Pathogen Effects on Plant

Pathogen Effects on Plant

Physiological Functions

Effect of Pathogens on Photosynthesis

P H O T O S Y N T H E S IS is a basic function of green plants that enables them to transform light energ y into energ y of chemical bonds which they can utilize in their cell activities. Photosynthesis is the ultimate source of all energ y u s ed in plant or animal cells, since, in a living cell, all activities except photosynthesis e x p e n d the energ y provided by photosynthesis.

In the basic reaction of photosynthesis, carbon dioxide from the atmosphere and water from the soil are brought together in the chloro- plasts of the green parts of plants and, in the p r e s e n ce of light, react to form glucose with concurrent release of oxygen:

light

6 CO, + 6 H^zO — — C⁶H^{1 2}0⁶ + 6 O,

2 2 chlorophyll ^{6 12 6 2}

83

All s u b s e q u e nt organic compounds p r o d u c ed by the plant either in the chloroplast or in nonphotosynthetic cells derive from the glucose molecules formed during photosynthesis. U p on degradation of glu- cose the energ y contained in the glucose molecule is released and transferred to high energ y phosphate bonds in adenosine triphosphate (ATP) from which it can be u s ed for various types of chemical reac- tions.

In view of the fundamental position of photosynthesis in the life of plants, it is apparent that any interference of pathogens with photo- synthesis results in a d i s e a s ed condition in the plant. That pathogens do interfere with photosynthesis is obvious from the chlorosis they cause on many infected plants, from the necrotic lesions or large ne- crotic areas they produce on green plant parts, from the r e d u c ed amounts of photosynthates (e.g., in growth, fruit) produced by many infected plants, and, in more subtle cases, from measurements of the photosynthetic rate of infected plants.

In leaf spot, blight, and other kinds of diseases in which there is destruction of leaf tissue, photosynthesis is obviously r e d u c ed be - cause of the reduction, through death, of the photosynthetic surface of the plant. E v en in other diseases, however, plant pathogens reduce photosynthesis, especially in the late stages of diseases, by affecting the chloroplasts and causing their degeneration. T h e overall chloro- phyll content of leaves in many fungal and bacterial diseases is re- duced, but the photosynthetic activity of the remaining chlorophyll seems to remain unaffected. H ow these pathogens attack the chloro- plast and destroy the chlorophyll is not known. S o me pathogens pro- duce toxins, but there is no proof that any of the toxins act on the chlo- rophyll molecules. In plants infected by vascular pathogens, chlorophyll is reduced and photosynthesis stops even before the eventual wilting of the plant.

In certain phases of some plant diseases the infected organs show an increased ability for photosynthetic uptake of C 02 in the dark, and at the same time noninfected leaves of the same plants exhibit an in- creased stimulation of C 02 fixation in the light. T h e se reactions ap- pear to b e either part of the defense reactions of the host plant, or they may, somehow, b e stimulated by the pathogen. As a result of the infec- tion also, many more nutrients move out of the healthy leaves and into the infected ones than in the opposite direction.

Most virus diseases induce varying d e g r e es of chlorosis, although s o me of them, e.g., phony peach, make the plants look greene r than the noninfected ones. In the majority of virus diseases photosynthesis

of infected plants is r e d u c ed greatly, in advanced stages of the d i s e a se the rate of photosynthesis b e i ng no more than one-fourth the normal rate. Although it appears that r e d u c ed amounts of chlorophyll and/or breakdown of the chloroplasts of leaves of infected plants are the main reasons for the r e d u c ed photosynthesis, it has also b e e n shown that, even in virus-infected plants that do not show chlorosis, the ability of the existing chlorophyll to carry on photosynthesis is impaired. Thus, photosynthesis by chloroplasts isolated from tobacco etch virus-in- fected plants is only half that carried on by chloroplasts from healthy plants. T h e r e is s o me evidence that the virus affects the dark reactions of photosynthesis and not the light ones. This indicates that at least some viruses may reduce the photosynthetic ability of their host plants without directly affecting the chlorophyll. In the d i s e a s es in which destruction of chlorophyll does take place, this s e e ms to b e brought about through increased activity of the enzyme chlorophyl- lase. T h e m e c h a n i sm responsible for activation of this e n z y me in the d i s e a s ed plant is not known. Chloroplasts usually break down follow- ing destruction of chlorophyll.

Viruses generally affect plants during their growth or affect only their growing parts. In chlorotic-appearing virus-infected plants, chlo- rosis is usually apparent while the plant is still growing b e c a u se the virus interferes, to a smaller or greater extent, with chlorophyll or chloroplast formation in the infected cell. In some diseases, e.g., maize dwarf mosaic, photosynthesis of inoculated leaves decreases by 2 5 % within 2-6 hours after inoculation and remains at that level. By the second day from inoculation the photosynthesis of leaves above the inoculated one decreases slightly, and by the third or fourth day it reaches the level of the inoculated leaf. In several viral diseases, how- ever, chlorosis appears late in the season regardless of the stage of growth of the plant at the time of infection. This may not necessarily indicate destruction of chlorophyll, but since chlorophyll is found in the leaf in a dynamic state, b e i ng formed a nd d e c o m p o s ed simultane- ously, the virus may in this case also interfere with chlorophyll syn- thesis which, upon natural destruction of the preexisting chlorophyll, will also lead to chlorosis.

Viruses not only decrease photosynthesis in infected plants, but they also change the relative amounts of c o m p o u n ds formed, affect the transport of the c o m p o u n ds out of the cells and of the leaf, and affect the ability of the cells to metabolize s o me of the compounds formed.

T h u s, curly top virus reduces starch by 2 5 - 3 0 % in infected tomato leaves but induces formation of several times more glucose and su-

crose in infected than in healthy leaves. Also, 100 times more trans- port materials m o v ed out of healthy tomato leaves to the rest of the plant than m o v ed out of infected leaves.

Since starch formation is d e p e n d e nt on photosynthesis, it is obvious that reduction of chlorophyll and destruction of chloroplasts by vi- ruses will also affect starch production. Although in many virus diseas- es, particularly mosaics, infected plants contain less starch in their leaves than do healthy plants, in others such as potato leaf roll and sugar b e e t yellows, virus-infected leaves contain more starch than healthy ones. In some diseases, starch accumulation in the leaves is probably d ue to r e d u c ed starch translocation b e c a u se of the destruc- tion of the p h l o em of the plant. This, however, is not always true since it occurs in diseases in which translocation is not impaired. E v en in some mosaic-infected plants, starch accumulation may b e shown on the infected or noninfected areas of the s a me leaves d e p e n d i ng on whether the plants are examined after they have b e e n kept in the dark or in the light, respectively, for several hours. It appears, therefore, that in s o me virus diseases, plants lose the ability to produce sufficient starch, in others they lose the ability to metabolize (degrade) it prop- erly and, in still others, the plants have a r e d u c ed capacity for synthe- sizing and for metabolizing starch. T h e mechanism of reduction in starch synthesis is apparently related to the r e d u c ed photosynthesis, but the mechanism of reduction in starch metabolism, although proba- bly d ue to interference with the activity of the enzymes involved, has not yet b e e n fully elucidated.

Selected References

Allen, P. J. 1942. C h a n g es in the m e t a b o l i sm of w h e at l e a v es i n d u c ed by infection with p o w d e ry m i l d e w. Am. J. Botany 2 9 : 4 2 5 - 4 3 5 .

B e c k m a n, C. H., W. A. Brun, a nd I. W. B u d d e n h a g e n. 1962. Water relations in b a n a na plants infected with Pseudomonas solanacearum. Phytopathology 5 2 : 1 1 4 4 - 1 1 4 8 . Gates, D. W., a nd R. T. G u d a u s k a s. 1967. Preliminary studies on the effect of m a i ze

dwarf m o s a ic virus on photosynthesis a nd respiration in corn. Phytopathology 5 7 : 4 5 9 (abstr.).

H o l m e s, F. O. 1931 . L o c al lesions of m o s a ic in Nicotiana tabacum. Contrib. Boyce Thompson Inst. 3 : 1 6 3 - 1 7 2 .

H o p k i n s, D. L., a nd R. E. H a m p t o n. 1967. Effect of tobacco etch virus on photosyn- thesis. Phytopathology 5 7 : 8 1 5 (abstr.).

L i v n e, A. 1964. Photosynthesis in healthy a nd rust-affected plants. Plant Physiol.

3 9 : 6 1 4 - 6 2 1 .

Mirocha, C. J., a nd P. D. Rick. 1967. C a r b on d i o x i de fixation in the dark as a nutritional factor in parasitism. In " T he D y n a m ic R o le of M o l e c u l ar Constituents in Plant-Par- asite I n t e r a c t i o n" ( C. J. Mirocha a nd I. Uritani, e d s . ), p p. 121-143. B r u c e, St. Paul, Minnesota.

P a n o p o u l o s, N., a nd A. H. G o l d. 1967. M e t a b o l ic aberrations in tomatoes i n d u c ed b y sugar b e e t curly top infection. Phytopathology 5 7 : 8 2 5 (abstr.).

Peterson, P. D., a nd Ç . H. M c K i n n e y. 1938. T h e influence of four m o s a ic d i s e a s es on the plastid p i g m e n ts a nd c h l o r o p h y l l a se in t o b a c co l e a v e s. Phytopathology 2 8 : 3 2 9 - 3 4 2 .

Roberts, D. Á., a nd Ì . K. Corbett. 1965. R e d u c ed p h o t o s y n t h e s is in tobacco plants in- fected with tobacco ringspot virus. Phytopathology 5 5 : 3 7 0 - 3 7 1 .

S e m p i o, C. 1959. T h e host is starved. In " P l a nt P a t h o l o g y ," (J. G. Horsfall a nd A. E . D i m o n d, eds.), Vol. I, p p. 2 7 8 - 3 1 2 . A c a d e m ic Press. N e w York.

Wynn, W. K., Jr. 1963. Photosynthetic phosphorylation by chloroplasts isolated from rust-infected oats. Phytopathology 5 3 : 1 3 7 6 - 1 3 7 7 .

Effect of Pathogens on Translocation of Water and Nutrients in the Host Plant

All living plant cells require an a b u n d a n ce of water and an a d e q u a te amount of organic and inorganic nutrients in order to live and to carry out their respective physiological functions. Plants absorb water and inorganic (mineral) nutrients from the soil through their root system.

T h e se are generally translocated u p w a rd through the xylem vessels of the stem and into the vascular b u n d l es of the petioles and leaf veins, from which they enter the leaf cells. T h e minerals and part of the wa- ter are utilized by the leaf and other cells for synthesis of the various plant substances, but most of the water evaporates out of the leaf cells into the intercellular spaces and from there diffuses into the atmo- sphere through the stomata. On the other hand, nearly all organic nu- trients of plants are p r o d u c ed in the leaf cells, following photosyn- thesis, and are translocated downward and distributed to all the living plant cells by p a s s i ng for the most part through the p h l o em tissues. It is apparent that interference by the pathogen with the u p w a rd move- men t of inorganic nutrients and water or with the downward move- men t of organic substances will result in d i s e a s ed conditions in the parts of the plant d e n i ed these materials. T h e se d i s e a s ed parts, in turn, will b e unable to carry out their own functions and will deny the rest of the plant their services or their products, thus resulting in dis- ease of the entire plant. For example, if water m o v e m e nt to the leaves

is inhibited, the leaves cannot function properly, photosynthesis is reduced or stopped, and few or no nutrients are available to m o ve to the roots, which, in turn, b e c o me starved, diseased, and may die.

Interference with Upward Translocation of Water and Inorganic Nutrients

Many plant pathogens interfere in one or more ways with the trans- location of water and inorganic nutrients through the plants. S o me pathogens affect the integrity or function of the roots and cause de- creased absorption of water by them; other pathogens, by growing in the xylem vessels or by other means, interfere with the translocation of water through the stem; and, in some diseases, pathogens also inter- fere with the water economy of the plant by causing excessive tran- spiration through their effects on leaves and stomata.

E F F E CT ON ABSORPTION OF W A T E R BY R O O TS

Many pathogens, such as the damping-off fungi, the root-rotting fungi and bacteria, most nematodes, and s o me viruses cause an exten- sive destruction of the roots before any symptoms appear on the above-ground parts of the plant. Root injury affects directly the amount of functioning roots and decreases proportionately the amount of water absorbed by the roots. S o me vascular parasites, along with their other effects, s e em to inhibit root hair production, which reduces water absorption. T h e se and other pathogens also alter the permeabil- ity of root cells, an effect that further interferes with the normal ab- sorption of water by roots.

E F F E CT ON TRANSLOCATION OF W A T E R THROUGH T H E X Y L EM Fungal and bacterial pathogens that cause damping-off, stem rots, and cankers may reach the xylem vessels in the area of the infection and, if the affected plants are young, may cause their destruction and collapse. Affected vessels may also b e filled with the bodies of the pathogen and with substances secreted by the pathogen or by the host in response to the pathogen, and may b e c o me clogged. Whethe r de- stroyed or clogged the affected vessels c e a se to function properly and allow little or no water to pass through them. Certain pathogens, such as the crown gall bacterium (Agrobacterium tumefaciens), the club- root fungus (Plasmodiophora brassicae), and the root-knot nematode (Meloidogyne sp.) induce gall formation in the stem and/or the roots.

T h e enlarged and proliferating cells near or around the xylem exert pressure on the xylem vessels, which may b e crushed and dislocated and, thereby, b e c o me less efficient in transporting water.

T h e most typical and complete dysfunction of xylem in translocat- ing water, however, is observed in the vascular wilts c a u s ed by fungi like Fusarium and Verticillium, and bacteria like Pseudomonas and Erwinia. T h e se pathogens invade the xylem of roots and stems and produce diseases primarily by interfering with the upward movement of water through the xylem. In many plants infected by these patho- gens the water flow through the stem xylem is r e d u c ed to a m e r e 2-4 % of that flowing through the stems of healthy plants. In general, the rate of flow through infected stems s e e ms to b e inversely proportional to the number of vessels blocked by the pathogen and by the substances resulting from the infection. Evidently, more than one factor is usually responsible for vascular disfunction in the wilt diseases. Although the pathogen is the single cause of the d i s e a s e, s o me of the factors respon- sible for the d i s e a se syndrome originate directly from the pathogen, while others originate from the host in response to the pathogen. T h e pathogen can reduce the flow of water through its physical p r e s e n ce in the xylem as mycelium, spores, or bacterial cells and by production of large molecules (polysaccharides) in the vessels. T h e infected host may reduce the flow of water through changes in the size of vessels after infection, d e v e l o p m e nt of tyloses in the vessels, release of large- molecule c o m p o u n ds in the vessels in reaction to pathogenic stimu- lants, and r e d u c ed water tension in the vessels d ue to pathogen-in- d u c ed alterations in foliar transpiration.

Mycelium, Spores, and Bacterial Cells in Vessels

Plugging of xylem vessels by wefts of mycelium and spores, in the case of the fungal wilts, and by slimy bacterial colonies, in the case of bacterial wilts, has often b e e n considered to b e a cause of vascular dysfunction. It is true that in infected plants at least some vessels be - come densely p a c k ed with mycelium or appear to b e filled with bac- teria. Neither mycelium nor bacteria, however, are usually sufficiently abundant through all vessels to justify their consideration as the only cause of the wilt, although they apparently help reduce the water flow in the affected vessels. T h e fungal microconidia produced in the ves- sels—and, in the bacterial wilts, the bacteria —are transported through conductive elements and b e c o me l o d g ed against en d walls or at nar- row constrictions, where they probably are more effective in reducing the flow of water than in the large lumen of the vessels. E v en so, the rather small portion of vessels affected makes their role in wilt appear contributory rather than causal.

Pathogenic Polysaccharides in Vessels

Most phytopathogenic bacteria and many fungi produce extracellu- lar polysaccharides. In the vascular diseases such polysaccharides are released into the transpiration stream. In some bacterial diseases the amount of polysaccharide p r o d u c ed by the pathogen s e e ms to b e pro- portional to the severity of the symptoms. Polysaccharides pass read- ily through open conductive elements but only slowly, if at all, through openings in cell walls. T h u s, polysaccharides may block the p a s s a ge of water from one vessel to another and from the vessels later- ally to other vessels and other types of cells. Although polysaccharides and similar macromolecular substances also increase the viscosity, and thereby decrease the rate of flow of the tracheal fluid, it appears that their primary effect in inducing wilt stems from the fact that they are entrapped, b e c a u se of their molecular size, in the channels through which water normally p a s s es from one cell to another.

Collapse of Vessels

Abnormal development of xylem vessels, even in areas of the plant not yet invaded by the pathogen, often follows infection by vascular pathogens. T h e walls of ne w vessels are thinner than normal and the vessels, instead of being circular in diameter, are flattened and appear collapsed. T h e functioning cross section of such vessels is greatly reduced in comparison to that of normal vessels, and the amount of water passing through is certainly reduced. Hypertrophy of paren- chyma cells surrounding vessels also occurs commonly in plants in- fected with vascular pathogens, and the increased pressure by these cells on the vessels is sufficient to crush the vessels.

Gels and Gums Released in Vessels by the Host Plant

Different kinds of plants respond to vascular infection by forming gels and gums in the vessels at or near the region of infection. G e ls appear to form first and appear to contain pectinaceous and other sub- stances. In some plants, gels accumulate in the region of infection within two to five days after inoculation, but in others they form in the later stages of the d i s e a se or not at all. G e ls usually form below the cross walls of the vessels, but in s o me cases (e.g., banana wilt) they form above the perforation plates. T h e formation of gels has b e e n at- tributed to the action of pathogenic pectinolytic and cellulolytic en- zymes on the middle lamella and primary cell walls of cells surround- ing the vessels. This results in the release of macromolecular fragments that are carried in the transpiration stream until they lodge

on the vessel e n d walls and accumulate to form gel plugs. A swelling effect exerted by certain toxic substances, e.g., fusaric acid, on the ves- sel cross walls may also b e responsible for gel formation. G u ms appear later, in the place where there w e re previously gels, probably from a reaction of the gels with melanoid pigments associated with vascular discoloration. T h e role of gels and g u ms in inducing wilting is not quite clear. In s o me cases they appear to b e present in large enough quantities to affect the flow of water through the vessels, but in others they appear so late in the d i s e a se as to suggest that they may b e a man- ifestation of existing dysfunction of the vessels rather than the cause of it.

Polymerized Melanoid Pigments in Vessels

Vascular diseases are accompanied by browning of the vessels d ue to melanoid pigments originating in vascular parenchyma cells adja- cen t to invaded vessels. T h e s e q u e n ce of events leading to browning of vessels is not clear, but it appears that several factors are involved.

T h e pathogen-secreted pectinolytic enzymes cause maceration and disorganization of host cells and thus initiate oxidation of phenolic compounds. F r e e or b o u nd phenolic c o m p o u n ds of affected paren- chyma cells are mobilized, while the activity of phenol-oxidizing en- zymes (polyphenoloxidases) increases markedly. Phenols b o u nd to sugar as phenolic glycosides may b e liberated by the action of the enzyme /3-glucosidase. This hydrolyzes the breakdown of the com- p o u nd into a sugar and the phenolic moiety. T h e latter is then avail- able for oxidation, probably by polyphenoloxidase. T h e oxidized products of phenols polymerize and form large p i g m e n t ed molecules which may also b e b o u nd to proteins. Whe n the permeability of the affected xylem parenchyma cells is altered or the cells are otherwise d a m a g e d, the large p i g m e n t ed molecules leak into the vessels where they are adsorbed to the vessel walls and impart to them the brown coloration. T h e melanoid pigments may also accumulate in pits be- tween vessels and obstruct lateral m o v e m e nt of water, or they can b e c o me trapped in gels when the vessel is already p l u g g ed and cause the darkening of the gels and their transformation into gums.

Tyloses

In many fungal, bacterial, and viral vascular diseases, tyloses de- velop. Tyloses are overgrowths of xylem parenchyma cells protruding into the lumen of adjacent v e s s e ls (Fig. 12). By their p r e s e n ce in the vessels, tyloses reduce the diameter of the vessels and, consequently, the flow of water. Plant species differ in their tendency or ability to

form tyloses. In some, many large tyloses may d e v e l op in a single ves- sel and may effectively block the m o v e m e nt of water through it. In some vascular diseases, extensive plugging of vessels by tyloses pre- cede s the appearance of wilt symptoms and is probably, at least in part, responsible for the wilting. Tyloses sometimes are abundant in invaded vascular bundles but are rare or absent in noninvaded ones.

Other kinds of plants wilt upon infection by vascular pathogens al- though tyloses are rare or absent. E v en w h en tyloses are produced, their importance in reducing the efficiency of the transport system as a whole d e p e n ds on the proportion of vessels containing tyloses, the extent of blocking of individual vessels, the position of tyloses along the vessel, and the timing of formation of tyloses in relation to the advance of the infection. It is apparent, therefore, that tyloses contrib- ute to wilting to a greater or lesser extent in s o me but not all vascular wilts.

E F F E CT ON TRANSPIRATION

In plant diseases in which the pathogen infects the leaves, transpi- ration is usually increased. This is the result of destruction of at least part of the protection afforded the leaf by the cuticle, increase in permeability of leaf cells, and dysfunction of stomata. D i s e a s es like the rusts, mildews, and apple scab destroy a considerable portion of the cuticle and epidermis and this results in uncontrolled loss of water from the affected areas. If water absorption and translocation cannot k e e p up with the excessive loss of water, loss of turgor and wilting of leaves follows. T h e suction force of excessively transpiring leaves is abnormally increased and may lead to collapse and/or dysfunction of underlying vessels through production of tyloses and gums.

In leaf spot or blight diseases, as well as in many virus and other diseases causing leaf deformities or defoliation, the proportion of healthy leaf cells per plant is r e d u c ed and this results in reduction of the suction force necessary to cause a flow of water into the leaves.

This in turn may reduce the rate of flow of water through the xylem.

Similar effects may b e c a u s ed by d e c r e a s ed permeability of leaf cells c a u s ed by deposition of polysaccharides or other macromolecules on the cell m e m b r a n es and by loss of the ability to regulate stomatal opening and to k e e p the stomata open during the normal periods of transpiration. On the other hand, increased permeability of cells has b e e n reported to occur in many vascular diseases, resulting in an ex- cessive loss of water from the plant. Several wilt toxins (e.g., fusaric

acid, lycomarasmin), and also auxin, are known to increase permeabil- ity of leaf cells, but their role in the d e v e l o p m e nt of wilt diseases is not yet clear.

Interference with the Translocation of Organic Nutrients through the Phloem

Organic nutrients p r o d u c ed in leaf cells through photosynthesis m o ve through p l a s m o d e s m a ta into adjoining p h l o em elements. F r om there, owing to differences in osmotic pressure, they m o ve down the p h l o em sieve tubes and eventually, again through plasmodesmata, into the protoplasm of living nonphotosynthetic cells, where they are utilized, or into storage organs, where they are polymerized. T h u s, in both cases, they are r e m o v ed from "circulation." Plant pathogens may interfere with the m o v e m e nt of organic nutrients from the leaf cells to the p h l o em or with their translocation through the p h l o em elements and, possibly, with their m o v e m e nt from the p h l o em into the cells that will utilize them.

Obligate fungal parasites, such as the rust and m i l d ew fungi, cause an accumulation of photosynthetic products, as well as inorganic nu- trients, in the areas invaded by the pathogen. In these diseases, the infected areas are characterized by r e d u c ed photosynthesis and in- creased respiration. However, synthesis of starch and of other com- pounds as well as dry weight are increased in the infected areas, indi- cating translocation of organic nutrients from uninfected areas of the leaves or from healthy leaves toward the infected areas. It is not known how obligate parasites bring about this abnormal accumulation of nutrients at the locus of infection. Part of the accumulation is proba- bly the result of d e c r e a s ed transport of nutrients out of the infected area. It has b e e n shown, however, that cytokinin increases the phloem transport of nutrients from leaves low in cytokinin to those containing high levels of cytokinin. It is possible, therefore, that the observed translocation of nutrients in the rusts and mildews is also d ue to in- creased cytokinin levels at the locus of infection.

On the other hand, no accumulation of nutrients appears to occur in plant areas infected with nonobligate parasites. This apparent differ- enc e b e t w e en obligate and nonobligate parasites has not yet b e e n investigated and still lacks a satisfactory explanation.

In some virus diseases, particularly the leaf curling type and some yellows diseases, starch accumulation in the leaves is a common phe- nomenon. In most of these diseases, starch accumulation in the leaves

is mainly the result of degeneration (necrosis) of the phloem of in- fected plants. Phloem necrosis is one of the first symptoms of these diseases and, apparently, inhibits the translocation of starch, in the form of its hydrolytic products, out of the leaf. It is also possible, how- ever, at least in s o me virus diseases, that the interference with translo- cation of starch stems from inhibition by the virus of the enzymes that hydrolyze starch into translocatable molecules. This possibility is suggested by the observation that in some mosaic diseases, in which there is no phloem necrosis, infected, discolored areas of leaves con- tain less starch than "healthy," greene r areas at the e n d of a period under light conditions favorable for photosynthesis; but the same leaf areas contain more starch than the " h e a l t h y" areas after a period in the dark, under conditions favorable for starch hydrolysis and transloca- tion. This suggests that virus-infected areas not only synthesize less starch then healthy ones, but also that starch is not easily d e g r a d ed and translocated from virus-infected areas, although no d a m a ge to the phloem is present.

Selected References

B e c k m a n, C. H. 1964. H o st r e s p o n s es to vascular infection. Ann. Rev. Phytopathol.

2 : 2 3 1 - 2 5 2 .

B u d d e n h a g e n, I., a nd A. Kelman. 1964. Biological a nd physiological a s p e c ts of bacterial wilt c a u s ed by Pseudomonas solanacearum. Ann. Rev. Phytopathol. 2 : 2 0 3 - 2 3 0 . C h a m b e r s, H., a nd Ì . E. C o r d e n. 1963. S e m e i o g r a p hy of Fusarium wilt of tomato. Phy-

topathology 5 3 : 1 0 0 6 - 1 0 1 0 .

D i m o n d, Á. E. 1967. Physiology of wilt d i s e a s e. In " T he D y n a m ic Role of Molecular Constituents in Plant-Parasite Interaction" (C. J. Mirocha a nd I. Uritani, eds.), p p.

100-120. B r u c e, St. Paul, Minnesota.

D u r b i n, R. D. 1967. O b l i g a te parasites: Effect on the m o v e m e nt of solutes a nd water. In

" T he D y n a m ic Role of M o l e c u l ar Constituents in Plant-Parasite I n t e r a c t i o n" (C. J.

Mirocha a nd I. Uritani, eds.), p p. 8 0 - 9 9 . B r u c e, St. Paul, Minnesota.

L i v n e, Á., and J. M. Daly. 1966. Translocation in healthy a nd rust-infected b e a n s. Phy- topathology 5 6 : 1 7 0 - 1 7 5 .

Pozsar, Â. I., a nd Z. Kiraly. 1966. Phloem-transport in rust infected plants a nd the cyto- kinin directed long-distance m o v e m e nt of nutrients. Phytopathol. Z. 5 6 : 2 9 7 - 3 0 9 . S e m p i o, C. 1959. T h e host is starved. In " P l a nt P a t h o l o g y" (J. G. Horsfall a nd A. E.

D i m o n d, eds.), Vol. I, p p. 2 7 7 - 3 1 2 . A c a d e m ic Press, N e w York.

Shaw, M., and D. J. Samborski. 1956. T h e physiology of the host-parasite relations. 1.

T h e accumulation of radioactive s u b s t a n c es at infections of facultative a nd obligate parasites i n c l u d i ng tobacco m o s a ic virus. Can. J. Botany 3 4 : 3 8 9 - 4 0 5 .

S u b r a m a n i a n, D., a nd L. Saraswathi-Devi. 1959. Water is deficient. In " P l a nt Patholo- g y" (J. G. Horsfall a nd A. E. D i m o n d, eds.), Vol. I, p p. 3 1 3 - 3 4 8 . A c a d e m ic Press, N ew York.

Effect of Pathogens on Host Plant Respiration

Respiration is the process by which cells, through enzymatically controlled oxidation (burning) of the energy-rich carbohydrates and fatty acids, liberate energ y in a form that can be utilized for the per- formance of various cellular processes. Plant cells carry out respira- tion in, basically, two steps. T h e first step involves the degradation of hexose sugars (glucose) to pyruvate and is carried out, either in the p r e s e n ce or in the a b s e n ce of oxygen, by enzymes found in the ground cytoplasm of the cells. T h e production of pyruvate from glucose fol- lows either the E m b d e n - M e y e r h of glycolytic pathway, otherwise known as glycolysis, or, to a lesser extent, the pentose pathway. T h e second step involves the degradation of pyruvate, however produced, to C 02 and water. This is accomplished by a series of reactions known as the Krebs cycle or tricarboxylic acid cycle and its companion, the glyoxalate shunt. T h e respiratory breakdown of pyruvate is carried out in the mitochondria and only in the p r e s e n ce of oxygen. Under normal (aerobic) conditions, that is, in the p r e s e n ce of oxygen, both steps are carried out and one molecule of glucose yields, as final products, six

C6H1 2Oe + 6 Oz 6 C 02 + 6 H20

molecules of C 02 and six molecules of water, with concomitant re- lease of energ y (678,000 calories). S o me of this energ y is lost, but al- most half is converted to 2 0 - 3 0 reusable high-energy b o n ds of adeno- sine triphosphate (ATP). T h e first step of respiration contributes two A T P molecules per mole of hexose, and the second step contributes the rest. Under unaerobic conditions, however—that is, in the ab- sence of oxygen —pyruvate cannot b e oxidized but it instead under- goes fermentation and yields lactic acid or alcohol. Since the main process of energ y generation is cut off, for the cell to secure the neces- sary energ y a much greater rate of glucose utilization by glycolysis is required in the a b s e n ce of oxygen than is in its presence. This is known as the Pasteur effect.

T h e energy-storing bonds of A T P are formed by the attachment of a phosphate ( P 0⁴) group to adenosine diphosphate (ADP), at the ex- p e n se of energ y released from the oxidation of sugars. T h e coupling of respiratory oxidation with the phosphorylation of A DP to A TP is called oxidative phosphorylation. Any cell activity that requires en- ergy utilizes the energ y stored in A T P by simultaneously breaking

down A TP to A DP and inorganic phosphate. T h e p r e s e n ce of A DP and phosphate in the cell, in turn, will stimulate the rate of respira- tion. If, on the other hand, A T P is not utilized sufficiently by the cell for some reason, there is little or no regeneration of A DP and respira- tion is slowed down. T h e amount of A DP (and phosphate) in the cell is determined therefore, by the rate of energ y utilization; this, in turn, determines the rate of respiration in plant tissues.

During respiration the released energ y is not always transformed directly to A T P bonds, but is often trapped by the reduction of coen- zymes, such as nicotinamide adenine dinucleotide (NAD), nicotinam- ide adenine dinucleotide phosphate (NADP), flavin, and the cyto- chromes, which upon oxidation release energ y and lead to formation of ATP. T h e successive transfer of two hydrogen ions ( H⁺) from re- d u c ed N AD through the other coenzymes and the cytochromes to the final receptor, oxygen, with formation of water is called terminal oxi- dation. This releases sufficient energ y for, and leads to the synthesis of, three molecules of A TP per mole of N A D H².

T h e types of oxidation and the intermediate compounds formed during respiration, i.e., during the oxidation of one glucose molecule to six molecules of C 0² and six molecules of H²0 , are summarized in S c h e me 1.

In the glycolytic phase of respiration, glucose b e c o m es phosphoryl- ated with phosphate derived from existing A T P and then is trans- formed to fructose. After an additional phosphorylation and the u se of another A T P molecule, fructose splits into phosphorylated glyceralde- hyde and dihydroxyacetone. T h e latter is continually transformed to glyceraldehyde, which, in the presence of N AD and phosphate, is oxidized to diphosphoglycerate with concomitant reduction of N AD to N A D H2. Diphosphoglycerate is then partially dephosphorylated to phosphoglycerate, the removed phosphate b e i ng received by A DP to form ATP. After an intramolecular transfer of the phosphate, phos- phoglycerate is converted to phosphoenol pyruvate by the removal of a molecule of water. Phosphoenol pyruvate finally reacts with A DP and produces pyruvate and ATP.

In the pentose pathway, phosphorylated glucose molecules are oxi- dized to phosphogluconate while N A DP is r e d u c ed to N A D P H2. Phosphogluconate is further oxidized with simultaneous decarboxyla- tion to form the pentose ribulose 5-phosphate and reduction of an- other N A DP molecule to N A D P H2. Ribulose 5-phosphate is then in- terconverted to xylulose 5-phosphate and ribose 5-phosphate which, through enzymatic condensation produce the three-carbon glyceral- d e h y de and a seven-carbon compound, sedoheptulose. Xylulose also

) t

A D P' G l u c o s e - 6 - P-

t

A T P \ F r u c t o s e - 6 -P

) t

A D P' F r u c t o s e - 1 , 6 -Ñ Dihydroxy- + Glyceralde

acetone-Ñ hyde-3-P 2 P-

2 A D PN 1, 3-Diphosphoglycerate

) t

2 A T P ' 3-Phosphoglycerate

t

2-Phosphogly cerate

t

2 ADP >v Phosphoenol pyruvate

J - *

NADPH2 NADPH2 ^ NADPH2

6-P-Gluconate 6-P-Gluconate 6-P-Gluconate

jVco² JS:O²

j^co

Ribulose-5-P Ribulose-5-P Ribulose-5-P

E r y t h r o s e - 5 -P « — R i b o s e - 5 -P PENTOSE PATHWAY

NADHo NAD

2 ATP>

GLYCOLYSIS

Pyruvate

Acetaldehyde

Oxalacetate / f ^ N A D H2

'Malate / ' / * J Fumarate / ] ^ F l a v i n - H2 /

Lactic acid

NADH2 NAD

Ethyl alcohol

ANAEROBIC FERMENTATION — * -2 ATP

Citrate

* \

ces-Aconitate

V

cina

ι

V

Succinate ATP ADP

NADHo NADH.

Reduced substrate (H2) Oxidized substrate

NAD NADH2

Flavin ( H ^ )^ ^ F ^ a v in

2 Cytochrome c 2 Cytochrome c (H²)

2 Cytochrome 2 Cytochrome

oxidase (H2) oxidase

Ë Keto- K ^ Oxalo- / ' J

\ \ g l u t a r a te )r s u c c i n a t e // /

co²

^ ^ "^ Glyoxalate — '

02 HzO

TERMINAL O X I D A T I O N — * - 22 ATP

-KREBS CYCLE.—»-2ATP GLYOXALATE SHUNT

S c h e me 1. Pathways of respiration in plants.

reacts with sedoheptulose to form two molecules of fructose. Both fructose and glyceraldehyde b e c o me part of the pool in the glycolytic pathway. E a ch molecule of glucose then, when metabolized through the pentose pathway, yields one C 02 molecule and the energ y trapped in two molecules of N A D P H2.

In the second step of respiration, the Krebs cycle, in which most of the energy of respiration is produced, pyruvate is first oxidized and de- carboxylated in the presence of N AD and yields acetate and N A D H2. Acetate reacts immediately with coenzyme A (CoA), forming acetyl CoA. Acetyl CoA, in turn, reacts with oxalacetate to form citric acid, which through a series of oxidations and decarboxylations, as shown in S c h e me 1, forms again oxalacetate. T h e energ y released in the process is trapped by the various coenzymes and is finally transformed to A TP b o nd energy.

Under certain conditions, some of the steps of the Krebs cycle may be b y p a s s ed by the glyoxalate shunt, in which isocitrate splits into succinate and glyoxalate, the latter reacting with acetyl CoA (from pyruvate) to form malate and then oxalacetate.

T h e various coenzymes, which accept the hydrogen ions ( H⁺) from the various intermediate compounds of the respiratory cycles and thus oxidize these compounds, cannot themselves react directly with oxy- gen. Instead, they pass the H⁺ on to other carriers, such as flavin, cyto- chrome c, and cytochrome oxidase, the last of which p a s s es the hydro- gen to molecular oxygen and forms water (see S c h e me 1), energ y being released and immediately trapped as A TP bonds in each hydrogen transfer.

T h e energ y produced through respiration is utilized by the plant for all types of cellular work, such as protoplasmic streaming, accumula- tion and mobilization of compounds, synthesis of proteins, organic phosphate, phenols, etc., activation of enzymes, cell growth and divi- sion, defense reactions, and a host of other processes. T h e complexity of the respiratory cycles, the number of enzymes involved in respira- tion, its occurrence in every single cell, and its far-reaching effects on the functions and existence of the cell, make it easy to understand why respiration of plant tissues is one of the first functions to b e affected during infection by plant pathogens.

Respiration of Diseased Plants G E N E R AL E F F E C TS

Whe n plants are infected by pathogens, the rate of respiration gen- erally increases. Obligate and facultative parasites alike cause an in-

crease in respiration of the affected tissues. Respiration even in- creases when plants are subjected to chemical or mechanical irritants.

T h e increase in respiration appears shortly after inoculation, certainly by the time of appearance of visible symptoms, and contin- ues to rise during the multiplication and sporulation of the pathogen.

After that, respiration declines to normal levels or to levels even lower than those of healthy plants. Respiration increases more rapidly in infections of resistant varieties, but it also declines quickly after it reaches its maximum. In susceptible varieties, respiration increases slowly after inoculation, but it continues to rise and it remains at a high level for much longer periods.

Several changes in the metabolism of the d i s e a s ed plant accompany the increase in respiration following infection. Thus, the activity or concentration of several enzymes of, or related to, the respiratory pathways s e em to b e increased. T h e accumulation and oxidation of phenolic c o m p o u n ds are also greater during increased respiration.

Increased respiration in d i s e a s ed plants is also accompanied by an increased activation of the pentose pathway and, sometimes, by the abolishment of the Pasteur effect.

M E C H A N I S MS OF I N C R E A S ED RESPIRATION IN D I S E A S ED P L A N TS Although considerable information is available on the respiration of the d i s e a s ed plant, neither the mechanism(s) of increase in respiration nor its relationship to the changes that accompany respiration are well understood. T wo main mechanisms of increase in respiration have b e e n proposed:

Uncoupling of Oxidative Phosphorylation

It is known that certain substances, e.g., 2,4-dinitrophenol ( D N P ), act as uncoupling agents in the respiration of healthy plants by pre- venting the phosphorylation of A DP to A T P, while they stimulate res- piration and its oxidative reactions. This results in d e c r e a s ed energ y (ATP) output in spite of the increased respiration and in continued increased respiration to provide, through other ways, the energ y re- quired by the cell for its vital processes. E v i d e n ce that pathogens may increase host respiration by inducing uncoupling of oxidative phos- phorylation is provided by the fact that D NP is much less effective in d i s e a s ed plants, where, presumably, uncoupling has already b e e n induced by the infection itself. An uncoupling-like effect on respira- tion may also b e brought about by pathogens which cause a shift from the glycolytic to the pentose pathway of respiration, the latter not b e i ng known to b e linked to oxidative phosphorylation.

Stimulation of Metabolism in Diseased Plants

T h e increased respiration of d i s e a s ed plants can also b e explained as the result of increased metabolism in the plant. In many plant dis- eases, growth is first stimulated, protoplasmic streaming increases, materials are translocated and accumulate in the d i s e a s ed areas, and even synthesis of ne w proteins and carbohydrates takes place. T h e energy required for these activities derives from A TP produced through respiration. T h e more A T P is utilized, the more A DP and in- organic phosphate are produced. Subsequently, the increased levels of A DP and phosphate further stimulate respiration. It is also possible that the plant, b e c a u se of the infection, utilizes A T P energ y less effi- ciently than a healthy plant. B e c a u se of the waste of part of the energy, an increase in respiration is induced and the resulting greater amount of energ y enables the plant cells to utilize sufficient energ y to carry out their accelerated processes.

C H A N G ES IN T H E RESPIRATORY P A T T E RN IN D I S E A S ED P L A N TS

Inhibition of the Pasteur Effect

T h e Pasteur effect provides that, in the p r e s e n ce of oxygen, fermen- tation is suppressed. An aspect of d i s e a s ed plant respiration, however, is the abolition of the Pasteur effect; that is, d i s e a s ed plants carry on considerably more fermentation than do healthy plants. Since fermen- tation is much less efficient in energ y yield than is aerobic respiration, it appears that d i s e a s ed plants break down greater amounts of carbo- hydrate but produce much less utilizable energ y (ATP). Inhibition of the Pasteur effect, however, w o u ld b e expected to occur under condi- tions of high A DP concentrations, regardless of whether these are the result of uncoupling or of stimulated metabolism in the d i s e a s ed plant.

Effect on the Glycolytic Pathway

T h e breakdown of carbohydrates through glycolysis s e e ms to b e generally increased in d i s e a s ed plants. T h e increase in glycolysis may or may not b e accompanied by an increase in the breakdown through the pentose pathway, although in s o me diseases the increased respira- tion may b e d ue to a greater increase in the pentose pathway than to the increase of the glycolytic pathway. In s o me cases, glycolysis may even b e d e c r e a s ed in favor of an increased pentose pathway.

Effect on the Pentose Pathway

T h e pentose pathway s e e ms to b e an alternate pathway of carbohy- drate metabolism to which plants resort to under conditions of an al- tered environment or physiological state of the plant. T h u s, the pen- tose pathway tends to replace the glycolytic pathway as the plants grow older and differentiate, and to increase upon treatment of the plants with hormones, toxins, wounding, starvation, etc. Infection of plants with pathogens also tends, in general, to activate the pentose pathway over the level at which it operates in the healthy plant. Since, the pentose pathway does not s e em to b e directly linked to A T P pro- duction, the increased respiration through this pathway fails to pro- d u ce as much utilizable energ y as the glycolytic pathway and is, therefore, a less efficient source of energ y for the functions of the dis- e a s ed plant. On the other hand, the p e n t o se pathway is the main source of phenolic c o m p o u n d s; these, as will b e seen in the section on biochemical defense (Chapter 6) play important roles in the defense mechanisms of the plant against infection.

Effect on the Krebs Cycle

F r om the information now available it w o u ld appear that in dis- e a s ed plants the oxidation of pyruvate through the Krebs cycle is in- hibited to a lesser or greater extent. This is supported by the fact that substances inhibiting certain steps of the Krebs cycle in healthy plants have little effect on d i s e a s ed plants, in which, presumably, the infec- tion has already exerted a similar inhibition; and also by the fact that in virus diseases virus multiplication is stimulated by feeding the in- fected plant organic acids which do not s e em to b e required or taken up by the virus for its multiplication.

It has b e e n reported, however, that in s o me d i s e a s es at least s o me of the components (e.g., coenzyme A) of the Krebs cycle are in higher concentration in the d i s e a s ed plant than in the healthy plant, indicat- ing a possible activation of the Krebs cycle in infected plants.

Effect on the Terminal Oxidation

Terminal oxidation in healthy plants involves the oxidation of N A D H², which is p r o d u c ed in the glycolytic pathway and in the Krebs cycle, through other intermediates, such as flavin, cytochrome c, and cytochrome oxidase, to atmospheric oxygen with formation of water.

T h e oxidation of N A D H2 is c o u p l ed with A T P formation and yields the major portion of energ y p r o d u c ed during respiration. Terminal

oxidation of N A D H² in the d i s e a s ed plant s e e ms to follow the same steps as in the healthy plants.

In d i s e a s ed plants, however, in which the pentose pathway is much more active than in healthy plants, considerable amounts of released energy are carried in molecules of N A D P H² rather than N A D H². So far it is not known whether oxidation of N A D P H² is through the cyto- chrome c system followed b y N A D H² or whether it is c o u p l ed with A TP production. T h e possibility that N A D P H² is oxidized through noncytochrome systems exists, and s o me experimental evidence has b e e n provided for at least two such systems. Thus, observed increased rates of oxidation of ascorbic acid with increased respiration suggest that oxidation of N A D P H² may proceed by the following steps:

NADPHo *- Glutathione - Ascorbic acid reductase

Ascorbic acid oxidase

On the other hand, the activation of polyphenoloxidase which has b e e n observed in many plant diseases suggests that N A D P H2 may b e oxidized in the following manner:

NADPHo »~ Glutathione • A s c o r b ic acid

°* ^{P O}r d a^esⁿe^{0 1}- ^ ^ —*

T h e operation of these two systems of terminal oxidation of N A D P H² in d i s e a s ed plants is not yet certain, but neither can it b e excluded.

T h e possibility for operation of the pathway via ascorbic acid oxidase is much stronger, at the present state of our knowledge, than for the pathway via polyphenoloxidase.

E F F E CT ON ACTIVITY OF E N Z Y ME S Y S T E MS OF T H E H O ST T h e behavior of several host enzyme systems upon infection has b e e n studied in several plant diseases but, in spite of the volume of the data available, few conclusions can be reached with a reasonable

d e g r ee of certainty as to the association of these enzymes with the respiration of the d i s e a s ed plant.

An increase in the activity of ascorbic acid oxidase and polyphenol- oxidases has b e e n reported, and their p o s s i b le role in respiration was d i s c u s s ed above. Increase in the activity of these enzymes is often associated with d e c r e a s ed levels of their substrates (ascorbic acid and phenols, respectively), but d i s e a s es in which the levels of the sub- strates also increase are known. This w o u ld s e em to preclude any generalizations on the effect of these enzymes on respiration.

Several enzymes operating in the pentose pathway appear to b e ac- tivated in many plant diseases and this, taken together with the in- creased rate of the pentose pathway in d i s e a s ed plants, indicates that a correlation exists b e t w e en the increased activity of these enzymes and the increased respiration.

E n z y m es of the Krebs cycle have also b e e n reported to occur in higher amounts in d i s e a s ed plants, but s o me of them are already pres- ent in rather high concentrations in healthy plants so that an addi- tional increase could have only a minor, if any, effect on respiration.

Among other enzymes activated in infected tissues are glycolic acid oxidase, glycolic acid b e i ng one of the early products of photosynthe- sis, glutamic-oxalacetic transaminase and glutamic acid dehydrogen- ase, both enzymes b e i ng involved in amino acid biosynthesis, and ribonuclease, responsible for breakdown of ribonucleic acid (RNA).

T h e importance of, or the connection b e t w e e n, the altered activity of these and many other enzymes to the respiration of the d i s e a s ed plant is not presently known.

Selected References

Akazawa, T., a nd I. Uritani. 1962. Pattern of carbohydrate b r e a k d o wn in s w e et potato roots infected with Ceratocystisfimbriata. Plant Physiol. 3 7 : 6 6 2 - 6 7 0 .

B a t e m a n, D. F., a nd J. M. Daly. 1967. T h e respiratory pattern of Rhizoctonia-infected b e an hypocotyls in relation to lesion maturation. Phytopathology 5 7 : 1 2 7 - 1 3 1 . Bonner, J., a nd J. E. Varner. 1965. T h e path of carbon in respiratory m e t a b o l i s m. In

" P l a nt B i o c h e m i s t r y" (J. B o n n er a nd J. Å. Varner, eds.), p p. 2 1 3 - 2 3 0 . A c a d e m ic Press, N e w York.

Bonner, W. D., Jr. 1965. Mitochondria a nd electron transport. In " P l a nt B i o c h e m i s t r y"

(J. B o n n er a nd J. E. Varner, eds.), p p. 8 9 - 1 2 3 . A c a d e m ic Press, N e w York.

Daly, J. M. 1967. S o me m e t a b o l ic c o n s e q u e n c es of infection by obligate parasites. In

" T he D y n a m ic Role of Molecular Constituents in Plant-Parasite Interaction" ( C . J.

Mirocha a nd I. Uritani, eds.), p p. 144-164. B r u c e, St. Paul, Minnesota.

Daly, J. M., A. A. Bell, a nd L. R. Krupka. 1961 . Respiratory c h a n g es during d e v e l o p m e nt of rust d i s e a s e s. Phytopathology 5 1 : 4 6 1 - 4 7 1 .

D i e n e r, T . O. 1961. Physiology of virus-infected plants. Ann. Rev. Phytopathol.

1:107-218.

Farkas, G. L., F. Solymosy, a nd L. L o v r e k o v i c h. 1965. T h e role of altered e n z y me levels in the regulation of m e t a b o l ic pattern in d i s e a s ed plant tissues. Deut. Dem. Rep.t

Deut. Akad. Landwir., Tagungsber. 7 4 , 7 1 - 8 1 .

Hirai, T., a nd T. T a k a h a s h i, 1967. Mitochondrial activities of d e t a c h ed tobacco l e a v es infected with tobacco m o s a ic virus. T h e p o s s i b le source of e n e r g y for virus multi- plication. In " T he D y n a m ic R o le of M o l e c u l ar Constituents in Plant-Parasite Inter- a c t i o n" (C. J. Mirocha a nd I. Uritani, eds.), p p. 2 7 0 - 2 8 2 . B r u c e, St. Paul, Minnesota.

Kuc, J. 1967. Shifts in oxidative m e t a b o l i sm during p a t h o g e n e s i s. In " T he D y n a m ic R o le of M o l e c u l ar Constituents in Plant-Parasite I n t e r a c t i o n" ( C. J. Mirocha a nd I.

Uritani, eds.), p p. 183-202. B r u c e, St. Paul, M i n n e s o t a.

Marre, E . 1961. Phosphorylation in higher plants. Ann. Rev. Plant Physiol. 1 2 : 1 9 5 - 2 1 8 . Maxwell, D. P., a nd D. F. B a t e m a n. 1967. C h a n g es in the activities of s o me oxidases in extracts of Rhizoctonia-infected b e an hypocotyls in relation to lesion maturation.

Phytopathology 5 7 : 1 3 2 - 1 3 6 .

Millerd, Á., a nd K. J. Scott. 1962. Respiration of the d i s e a s ed plant. Ann. Rev. Plant Physiol. 13:559-574 .

Scott, K. J., J. S. C r a i g i e, a nd R. M. S m i l l i e. 1964. Pathways of respiration in plant tu- mors. Plant Physiol. 3 9 : 3 2 3 - 3 2 7 .

Scott, K. J. 1965. Respiratory e n z y m ic activities in the host a nd p a t h o g en of barley l e a v es infected with Erysiphe graminis. Phytopathology 5 5 : 4 3 8 - 4 4 6 .

S h a w, M. 1963. T h e p h y s i o l o gy a nd host-parasite relations of the rusts. Ann. Rev. Phyto- pathol. 1:259-294.

Uritani, I., a nd T. Akazawa. 1959. Alteration of the respiratory pattern in infected plants.

In " P l a nt P a t h o l o g y" (J. G. Horsfall a nd A. E . D i m o n d, eds.), Vol. I, p p. 3 4 9 - 3 9 0 . A c a d e m ic Press, N e w York.