Increase in PAL and Lignin-like Compounds as Race-specific Resistance Responses of Potato Tubers to...

Phytopath. Z., 94, 323—334 (1979)© 1979 Verlag Paul Parey, Berlin und HamburgISSN 0031-9481 / ASTM-Coden: PHYZA3

Department of Plant Biology, University of Hull, England

Increase in PAL and Lignin-like Compoundsas Race-specific Resistance Responses

of Potato Tubers to Phytophthora infestans

By

SUSAN J. HENDERSON and J. FRIEND

Received February 8, 1978

The deposition of insoluble lignin-like material occurs during the infec-tion of potato tuber discs by Phytophthora infestans (FRIEND and KNEE 1969).When the responses of tuber discs of a resistant (Orion — Ri) were comparedwith those of a susceptible (Majestic — r) cultivar, it was found that deposi-tion of the lignin-like material and an associated increase in phenylalanineammonia lyase (PAL) activity occurred more rapidly and initially to agreater extent in the resistant than in the susceptible tuber tissue (FRIEND1973, FRIEND, REYNOLDS and AVEYARD 1973). In later experiments, a correla-tion was also found between caffeic acid O-methyl transferase activity andthe resistant reaction (FRIEND and THORNTON 1974). A hypothetical basis forthe role of lignification in the resistance of potato tuber tissue to P. infestanshas been proposed (FRIEND 1973, 1976). We have now extended the observa-tions on "lignification" and PAL activity by using two races of the pathogen(race 4 and race 1,3,5) on two cultivars cf potato (Orion — Ri and PentlandBeauty — Rj) on which the fungal races produce a resistant and a susceptiblereaction respectively. The evidence further supports the proposition that rapidincrease in PAL and the associated lignification are components of the resistantrather than the susceptible reaction.

In addition we present evidence to show that the race-specific responseis not manifested when tuber discs are treated with fungal culture filtrates,extracts or sonicates or when the tuber tissue is treated with chloramphenicolshortly after mycelial inoculation.

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324 HENDERSON and FRIEND

Materials and Methods

Fungal culture

Two strains of Phytophthora infestans, race 4 and race 1,3,5 were provided by Dr.JEAN MALCOLMSON of the Scottish Plant Breeding Station. They were maintained as stockcultures on pea agar at 15° (FRIEND and KNEE 1969), and regularly re-isolated from KingEdward tuber tissue to maintain their infectivity using the antibiotic agar medium of HOLLO-MON (1965). For infection experiments the fungus was cultured on pea agar, and sporangiaisolated by the method of FRIEND and KNEE (1969). If separated mycelium was required,the surface of a pea agar plate was' covered with a circle of cellophane (British CellophaneLtd.) which was then inoculated with the fungus. After 7 days' growth on the surface ofthe cellophane, the mycelium could be stripped off and discs prepared for infection ex-periments.

For preparation of culture filtrates the fungus was grown on the liquid culture mediumof FEHRMANN (1966); the sterilized medium was inoculated with a mycelial disc of theappropriate race of the fungus. The mycelium was removed by filtration.

Mycelial extracts were made from both mycelium grown on liquid medium and grownon cellophane on pea agar. The mycelium was removed either by filtration from the mediumor by stripping from the cellophane and 1.5 g of mycelium homogenized in 20 ml sterile0.1 M phosphate buffer pH 5.5 for 2 minutes in a top-drive homogenizer (MSE Ltd.). Thehomogenate was then centrifuged for 10 minutes at 3,000 X g and the supernatant used asthe mycelial extract.

To obtain sonicated mycelium, only mycelium grown on cellophane on pea agar wasused. The 3,000 X g pellet, after removal of the buffer extract, was washed by resuspendingin buffer and recovered by centrifugation; this washing process was repeated twice. Thewashed residue was resuspended in 20 ml phosphate buffer pH 5.5 and the suspension pouredon to a 2 mm layer of Ballotini No. 8 glass beads and disintegrated in an MSE 60 W ultra-sonic disintegrator for 10 minutes; the sample container was kept in an ice bath during theperiod of treatment. The suspension was poured from the glass beads, and centrifuged for10 minutes at 3,000 X g. It was washed three times with sterile 0.1 M phosphate buffer byresuspension and centrifugation and finally suspended in 20 ml buffer.

Preparation of tuber tissue and inoculation

Potatoes of the varieties Majestic (r). King Edward (r), Orion (Rj) and PentlandBeauty (R,,) were either grown at the University Botanic Garden or bought from suppliersin Scotland and stored at 4 °C in the dark until required. Tuber discs' were prepared accord-ing to the method of FRIEND and KNEE (1969).

Inoculations with sporangia or mycelial discs were made by the method of FRIEND,REYNOLDS and AVEYARD (1973). Treatment with culture filtrates, buffer extracts andsonicated mycelia were made using essentially the same method.

In order to reduce the possibility of bacterial infection in those experiments' whereculture filtrates and mycelial extracts were used, 2 drops of neomycin sulphate solution(50 ppm) were added to the tuber discs. Neomycin sulphate is antibacterial but has no effecton P. infestans (ECHERT and TsAO 1962) and in preliminary experiments was shown to havelittle effect on PAL production induced in potato tuber discs by light.

Estimation of PAL activity, lignin and chlorogenic acid

These estimations were carried out as described by FRIEND, REYNOLDS and AVEYARD(1973).

Increase in PAL and Lignin-like Compounds

Results

325

In the first series of three experiments, Orion (Ri) and Pentland Beauty(R3) were used as examples of potato varieties containing major genes forresistance to race 4 of P. infestans. Each of them shows a susceptible reactionwith race 1,3,5 of P. infestans. The potato variety King Edward does not con-tain any major gene for resistance and is consequently susceptible to bothrace 4 and race 1,3,5 of P. infestans.

In these experiments, the levels of PAL, of a lignin-like material and ofchlorogenic acid were measured on the first 3 days after inoculation of tuberdiscs with sporangia.

Levels of PAL in tuber discs

Days afterinoculation

1 inoculateduninoculateddifference



Table 1ifter inoculation with sporangial suspensions of P. infestans

PAL activityexpressed as fi% cinnamate g-i fresh weigh

incompatible reaction

Orionrace 4

504010

793841

5852

6

PentlandBeautyrace 4

713041

824141

634716

ch-i

compatible reaction

Orionrace 1,3,5

6355

8

654817

4055

—15

PentlandBeauty

race 1,3,5

392316

443212

2340

—17

KingEdwardrace 4

422814

4033

7

2834

— 6

The first two days after inoculation levels of PAL are all higher in theinoculated than in the uninoculated tuber discs (Table 1). On the first day,apart from the incompatible reaction on Pentland Beauty, the differencesbetween inoculated and uninoculated show little difference when the incom-patible and compatible reactions are compared. However, on day 2 the dif-ferences (inoculated minus uninoculated) are much larger for the two incom-patible reactions and on day 3 there is a positive difference only in the incom-patible reactions since the compatible reactions are showing a negative dif-ference, that is the levels of PAL in the inoculated are lower than those in theuninoculated tuber discs.

The absorbance values at 355 nm, used for determination of the lignin-like polymer, show similar small differences between inoculated and un-inoculated on the first day after inoculation (Table 2). On the second dayafter inoculation, the differences although larger are still not very great


between the incompatible and compatible interactions. By the third day thedifferences are markedly higher in the incompatible reactions.

In Table 3 the levels of chlorogenic acid are given for the first three daysafter inoculation. It will be seen that in the case of both compatible and in-compatible reactions with Orion and Pentland Beauty the level is lower inthe inoculated than in the uninoculated discs whereas in the susceptible variety

Table 2Lignification in tuber discs after inoculation with sporangial suspensions of P. infestans





Absorbance at 360 nm per g fresh weight

incompatible

Orionrace 4

0.0900.0800.010

0.1700.0900.080

0.4100.1500.260


0.1300.1200.010

0.2800.2000.080

0.3500.2300.120

compatible

Orionrace 1,3,5

0.0400.0300.010

0.1400.0600.080

0.1700.1400.030

PentlandBeauty

race 1,3,5

0.0700.050 •0.020

0.2400.1900.050

0.2900.2300.060

Table 3Chlorogenic acid levels in potato tuber discs

following inoculation with sporangial suspensions of P. infestans





Chlorogenic acic

incompatible

Orionrace 4

81105

— 24

63117

— 54

95137

— 42


82119

— 37

53137

— 84

154185

— 31

1. A<g per g<

Orionrace 1,3,5

104122

— 18

179213

— 34

80249

—169

"resh weight

-ompatible

PentlandBeauty

race 1,3,5

5371

— 18

5364

— 11

2485

— 61

KingEdwardrace 4

715318

1059114

623131

Increase in PAL and Lignin-like Compounds 327

King Edward there is, on all three days, more chlorogenic acid in the inoculat-ed than in the uninoculated discs. The difference in chlorogenic acid levelsbetween inoculated and uninoculated tuber discs depends more upon tubervariety rather than whether the tuber is inoculated with a compatible or anincompatible race of fungus.

Table 4PAL levels expressed as difference between treated and untreated tuber discs of Orion andMajestic, treated with myceiial discs of fungus, culture filtrate or myceiial sonicate. Allsolutions or extracts with which the tuber discs were treated contained 50 ppm of neomyfcinsulphate. The discs inoculated with mycelium were also treated with 50 ppm neomycin

sulphate

PAL levelsmeasured as nmoles cinnamate mg~i protein h-'

Oriondays after treatment

1

32.410.8

16.223.7

61.539.2

2

25.016.9

10.815.5

43.938.5

3

10.86.8

—15.5—14.9

56.142.6

Majesticdays after treatment

1

6.86.1

34.522.3

11.519.6

2 3

8.8 — 8.110.8 —10.1

52.0 31.857.4 38.5

73.0 41.975.0 40.5

Myceiial inoculation Race 4Race 1,3,5

Buffer extract of my- Race 4celium Race 1,3,5

Resuspended sonicated Race 4mycelium Race 1,3,5

The specificity of PAL accumulation in R-gene containing resistantvarieties is only shown after inoculation with either sporangial suspensions ofthe fungus or with myceiial discs, and not with extracts of the fungus. Whenthe fungus is extracted with buffer, treatment of tuber discs with buffer extractscauses a large increase in PAL in the susceptible cultivar Majestic (Table 4);on the first day higher levels are achieved with discs treated with extract fromrace 4 than from race 1,3,5 but on days 2 and 3 extracts from race 1,3,5 givehigher levels than those from race 4. In the case of the Ri cultivar Orion,treatments with buffer extract of either race of the fungus give lower levelsof PAL than in Majestic. On days 1 and 2 the extracts from the compatiblerace 1,3,5 give higher PAL levels than those from the incompatible race 4 andon day 3 there is, with each extract, less PAL in the treated than in the controldiscs. Using a sonicate of residual myceiial material after buffer extractionthere is, in both varieties of potato, a greater increase in PAL than withsporangial inoculation and the increased levels are maintained on the thirdday, when the values obtained with sporangial inoculation have dropped.There is little difference between the effects of the sonicates of the two raceson Majestic but with Orion there is some race specificity since the race 4 soni-cates give a higher level of PAL than those of race 1,3,5.


It was also found that the PAL level in susceptible (Majestic) discs couldbe increased considerably by treating them with either fungal culture filtrateor by adding chloramphenicol after mycelial inoculation.

Table 5 gives the results of an experiment from which it can be seen thattreatment of tuber discs with culture filtrates from race 4 of P. infestans alsocauses an increase in PAL which is non race specific. The PAL levels inducedin Majestic are much higher than those induced in Orion (day 1, 4.1 X; day 2,2.4; day 3, 3.4). It should be noted that the results in this table are expressedon a unit fresh weight basis whereas those in Table 4 are expressed on a unitprotein basis. Once again buffer extract of race 4 mycelium increases the levelof PAL in tuber discs; it is particularly effective on Majestic discs 2 and3 days after inoculation, although not as effective on day 1 as in the previousexperiment (Table 4).

Table 5The effects of culture filtrate from P. infestans (race 4) and of buffer extracts of myceliumof P. infestans (race 4) grown on liquid medium on PAL levels in discs of Majestic and Orion

tubers. Neomycin sulphate 50 ppm was added to all treatments

Culture filtrateUninocuiated mediumDifference

Buffer extract-Distilled waterDifference

PALle

days

1

40.330.2

10.1

41.826.3

15.5

vels measured as jUg cinnamate \

Orion

after inoculation

2

63.847.6

16.2

56.543.4

13.1

3

48.136.4

11.7

28.639.1

—10.5

;~i fresh weight

Majestic

h-i

days after inoculation

1

53.312.241.1

27.415.8

11.6

2

58.620.4

38.2

37.518.3

19.2

3

56.316.4

39.9

32.217.1

15.1

Table 6The effect of adding 400 ppm chloramphenicol (CAP) 2 hours after inoculating

discs of Majestic tubers with mycelial discs of P. infestans

PA.L levelsexpressed as nmoles cinnamate mg~' protein h-'

no CAPdays after inoculation

CAP presentdays after inoculation

Inoculated race 4 14.9 23.6 37.8 25.0 53.4 43.2Inoculated race 1,3,5 19.6 26.4 34.5 38.5 58.1 50.0Uninocuiated 12.2 32.4 33.8 7.4 25.7 31.8

Difference race 4 2.7 —8.8 4.0 17.6 27.7 11.4Difference race 1,3,5 7.4 —6.0 0.7 31.1 32.4 18.2


It can also be shown (Table 6) that treatment of discs of the susceptiblevariety Majestic with chloramphenicol 2 h after inoculation with myceliumincreases the PAL level considerably compared with that in discs which werenot treated with chloramphenicol; this effect is found with both race 4 andrace 1,3,5.

Thus, treatment of tuber discs of the susceptible variety Majestic witheither fungal culture filtrate or with chloramphenicol before mycelial inocula-tion causes them to show an increase in PAL characteristic of a resistant ratherthan a susceptible response.

Discussion

In an earlier paper (FRIEND, REYNOLDS and AVEYARD 1973) it was shownthat after inoculation of the resistant variety Orion (Ri) with either sporangiaor mycelial discs of race 4 of P. infestans there was a relatively rapid rise inthe level of PAL and the deposition of a lignin-like material. The rises werelower and less rapid when the susceptible variety Majestic (r) was inoculated.In the present series of experiments it has been shown that following in-oculation by race 4 of P. infestans the resistant variety Pentland Beauty (R3)shows a similar response to that of Orion. When either Orion or PentlandBeauty is inoculated with race 1,3,5 of P. infestans to which they are bothsusceptible, the responses in respect of levels of PAL and the lignin-likematerial are similar to those shown by the susceptible varieties Majestic (r) andKing Edward (r) when they are inoculated with either race 4 or 1,3,5. It seemsreasonable to state, therefore, that the relatively rapid increase in the level ofPAL and the deposition of lignin-like material are specific responses associatedwith major gene resistance of potato tuber discs of either Rt or R3 genotype toinoculation with an incompatible race of P. infestans. Since the rise in PALactivity is manifested about 1 day earlier than the rise in the lignin-likematerial, it is convenient to use the rise in PAL over the first two or threedays following inoculation as an indication of resistance. It is also more con-venient to use the rise in PAL than the rise in caffeic acid O-methyl transferase(COMT) activity (FRIEND and THORNTON 1974) as an indicator, since it hasalso been shown that the peak level of PAL occurs 1 or 2 days earlier thanthe peak of COMT activity (FRIEND and THORNTON 1974).

In earlier experiments (FRIEND et al. 1973) the levels of chlorogenic acidin the susceptible variety Majestic were higher in the inoculated than the un-inocuiated discs. In the present experiments, the susceptible variety KingEdward behaves in a similar manner to Majestic. The chlorogenic acid levelsin both Orion and Pentland Beauty inoculated with race 4 follow the patternshown by Orion in the previous experiment, that is the levels in inoculatedare lower than in unincculated discs. The same chlorogenic acid pattern isfound in these 2 varieties regardless of whether they are inoculated with theincompatible race 4 or the compatible race 1,3,5. It may be construed, therefore.


that the pattern of chlorogenic acid levels depends upon tuber variety ratherthan on whether it is giving a resistant or a susceptible reaction to a particularrace of fungus.

In the later experiments it is seen that the race-specificity of increasedPAL levels as an indication of a resistant reaction of tuber discs is shown onlywhen they are inoculated with either sporangial suspensions or mycelial discsof an incompatible race of fungus.

When tuber discs are treated with buffer extracts of mycelium, the reac-tion pattern is more or less the reverse of that shown by either a mycelial orsporangial inoculation. In the case of Majestic treated with buffer extract ofeither race 4 or race 1,3,5 the responses are similar to the resistant responseof major-gene containing tubers to fungal inoculation. On the other handOrion treated with either race tends to give a susceptible-type response, slightlymore marked with race 4 than with race 1,3,5.

Treatment with resuspended ultrasonicated mycelium of either race giveshigh levels of PAL activity in tuber discs of both varieties, which would cor-respond to a resistant response which is more intense than that found withfungal inoculation of a resistant variety.

Treatment with fungal culture filtrates shows an incompatible type reac-tion in both varieties, with a more marked response in the susceptible varietyMajestic. The experiment with culture filtrates is interesting because it isknown that culture filtrates of P. infestans contain a toxin whidi causes necro-sis of potato leaf tissue (SEIDEL 1961, HENDERSON 1975). However, the pre-cipitated toxin causes less necrosis (SEIDEL 1961), but gives an increase inchlorogenic acid and o-diphenols in tuber slices of cultivars resistant to theraces of P. infestans used, but not in those of susceptible cultivars (FEHRMANN1966). It is likely that there are several factors excreted into the culturemedium.

The present experiments, using the increase in PAL as an indication ofresistance, show that a resistant-type response can be induced in tuber discs ofeither susceptible or resistant potato cultivars by sonicates and culture filtratesof P. infestans. They confirm, and extend, the earlier observations of VARNSand Kuc (1971) and KIRALY, BARNA and £RSEK (1972) that mycelial sonicatesof either compatible or incompatible races of P. infestans will induce aresistant-type response in a range of potatoes of either R or r genotypes. Inthe earlier experiments, resistance was indicated either by rishitin accumula-tion (VARNS and Kuc) or by both rishitin accumulation and tissue necrosis(KIRALY et al. 1972). Our results therefore support the conclusion drawn byVARNS, CURRIER and Kuc (1971) that all tubers, regardless of whether theyare R or r genotypes, contain the genetic information for the hypersensitiveresponse. It is also clear, from all three sets of experiments, that in potatoeswhich are susceptible to P. infestans, resistance is not expressed in the presenceof the fungus.

KIRALY et al. (1972) and ERSEK et al. (1973) also found that treatmentof tuber slices of a susceptible variety Gulbaa (r), with chloramphenicol causedthem to show a hypersensitive reaction (with phytoalexin accumulation and


necrosis) to a compatible race of P. infestans. Using a different biochemicalcriterion of resistance and a different susceptible tuber cultivar, the presentexperiments confirm that diloramphenicol treatment can induce a resistantreaction in tuber discs of a susceptible cultivar.

Two different explanations have been proposed both for the medianismof resistance and for the experiments in which potatoes with a susceptiblegenotype can be induced to give a resistant reaction to homogenates of com-patible races of P. infestans. KIRALY et al. (1972) and ERSEK et al. (1973)postulate that all races of the fungus contain a toxin which causes the hyper-sensitive response but that the toxin is only released when the fungus isdamaged. In a normal incompatible reaction they postulate that a resistancemechanism, presumably acting in the tuber, inhibits or damages the fungus,causing it to release its toxin which then causes hypersensitive death of thetuber cells. They assume that in a susceptible host, the compatible fungus isnot damaged and the toxin is consequently not released from the fungus. Theyhave therefore proposed that hypersensitivity is a consequence, and not thecause of resistance. In support of this hypothesis, they propose that the induc-tion by chloramphenicol of a resistant reaction in a susceptible tuber sliceinoculated by a compatible race of the fungus is the result of the fungus beingkilled by the antibiotic, then releasing its toxin and so killing the host cells.

However, it is difficult to accept the hypothesis of KIRALY et al. (1972).It has been clearly shown by ultra-microscopic examination of both leavesand tubers of the Ri variety Orion that hypersensitive cell death of hosttissue occurs before fungal death (SHIMONY and FRIEND 1975, 1976, FRIENDand SHIMONY 1978), and it seems that at least in this cultivar resistanceprimarily depends upon hypersensitive cell death. In other incompatiblepotato-P. infestans combinations, it has been shown by light microscopy thathost cell death is the trigger for rishitin synthesis; this occurs about 8 or 9 hafter death of 20 % of the tuber cells and the lesion continues developing for afurther 10 h (SATO, KITAZAWA and TOMIYAMA 1971).

A more recent explanation is that of Kuc, CURRIER and SHIH (1976) whoproposed that all races of the fungus contain a compound which will elicit aresistant response in any potato cultivar irrespective of whether it is of R orr genotype. The elicitor or rishitin-inducing activity (RIA) causes diversionof terpenoid metabolism into the fungitoxic phytoalexins rishitin and phy-tuberin. The activity of this RIA can be modified by a labile blocker which israce specific and which can interact with a specific "site" in the tuber to sup-press the accumulation of rishitin; under these circumstances non-fungitoxicterpenoids such as the glycoalkaloids solanine and chaconine accumulate. It isnow possible to extend the hypothesis of Kuc et al. (1976) to suggest that RIAelicits not only the synthesis of rishitin but also a range of biochemicalresponses, one of which is an increase in PAL followed by the deposition oflignin-like material; this is also probably accompanied by increases in otherenzymes such as COMT. If this hypothesis is accepted, then RIA should bere-named, but for the purposes of this discussion it is probably best to term itmerely an "elicitor". We also need to retain the concept of a "blocker" or


"inhibitor" of the resistant reaction. This blodier or inhibitor would be activein the susceptible response of potatoes inoculated with either sporangia ormycelial discs of a compatible race of P. infestans. We can then explain theelicitation of the increased levels of PAL in both Majestic and Orion by theresuspended sonicates as an indication of the action of the elicitor. Further-more the similar response of tuber slices of the same two varieties to fungalculture filtrates indicates that elicitor-activity is excreted into the culturemedium. Whether this is the same elicitor which is found in the resuspendedsonicates, or is a smaller molecule containing the same structural featuresnecessary for elicitor-like activity cannot be determined by our present results.The results of the experiments with the buffer extracts of mycelium can beexplained on the basis that the extracts of the compatible race contain theelicitor but that extracts of the incompatible race contain excess amounts of ablocker or inhibitor.

In the explanation of Kuc et al. (1976) it is assumed that the blocker orinhibitor of the hypersensitive reaction is produced by compatible races of thefungus and manifested after inoculation of susceptible varieties. On the basisof this hypothesis, an explanation for the action of chloramphenicol is that itis interfering with, or inhibiting, the action of the blocker. However, there is apossible alternative, namely that the blocker is produced by the potato after asuitable "signal" is received indicating that a compatible interaction will takeplace. If the synthesis of the blocker is dependent upon protein synthesis,chloramphenicol would stop the synthesis of the blocker; the elicitor presentin the fungus would then induce a hypersensitive reaction in the tuber discs.

Summary

The changes in levels of phenylalanine ammonia lyase (PAL) activity,deposition of lignin-like material and the levels of chlorogenic acid have beenexamined in tuber discs of potato cultivars after inoculation with two races ofPhytophthora infestans. Orion (R,) and Pentland Beauty (R3) are resistant torace 4 and susceptible to race 1,3,5 of the fungus; Majestic (r) and KingEdward (r) are susceptible to both races. It has been shown that a resistant,but not a susceptible, reaction of tuber discs of Ri and R3 genotypes to in-oculation with spores or mycelium of P. infestans was accompanied by arelatively rapid rise in the activity of PAL and the deposition of lignin-like material. However, the specificity of the PAL responses was lost whenfungal extracts were used. Culture filtrates and resuspended ultrasonicallydisintegrated mycelium of either race gave a resistant-type response with eitherresistant or susceptible tuber cultivars and buffer extracts of mycelium gavethe opposite reaction to that of intact mycelium or spores. Addition of chlor-amphenicol to tuber discs of a susceptible (r) variety caused them to give aresistant type reaction to mycelium of a compatible race. Possible explana-tions are discussed.


Zusammenfassung

Zunahme von PAL und ligninartigen Verbindungenals rassespezifische Resistenzreaktion von Kartoffelknollen

gegen Phytophthora infestans

Nach Inokulation von Kartoffelscheiben mit zwei Rassen von Phyto-phthora infestans wurden die Veranderungen der Phenylalanin-Ammonium-Lyase-Aktivitat (PAL), die Einlagerung von ligninartigem Material und dieGehalte von Chlorogensaure untersucht. Orion (Ri) und Pentland Beauty(Rs) sind gegenliber Rasse 4 resistent und gegeniiber Rasse 1,3,5 des Pilzesanfallig; Majestic (r) und King Edward (r) sind gegenuber beiden Rassenanfallig. Es konnte gezeigt werden, dafi eine resistente, nicht jedoch eine an-fallige Reaktion der Kartoffelscheiben von Ri- und R3-Genotypen auf Inoku-lationen mit Sporen oder Mycelien von P. infestans von einem relativ schnellenAnstieg der PAL-Aktivitat und der Einlagerung von ligninartigem Materialbegleitet war. Die Spezifitat der PAL-Reaktion ging jedoch verloren, wennPilzextrakte benutzt wurden. Kulturfiltrate und resuspendierte ultrasdiall-zerstorte Mycelien jeder Rasse ergaben eine resistenztypische Reaktion mitentweder resistenten oder anfalligen Kartoffelscheibenkulturen; Pufferextraktevon Mycelien zeigten eine gegensatzliche Reaktion gegenuber jenen von intak-ten Mycelien oder Sporen. Die Zugabe von Chloramphenicol zu den Kar-toffelscheiben einer anfalligen (r) Sorte rief eine resistenztypische Reaktiongegenuber Mycelium einer kompatiblen Rasse hervor. Moglidie Erklarungenwurden diskutiert.

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Authors' address: Department of Plant Biology, University of Hull, Hull HU6 7RX(U.K.).

Increase in PAL and Lignin-like Compounds as Race-specific Resistance Responses of Potato Tubers to...

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