Biochemical Mode of Action of Fungicides - American Chemical

2 Biochemical Mode of Action of Fungicides Ergosterol Biosynthesis Inhibitors Dieter Berg

Downloaded by UNIV OF PITTSBURGH on October 7, 2015 | http://pubs.acs.org Publication Date: April 22, 1986 | doi: 10.1021/bk-1986-0304.ch002

Bayer AG, Agrochemical Division, Research Biochemistry, Plant Protection Center Monheim, D5090 Leverkusen, Federal Republic of Germany

One or two decades ago the biochemical mode of action of a fungicide normally was not known. A great part of this was due to the fact that most of the fungicides were "multi-site effectors" and thus not accessible to relevant biochemical studies. The situation changed drastically with the finding of the so-called "single-site effectors" which could be studied mechanistically. Biochemistry then became an important assistance for the chemist during optimization of efficacy within a defined chemical group. By now a tremendous number of dif ferent mechanisms have been described. A crude classification of the modes of action leads to three groups of fungicides: 1) those which inhibit energy production by blocking SH-groups, the glycolysis/ci trate cycle, or the respiratory chain, 2) those that inhibit biosyntheses of proteins, nucleic acids, cells walls, and membrane lipids, or interfere with mitosis, and 3) those which induce indirect effects which change host/pathogen interactions. An example of this last group is the induction of phytoalexin production by dichlorodimethylcyclopropane-carboxylic acid (1). There are numerous examples of the inhibition of biosynthesis by fungicides. Some fungicidal secondary metabolites, like cycloheximide and blasticidine, interfere with synthesis of peptide bonds at the ribosomal site (2). Another, kasugamycine, influences aminoacylt-RNA/ribosome interactions (3). Finally, another mechanism inhibit ing protein biosynthesis is realized on the DNA/RNA- level by the 0097-6156/86/0304-0025$08.00/0 © 1986 American Chemical Society

In Fungicide Chemistry; Green, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

FUNGICIDE CHEMISTRY

26

acylanilides.

F o r example, m e t a l a x y l

i n t e r f e r e s w i t h RNA- polymerase

(4) . The f i r s t group o f s y s t e m i c inhibitors (5) .

of mitosis

f u n g i c i d e s , the benzimidazoles, are

by i n t e r f e r e n c e w i t h

Thus, they p r e v e n t

an arrangement

The t u b u l i n e - b e n z i m i d a z o l e - i n t e r a c t i o n s (6) .

polymerization

of the spindle

apparatus.

have been s t u d i e d

i n detail

I t i s known t h a t carbendazim, f o r example, a f t e r e n t e r i n g t h e

nucleus, Downloaded by UNIV OF PITTSBURGH on October 7, 2015 | http://pubs.acs.org Publication Date: April 22, 1986 | doi: 10.1021/bk-1986-0304.ch002

tubuline

specifically

binds

t o the

3 -subunit

t h i s i n h i b i t s the d i m e r i z a t i o n of the α t i o n a l tubuline u n i t .

and

3 -subunits

f o r benzimidazoles

tion

of mitosis, l i k e

inhibitors

hydrocarbons, or dithiocarbamates,

(7) ·

and by

to a func

R e s i s t a n t s t r a i n s possess a l t e r e d

w i t h a decreased a f f i n i t y of other

of tubuline

β -subunits

The modes o f a c

dicarboximides,

aromatic

have n o t y e t been p r e c i s e l y de

s c r i b e d on a m o l e c u l a r b a s i s . Another group o f f u n g i c i d e s i n t e r f e r e s w i t h c e l l w a l l Two examples o f t h i s group i n c l u d e : chitin

formation

by c o m p e t i t i v e l y

formation.

1) t h e p o l y o x i n e s , w h i c h p r e v e n t inhibiting

c h i t i n - s y n t h a s e , the

f i n a l enzyme i n v o l v e d i n c h i t i n b i o s y n t h e s i s , and 2) mélanine thesis this

inhibitors,

especially i n Pyricularia

type o f f u n g i c i d e

oryzae.

Examples o f

a r e T r i c y c l o a z o l e and L i l o l i d o n e ,

that i n t e r f e r e with pentaketide

tors w i l l

biosynthesis

The modes o f a c t i o n o f s t e r o l s y n t h e s i s

be d i s c u s s e d

Kitazin,

Isoprothiolane

sively.

I t could

methylation

compounds

synthesis ( 8 ) .

The n e x t l a r g e group i n c l u d e s compounds i n h i b i t i n g of membrane l i p i d s .

biosyn

i n detail

later,

inhibi

b u t t h e mode o f a c t i o n o f

and E d i f e n p h o s has a l s o been s t u d i e d

be shown t h a t

t h e S-adenosylmethionine

inten

dependent

of phosphatidyl-ethanolamines t o the corresponding

leci

t h i n s i s a f f e c t e d by these compounds ( 9 ) . Membranes cannot o n l y be disturbed well.

by r a t h e r

specific

F o r example,

effect.

dodine

mechanisms b u t by g e n e r a l damages membranes

by a

a c t i o n s as

detergent-like

An example o f a d i f f e r e n t mechanism i s t h e i n h i b i t i o n o f

adenosine-deaminase by t h e p y r i m i d i n e d e r i v a t i v e s e t h i r i m o l and dimethyrimol (10). I n t h e d i s c u s s i o n o f t h e mode o f a c t i o n o f e r g o s t e r o l thesis

inhibitors,

the question

biosyn

o f t h e f u n c t i o n o f s t e r o l s i n mem-


2.

branes has t o be asked. et

27

Ergosterol Biosynthesis Inhibitors

BERG

a l . (11),

stabilized

A model e x p e r i m e n t , conducted by Ladbrooke

demonstrates

that

phospholipid

by a d d i t i o n o f a s t e r o l ,

fungi, c h o l e s t e r o l i s replaced "quasiplanar"

phases a r e p h y s i c a l l y

i nthis

case c h o l e s t e r o l . I n

by e r g o s t e r o l , b u t b o t h s t e r o l s a r e

and thus a r e a b l e

t o f u n c t i o n as membrane components.

I f one s i m u l a t e s a d e f i c i e n c y o f a p l a n a r s t e r o l , i t can be shown by differential sions

c a l o r i m e t r y measurements

(Figure

of phosphatidylcholine/cholesterol

1) t h a t w i t h

mixtures

disper

i n water,


t r a n s i t i o n s between d i f f e r e n t l i p i d phases a r e i n d u c e d ( 1 1 ) . the

case o f s t e r o l

d e f i c i e n c y , t h e phase t r a n s i t i o n

phase If, i n

temperature i s

passed between t h e " q u a s i - c r y s t a l l i n e " and l i q u i d phase o f phospha t i d y l c h o l i n e s , an energy consuming r e a c t i o n i s o b s e r v e d .

This

isa

q u a n t i f i a b l e i n d i c a t i o n o f d r a s t i c changes i n l i p i d s t r u c t u r e . Such changes i n membrane s t r u c t u r e do n o t o n l y

induce d r a s t i c

changes i n the p h y s i c a l s t a b i l i t y o f membranes, b u t a l s o a f f e c t the specific

activities

strated with mycoides

This

can be demon

the example o f membrane ATP-ase a c t i v i t y o f Mycoplasma

(Figure

temperature,

o f membrane-bound enzymes.

2).

When t h e enzyme a c t i v i t y

the wild-type

Arrhenius-diagram.

shows

no

phase

i s plotted transition

However, i n t h e case o f a s t e r o l

against in

the

d e f i c i e n t mu

t a n t , the s p e c i f i c a c t i v i t y o f the membrane ATP-ase changes a t 18°C (12).

T h i s c l e a r l y i n d i c a t e s t h a t a temperature-dependent change o f

the membrane c o n f o r m a t i o n

causes a t e m p e r a t u r e - i n d u c e d change i n t h e

s p e c i f i c a c t i v i t y o f membrane-bound enzymes. I f s t e r o l content s t a b i l i t y , we s h o u l d The

and c o n f o r m a t i o n study

a r e so important

the b i o s y n t h e s i s

f o r membrane

of s t e r o l s (Figure 3 ) .

f i r s t enzyme i n t e r p e n o i d b i o s y n t h e s i s i s t h e 3-Hydroxy-3-Methyl-

Glutaryl-Coenzyme Α-reductase (HMG-CoA-reductase) t h a t c a t a l y z e s t h e s y n t h e s i s o f mevalonate. mevalonate sterol

lead

synthesis.

Two p h o s p h o r y l a t i o n s

t o isopentenylpyrophosphate,

thebasic

C^-unit i n

Isopentenylpyrophosphate reacts w i t h

i t s isomer,

the d i m e t h y l a l l y l - p y r o p h o s p h a t e ,

i n a head/tail-reaction to geranyl-

pyrophosphate; r e a c t i o n w i t h a n o t h e r C ^ - u n i t phosphate, t h a t d i m e r i z e s expoxidation

and d e c a r b o x y l a t i o n o f

of i t s Δ

leads t o farnesyl-pyro-

i n a t a i l / t a i l - r e a c t i o n to squalene. -double

bond,

squalene

After

c y c l i z e s t o lano-


FUNGICIDE CHEMISTRY

28

endothermic


Rate heat flow

exothermic

290

π—ι—ι—ι—I 320 350

Average temperature (°K)

F i g u r e 1. D i f f e r e n t i a l scanning c a l o r i m e t r y curves f o r 50 wt% dispersions of 1,2-dipalmitoylphosphatidyl-choline-cholesterol m i x t u r e s i n water c o n t a i n i n g ( a ) 0; ( b ) 5.0; ( c ) 12.5; (a) 20.0; (e) 32.0; and ( f ) 50.0 m o l % c h o l e s t e r o l . (Reproduced w i t h p e r m i s s i o n from Ref. 11. C o p y r i g h t 1968 E l s e v i e r S c i e n c e P u b l i s h i n g Company, I n c . ) Adenosine triphosphatase activity (pmol Pi released/mg protein/5 min) 5.0 π

1

1

1

33 34 1 0 χ 1 / Τ ( · Κ" ) 3

35

1

F i g u r e 2. A r r h e n i u s p l o t s o f membrane ATP-ase a c t i v i t y of n a t i v e (-·-) and s t e r o l d e f i c i e n t (-o-) Mycoplasma mycoides v a r . mycoides. (Reproduced w i t h p e r m i s s i o n from Ref. 12. C o p y r i g h t 1973 E l s e v i e r S c i e n c e P u b l i s h i n g Company, I n c . )


2.

BERG

29


sterol,

the f i r s t

sterol

b u t the l a s t

common i n t e r m e d i a t e

o f both

c h o l e s t e r o l and e r g o s t e r o l s y n t h e s i s . In ergosterol biosynthesis, side chain a l k y l a t i o n of l a n o s t e r o l normally itself C^.

takes

place

to build

i s then t h e s u b s t r a t e

The C ^ - d e m e t h y l a t i o n

first

step

involved

f o r demethylation has been s t u d i e d

1

o x i d a t i v e demethylation

24-methylenedihydrolanosterol, reactions at i ndetail.

c a t a l y z e d by a cytochrome P ^ ^ - s y s t e m . The

i nthis

reaction

i s the hydroxylation


h y d r o x y l a t i o n and l o s s o f water l e a d t o t h e C ^ - f o r m y l

Decarboxylation

sequently, demethylation

carboxy-

from C ^ , f o l l o w e d by e l i m i n a t i o n

o f a Δ ^ - d o u b l e bond.

The NADPH-dependent r e d u c t i o n

Δ ^ - d o u b l e bond f i n i s h e s t h e d e m e t h y l a t i o n

of t h e

the

does n o t d i r e c t l y take p l a c e , b u t proceeds

i n s t e a d by a b s t r a c t i o n o f a p r o t o n formation

of

A second

intermediate,

w h i c h i s h y d r o x y l i z e d a t h i r d time t o form t h e c o r r e s p o n d i n g

and

and

I t i s an

C ^ - m e t h y l - g r o u p t o form the C ^ - h y d r o x y m e t h y l d e r i v a t i v e .

l i c acid.

which

at

reaction.

Sub

has t o take p l a c e t w i c e , f o l l o w e d by a

d e h y d r o g e n a t i o n r e a c t i o n i n Δ ^ - p o s i t i o n and i s o m e r i z a t i o n from

Δ

Δ

to

and £24(28)

Δ

2 ί

respectively. I n r e s e a r c h f o r new f u n g i c i d e s and a n t i m y c o t i c s , one has t o l o o k for

a concept

f o r pathogen-specific

i n h i b i t i o n o f s t e r o l synthesis

should

inhibitors. n o t take

place

s t e p ; t h e aim i s t o o n l y i n h i b i t p a t h o g e n - s p e c i f i c thesis. city.

This

means

that

a t any common

steps

i n biosyn

The r e a s o n f o r t h i s i s t o m i n i m i z e the r i s k o f human t o x i I f one compares t h e b i o s y n t h e s i s o f mammalian c h o l e s t e r o l t o

t h a t o f e r g o s t e r o l , the main s t e r o l o f p a t h o g e n i c f u n g i , i t becomes obvious that there

are a t l e a s t four pathogen-specific

steps

t o be

inhibited. The

first

pathogen-specific

r e a c t i o n i s t h e S-adenosylmethio-

nine-dependent s i d e c h a i n a k l y l a t i o n o f l a n o s t e r o l . T h i s i s pathogen s p e c i f i c s i n c e i n c h o l e s t e r o l s y n t h e s i s , a s i d e c h a i n a l k y l a t i o n does not

take

place.

C^-positions as

well.

substrate

Secondly, the demethylation

o f 24-methylene-dihydrolanosterol

I n mammals d e m e t h y l a t i o n i s not side chain

reactions

r e a c t i o n s a t C ^ - and are pathogen-specific take

place,

a l k y l a t e d , so t h e c o r r e s p o n d i n g

but

the

enzyme

should possess d i f f e r e n t b i n d i n g s i t e s f o r the d i f f e r e n t s u b s t r a t e s .


30

FUNGICIDE CHEMISTRY

A t h i r d pathogen-specific Λ 22 drogenation

in

Δ

step

i n ergosterol synthesis

- p o s i t i o n of the s i d e c h a i n .

e s t i n g t a r g e t i s the

Δ

Δ^-dehydrogenation.

8

7

Δ-

to

The

i s the

dehy-

fourth

i s o m e r i z a t i o n r e a c t i o n or

These f o u r r e a c t i o n s e i t h e r do not

inter

the

take

place

i n mammals, or u t i l i z e d i f f e r e n t s u b s t r a t e s . Another t a r g e t , t h a t a t f i r s t seems t o be u n f a v o r a b l e

since i t

i s p r i n c i p a l l y common f o r a l l o r g a n i s m s , i s the enzyme HMG-CoA-reducDownloaded by UNIV OF PITTSBURGH on October 7, 2015 | http://pubs.acs.org Publication Date: April 22, 1986 | doi: 10.1021/bk-1986-0304.ch002

t a s e w h i c h i s the r e g u l a t o r y enzyme i n t e r p e n o i d b i o s y n t h e s i s .

Re

s u l t s from t r i a l s w i t h n a t u r a l l y produced i n h i b i t o r s f o r t h a t enzyme, such as Compactine and M e v i n o l i n e , able

t o lower the

c h o l e s t e r o l content

depress s t e r o l s y n t h e s i s i n f u n g i There are biosynthesis

i n d i c a t e t h a t these compounds are i n mammals, but

at l e a s t four d i s t i n c t chemical

inhibitors

(EBI's).

The

4).

The

first

commercial

seed d r e s s i n g agent.

The

group,

triadimefon,

the

of

bitertanol.

the

benzyl

ketals,

compound

that

are

Analogous t o these

d i c l o b u t r a z o l e has

propicanozole

activities

and

etaconazole,

slightly

been

those

Topas does not might c o n s i d e r

analogue.

Fluotrimazole

compounds, but has tic

The s u b s t i t u t i o n

still

active

show s p e c t r a

d i f f e r e n t from

is significantly

of of

to

against

distinct

two

biological

contain it a

The

triadimefon, the

ketal

propiconazole

from

the

remarkable homology t o c l o t r i m a z o l e , an

other

antimyco

azole. A p a r t from the t r i a z o l e d e r i v a t i v e s , a s m a l l group o f

is

a

systemic

developed.

t r i a d i m e n o l , or d i c h l o b u t r a z o l e . one

be

phenoxy-derivatives,

partial

s t r u c t u r e , but

can

a phenyl substituent l e d

B i t e r t a n o l i s mainly

r u s t and scab i n t r e e f r u i t .

of

(Figure

used as

main i n d i c a t i o n of b o t h of these

c h l o r i n e atom i n t r i a d i m e n o l by

synthesis

terms

that

t r i a d i m e n o l , which i s m a i n l y

compounds i s f o r powdery mildew c o n t r o l i n c e r e a l s . of the

in

triazole derivatives

compound was

reduced to y i e l d

markedly

groups of e r g o s t e r o l

largest

number of commercial compounds, i s the

chemically

not

(13).

active against

plant

pathogens

(Figure

5).

The

compounds i n t h i s group are

i m a z a l i l and

of the important

EBI's o r i g i n a t e from the

ries.

Two

of

antimycotic these

antimycotics

w h i c h resemble f l u o t r i m a z o l e .

are

prochloraz.

bifonazole

These two

and

antimycotics

imidazoles

most

important

However, most imidazole

se

clotrimazole, will


be

dis-

BERG


ÇH HO-CO-CH -Ç-CH -CO-S-CoA 3

2

HMG - COA

2

OH HMG-CoA-reductase

I

ÇH HO-CO-CH -Ç-CH2-CH -OH OH 3

2

mevalonic acid

2

i 4 lanosterol

I

HO ^


4.4 -dimethyl-δ

8 5 4

cholestadien )lestadien - ol (3)

-

V

X 4,4-dimethyl -

/

f~*i\* (^\\)—*

- o l (3)

01(3)

HO

*

(3)

HO

HO ergosterol (yeasts, derenatophytes, bacteria)

cholesterol (mammals)

F i g u r e 3. sis.

Strategies

forinhibition

•CH-CO-C(CH ) 3

of ergosterol

OH I 3

|-CH-CH-C(CH ) 3

Ν —

biosynthe

3

u

Triadimenol Ν CI U

0-ÇH-CH-C (CH ) 3

JÎ

3

C I - ^ ~ ^ - G H - C H - C H - C (CH3) 2

3

II

II

II U

Ν Oiclobutrazol -CI V=/

ο

Ο

\=N

\ = / Ο

Ο

CHn 3

^=N C H

7

2

5

Propiconazol

Q

-

r

O

\ = /

I CHn 3

\=N

7

Ν U Fluotrimazol

F i g u r e 4.

Triazolyl ergosterol biosynthesis

inhibitors.


FUNGICIDE CHEMISTRY

32 cussed l a t e r w i t h r e s p e c t

t o t h e i r a b i l i t y t o i n h i b i t HMG-CoA-reduc-

t a s e i n dermatophytes. The mol,

third

group o f EBI's a r e t h e p y r i m i d i n e

f e n a r i m o l , and t r i a r i m o l ( F i g u r e 6 ) .

closely related chemically. zoles

and i m i d a z o l e s

the morpholines

The commonality they have w i t h t h e t r i a

i s the nitrogen

heteroatom

i n the 3 - p o s i t i o n f

from t h e c e n t r a l c a r b o n . are

derivatives nuari-

A l l o f t h e s e compounds a r e

The f o u r t h most i n t e r e s t i n g group o f E B I s

which

are represented

by j u s t

two compounds,


t r i d e m o r p h and fenpropimorph ( F i g u r e 7 ) . When we s t a r t e d t o l o o k E B I ' s , we d e c i d e d

f o r t h e mode o f a c t i o n o f d i f f e r e n t

t o use s e v e r a l pathogens as t e s t organisms.

These

were P y r i c u l a r i a o r y z a e , B o t r y t i s c i n e r e a , C e r c o s p o r a musae, F u s a r i u m nivale,

and D r e c h s l e r a

possible

antimycotic

teres.

To g e t r a p i d

information

about t h e

e f f i c a c y o f a t e s t compound, we a l s o

used

a

non-pathogenic y e a s t , Saccharomycopsis l i p o l y t i c a . I n t h e n e x t p a r t o f o u r r e s e a r c h on E B I - f u n g i c i d e s , we r e s t r i c t ed o u r s e l v e s

t o P y r i c u l a r i a o r y z a e s i n c e from o u r p o i n t o f v i e w t h e

i n v i t r o r e s u l t s w i t h t h a t organism a r e r e p r e s e n t a t i v e and t h e t e s t procedure i s r a t h e r simple.

The t e s t c h e m i c a l

i s applied i n a s u i t

able

concentration

t o t h e c u l t u r e medium w h i c h

from

an u n t r e a t e d

pre-culture.

cells

are separated

from

After

a 24-hour

the c u l t u r e

filtrate

i s then

inoculated

fermentation, the by c e n t r i f u g a t i o n ,

resuspended i n a c h l o r o f o r m / m e t h a n o l - m i x t u r e and homogenized u s i n g an u l t r a t u r r a x treatment.

A f t e r t h i s e x t r a c t i o n procedure, the s t e r o l -

c o n j u g a t e s a r e s p l i t t o f r e e s t e r o l s by a p o t a s s i u m - h y d r o x i d e ment.

Adsorption

desorption

treat

o f t h e s t e r o l s t o a Sep-pak column and

step-wise

l e a d s t o a s t e r o l f r a c t i o n w h i c h c a n be a n a l y z e d

directly

by gas chromatography on a SE-30 c a p i l l a r y column. An example f o r a G C - a n a l y s i s i s shown i n F i g u r e 8.

o f i s o l a t e d s t e r o l s from P. o r y z a e

An e l u t i o n diagram o f an u n t r e a t e d

control i s

compared t o t h e s t e r o l a n a l y s i s a f t e r a p p l i c a t i o n o f 10 ppm t r i a d i menol.

I t c a n e a s i l y be seen t h a t i n t h e r e g i o n where t h e s t e r o l s

a r e e l u t e d (framed a r e a ) , t h e p a t t e r n becomes t o t a l l y d i f f e r e n t . elution

index

accumulating

i s the f i r s t sterol.

criterion

f o r the c h e m i c a l

nature

The o f an

However, s t r u c t u r e e l u c i d a t i o n has been p e r -


BERG


κ5-

y— C H - O - C H 2 - C H - C H 3

C\—(/

CH

2

0-CH2-CH -N-C3H n 2

7

CO I

CI

Ο U

Ν


antimycotic imidazoles (2 examples)

Ô

NΜ Bifonazole ϋ

F i g u r e 5. I m i d a z o l y l e r g o s t e r o l b i o s y n t h e s i s

F i g u r e 6.

inhibitors.

Pyrimidine ergosterol biosynthesis

/CH

/ Ci H 3

2 7

CH

3

\

-N

tridemorph

/

CH

3

/Γ"λ

Ο ^CH

3

inhibitors.

tC H -^ 4

9

I / ~ \ y-CH -CH-CH -N Ο 2

2

N 3

^CH

3

fenpropimoφh

F i g u r e 7. M o r p h o l i n e e r g o s t e r o l b i o s y n t h e s i s

inhibitors.



34

FUNGICIDE CHEMISTRY

control

10ppm triadimenol

F i g u r e 8. Gas chromatography o f s t e r o l s oryzae t r e a t e d w i t h t r i a d i m e n o l .

from P y r i c u l a r i a


2.

35


BERG

formed by GC/MS-coupling e x p e r i m e n t s ; t h e m a s s - s p e c t r a b e i n g compared t o those o f a u t h e n t i c m a t e r i a l s . was

Q u a n t i f i c a t i o n o f the accumulation

performed by t h e 100% method w i t h

t h e assumption t h a t

the FID-

response i s t h e same f o r a l l s t e r o l s . The

quantity

and d i s t r i b u t i o n o f t h e s t e r o l s t r u c t u r e s

o r y z a e a f t e r t h e fungus had been t r e a t e d w i t h listed

i n Figure

9.

The u n t r e a t e d

10 ppm t r i a d i m e n o l a r e

control contains,

as e x p e c t e d , a


l a r g e amount o f e r g o s t e r o l a s t h e main membrane component. minor c o n c e n t r a t i o n s

o f Δ ^ - e r g o s t e n o l and Δ

P y r i c u l a r i a contains

about 15% Δ ^ - s t i g m a s t e n o l ,

the

latter

compound i s n o t a f f e c t e d

As a consequence o f t r e a t m e n t w i t h 4- f o l d

decrease

i n ergosterol

i n P.

Besides

^'^-ergostadiene-ol, but the content o f

by a p p l i c a t i o n o f t r i a d i m e n o l .

10 ppm t r i a d i m e n o l , we observed a

c o n t e n t and an a c c u m u l a t i o n o f i t s

precursor

2 4 - m e t h y l e n e d i h y d r o l a n o s t e r o l , a n o n p l a n a r compound due t o

the

m e t h y l groups a t

three

bond.

Therefore,

and

the accumulating

and l a c k o f t h e

-double

s t e r o l i s not able

to function

inhibits

biosynthesis

p r o p e r l y as a membrane component. The is

s i t e a t which t r i a d i m e n o l

illustrated

This

i n Figure

ergosterol

10 and t h e a c c u m u l a t i n g s t e r o l i s framed.

result clearly indicates

that

chrome P^Q-dependent o x i d a t i v e

triadimenol

inhibits

the cyto

removal o f t h e C ^ - m e t h y l group o f

24-methylenedihydrolanosterol, which i s a pathogen-specific

precursor

of e r g o s t e r o l . I n t h e same t e s t system, s e v e r a l o t h e r t r i a z o l e s were examined, i n c l u d i n g t r i a r i m o l a s an example o f t h e p y r i m i d i n e i m a z a l i l as a r e p r e s e n t a t i v e is

of the imidazole

common t o a l l these compounds t h a t

d e r i v a t i v e s , and

s e r i e s ( F i g u r e 11). I t

24-methylenedihydrolanosterol

accumulates i n d i c a t i n g an i d e n t i c a l p r i m a r y mode o f a c t i o n . I n the 5 22 , __ ' -ergosta-

Δ

d i e n e - o l c a n be o b s e r v e d , i n d i c a t i n g t h a t i n P y r i c u l a r i a o r y z a e Δ

-dehydrogenation i s a l s o

not

necessarily

affected

correspond with

by t r i a r i m o l .

These d a t a do

b i o l o g i c a l e f f i c a c y since

most o f

these compounds a r e used t o c o n t r o l powdery mildews, w h i c h a r e o b l i gate p a r a s i t e s Another

and a r e n o t e a s i l y a c c e s s i b l e

important

condition

cannot

to i n vitro

be f u l f i l l e d

i n such

studies. i n vitro


36

FUNGICIDE CHEMISTRY

sterol [%]


ergosterol

D 5

A -ergostene-ol (3)

5

22

Δ · -ergostadiene-ol (3)

5

A -stigmastene-ol (3)

— 24-methylenedihydrolanosterol

control

triadimenol [10ppm]

74.7

18.0

5.5

4.2

4.4

7.9

15.4

15.7

-ergostadiene-ol (3)

8

Δ ·

2 4

< >-efB08tadiene-ol (3) 28

5.8.24 — - e r g o s t a d i e n o l (3)

Biochemical Mode of Action of Fungicides - American Chemical

Recommend Documents