F. carica

2

+ + + +

5-MOP

8-MOP

+ + + + + + +

•

+ detected from species; - not detected from species.

two herbaceous genera in the Moraceae. In addition to psoralen and 5-MOP, a new furanocoumarin was detected. After NMR and mass spectral analysis, the compound was identified as the furanocoumarin 5-EDOP (III) (4g). Unlike psoralen and 5-MOP which are highly phototoxic, 5-EDOP was only slightly antibiotic, and the activity was not enhanced by U V A irradiation. Table IV displays the distribution of furanocoumarins in Dorstenia and Fatoua. Psoralen and 5-MOP, the two highly phototoxic furanocoumarins, are mainly found in root and flowers, particularly in roots. 5-EDOP, the nonphototoxic furanocoumarin is the major furanocoumarin in Dorstenia and is particularly concentrated in leaf tissue. In addition, 5-EDOP was identified in all species of Dorstenia but was absent from Fatoua.


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Table IV. Distribution of furanocoumarins in Dorstenia and Fatoua.

Taxon

psoralen

Dorstenia contrajerva leaves flowers roots Dorstenia foetida leaves roots Dorstenia zanzibrica leaves flowers roots Dorstenia sp. (FTG 80-207) leaves flowers roots Dorstenia sp. (FTG 80-506) leaves flowers roots Fatoua villosa leaves flowers roots

5-MOP

5-EDOP

+ ++

++ +++

++ + ++ ++ +

+

+ +

++ +

+ + ++

++ ++ +++

++ ++ ++

+

+ ++ +++

++ + ++ + ++

+

+ ++

++ + ++ + ++

-

+ ++

- = not detected; + = < 10 ug/gdw; + + = 10-100 ug/gdw; + + + = > 100 ug/gdw.



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The distribution of furanocoumarins in the Moraceae raises an important point. While all of the species of Dorstenia produce furanocoumarins, this ability is found in only a small percentage of Ficus species. The evolutionary relationships within the Moraceae are still in question (42), but it is generally agreed that Dorstenia is an evolutionary advanced genus in the family. It is possible and even probable that the ancestor to this genus possessed the ability to synthesize furanocoumarins. It may even be possible that Dorstenia evolved from a furanocoumarin-producing species of Ficus. From an ecological perspective, the presence of furanocoumarins in Dorstenia and Fatoua is interesting as well. In contrast to the rest of the Moraceae, both Dorstenia and Fatoua are herbaceous. This might suggest that furanocoumarins serve a more important purpose in small, herbaceous plants than they do in large, woody plants. Even limited herbivory would have a severe effect on plants with small leaf areas such as Dorstenia and Fatoua due to loss of limited nutrients and photosynthetic ability. Although 5-EDOP does not demonstrate the phototoxic ability of either psoralen or 5-MOP, it may serve as feeding deterrent to potential herbivores. We are currently preparing to test this hypothesis. A final observation brought to light by these data involves the distribution of furanocoumarins throughout the individual Dorstenia plants. The phototoxic furanocoumarins, psoralen and 5-MOP are more concentrated in the roots of Dorstenia than above ground parts while the highest concentrations of the nonphototoxic furanocoumarin, 5-EDOP, are in the leaves. A first glance, it would seem logical for phototoxic chemicals to be concentrated in an environment where light was present in order to utilize the full potential of their toxicity. Although some light is transmitted from leaves to other parts of the plant (49). the levels of activating wavelengths are probably too low to elevate furanocoumarins to their reactive state. Recently, however, it was suggested that mechanisms other than light may activate these phototoxins (5Q). Photochemicaltype reactions can occur in the absence of light (51-52^ and it has been suggested that enzymes such as peroxidase may be capable of catalyzing such reactions. Peroxidase is a common enzyme in plants and levels of peroxidase increase drastically in the roots of many plants that have been infected or wounded by invaders. This enzyme may provide the means by which furanocoumarins could be activated to their most toxic state in these tissues. Conclusions Plants are capable of producing a seemingly limitless number of compounds, many of which have proven to be toxic to one or more types of organisms. Phototoxic metabolites are more limited in their distribution, but have a broad spectrum of biocidal activity and appear to provide their hosts with formidable chemical defenses against potential invaders. Certain plant families such as the Rutaceae, Asteraceae, and Apiaceae are well known for their phototoxic abilities while this characteristic is less common in others. The distribution of phototoxic furanocoumarins in the Moraceae provides us with an opportunity to study what role these chemicals may have played in the evolution of this family as well as what function they may serve today. The


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concentration of phototoxins in the roots of Dorstenia is by no means unique. Phototoxic thiophenes and polyacetylenes are commonly found in the roots of members of the Asteraceae, so similar roles for these compounds appears plausible. Further investigations are needed to clarify their importance to the host plants, and plants such as Dorstenia can serve as useful tools in this venture. Acknowledgments We thank Dr. J.M.E. Quirke, Dr. Stephen A. Winkle, Dr. Mehrzad Mehran, and Lavina Faleiro for their technical assistance. We also thank Dr. Robert Knight and the USDA Plant Induction Station at Chapman Field for many of the plant specimens. This work was supported by grants from the Whitehall Foundation and NSF (BBS-8613863) to KRD. Literature cited 1. 2. 3. 4.

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6. 7.

8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

18.

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Phototoxic Metabolites of Tropical Plants - ACS Publications

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