The Role of Natural Photosensitizers in Plant Resistance to Insects

The Role of Natural Photosensitizers in Plant Resistance to Insects T. ARNASON—University of Ottawa, Biology Department, Ottawa, Ontario K 1N 6N5, Canada

Downloaded by UNIV OF PITTSBURGH on October 17, 2015 | http://pubs.acs.org Publication Date: January 20, 1983 | doi: 10.1021/bk-1983-0208.ch008

G. H. N. TOWERS and B. J. R. PHILOGÈNE—University of British Columbia, Botany Department, Vancouver, British Columbia V6T 1W5, Canada J. D. H. LAMBERT—Carleton University, Biology Department, Ottawa, Ontario K15 5T6, Canada

Recent work has suggested that certain secondary metabolites from plants are capable of photosensitizing insects. With the recognition of increasing numbers of natural photosensitizers including polyacetylenes, furanocoumarins, β-carbolines and extended quinones, this unusual mechanism of plant defense now appears to be present i n a wide variety of plant families. Polyacetylenes, a group, of over 500 diverse compounds which are especially widespread in the Asteraceae are powerful photosensitizers. At least one sulphur derivative, α-terthienyl i s more toxic to mosquito larvae than DDT. The t o x i c i t y of some of these compounds is mediated by the production of singlet oxygen although free radicals may also be involved. Furanocoumarins, furanoquinolines and β-carbolines on the other hand interact with DNA and cause gross chromosomal abnormalities i n v i v o . The furanocoumarin, 8-methoxypsoralen has been shown to be highly phototoxic to herbivorous Spodoptera larvae. I t has also been suggested that the leaf r o l l i n g habit of certain microlepidopteran larvae on leaves containing photo sensitizing furanocoumarins i s related to l i g h t avoidance. Although only limited information is a v a i l able about the effects of naturally occurring compounds on insects, work with synthetic dye photosensitizers provides a broader basis for understanding the photosensitization of insects.

0097-6156/83/0208-0139$06.00/0 © 1983 American Chemical Society In Plant Resistance to Insects; Hedin, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

PLANT RESISTANCE TO INSECTS


140

L i g h t i s o f t e n a f o r g o t t e n or underestimated f a c t o r i n t h e study of i n s e c t s (1) and u n t i l r e c e n t l y l i t t l e a t t e n t i o n has been paid t o i t s r o l e i n p l a n t i n s e c t r e l a t i o n s . I n p a r t i c u l a r we wish t o address the question of a c t i v a t i o n of p l a n t secondary substances by l i g h t and t h e i r subsequent p h o t o s e n s i t i z i n g e f f e c t s on i n s e c t s . At l e a s t two examples of i n s e c t p h o t o s e n s i t i z i n g secondary m e t a b o l i t e s , furanocoumarins and polyacetylenes have now been reported. These s t u d i e s suggest that t h i s mode of p l a n t defense has p a r t i c u l a r adaptive advantages and may be more widespread than p r e v i o u s l y imagined. F o r t u n a t e l y the mode of a c t i o n of p h o t o s e n s i t i z e r s from p l a n t s i s understood from work w i t h t a r g e t organisms other than i n s e c t s . I n a d d i t i o n some d e t a i l e d i n f o r m a t i o n on the e f f e c t s of p h o t o s e n s i t i z e r s on i n s e c t s i s suggested from work w i t h s y n t h e t i c dye s e n s i t i z e r s . These l a t t e r aspects a r e c r u c i a l t o the understanding of the e f f e c t s of n a t u r a l p h o t o s e n s i t i z e r s and a r e t r e a t e d f i r s t i n t h i s review. Photosensitization For p h o t o s e n s i t i z a t i o n t o occur l i g h t must be absorbed by the p h o t o s e n s i t i z e r and cause a r e a c t i o n t o take p l a c e i n t h e b i o l o g i c a l system ( 2 ) . Two main types o f p h o t o s e n s i t i z a t i o n a r e known t o occur. Those i n v o l v i n g O2 a r e c a l l e d photodynamic s e n s i t i z a t i o n s and are mediated by the bulk of the 4 0 0 s y n t h e t i c and n a t u r a l p h o t o s e n s i t i z e r s known ( 3 ) . The second type of p h o t o s e n s i t i z a t i o n does not i n v o l v e O2 and i s a s s o c i a t e d w i t h the furanocoumarins. Photodynamic s e n s i t i z a t i o n s are photo-oxidations which proceed by one of two pathways (3,4_) . Type I p h o t o s e n s i t i z a t i o n s ( l e s s common) y i e l d superoxide r a d i c a l s ( 0 j ) through a s e r i e s of e l e c t r o n t r a n s f e r processes f o l l o w i n g l i g h t a b s o r p t i o n . Anthraquinone dyes and f l a v i n s operate through t h i s mechanism. I n type I I s e n s i t i z a t i o n s , the s e n s i t i z e r i s e x c i t e d from the ground s t a t e , S Q , t o the f i r s t e x c i t e d s t a t e , * S by a b s o r p t i o n of a photon. Crossing over leads t o the formation o f the e x c i t e d t t i p l e t state, which i n t e r a c t s w i t h ground s t a t e oxygen, ^02The t r a n s f e r of e x c i t a t i o n energy leads t o the production of s i n g l e t oxygen (^02) and the s e n s i t i z e r r e t u r n s t o i t s ground state:

(1) (2)

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In Plant Resistance to Insects; Hedin, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


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ARNASON ET A L .

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Because the ground s t a t e o f t h e s e n s i t i z e r i s regenerated the r e a c t i o n i s c a t a l y t i c . Once formed the a c t i v a t e d species o f oxygen, from type I I r e a c t i o n s Oj from type I mechanisms cause b i o l o g i c a l damage by o x i d a t i o n o f b i o l o g i c a l molecules. They are a l s o p o t e n t i a l l y i n t e r c o n v e r t i b l e ( 5 ) . Recent advances have allowed the i d e n t i f i c a t i o n o f ^2 i n photochemical damage of b i o l o g i c a l systems by the use o f quenchers (6) o r D2O which enhances the l i f e t i m e o f ^ 2 (7) Superoxide i s detected i n photoo x i d a t i o n s by the use o f the scavenging enzyme superoxide dismutase ( 8 ) . Important t a r g e t molecules are p r o t e i n s , l i p i d s ( e s p e c i a l l y c h o l e s t e r o l ) and n u c l e i c a c i d s (_3, 4_, 9) but the e f f e c t s i n v i v o a r e l a r g e l y dependant on the s i t e t o which the p h o t o s e n s i t i z e r b i n d s . Thus rose bengal (compound I ) binds t o and l y s e s membranes w h i l e a c r i d i n e orange penetrates t o the nucleus and causes damage t o DNA (6, 1 0 ) . The second group o f p h o t o s e n s i t i z e r s bind and c r e a t e photochemical damage a t the l e v e l o f DNA without any O2 requirement f o r a c t i v i t y . These i n c l u d e the w e l l s t u d i e d furanocoumarins (11) and more r e c e n t l y the f u r o q u i n o l i n e a l k a l o i d s (12). Work w i t h the furanocoumarins suggests that the f i r s t step i s i n t e r c a l a t i o n o f the p h o t o s e n s i t i z e r i n t o DNA i n such a way that one o r two o f the double bonds o f the r i n g s t r u c t u r e a l i g n w i t h the p y r i m i d i n e double bonds. The e x c i t e d s t a t e o f the photosens i t i z e r undergoes c y c l o a d d i t i o n t o the p y r i m i d i n e ( u s u a l l y thymine) forming a photoadduct ( I I ) . Compounds such as the f u r o q u i n o l i n e , dictamnine (X) and hindered furanocoumarins form o n l y monofunctional adducts (11, 12) w h i l e the l i n e a r furanocoumarins such as 8-methoxypsoralen (8-MOP) (IV) form d i f u n c t i o n a l adducts ( I I I ) l e a d i n g t o i n t e r s t r a n d c r o s s - l i n k a g e of DNA. Both types o f damage but e s p e c i a l l y the l a t t e r have s e r i o u s consequences f o r DNA t r a n s c r i p t i o n o r d u p l i c a t i o n and can l e a d t o c e l l death o r mutagenesis ( n ) . E f f e c t o f P h o t o s e n s i t i z e r s on I n s e c t s Because of the a v a i l a b i l i t y o f s y n t h e t i c dye s e n s i t i z e r s , t h e i r e f f e c t on i n s e c t s i s somewhat b e t t e r understood than photos e n s i t i z i n g p l a n t secondary substances. The f i r s t r e p o r t o f the e f f e c t of p h o t o s e n s i t i z e r s on i n s e c t s was made by B a r b i e r i i n 1928 (13), 28 years a f t e r the d i s c o v e r y o f p h o t o s e n s i t i z a t i o n by Raab. A 1972 review by Graham (14) reported o n l y f i v e i n v e s t i gations i n t o the p h o t o s e n s i t i z a t i o n o f i n s e c t s , but the number has more than doubled s i n c e that time, p r i n c i p a l l y due t o the work a t M i s s i s s i p p i S t a t e . Some o f these r e p o r t s a r e summarized i n TABLE I . S t u d i e s have concentrated mainly on the t h i a z i n e dye methylene b l u e ( I I ) and a range o f xanthene dyes o f which rose bengal ( I ) was the most prominant. Four orders of i n s e c t s are represented i n c l u d i n g eggs, l a r v a e , pupae and a d u l t s . No u n u s u a l l y r e s i s t a n t species have been found and the r e s u l t s w i t h the b o l l w e e v i l and f i r e ant a r e p a r t i c u l a r l y s i g n i f i c a n t s i n c e






water

diet

methylene blue

Lepdioptera P i e r r i s b r a s s i c a e

injection

mythylene blue

diet

diet

diet

benzpyrene, methyl cholanthrene, dibenzanthrene

treatment

xanthene dyes

diet

xanthene dyes

fl diet

bengal

H

it

xanthene dyes

rose

xanthene dyes

uranin,erythrosin a c r i d i n e red e o s i n , rhodamine, rose bengal

rose bengal

Photosensitizer

xanthene dyes

Anthonomus grandis (adult b o l l w e e v i l ) Tenebrio m o l i t o r (meal worm l a r v a e )

Anopheles sp. & Aedes sp. (mosquito l a r v a e ) Aedes t r i s e r a t u s and Culex p i p i e n s quinquefasciatus (mosquito l a r v a e ) Musca domes t i c a (adult house f l y ) Musca autumnalis (adult face f l y ) D r o s o p h i l a melanogaster

Culex p i p i e n s q u i n q u e f a s c i a t u s (mosquito l a r v a e ) Aedes a e g y p t i & Anopheles spp. (mosquito l a r v a e )

Organism

Hymenoptera Solenopsis r i c h t e r i ( f i r e ant)

Coleoptera

Diptera

Order

Reference

(23)

(22)

(21)

(20)

(19)

(18)

(17)

(13)

(16)

(15)

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3

2

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Studies of the E f f e c t o f Photodynamic Dye S e n s i t i z e r s on Insects Mode o f Administration

oo

TABLE I



144


they i n d i c a t e that even hard bodied i n s e c t s are s u s c e p t i b l e to photodynamic a c t i o n . M o r t a l i t y i n most cases was found to be d i r e c t l y p r o p o r t i o n a l to the photon fluence and dye concentrat i o n . Except f o r the more recent work, t o x i c i t i e s are d i f f i c u l t to compare from one study to the next because of the range of c o n d i t i o n s p o s s i b l e and the d i f f i c u l t y i n e s t i m a t i n g the fluence of l i g h t absorbed w i t h lamps of d i f f e r e n t s p e c t r a l q u a l i t y and dyes w i t h d i f f e r e n t a b s o r p t i o n maxima. An attempt a t q u a n t i t a t i o n of the r e s u l t s has been made by d e r i v a t i o n of second (22) and t h i r d order (24) r a t e constants f o r p h o t o - o x i d a t i o n . Using the second order r a t e constant i n s e c t s can be ranked i n order of t h e i r s u s c e p t i b i l i t y : Aedes t r i s e r a t u s ( l a r v a e ) > Anthonomus grandis ( a d u l t ) > Musca autumnalis ( a d u l t ) > Solenopsis r i c h t e r i ( a d u l t ) . With mosquito l a r v a e s u s c e p t i b i l i t y f a l l s w i t h the number of i n s t a r s . The r a t e constant can a l s o be used to rank dye t o x i c i t y (18). Both dye t o x i c i t y to house f l i e s and quantum y i e l d f o r dye phosphorescence i n c r e a s e i n the order: f l u o r o s c e i n , e o s i n y e l l o w , p h l o x i n B, e r y t h r o s i n B and rose bengal. This r e s u l t i s explained by the c u r r e n t view that phosphorescence and the production of I-O2 i n the p h o t o s e n s i t i z a t i o n r e a c t i o n are dependant on the p r o p o r t i o n of dye molecules i n t h e i r t r i p l e t excited state (3). At the p h y s i o l o g i c a l l e v e l i t i s w e l l e s t a b l i s h e d that v i t a l dyes such as n i l e b l u e , n e u t r a l red and methylene blue r e t a r d l a r v a l development under normal l i g h t i n g c o n d i t i o n s (12L/12D w i t h source u n s p e c i f i e d ) (25~27). Female but not male pupal weights are a l s o reduced. U n f o r t u n a t e l y experiments were conducted without dark c o n t r o l s so that i t i s d i f f i c u l t to evaluate the r o l e of p h o t o s e n s i t i z a t i o n i n these e f f e c t s . As house f l i e s and f i r e ants succumb to p h o t o s e n s i t i z a t i o n , they l o s e motor c o n t r o l and become more e x c i t a b l e (28). This suggested a neurotoxic e f f e c t and i n v e s t i g a t i o n of f i r e ant a c e t y l c h o l i n e s t e r a s e i n v i t r o revealed that t h i s enzyme was s e n s i t i v e to photo-oxidation. I n v i v o r e s u l t s , however, revealed no e f f e c t on the enzyme which suggests another mode of a c t i o n . Epoxidation of c h o l e s t e r o l and membrane l y s i s may be a l t e r n a t i v e primary s i t e s . I f t h i s were the case ecdysone metabolism of i n s e c t s would probably a l s o be e f f e c t e d . Furanocoumarins Although furanocoumarins are w e l l s t u d i e s f o r t h e i r e f f e c t s on human s k i n , recent work has suggested t h e i r JuLc&on d H&id i n p l a n t s may be l i n k e d to t h e i r r o l e as p r o t e c t i v e agents that are e f f e c t i v e against i n s e c t s or pathogenic f u n g i (29, 30). This group of compounds i s reported i n 8 f a m i l i e s but f i n d t h e i r g r e a t e s t d i v e r s i t y i n the Apiaceae and Rutaceae. Berenbaum has demonstrated t h e i r a c t i v i t y a g a i n s t i n s e c t herbivores i n feeding t r i a l s w i t h a polyphagous h e r b i v o r e , the f a l l armyworm Spodoptera e r i a n i a which w i l l feed on c a r r o t which does not 1



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c o n t a i n furanocoumarins but not on parsnip that does c o n t a i n these compounds. Larvae were administered 0,1% 8-methoxypsoralen (IV), a furanocoumarin compound which occurs w i d e l y i n the Aplaceae by treatment of a r t i f i c i a l d i e t . These l e v e l s are compar a b l e t o those i n p l a n t s and caused 100% m o r t a l i t y i n l a r v a e that were a l s o t r e a t e d w i t h near UV, the a c t i v a t i n g wavelength range for these s e n s i t i z e r s . Dark c o n t r o l s and i n s e c t s fed untreated d i e t s but i r r a d i a t e d w i t h near UV showed a s l i g h t r e d u c t i o n i n s u r v i v o r s h i p . The e f f e c t of near-UV and 8-M0P together was h i g h l y s i g n i f i c a n t and i s presumably due t o the e f f e c t of the compound on l a r v a l DNA. Thus t o feeding g e n e r a l i s t s 8-M0P represents a formidable b a r r i e r t o g r a z i n g on the p l a n t s that c o n t a i n i t . Since the precursor t o p s o r a l e n s , u m b e l l i f e r o n e , i s not t o x i c t o armyworms, Berenbaum hypothesizes that these p l a n t s have escaped t h e i r enemies by the a l t e r a t i o n of t h e i r chemical phenotype. I n s e c t s have i n some cases responded by overcoming these defenses. For example aphids that feed e x c l u s i v e l y on the Apiaceae take up and bind 8-M0P b u t are u n a f f e c t e d by i t , perhaps due t o some d e t o x i f i c a t i o n mechanism (29). Yu, S.J., Berry, R.E., and T e r r i e r e , L.C.,(1979) showed that phytophagous i n s e c t s possess enzymes that are induced by p l a n t secondary substances and that these enzymes are i n v o l v e d i n r e s i s t a n c e t o otherwise t o x i c compounds (31). M i c r o l e p i d o p t e r a appear t o have adapted t o l i f e on p h o t o t o x i c Apiaceae by a l e a f r o l l i n g h a b i t that screens out near UV. (30). C l u s t e r a n a l y s i s of the fauna of the Apiaceae i n d i c a t e s that the i n s e c t assemblages of p l a n t s c o n t a i n i n g phototoxic furanocoumarins are s i m i l a r and d i f f e r e n t from p l a n t s i n the f a m i l y t h a t do not c o n t a i n phototoxic furanocoumarins (32). This lends support t o the hypothesis that a s p e c i a l i s t group of i n s e c t s e x i s t s that i s adapted t o p h o t o t o x i c Apiaceae. Polyacetylenes P o l y a c e t y l e n e s are a v e r y l a r g e group of secondary compounds whose p h o t o s e n s i t i z i n g p r o p e r t i e s have r e c e n t l y been e s t a b l i s h e d by our research team a t U.B.C. (33). These compounds have conjugated double and t r i p l e bond systems (e.g. compound V I I and V I I I ) or may be b i o s y n t h e t i c a l l y c y c l i z e d i n t o thiophene compounds such as alpha t e r t h i e n y l (a-T) (compound V I ) . P o l y a c e t y l e n e s and t h e i r thiophene d e r i v a t i v e s occur i n s e v e r a l f a m i l i e s but f i n d t h e i r g r e a t e s t d i v e r s i t y i n the Asteraceae, the l a r g e s t p l a n t f a m i l y C34). As i n the case of the furanocoumarins, p h o t o s e n s i t i z a t i o n i s mediated by the near-UV r e g i o n of the spectrum but the mechanism of a c t i o n does not i n v o l v e c r o s s - l i n k i n g of DNA (35). Some compounds such as a-T are c l e a r l y photodynamic i n t h e i r mode of a c t i o n (36) but the s i t u a t i o n i s l e s s c l e a r f o r compounds c o n t a i n ing t r i p l e bond systems such as phenylheptatriyne (PHT) ( V I I I ) . P h o t o - o x i d a t i o n of c h o l e s t e r o l and a c e t y l c h o l i n e s t e r a s e has been observed (37) w i t h t h i s and other polyacetylenes but l y s i s of r e d blood c e l l s and p h o t o s e n s i t i z a t i o n _E. c o l i were not 0 dependant ?




146

C38). These r e s u l t s suggest a novel mechanism of a c t i o n and are an a c t i v e area of research. P o l y a c e t y l e n e s are t o x i c to a broad range of organisms (39) but are e s p e c i a l l y t o x i c t o i n s e c t s . At 0.5 ppm, 9 of 14 compounds t e s t e d were t o x i c to f i r s t i n s t a r mosquito l a r v a e (Aedes aegypti) i n 30 min treatments w i t h sources of near UV (15 W/m^) (40). The compounds were more a c t i v e i n s u n l i g h t . For example a-T k i l l e d second i n s t a r l a r v a e i n s t a n t a n e o u s l y at 4 ppm. Compounds V I I , a-T and PHT were e s p e c i a l l y a c t i v e and were s e l e c t e d f o r f u r t h e r t e s t i n g i n dose response experiments ( 4 1 ) . For s i m i l a r near UV treatments, the LC50 f o r a-T was 19 ppb, 79 ppb f o r compound V I I and 1.0 f o r PHT. Alpha T had some a c t i v i t y i n the dark (LC50 =0.74 ppm) i n d i c a t i n g t h a t i t may bind to a s e n s i t i v e c e l l s i t e and d i r e c t p h o t o s e n s i t i z a t i o n to t h i s s i t e . This compound was so a c t i v e i n the presence of near UV t h a t i t s p o t e n t i a l as a commercial l a r v i c i d e was evaluated i n simulated pond t r i a l s . For 500 l a r v a e placed i n 200 1 of water i n summer s u n l i g h t , 100% m o r t a l i t y was observed i n 15 min a t 200 ppb and 120 min at 20 ppb, A d e t a i l e d a c t i o n spectrum f o r p h o t o s e n s i t i z a t i o n undertaken w i t h narrowband i n t e r f e r e n c e f i l t e r s i n d i c a t e d that there was c l o s e agreement between a c t i o n and a b s o r p t i o n . This suggested t h a t the p o l y a c e t y l e n e was the absorbing species and was not i n t e r f e r i n g w i t h metabolism i n such a way as to cause a p h o t o s e n s i t i z i n g product to be formed. Such mechanisms do occur w i t h i c t e r o g e n s produced by the genus T r i b o l u s which are ingested by range animals ( 4 2 ) . The r e s u l t w i t h p o l y a c e t y l e n e s a l s o d i f f e r s from t h a t w i t h psoralens where a DNA-psoralen complex i s thought to r e s u l t i n the d e v i a t i o n of the a c t i o n spectrum from the a b s o r p t i o n spectrum (43). We are c u r r e n t l y i n v e s t i g a t i n g the e f f e c t of p o l y a c e t y l e n e s and near-UV i n s u b l e t h a l doses during feeding t r i a l s w i t h Euxoa messoria (Lepidoptera, n o c t u i d a e ) . P o t e n t i a l f o r f u r t h e r work a l s o e x i s t s w i t h the adapted i n s e c t , the s o l d i e r b e e t l e (Coleoptera, Cantheridae), which apparently uses a p o l y a c e t y l e n e as a defense compound (44), Other compounds Despite the hypothesis t h a t the e v o l u t i o n a r y s i g n i f i c a n c e of p h o t o t o x i c secondary substances may be l i n k e d to t h e i r a b i l i t y to discourage i n s e c t h e r b i v o r e s , most research has been d i r e c t e d toward t h e i r e f f e c t s on human s k i n and range animals (42). I n an attempt to extend our knowledge of i n s e c t p h o t o s e n s i t i z e r s we have screened a number of p l a n t secondary substances (TABLE I I ) f o r t h e i r p h o t o s e n s i t i z i n g a c t i v i t y to 4 t h i n s t a r mosquito l a r v a e Aedes atropalpus under s o l a r s i m u l a t i n g lamps. H y p e r i c i n (IX) i s an extended quinone from S t . John's wort (Hypericum spp), a common pasture weed throughout the w o r l d . T h i s photodynamic compound causes a p h o t o s e n s i t i v e disease c a l l e d 'bighead i n sheep t h a t consume i t (42). Related compounds, the 1


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American Chemical Society Library 1155 16th st. N. w. In Plant Resistance to Insects; Hedin, P.; WatUngtofi. D. C. Society: 2003SWashington, DC, 1983. ACS Symposium Series; American Chemical

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TABLE I I


P h o t o t o x i c i t y of P l a n t Secondary M e t a b o l i t e s to Mosquito Larvae Compound

Type

Phototoxic Activity

Hypericin

Extended quinone

+ +

Dictamnine

Furo q u i n o l i n e a l k a l o i d

+ +

Harmaline

3-Carboline a l k a l o i d

+ +

Methoxyharmaline

g-Carboline a l k a l o i d

+ +

Harman

(3-Carboline a l k a l o i d

Norharman

$-Carboline a l k a l o i d

+

Harmalol

$-Carboline a l k a l o i d

+

Berberine

Isoquinoline alkaloid

+

Note;

Compounds were screened a t s e v e r a l c o n c e n t r a t i o n s i n 24-hr acute t o x i c i t y t e s t w i t h f o u r t h i n s t a r Aedes a t r o p a l p u s l a r v a e . Tests were run w i t h p a r a l l e l t r i a l s i n the dark and under s o l a r s i m u l a t i n g " v i t a l i t e s " w i t h an i n t e n s i t y of 400 w/m^. A c t i v i t i e s were r a t e d as f o l l o w s (-) no d i f f e r e n c e between l i g h t and dark t r i a l , (+) enhancement of l i g h t t o x i c i t y over dark, (+ +) l a r g e r enhancement of l i g h t t o x i c i t y over dark. R



8.

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Role of Natural

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fagopyrins occur i n buckwheat (fagopyrum s p ) . I n a d d i t i o n we have included two groups o f a l k a l o i d s , the g-carbolines (of which hannaline (X) i s an example) and the f u r o q u i n o l i n e a l k a l o i d dictamnine (V) which were r e c e n t l y discovered by one o f us (Towers) t o be p h o t o t o x i c t o yeast (45, 46). The a c t i v i t y o f berberine was suggested by i t s f l u o r e s c e n c e . Obviously many compounds a r e phototoxic o r have enhanced t o x i c i t y i n l i g h t as compared t o dark. We b e l i e v e t h i s demonstrates the p o t e n t i a l f o r f u r t h e r work i n t h i s f i e l d and suggests that t h i s i s an area that has been overlooked. One o f the reasons that many p h o t o s e n s i t i z i n g compounds have been overlooked i s because o f t h e i r apparent l a c k o f c o l o r . F o r example the p o l y a c e t y l e n e PHT i s a potent i n s e c t p h o t o s e n s i t i z e r i n n a t u r a l s u n l i g h t i s completely c o l o r l e s s i n s o l u t i o n . I t s spectrum r e v e a l s (Figure 1) strong a b s o r p t i o n bands i n the near UV which are w e l l beyond the human v i s u a l l i m i t (380 nm), but w i t h i n the range o f wavelengths t r a n s m i t t e d by the atmosphere ( g e n e r a l l y > 300 nm) (47). I n a d d i t i o n i t should be noted that the energy absorbed a t these wavelengths i s c o n s i d e r a b l y higher than i n the v i s i b l e range and may be a f a c t o r i n the h i g h t o x i c i t y of compounds l i k e alpha T. I n c o n c l u s i o n i t i s evident that many research o p p o r t u n i t i e s e x i s t i n the i d e n t i f i c a t i o n and c h a r a c t e r i z a t i o n of new substances and e v a l u a t i o n o f t h e i r e c o l o g i c a l e v o l u t i o n a r y and p h y s i o l o g i c a l s i g n i f i c a n c e . I n a p r a c t i c a l sense i t can be hoped that some o f these new compounds because o f t h e i r n o v e l mode o f a c t i o n may be u s e f u l f o r the c o n t r o l o f phytophagous i n s e c t s as p a r t o f i n t e g r a t e d pest management programs.

2.0-

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The Role of Natural Photosensitizers in Plant Resistance to Insects

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