Cell Wall Alterations and Antimicrobial Defense in Perennial Plants

Chapter 25

Cell Wall Alterations and Antimicrobial Defense in Perennial Plants

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on July 15, 2016 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0399.ch025

R. B. Pearce Oxford Forestry Institute, Department of Plant Sciences, University of Oxford, South Parks Road, Oxford OX1 3RB, England

Cell wall alterations, especially suberization responses, appear particularly important as disease resistance mechanisms in the perennial tissues of plants. Antimicrobial defence in such tissues may entail the durable operation of resistance mechanisms during prolonged host-parasiteinteractions: wall alterations are well suited to this requirement. In the bark of many trees, the suberized phellem forms a constitutive barrier to invasion, and suberization is an early and important part of the wound and defence responses restoring an intact periderm surface. This is exemplified by the response of Sitka spruce to attack by Armillaria obscura. Induced suberization responses also occur in sapwood, conferring enhanced resistance to degradation by decay fungi and contributing to the formation of barriers which limit the spread of pathogens in the living tree. A n t i m i c r o b i a l defence i n plants has been s t u d i e d most extensively i n a n n u a l , herbaceous species, a category i n c l u d i n g the m a j o r i t y o f i m p o r t a n t a g r i c u l t u r a l crop p l a n t s . A range of different disease resistance m e c h a n i s m s have been described for such p l a n t s , i n c l u d i n g b o t h c o n s t i t u t i v e a n d i n d u c e d a n t i m i c r o b i a l c o m p o u n d s a n d cell w a l l a l t e r a t i o n s ( 1 , 2 ) . A s the g l o b a l emphasis i n w o o d p r o d u c t i o n , for t i m b e r , p u l p , fuel a n d c h e m i c a l p r o d u c t s , shifts f r o m the h a r v e s t i n g o f n a t u r a l o r s e m i - n a t u r a l forests t o p r o d u c t i o n i n increasingly intensively m a n a g e d p l a n t a t i o n s , the p a t h o l o g y of these w o o d y perennials is a t t a i n i n g increasing i m p o r t a n c e . T h e a d o p t i o n o f such practices as the creation o f h i g h l y u n i f o r m ( p h y s i c a l l y a n d genetically) m o n o c u l t u r e stands is likely t o increase the p o t e n t i a l for t h e development o f disease epidemics, as c o m m o n l y occur i n a g r i c u l t u r a l crops ( 3 , 4 ) . I n consequence increased a t t e n t i o n has been g i v e n t o disease resistance i n w o o d y species. A l t h o u g h o u r u n d e r s t a n d i n g o f resistance i n 0097-6156/89/0399-0346$06.00/0 © 1989 American Chemical Society

Lewis and Paice; Plant Cell Wall Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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these w o o d y p e r e n n i a l s s t i l l lags b e h i n d t h a t i n a n n u a l p l a n t s , significant advances have been m a d e d u r i n g the past fifteen years. Sufficient studies have been c a r r i e d o u t t o i n d i c a t e t h a t the m a j o r defence m e c h a n i s m s r e p o r t e d f r o m herbaceous species occur also i n w o o d y p l a n t s . H o w e v e r , i n the m o r e e x t e n s i v e l y developed secondary tissues o f these p l a n t s defence m e c h a n i s m s m a y b e o r g a n i z e d t o f o r m clearly defined barriers ( 5 ) . T h i s chapter w i l l concentrate o n these s t r u c t u r a l b a r r i e r s , w h i c h a p p e a r p a r t i c u l a r l y i m p o r t a n t i n the c o n t a i n m e n t o f pathogens o r p o t e n t i a l pathogens w i t h i n these secondary tissues. Requirements for Defence i n Perennial Plant Tissues In a n n u a l plants t h e d u r a t i o n o f the h o s t - p a t h o g e n i n t e r a c t i o n is genera l l y r e l a t i v e l y brief; i n p e r e n n i a l p l a n t s , however, these i n t e r a c t i o n s m a y continue for g r e a t l y e x t e n d e d periods i n the l o n g - l i v e d secondary tissues. S u c h l o n g - t e r m infections i n c l u d e p e r e n n i a l cankers a n d w o o d d e c a y i n g pathogens, i n c l u d i n g m a n y o f the i m p o r t a n t root- a n d b u t t - r o t t i n g forest pathogens s u c h as Armillaria s p p . a n d Heierobasidion annosum. These p e r e n n i a l pathogens colonize ( a n d frequently k i l l ) a v o l u m e o f host tissue, w h i c h provides a f o o d base t h a t c a n s u s t a i n the fungus d u r i n g a p r o l o n g e d i n t e r a c t i o n w i t h the l i v i n g tissues o f the host. I n the case o f Armillaria spp. n u t r i e n t s m o b i l i z e d f r o m one infected host m a y be t r a n s l o c a t e d a l o n g r h i z o m o r p h s , s u p p o r t i n g their g r o w t h a n d the i n f e c t i o n process where they e n counter a new p o t e n t i a l host (6). I n b o t h p e r e n n i a l cankers a n d p a t h o g e n i c w o o d decays there is a n i n t e r n a l interface between the l i v i n g host a n d the p a t h o g e n at the lesion m a r g i n . T h e advance o f the p a t h o g e n at these i n terfaces is t y p i c a l l y very slow. Defences effective i n l i m i t i n g pathogen advance under these c i r c u m stances must be capable o f m a i n t a i n i n g their f u n c t i o n for a n e x t e n d e d t i m e . A l t h o u g h t h i s r e q u i r e m e n t for d u r a b i l i t y does n o t p r e c l u d e the i n v o l v e m e n t of c h e m i c a l defences, s t r u c t u r a l defence m e c h a n i s m s are p a r t i c u l a r l y well s u i t e d t o the p r o t e c t i o n o f the host under these c i r c u m s t a n c e s . W h i l s t a n t i m i c r o b i a l chemicals m a y be l a b i l e or diffusible, s t r u c t u r a l defences, c o m p r i s i n g either w a l l a l t e r a t i o n s t o p r e - e x i s t i n g cells or tissues f o r m e d de novo b y renewed cell d i v i s i o n , are m u c h more stable. S u c h barriers c a n protect the u n d e r l y i n g tissues indefinitely against m i c r o o r g a n i s m s l a c k i n g a specific a b i l i t y t o penetrate or c i r c u m v e n t t h e m . S t r u c t u r a l defences, p a r t i c u l a r l y those r e q u i r i n g the p r o d u c t i o n o f new tissues b y the p l a n t m a y take some t i m e t o f o r m ( 5 , 7 - 9 ) . A n t i m i c r o b i a l c o m p o u n d s m a y be i m p o r t a n t d u r i n g the i n i t i a l stages o f the host-parasite i n t e r a c t i o n , before b a r r i e r development has o c c u r r e d , or m a y occur c o n c o m i t a n t l y w i t h cell w a l l a l t e r a t i o n s . I n S i t k a spruce (Picea sitchensis) the a n t i f u n g a l stilbenes a s t r i n g e n i n a n d i s o rhapontigenin accumulate around bark wounds inoculated w i t h potentially p a t h o g e n i c f u n g i . These are released o n i n f e c t i o n f r o m the c o r r e s p o n d i n g stilbene glucosides a s t r i n g i n a n d r h a p o n t i c i n w h i c h are c o n s t i t u t i v e i n t h e b a r k tissues, a n d provide a r a p i d response t o infection t h a t m a y i n h i b i t p o t e n t i a l pathogens u n t i l more d u r a b l e s t r u c t u r a l barriers have been f o r m e d (10).

American Chemical Society Library 1155 16th St., N.W. Lewis and Paice; Plant Cell Wall 20036 Polymers Washington, O.C. ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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Structural Defence Mechanisms in Plants S t r u c t u r e s interprétable as h a v i n g a defensive role m a y be either n o r m a l features of the p l a n t , or t h e i r f o r m a t i o n m a y be i n d u c e d as a result of m i c r o b i a l challenge or w o u n d i n g . I n the case of i n d u c e d s t r u c t u r a l defences the responses m a y comprise either changes i n p r e - e x i s t i n g cells, or m a y result i n the f o r m a t i o n of specific b a r r i e r tissues de novo b y renewed or m o d i f i e d cell d i v i s i o n . O c c a s i o n a l l y n o n - c e l l u l a r s t r u c t u r a l defences m a y operate. T h e resin flow t h a t accompanies w o u n d i n g or i n f e c t i o n i n m a n y coniferous trees m a y act as a m e c h a n i c a l b a r r i e r , h a r d e n i n g to f o r m a n a m o r p h o u s v a r n i s h i m p e n e t r a b l e to f u n g a l i n v a s i o n (11). M o r e c o m m o n l y , however, s t r u c t u r a l defences are effected b y cell w a l l a l t e r a t i o n s . T h e s e i n c l u d e l i g n i f i c a t i o n (12-14), s u b e r i z a t i o n ( 1 4 , 1 5 ) , callose d e p o s i t i o n (16), and silicification (17,18). S u c h w a l l a l t e r a t i o n s m a y operate i n several ways. P r o b a b l y the most o b v i o u s a n d w i d e l y suggested m e c h a n i s m is b y p h y s i c a l blockage o f the p a t h o g e n . F o r a b a r r i e r to prevent the advance of a p a t h o g e n , i t m u s t be m o r e resistant to the cell w a l l - d e g r a d i n g enzymes of the pathogen t h a n u n m o d i f i e d walls. S u c h enhanced resistance to d e g r a d a t i o n has been d e m o n s t r a t e d b o t h for lignified p a p i l l a e a n d halos i n wheat leaf e p i d e r m a l cells (19) a n d for s u b e r i z e d barriers i n oak x y l e m (15). A l t e r n a t i v e l y , s t r u c t u r a l responses of a p l a n t c o u l d e s t a b l i s h a p e r m e a b i l i t y b a r r i e r , i s o l a t i n g the p a t h o g e n f r o m the l i v i n g tissues of its host. S u c h a p e r m e a b i l i t y b a r r i e r c o u l d protect h e a l t h y cells f r o m f u n g a l t o x i n s or enzymes, i t c o u l d reduce the rate o f diffusion of host a n t i f u n g a l c o m p o u n d s f r o m the infection c o u r t , p e r h a p s e n h a n c i n g the efficacy of these c h e m i c a l defences, or i t c o u l d reduce the flow of n u t r i e n t s or water to the p a t h o g e n (20). A test for tissue p e r m e a b i l i t y to ions, u s i n g ferric chloride a n d p o t a s s i u m f e r r i c y a n i d e , t e r m e d the F - F test (21), has d e m o n s t r a t e d the i m p e r m e a b i l i t y of s u b e r i z e d b a r rier tissues at lesion m a r g i n s i n the b a r k o f various w o o d y species ( 7 , 1 4 ) , a l t h o u g h i n i t i a l l y the s u b e r i z a t i o n of these i m p e r v i o u s tissues was not recognized (7). I n a d d i t i o n , the processes l e a d i n g to the f o r m a t i o n of s t r u c t u r a l barriers m a y have a direct effect o n the pathogen itself, either as a result o f c h e m i c a l i n h i b i t i o n by n o n - p o l y m e r i z e d monomers of the w a l l - m o d i f y i n g m a t e r i a l , or b y d e p o s i t i o n of t h i s m a t e r i a l o n t o the w a l l of the p a t h o g e n , w i t h consequent interference w i t h i t s g r o w t h (22). L i g n i f i c a t i o n responses have been i m p l i c a t e d i n disease resistance i n p e r e n n i a l p l a n t s . I n the leaves o f oak species a lignified p a p i l l a response appears to be i m p o r t a n t i n the resistance of older leaves to oak m i l d e w , Microsphaera alphitoides (23). Leaves of a deciduous tree are, however, e p h e m e r a l organs, c o m p a r a b l e to those o f herbaceous species. A s discussed i n t h i s p a p e r , s u b e r i z a t i o n of cell walls (sometimes a c c o m p a n i e d b y l i g n i f i c a t i o n also) is perhaps more t y p i c a l of the barriers f o u n d i n the secondary tissues of w o o d y ( 8 , 1 4 ) a n d herbaceous (24) p l a n t s . S u b e r i n , a l t h o u g h s t i l l i m p e r f e c t l y c h a r a c t e r i z e d , is p r e d o m i n a n t l y a polyester c o m p o s e d of l o n g c h a i n ( C i s — C 3 0 ) h y d r o x y - a n d h y d r o x y e p o x y - f a t t y acids. L i g n i n - l i k e a r o m a t i c d o m a i n s m a y also be associated w i t h t h i s p o l y m e r (25). It therefore differs m a r k e d l y f r o m the a r o m a t i c a n d c a r b o h y d r a t e p o l y m e r s t h a t c o m -


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prise the b u l k of the secondary p l a n t tissues. A l s o , because of its h y d r o p h o bic properties i t is h i g h l y effective as a p e r m e a b i l i t y b a r r i e r to w a t e r . A s water content has been i m p l i c a t e d i n the e n v i r o n m e n t a l e x c l u s i o n of m a n y w o o d - i n h a b i t i n g f u n g i f r o m the l i v i n g secondary x y l e m (sapwood) of trees (26-28), t h i s m a y be p a r t i c u l a r l y significant i n the p r o t e c t i o n of these t i s sues. W h i l e s u b e r i n c a n be degraded b y some f u n g i , i n c l u d i n g Armillaria sp. (29) a n d Rosellinia desmazieresii (30), the rates of b r e a k d o w n were not high.


C o n s t i t u t i v e S t r u c t u r a l Defences A l t h o u g h m a n y s t r u c t u r a l features of a p l a n t , such as f o r m a n d a r r a n g e m e n t of s t o m a t a , can be i n t e r p r e t e d i n terms of defence (31), o n l y the secondary surface w i l l be considered i n d e t a i l here. T h i s comprises a p e r i d e r m , or series of sequent p e r i d e r m s f o r m i n g a r h y t i d o m e . P e r i d e r m s are sites of c a m b i a l a c t i v i t y , suberized p h e l l e m cells being p r o d u c e d e x t e r n a l l y b y the phellogen. T h e thickness of p h e l l e m a c c u m u l a t i n g varies g r e a t l y between species. Few pathogens are capable of p e n e t r a t i n g the i n t a c t p e r i d e r m surface of w o o d y p l a n t s . M o s t , i f not a l l , of the f u n g i t h a t infect b y t h i s route (rather t h a n b y b y p a s s i n g the surface b a r r i e r , i n f e c t i n g via w o u n d s a n d n a t u r a l d i s c o n t i n u i t i e s i n the p e r i d e r m ) do so f r o m established infections o n a n e i g h b o r i n g f o o d base, p e r m i t t i n g a prolonged a t t a c k . C o m m o n l y (e.g., w i t h Armillaria spp., Rosellinia spp.) p e n e t r a t i o n is effected b y m y c e l i a l aggregations, s u p p o r t e d m e t a b o l i c a l l y b y m y c e l i a l cords or r h i z o m o r p h s ( 3 0 , 3 2 , 3 3 ) . W i t h o u t t h i s c a p a c i t y to m o u n t a prolonged a t t a c k , i n w h i c h b o t h m e c h a n i c a l a n d e n z y m i c a c t i v i t y m a y be i n v o l v e d , p o t e n t i a l pathogens appear u n a b l e to overcome the p e r i d e r m b a r r i e r . P e r h a p s because of its u b i q u i t y , there have been few detailed studies of the role of surface p e r i d erms i n defence: c o m m o n l y the p r o t e c t i o n p r o v i d e d is assumed (e.g., 34), a n d is a t t r i b u t e d to the resistance of the p h e l l e m cells t o d e g r a d a t i o n . P e r i d e r m (or r h y t i d o m e ) surfaces of some tree species persist w i t h o u t erosion for l o n g periods, surface features r e m a i n i n g v i s i b l e for several decades at least (35). T h e suberized p h e l l e m cells c o m p r i s i n g a n i m p o r t a n t c o m p o n e n t of these tissues are n o n - l i v i n g a n d i n c a p a b l e of any active r e p a i r , b u t are clearly more d u r a b l e t h a n other p l a n t tissues. In c e r t a i n instances, however, factors other t h a n the cell w a l l p o l y mers of the p h e l l e m m a y be i m p o r t a n t i n the p r o t e c t i o n p r o v i d e d by the secondary surface. Rosellinia desmazieresii i n o c u l a t e d i n a food base o n t o the u n d e r g r o u n d stems of a resistant Salix repens h y b r i d ( 5 . χ Friesiana) e x h i b i t e d g r e a t l y reduced e p i p h y t i c g r o w t h a n d cord f o r m a t i o n c o m p a r e d w i t h i n o c u l a t i o n s o n t o susceptible S. repens itself. A t t e m p t e d p e n e t r a t i o n was not observed o n the resistant h y b r i d (30). T h i s b e h a v i o u r suggests t h a t diffusible c h e m i c a l i n h i b i t o r s at the s t e m surface m a y be i m p o r t a n t i n resistance to t h i s p a t h o g e n , w h i c h has a d e m o n s t r a t e d a b i l i t y to degrade s u b e r i n a n d penetrate the surface p e r i d e r m (30). A l t h o u g h i n a n u n w o u n d e d , healthy, p l a n t p e r i d e r m s are n o r m a l l y s u p e r f i c i a l , i n t e r x y l a r y cork has been r e p o r t e d f r o m at least 40 species of d i -



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c o t y l e d o n o u s plants (36). These i n t e r x y l a r y p e r i d e r m s are associated w i t h the fission of p e r e n n a t i n g axes a n d the d y i n g back of a n n u a l organs such as a e r i a l shoots. I n Epilabium angustifolium an i n t e r x y l a r y p e r i d e r m is f o r m e d each year over the surface of the x y l e m connected w i t h the previous year's aerial shoots. T h e older, r e d u n d a n t , vascular tissue is thus w a l l e d off f r o m the f u n c t i o n a l tissues of the p l a n t , w h i c h are then protected b y b o t h inner a n d outer suberized barriers. It has been suggested t h a t such i n t e r n a l p e r i d e r m s m a y be i m p o r t a n t i n affording p r o t e c t i o n against the ingress of m i c r o o r g a n i s m s associated w i t h the d y i n g d o w n of the a n n u a l a e r i a l shoots (37), a f u n c t i o n i n accordance w i t h the h a b i t of m a n y of the species f r o m w h i c h they have been recorded. Defence in B a r k

Tissues

S h o u l d the b a r r i e r presented b y the secondary p l a n t surface be breached, defence m e c h a n i s m s m a y operate i n the b a r k (i.e., secondary cortex a n d p h l o e m ) of w o o d y p l a n t s , l i m i t i n g pathogen development. These i n c l u d e c h e m i c a l defences a n d s t r u c t u r a l barriers r e s u l t i n g f r o m cell w a l l a l t e r a t i o n s (5). B o t h p r o b a b l y act i n concert, a l t h o u g h i n a n o n - w o o d y p l a n t (carrot) it has been concluded t h a t the i m p o r t a n c e of cell w a l l a l t e r a t i o n s was seco n d a r y to t h a t of c h e m i c a l defence (24). O n l y the cell w a l l a l t e r a t i o n s i n v o l v e d i n defence w i l l be considered further here. Cell Wall Alterations in the Bark of Gymnosperms. Non-specific defence responses to w o u n d i n g , insect a n d f u n g a l a t t a c k i n the b a r k of conifers, l e a d i n g to a r e s t o r a t i o n of the p e r i d e r m surface, have been described. In essence, n e c r o p h y l a c t i c ( w o u n d ) p e r i d e r m s are formed continuous w i t h the n o r m a l s t r u c t u r a l surface p e r i d e r m , w a l l i n g off the lesion a n d effectively e x c l u d i n g it f r o m the p l a n t (7). P r i o r to the development of this p e r i d e r m b y dedifferentiation a n d renewed d i v i s i o n of cells i n the bark s u r r o u n d i n g the lesion, cell w a l l alterations take place i n p r e - e x i s t i n g tissues b o r d e r i n g the lesion. P r o m i n e n t a m o n g these a l t e r a t i o n s is the f o r m a t i o n of a n i m p e r v i o u s zone i m m e d i a t e l y o v e r l y i n g the site of p e r i d e r m r e s t o r a t i o n . T h i s was i n i t i a l l y described as non-suberized i m p e r v i o u s tissue ( N I T ) (7). I n a m o r e recent s t u d y of defence responses f o l l o w i n g w o u n d i n g a n d a r t i f i c i a l i n o c u l a t i o n of Picea sitchensis root b a r k w i t h the weakly pathogenic b u t t rot fungus Phaeolus schweinitzii, use of i m p r o v e d h i s t o c h e m i c a l techniques d e m o n s t r a t e d t h a t s u b e r i z a t i o n of cell walls occurred i n these tissues (9). C e l l walls i n the necrotic tissue of these wounds were b r o w n e d . S t a i n i n g w i t h d i a z o t i z e d o-tolidine a n d t o l u i d i n e blue confirmed the p o l y p h e n o ls n a t u r e of these b r o w n depositions, w h i c h m a y have resulted f r o m the p o l y m e r i z a t i o n of the stilbenes present i n large quantities i n spruce b a r k . P h e n o l i c residues were deposited o n the walls of c e r t a i n cells i n t e r n a l to the necrotic tissues b y 10 days after w o u n d i n g . B y 36 days these cells h a d become t h i c k - w a l l e d . T h e precise nature of substances responsible for this t h i c k e n i n g has not been d e t e r m i n e d , v a r i a b l e responses b e i n g o b t a i n e d w i t h h i s t o c h e m i c a l tests for l i g n i n (cf. T a b l e I). S u b e r i n was detectable i n cells i m m e d i a t e l y u n d e r l y i n g the t h i c k walled cells, w h i c h corresponded to the


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( " n o n - " ) suberized i m p e r v i o u s tissue, a n d i n the t h i n w a l l e d p h e l l e m cells f o r m e d later b y the developing n e c r o p h y l a c t i c p e r i d e r m (9). These b a r k responses are i l l u s t r a t e d d i a g r a m m a t i c a l l y i n F i g u r e 1. T a b l e I. H i s t o c h e m i c a l responses of cell walls associated w i t h the necrop h y l a c t i c p e r i d e r m response i n Picea sitchensis challenged w i t h Armillaria obscura


Wall Polymer

Stain

Pectic Materials

Ruthenium red

Cellulose

Zinc-chloriodine Toluidine blue

Callose

R e a c t i o n i n Tissues

a

Resorcinol blue

Healthy Bark Parenchyma

Necrophylactic Phellem

Thick Walled Tissue

Necrotic Tissues

+

-

±

+

+

—

—

—

+

-

-

-

±

—

—

—

—

—

+

+

—

±

+

+

— —

+ —

+ —

— —

-

+

-

-

±

(locally) Lignin/ Wall-bound phenolics

Suberin a

Zinc-chloriodine Toluidine blue Phloroglucinol-HCl M a u l e test Lignin pink Sudan I V

-

S p e c i m e n p r e p a r a t i o n a n d s t a i n i n g methods as described p r e v i o u s l y (15).

E s s e n t i a l l y i d e n t i c a l processes occur i n u n wounded spruce b a r k , c h a l lenged by, a n d resisting successfully, at least i n the short t e r m , the root- a n d b u t t - r o t pathogen Armillaria obscura (Secretan) H e r i n k . A t r h i z o m o r p h contact a n d a t t e m p t e d i n f e c t i o n sites o n the b a r k of buttress roots of c 60-year-old trees of Picea sitchensis (Bong.) C a r r . , p e n e t r a t i o n of the s u r face p e r i d e r m h a d o c c u r r e d . S m a l l (c 10 m m . diameter) necrotic regions resulted, confined to superficial b a r k tissues only. M e c h a n i c a l a t t a c h m e n t of these lesions ( w h i c h h a d been i n i t i a t e d some t i m e p r e v i o u s l y so t h a t host responses to the challenge were essentially complete) was weak, a n d they were r e a d i l y detached f r o m the roots as b a r k scales. Cleavage o c c u r r e d i n the plane of a n e c r o p h y l a c t i c p e r i d e r m , f o r m e d at the m a r g i n between tissue colonized b y A. obscura a n d the h e a l t h y bark ( F i g u r e 2). C e l l w a l l



352


F i g u r e 1. S t r u c t u r a l responses of the b a r k of Picea sitchensis to w o u n d i n g a n d i n o c u l a t i o n w i t h Phaeolus schweinitzii. I W , inoculated wound; S P , surface p e r i d e r m ; N T , necrotic tissue; T C , thickened cells; S I T , relic of suberized i m p e r v i o u s tissue; N P , n e c r o p h y l a c t i c p e r i d e r m ; P , p h l o e m ; V C , vascular c a m b i u m .


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a l t e r a t i o n s , i d e n t i c a l to those associated w i t h a r t i f i c i a l wounds (9) were detectable i n a n d a r o u n d the lesion u s i n g a range o f h i s t o c h e m i c a l tests ( T a b l e I). T h e u n d e r l y i n g h e a l t h y b a r k was separated f r o m the f u n g a l l y c o l o n i z e d tissues b y a n e c r o p h y l a c t i c p e r i d e r m , essentially s i m i l a r to the n o r m a l root surface p e r i d e r m . T h i s c o m p r i s e d layers of t h i n w a l l e d , s u b e r i z e d p h e l l e m cells a n d t h i c k w a l l e d cells s t a i n i n g p o s i t i v e l y w i t h the p h l o r o g l u c i n o l H C 1 reagent. O u t s i d e the outermost p h e l l e m cells there existed a single layer o f i r r e g u l a r s u b e r i z e d cells, c o r r e s p o n d i n g to the cells f o r m i n g the suberized i m p e r v i o u s tissue p r i o r to phellogen f o r m a t i o n . T h e t h i c k e n e d , p h l o r o g l u c i n o l - p o s i t i v e cells between these a n d the necrotic tissue were not s u b e r i z e d . F u n g a l h y p h a e were v i s i b l e i n these cells ( F i g u r e 3), i n d i c a t i n g t h a t t h i s w a l l t h i c k e n i n g d i d not itself pose a m a j o r i m p e d i m e n t to the g r o w t h of the fungus, a n d were present also i n the necrotic tissues, f r o m w h i c h evidence of w a l l - b o u n d phenolic c o m p o u n d s was also o b t a i n e d . N o h y p h a e were seen, however, i n the suberized tissues, suggesting t h a t these were c r u c i a l to the effectiveness of t h i s b a r r i e r response. Cell Wall Alterations in the Bark of Woody Angiosperms. Responses to w o u n d i n g a n d i n f e c t i o n closely s i m i l a r to those f o u n d i n G y m n o s p e r m s occur also i n the b a r k of A n g i o s p e r m s . These responses have been described i n d e t a i l f r o m a n u m b e r o f w o o d y species ( 8 , 1 4 , 3 8 , 3 9 ) a n d a p p e a r essentially s i m i l a r i n a l l . F o r m a t i o n o f a suberized i m p e r v i o u s tissue i n u n d a m a g e d tissues b e n e a t h the lesion is followed by the r e s t o r a t i o n of the p l a n t surface b y development of a n e c r o p h y l a c t i c p e r i d e r m . W h e r e a r a p i d l y s p r e a d i n g lesion results f r o m p o p l a r infections by Cytospora chrysosperma, there m a y be c o l o n i z a t i o n of the host b a r k tissues w i t h o u t expression of these defensive responses. However, w h e n s l o w l y e x t e n d i n g p e r e n n i a l cankers are f o r m e d b y t h i s p a t h o g e n a n e c r o p h y l a c t i c p e r i d e r m is p r o d u c e d b y the p l a n t , a n d advance of the fungus appears to take place o n l y where there are d i s c o n t i nuities i n t h i s b a r r i e r , e.g., where i t is traversed b y bundles of p h l o e m fibres (38). Responses in Non-woody Perennial Plants. A l t h o u g h pathogens capable of m o u n t i n g a p r o l o n g e d a t t a c k f r o m a colonized food base are less u s u a l o n n o n - w o o d y species, such a t t a c k s c a n sometimes o c c u r . In these instances p e r i d e r m r e s t o r a t i o n responses c o m p a r a b l e to those described f r o m w o o d y plants m a y operate to restrict pathogen ingress. T h i s m a y be exemplified b y the response of h o r s e r a d i s h , Armoracia rusticana G a e r t i n , M a y & Scherb. to a t t a c k b y a n Armillaria species ( p r o b a b l y A. bulbosa ( B a r l a ) , K i l e a n d W a t l i n g ) . A l t h o u g h not a n i m p o r t a n t p a t h o g e n of t h i s p l a n t , Armillaria r h i z o m o r p h s m a y a t t e m p t t o penetrate a n d colonize its large, fleshy roots. Sections of infected roots show colonized, necrotic tissues b o u n d e d b y suberized p h e l l e m cells. P e n e t r a t i o n of these n e c r o p h y l a c t i c p e r i d e r m s was effected b y r h i z o m o r p h - l i k e aggregations of h y p h a e ( F i g u r e 4). C e l l s i n the necrotic zone s t a i n e d green w i t h t o l u i d i n e blue a n d gave a p o s i t i v e response w i t h the p h l o r o g l u c i n o l - H C l reagent, b u t d i d not color w i t h the M a u l e test, suggesting the presence o f c e l l - w a l l b o u n d phenolics i n these tissues, a l -



354


F i g u r e 2. N e c r o p h y l a c t i c p e r i d e r m i n the b a r k of Picea sitchensis f o l l o w i n g a t t e m p t e d infection b y Armillaria obscura. B a r k sectioned a n d s t a i n e d w i t h S u d a n I V , after e x t r a c t i o n w i t h chlorine dioxide to remove a r o m a t i c c o m p o u n d s , essentially as p r e v i o u s l y described (15). N T , necrotic tissue; T C , thickened cells; S I T , relic of suberized i m p e r v i o u s tissue; N P , n e c r o p h y l a c t i c p e r i d e r m . Scale bar = 25 μπι.

F i g u r e 3. H y p h a e (arrowed) of Armillaria obscura i n t h i c k walled cells ( T C ) o v e r l y i n g the n e c r o p h y l a c t i c p e r i d e r m ( N P ) i n Picea sitchensis. B a r k sec t i o n e d a n d s t a i n e d w i t h t o l u i d i n e blue as p r e v i o u s l y described (15). Scale b a r = 25 μηα.


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t h o u g h l i g n i n f o r m a t i o n itself m a y n o t have o c c u r r e d . H y p h a e were present i n these necrotic tissues b u t none were seen p e n e t r a t i n g the s u b e r i z e d cells. A l t h o u g h the role o f c h e m i c a l defences i n t h i s i n t e r a c t i o n was n o t e x a m i n e d (cf. 24), i t w o u l d appear t h a t the suberized p e r i d e r m barriers p r o d u c e d i n response to i n f e c t i o n posed a more effective defence against Armillaria t h a n the w a l l - b o u n d phenolics o f the necrotic tissues. T h e p e r i d e r m s were, however, repeatedly breached b y the aggregated m y c e l i u m o f t h e fungus, r e s u l t i n g i n a c o m p l e x lesion c o n t a i n i n g m a n y defeated p e r i d e r m b a r r i e r s e x t e n d i n g i n t o the root x y l e m . Defence i n L i v i n g W o o d T h e s a p w o o d o f a l i v i n g tree, c o m p r i s i n g the x y l e m tissues f u n c t i o n a l i n t r a n s p o r t a n d storage, contains l i v i n g cells a n d is capable of active responses to i n f e c t i o n . I n contrast the h e a r t w o o d ( i n those species t h a t f o r m i t ) is n o n - l i v i n g a n d i n c a p a b l e of further m e t a b o l i c a c t i v i t y . A c t i v e defensive r e sponses are thus impossible i n h e a r t w o o d , a l t h o u g h a n t i f u n g a l c o m p o u n d s or cell w a l l a l t e r a t i o n s l a i d d o w n when the w o o d was s t i l l l i v i n g m a y c o n t i n u e t o f u n c t i o n . Indeed there are s t r i k i n g s i m i l a r i t i e s between m a n y o f the defences i n d u c e d i n s a p w o o d b y f u n g a l a t t a c k a n d the changes o c c u r r i n g d u r i n g h e a r t w o o d f o r m a t i o n , suggesting t h a t these processes m a y be related (5). B o t h a n t i m i c r o b i a l c h e m i c a l defences (40) a n d cell w a l l a l t e r a t i o n s have been r e p o r t e d f r o m x y l e m of w o o d y p l a n t s . A s w i t h defences i n t h e b a r k , o n l y cell w a l l a l t e r a t i o n s w i l l be discussed. T w o types o f b a r rier c a n be i d e n t i f i e d — t h o s e c o m p r i s i n g specialized tissues f o r m e d de novo i n response to w o u n d i n g a n d i n f e c t i o n , a n d those f o r m e d i n p r e - e x i s t i n g xylem. Tissue Barriers Formed de novo. M a n y w o o d y perennials r e s p o n d to w o u n d s r e s u l t i n g i n c a m b i a l damage, a n d c o n c o m i t a n t f u n g a l infection o f u n d e r l y i n g x y l e m , b y the p r o d u c t i o n of a d i s t i n c t i v e b a r r i e r tissue. T h i s b a r r i e r , w h i c h has been termed the c o m p a r t m e n t a l i z a t i o n w a l l 4 (41), c o m prises a sheet o f t r a u m a t i c a x i a l p a r e n c h y m a cells, formed b y the c a m b i u m i n t h e v i c i n i t y o f t h e w o u n d , a n d l a i d d o w n as the first x y l e m tissue p r o duced after w o u n d i n g ( 1 5 , 4 2 ) . I n conifers t h i s p a r e n c h y m a is c o m m o n l y a c c o m p a n i e d by resin ducts (43). I n most, b u t n o t a l l , species so far e x a m i n e d t h e p a r e n c h y m a cells o f the w a l l 4 b a r r i e r zone are s u b e r i z e d , at least where they overlie fungally colonized w o o d (5, Pearce, R . B . , u n p u b l i s h e d d a t a ) . E v i d e n c e has been o b t a i n e d f r o m oak (Quercus robur) t o suggest t h a t the s u b e r i z a t i o n response o f these tissues, b u t n o t necessarily their f o r m a t i o n b y the c a m b i u m , is f u n g a l l y elicited (5). S u b e r i z a t i o n o f these cells renders t h e m resistant t o d e g r a d a t i o n b y w o o d d e c a y i n g f u n g i . Sections t h r o u g h the c o m p a r t m e n t a l i z a t i o n w a l l 4 b a r r i e r region i n Q. robur, i n c u b a t e d for several weeks w i t h the w o o d decay p a t h o g e n Stereum gausapatum, were extensively degraded b y the fungus, except for the suberized cells w h i c h r e m a i n e d essentially i n t a c t (15). S u b e r ized x y l e m cells are clearly able t o resist d e g r a d a t i o n b y w o o d decay f u n g i for prolonged periods: the relics o f suberized cells a n d tyloses, f o r m e d as a



356


n o r m a l h e a r t w o o d c o m p o n e n t i n Q. robur (44), r e m a i n e d i n w o o d otherwise more or less t o t a l l y decayed b y n a t u r a l infection w i t h the w h i t e - r o t fungus Ganoderma adspersum ( F i g u r e 5). I n some species, however, e.g. a s h , Fraxinus excelsior, cells of the t r a u m a t i c a x i a l p a r e n c h y m a of the c o m p a r t m e n t a l i z a t i o n w a l l 4 m a y show n o evidence of cell w a l l a l t e r a t i o n s , yet appear to act n o r m a l l y as a f u n c t i o n a l b a r r i e r t o decay (Pearce, R . B . , u n p u b l i s h e d d a t a ) . It is t o be p r e s u m e d t h a t the spread of decay f u n g i is arrested either b y c h e m i c a l defences or by e n v i r o n m e n t a l c o n s t r a i n t s (cf. 26-28) i n such species. C l e a r l y , a c o n t r i b u t i o n m a y be m a d e b y these defences i n s u b e r i z i n g species also: p h y t o a l e x i n - l i k e a n t i f u n g a l c o m p o u n d s have been detected i n association w i t h a s u b e r i z e d w a l l 4 b a r r i e r i n Acer saccharinum (42). M o r e work w i l l be required to e l u cidate the l o n g - t e r m effectiveness of the various m e c h a n i s m s m a i n t a i n i n g the f u n c t i o n of these b a r r i e r w a l l s . Cell Wall Alterations in Pre-existing Xylem. W h e r e x y l e m c o l o n i z i n g f u n g i interface w i t h p r e - e x i s t i n g s a p w o o d , responses of the l i v i n g p a r e n c h y m a cells can result i n the f o r m a t i o n of a r e a c t i o n zone (45-47). R e a c t i o n zones present a less w e l l defined b a r r i e r t o i n f e c t i o n t h a n the c o m p a r t m e n t a l i z a t i o n w a l l 4 a n d have been envisaged as a d y n a m i c response, h e a l t h y s a p w o o d being converted to reaction zone tissue ahead of the a d v a n c i n g fungus ( 4 5 , 4 7 ) . E v i d e n c e has recently been o b t a i n e d t h a t t h i s is not a c o n tinuous process, b u t t h a t reaction zones act as s t a t i c b o u n d a r i e s t o decay, w h i c h advance d i s c o n t i n u o u s l y w i t h i n the tree, periods of r a p i d f u n g a l a d vance e l i c i t i n g l i t t l e or no host response b e i n g followed by periods of stasis, a c c o m p a n i e d b y the f o r m a t i o n of clearly defined reaction zones (5, Pearce, R . B . , unpublished data). T h e a c c u m u l a t i o n of p h y t o a l e x i n - l i k e a n t i f u n g a l c o m p o u n d s c o m m o n l y occurs i n r e a c t i o n zone tissues ( 4 0 , 4 2 ) , a n d b r o w n deposits, g i v i n g s t a i n i n g responses p o s i t i v e for phenolic c o m p o u n d s , are frequently present i n cells i n t h i s region. Vessels m a y be o c c l u d e d b y these deposits (gummosis) or b y the f o r m a t i o n of tyloses, b a l l o o n - l i k e o u t g r o w t h s f r o m the walls of vessel p a r e n c h y m a cells (5). C e l l w a l l s u b e r i z a t i o n responses c o m m o n l y o c c u r i n r e a c t i o n zones ( 5 , 4 8 ) , a l t h o u g h they are not always present, even i n species h a v i n g a d e m o n s t r a b l e s u b e r i z a t i o n response i n the w a l l 4 b a r r i e r , e.g. Acer saccharinum (42). Tyloses are u s u a l l y suberized ( 4 4 , 4 9 , Pearce, R . B . , u n p u b l i s h e d d a t a ) . In those species i n w h i c h a b u n d a n t tylosis form a t i o n o c c u r r e d i n r e a c t i o n zones, s u b e r i z a t i o n of x y l e m p a r e n c h y m a cells, i n c l u d i n g the r a y p a r e n c h y m a , was n o r m a l l y observed. In species i n w h i c h vessel o c c l u s i o n was p r e d o m i n a n t l y b y g u m m o s i s , s u b e r i z a t i o n responses were often l a c k i n g (Pearce, R . B . , u n p u b l i s h e d d a t a ) . U n l i k e the c o m p a r t m e n t a l i z a t i o n w a l l 4, the s u b e r i z e d tissues i n rea c t i o n zones do not f o r m a continuous b a r r i e r , p o t e n t i a l l y i m p r e g n a b l e b y pathogens i n c a p a b l e of d e g r a d i n g suberized walls. However, suberized t y loses a n d p a r e n c h y m a cell walls block the easiest routes of f u n g a l spread i n the x y l e m — a x i a l l y a l o n g vessels a n d r a d i a l l y i n the rays ( 5 0 ) — a n d m a y thus g r e a t l y h i n d e r f u n g a l spread. U n d o u b t e d l y these cell w a l l a l t e r a t i o n s n o r m a l l y act i n concert w i t h other defences—antifungal c o m p o u n d s a n d / o r



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F i g u r e 4. P e n e t r a t i o n of n e c r o p h y l a c t i c p e r i d e r m s ( N P ) i n the root of Armoracia rusticana b y r h i z o m o r p h - l i k e aggregations of h y p h a e ( R ) . Scale bar - 100 μτη.

F i g u r e 5. H e a r t w o o d of Qutrcus robur w i t h advanced decay caused by Ganoderma adspersum. A l t h o u g h the wood has been extensively degraded, suberized tyloses a n d vessel linings r e m a i n recognizable. W o o d sectioned a n d s t a i n e d w i t h S u d a n I V as p r e v i o u s l y described (15). Scale bar = 100 μπι.


358


e n v i r o n m e n t a l c o n s t r a i n t s o n pathogen g r o w t h (as w i t h the w a l l 4 b a r r i e r also). W h e r e cell w a l l a l t e r a t i o n s are not detectable i t is to be p r e s u m e d t h a t adequate p r o t e c t i o n is afforded b y these a l t e r n a t i v e m e c h a n i s m s .


Concluding Discussion A c o m m o n requirement for the successful p r o t e c t i o n of p e r e n n i a l p l a n t s against m a n y of their pathogens is for defence to be d u r a b l e . S e c o n d a r y tissues m a y persist for decades or centuries, a n d pathogens s u p p o r t e d b y a s u b s t a n t i a l food base m a y be capable of m o u n t i n g a prolonged assault o n the p o t e n t i a l host. C e l l w a l l a l t e r a t i o n s , c o m p r i s i n g either the depos i t i o n of new m a t e r i a l o n p r e - e x i s t i n g cell w a l l s , or the f o r m a t i o n of new tissues w i t h a t y p i c a l or specialized cell w a l l s , have been i m p l i c a t e d as defence mechanisms i n a wide range of circumstances i n p e r e n n i a l p l a n t s . A role i n a n t i m i c r o b i a l defence can be ascribed to the s u b e r i z e d p h e l l e m f o r m i n g the surface of the secondary tissues of such p l a n t s . I n t e r n a l p e r i d e r m s w h i c h m a y have a defensive f u n c t i o n occur i n some species also. W o u n d i n g or i n f e c t i o n of c o r t i c a l a n d p h l o e m ( b a r k ) tissues n o r m a l l y results i n a sequence of cell w a l l a l t e r a t i o n s , c o m m e n c i n g w i t h changes to w a l l s of p r e - e x i s t i n g cells, a n d c u l m i n a t i n g w i t h the r e s t o r a t i o n of the i n t a c t p l a n t surface b y means of the f o r m a t i o n of a n e c r o p h y l a c t i c p e r i d e r m . W o u n d s to the c a m b i u m a n d x y l e m result i n the p r o d u c t i o n of a t r a u m a t i c b a r r i e r tissue, often w i t h suberized w a l l s , w h i c h prevents damage t o the v i t a l c a m b i u m a n d the youngest, m e t a b o l i c a l l y most i m p o r t a n t , secondary tissues b y pathogens b e c o m i n g established i n the exposed x y l e m . S u b e r i z a t i o n responses also often occur i n the reaction zone m a r g i n s of infection a n d decay in living wood. A s defence mechanisms i n plants c o m m o n l y act i n concert, i t w o u l d be w r o n g to ascribe a p r i m a r y role to these various w a l l a l t e r a t i o n s w i t h o u t a d d i t i o n a l i n v e s t i g a t i o n : i n v i r t u a l l y a l l cases evidence for the c o n c o m i t a n t o p e r a t i o n of c h e m i c a l defences is also available. However, cell w a l l a l t e r ations are p a r t i c u l a r l y w e l l s u i t e d to l o n g t e r m defence, p r o v i d i n g stable a n d d u r a b l e barriers t h a t are not dependant o n the c o n t i n u i n g m e t a b o l i c a c t i v i t y of l i v i n g cells to m a i n t a i n t h e m , a n d w h i c h are not subject to c h e m i c a l i n s t a b i l i t y or l e a c h i n g . In the cell w a l l a l t e r a t i o n s discussed s u b e r i z a t i o n has played a p r o m i nent role. T h i s is p r o b a b l y not c o i n c i d e n t a l . B o t h c a r b o h y d r a t e p o l y mers a n d l i g n i n are a b u n d a n t i n the secondary tissues of p e r e n n i a l p l a n t s , especially w o o d y species. Pathogens a d a p t e d to i n h a b i t t h i s h i g h l y l i g nified e n v i r o n m e n t are likely to possess effective e n z y m e systems for the m e t a b o l i s m of t h i s phenolic p o l y m e r , w h i c h is therefore u n l i k e l y to present a serious i m p e d i m e n t to these f u n g i . T h i s is i n m a r k e d contrast to the pathogens of p r i m a r y plant tissues (leaves, etc.) i n w h i c h l i g n i n is not a b u n d a n t l y present, against w h i c h cell w a l l l i g n i f i c a t i o n m a y present a n effective defence (51). Indeed, f u n g a l c o l o n i z a t i o n of cells w i t h walls altered b y l i g n i f i c a t i o n (or at least deposition of w a l l - b o u n d phenolics) was seen i n most of the interactions discussed here. T h e p r e d o m i n a n t l y a l i p h a t i c p o l y mer s u b e r i n is not a p r i m a r y constituent of these secondary tissues a n d is


25.

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359

thus likely (as has been demonstrated) t o present more o f a n i m p e d i m e n t to s u c h pathogens. A l s o , i f as has been suggested (26-28), t h e m a i n t e n a n c e of a n a p p r o p r i a t e m o i s t u r e content i n f u n c t i o n a l s a p w o o d is i m p o r t a n t i n l i m i t i n g f u n g a l c o l o n i z a t i o n , s u b e r i n m a y provide the necessary p e r m e a b i l i t y b a r r i e r t o m a i n t a i n t h i s a n d m i n i m i z e t h e extent o f d i s r u p t i o n t o x y l e m t r a n s p o r t r e s u l t i n g f r o m w o u n d i n g o r m i c r o b i a l infection.


Literature Cited

1. Callow, J.A ., Ed. Biochemical Plant Pathology; John Wiley & Sons: Chichester, 1983. 2. Horsfall, J. G.; Cowling, Ε. B., Eds. Plant Disease: An Advanced Trea tise; Vol. V; Academic Press: New York, 1980. 3. Gibson, I. A. S.; Burley, J.; Speight, M. R. C.F.I Occasional Papers, No. 18., 1980. 4. Woodward, S.; Pearce, R. Biomass News International 1986, 16, 2-3. 5. Pearce, R. B. In Fungal Infection of Plants; Pegg, G.F.; Ayres, P.G., Eds.; Cambridge University Press: Cambridge, 1987; pp 219-238. 6. Sinclair, W. Α.; Lyon, Η. H.; Johnson, W. T. Diseases of Trees and Shrubs; Comstock Publ. Assoc.: Ithaca, 1987; pp 308-313. 7. Mullick, D. B. In The Structure, Biosynthesis and Degradation of Wood; Loewus, F. Α.; Runeckles, V. C., Eds.; Plenum Press: New York, 1977; pp 395-442. 8. Biggs, A. R. Phytopathology 1985, 75, 1191-1195. 9. Woodward, S.; Pearce, R. B. Physiol. Mol. Plant Pathol. 1988, 33, 151-162. 10. Woodward, S.; Pearce, R. B. Physiol. Mol. Plant Pathol. 1988, 33, 127-149. 11. Prior, C. Ann. Bot. 1976, 40, 261-279. 12. Sherwood, R. T.; Vance, C. P. Phytopathology 1976, 66, 503-510. 13. Ride, J. P.; Pearce, R. B. Physiol. Plant Pathol. 1979, 15, 79-92. 14. Biggs, J.R. Can. J. Bot. 1984, 62, 2814-2821. 15. Pearce, R. B.; Rutherford, J. Physiol. Plant Pathol. 1981, 19, 359-369. 16. Aist, J. R. Ann. Rev. Phytopathol. 1976, 14, 145-163. 17. Kunoh, H.; Ishizaki, H. Physiol. Plant Pathol. 1975, 5, 283-287. 18. Heath, M. C. Physiol. Plant Pathol. 1979, 15, 141-148. 19. Ride, J. P. Physiol. Plant Pathol. 1980, 16, 187-196. 20. Aist, J. R. In The Dynamics of Host Defence; Bailey, J. Α.; Deverall, B. J., Eds.; Academic Press: Sydney, 1983; pp.33-70. 21. Mullick, D. B. Can. J. Bot. 1975, 53, 2443-2457. 22. Hammerschmidt, R.; Kuc, J. Physiol. Plant Pathol. 1982, 20, 61-71. 23. Edwards, M. C.; Ayres, P. G. New Phytol. 1981, 89, 411-418. 24. Garrod, B.; Lewis, B. G.; Brittain, M. J.; Davies, W. P. New Phytol. 1982, 90, 99-108. 25. Kolattukudy, P. E. Can. J. Bot. 1984, 62, 2918-2933. 26. Boddy, L.; Rayner, A. D. M. New Phytol. 1983, 94, 623-641. 27. Cooke, R. C.; Rayner, A. D. M. Ecology of Saprotrophic Fungi; Long man: London, 1984; pp. 196-237.



360

PLANT C E L L W A L L

POLYMERS

28. Rayner, A. D. M. In Water, Fungi and Plants; Ayres, P.G.; Boddy, L., Eds.; Cambridge University Press: Cambridge, 1986; pp. 321-341. 29. Zimmermann, W.; Seemüller, E. Phytopath. Z. 1984, 110, 192-199. 30. Ofong, A. U. Ph.D. Thesis, Oxford University, Oxford, UK, 1988. 31. Royle, D. J. In Biochemical Aspects of Plant Parasite Relationships; Friend, J.; Threlfall, D.R., Eds.; Academic Press: London, 1976; pp. 161-193. 32. Thomas, H. E. J. Agric. Res. 1934, 48, 187-218. 33. Tourvieille de Labrouhe, D. Agronomie 1982, 2, 553-560. 34. Campbell, C. L.; Huang, J. S.; Payne, G. A. In Plant Disease: An Advanced Treatise; Vol. V; Horsfall, J. G.; Cowling, Ε. B., Eds.; Academic Press: New York, 1980; pp. 103-120. 35. Hepper, F. N. Arboricultural J. 1981, 5, 39-43. 36. Moss, E. H.; Gorham, A. L. Phytomorphology 1953, 3, 285-294. 37. Moss, E. H. Amer. J. Bot. 1936, 23, 114-120. 38. Biggs, A. R.; Davis, D. D.; Merrill, W. Can. J. Bot. 1983, 61, 563-574. 39. Biggs, A. R.; Merrill, W.; Davis, D. D. Can. J. For. Res. 1984, 14, 351-356. 40. Kemp, M. S.; Burden, R. S. Phytochemistry 1986, 25, 1261-1269. 41. Shigo, A. L.; Marx, H. G. USDA For. Serv. Agric. Inf. Bull. No. 405, 1977. 42. Pearce, R. B.; Woodward, S. Physiol. Mol. Plant Pathol. 1986, 29, 197-216. 43. Tippett, J. T.; Shigo, A. L. Eur. J. For. Pathol. 1981, 11, 51-59. 44. Pearce, R. B.; Holloway, P. J. Physiol. Plant Pathol. 1984, 24, 71-81. 45. Shain, L. Phytopathology 1967, 57, 1034-1045. 46. Shain, L. Phytopathology 1971, 61, 301-307. 47. Shain, L. Phytopathology 1979, 69, 1143-1147. 48. Biggs, A.R. Phytopathology 1987, 77, 718-725. 49. Parameswaran, N.; Knigge, H.; Liese, W. IAWA Bull. N.S. 1985, 6, 269-271. 50. Greaves, H.; Levy, J. F. J. Inst. Wood Sci. 1965, 15, 55-63. 51. Ride, J. P. In Biochemical Plant Pathology; Callow, J. Α., Ed.; John Wiley & Sons: Chichester, 1983; pp. 215-236. RECEIVED March 10, 1989


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