Effects of host and pathogen genotypes on inducibility of resistance in tomato (Solanum lycopersicum) to Phytophthora infestans

Authors


E-mail: kalpana@mail.wiz.uni-kassel.de

Abstract

Thirteen tomato (Solanum lycopersicum) accessions were tested for inducibility of resistance against two isolates of Phytophthora infestans using BABA (dl-3-amino butyric acid) as the inducing agent. In a more detailed trial, six of the accessions were assessed for inducibility of resistance to six P. infestans isolates on three leaves of different age per plant. Plants were inoculated 1 week after treatment with BABA. Area under the disease progress curve (AUDPC), sporulation capacity (SC) and infection efficiency (IE) were all affected by treatment with BABA. On leaves of all ages AUDPC was most affected by induction (43–100% reduction on the youngest leaves) followed by SC (14–100%) and IE (0–100% reduction). Tomato genotypes varied significantly in inducibility of resistance against P. infestans and the degree of induction generally decreased with increasing leaf age, whilst the absolute susceptibility with respect to AUDPC and SC rarely changed. The level of induction was not always related to the resistance level of the tomato accession and it was significantly influenced by the pathogen isolate used for challenge inoculation. The results show that inducibility of resistance is a selectable trait that is, however, isolate-specific.

Introduction

Exploitation of induced resistance (IR) is a potentially desirable strategy in plant protection since it involves enhancing natural defence mechanisms in plants (Walters et al., 2005). Different biological (Pozo et al., 1998; Zehnder et al., 2001; Yan et al., 2002), physical (i.e. abiotic, e.g. wounding, water drops or various types of stress) (Walters et al., 2005) and chemical inducers (Dann et al., 1998; Cohen, 2002; Faoro et al., 2008) can be used to activate or boost natural disease resistance in non-infected plant tissue.

Despite the numerous times in which induced plant responses to pathogens have been demonstrated (see Vallad & Goodman, 2004; for review), only a few studies have investigated differences in inducibility of resistance among genotypes (e.g. Steiner et al., 1988; Martinelli et al., 1993; Hijwegen & Verhaar, 1995; Dann et al., 1998; Olivieri et al., 2009).

Tomatoes have served as a successful model system for induction of resistance to many pathogens, including Phytophthora infestans (Oka et al., 1999; Cohen, 2002; Silva et al., 2004; Atia et al., 2005; Romerio et al., 2005; Thuerig et al., 2006). With respect to P. infestans, reductions of between 36% and 95% in late blight severity have been reported, depending on tomato genotype, inducing agent used and study (Heller & Gessler, 1986; Cohen et al., 1993; Enkerli et al., 1993; Cohen, 1994; Anfoka & Buchenauer, 1997; Pozo et al., 1998; Jeun et al., 2000; Jeun & Buchenauer, 2001; Yan et al., 2002; Atia et al., 2005; Thuerig et al., 2006; Unger et al., 2006). It is unclear, however, if the different protection levels reported resulted from differences in the inducers and experimental conditions or from the genetic background of the tomato accessions and/or pathogen isolates used. In two studies where more than one tomato genotype was used, different levels of disease reduction through IR were found (Enkerli et al., 1993; Cohen, 1994). Overall, no systematic information is available on the genetic variation in inducibility of resistance in tomato.

In a preliminary trial, inducibility of resistance against two isolates of P. infestans was tested on detached leaves of 32 tomato accessions using BABA (dl-3-amino butyric acid) as the inducer. The data indicated considerable accession-by-treatment interactions. Disease reductions through BABA were not the same on accessions of the same level of susceptibility and both leaf position and isolate interacted with inducibility (Sharma et al., 2009). In that trial, only eight accessions could be handled at a time and inoculations were carried out over several months. This, plus variations in leaf size, made direct comparisons across all accessions difficult, however.

The objective of this research was to quantify the extent of variation for inducibility of resistance in tomatoes in detail. The following questions were addressed in this study: (i) to what extent does inducibility of resistance vary? (ii) is inducibility affected by different isolates of P. infestans? and (iii) how is inducibility affected by leaf age? Results of two fully repeated trials using a standardized setup with leaf discs are presented. The previously observed host and isolate effects were confirmed using 13 accessions and two isolates, whilst effects of leaf position and isolate were determined for six of the accessions challenged with six different pathogen isolates.

Materials and methods

Growing of plants and BABA treatment

For simplicity, the term accession is used in this paper for cultivars and gene-bank and breeding materials. The tomato accessions used were selected from a collection of more than 100 accessions that were obtained from breeders, gene banks and commercial shops. Little information is generally available on the pedigrees or specific resistances of tomatoes as tomato breeding is an exclusively private business. The selection was therefore based on the reactions of the accessions to 10 P. infestans isolates tested in the present laboratory (A.F. Butz & M.R. Finckh, unpublished). Only accessions compatible with the isolates used were selected. Thus, the isolates varied in their aggressiveness to the tomato accessions used (A.F. Butz & M.R. Finckh, unpublished data), but there were no qualitative avirulent reactions.

Plants were grown in a peat substrate containing no added nutrients in the glasshouse under controlled climatic conditions (day/night temperatures of 23/18°C). For each experiment, six plants were used per treatment and accession. Ten-day-old seedlings were transplanted (one plant per pot) into square containers 9 × 9 × 9·5 cm3 and fertilized weekly with 50 mL mineral fertilizer (8:8:6 NPK, 3 mL L−1) each. The plants were randomly distributed on the tables.

BABA was used for resistance induction in all experiments. Twenty days after transplanting, when plants had 5–6 fully developed compound leaves and 7 days before inoculation, plants were sprayed to near runoff with a solution of 1 g L−1 BABA in demineralized water, whilst control plants were sprayed with demineralized water only. The youngest fully expanded leaf at that time was marked by hanging a plastic clip on the leaf petiole.

Preparation of pathogen inoculum and inoculations

Phytophthora infestans was grown and maintained at 17°C in Petri dishes on pea agar (125 g frozen peas L−1 H2O, 1·5% agar) in the dark. All isolates used were re-isolated before use from lesions on tomato leaves to maintain vigour and pathogenicity. For this, a 1-cm2 piece was excised from the edge of a sporulating lesion on a tomato leaf and sandwiched between two potato tuber slices of the cultivar Nicola, which is susceptible to all isolates used. Seven days later, mycelia with sporangia which appeared on the upper and lower surfaces of the sandwiches were re-isolated onto pea agar containing 100 mg ampicillin, 30 mg rifamycin, 10 mg benomyl and 0·4 mL pimaricin L−1. Once free of contaminants, cultures of P. infestans were transferred onto pea agar without antibiotics.

Sporangial suspensions were prepared from 21-day-old cultures by adding 3 mL sterile water into the Petri plate. The thin end of a Pasteur pipette was bent at a right angle over a flame and was then used to dislodge the sporangia from the mycelium. The sporangial concentration was determined with a haemocytometer and adjusted to 5 × 104 sporangia mL−1. The suspensions were incubated for 2 h at 5°C to induce the release of zoospores. Whilst the number of zoospores per sporangium was not verified, typically between two and 25 zoospores are released per sporangium (Ullrich & Schöber, 1972). Depending upon the test, isolates 66, 75, 85, 101 and 108, which were locally collected from tomatoes in 2004, and isolate 19, collected from potatoes, were used.

As resistance might be induced by wounding, before starting the trials reported here, detached leaves and whole plants of two tomato cultivars were inoculated with two isolates in four independent trials with four to six replications each. Overall, infection success was higher on detached leaves and sporulation was observed a day earlier, but the relative difference in diseased leaf area among the two isolates and the two cultivars were very much alike. Additional experiments were conducted to confirm that standardized leaf discs also resulted in qualitatively identical results (data not shown). Only the two lateral leaflets nearest the tip of the compound leaves were used for inoculations.

Leaf discs were prepared using an 18-mm-diameter cork borer to standardize the inoculated leaf area and placed in square Petri plates (10 × 10 cm2) lower side up on moistened sterilized filter paper. Each leaf disc was inoculated with a 20-μL drop of the sporangial solution. Inoculated leaf discs were kept in the dark for 24 h at 17°C and afterwards a 16-h photoperiod was maintained and leaf discs were sprayed with sterile demineralized water every 2 days. From 3 days after inoculation (DAI), the leaf discs were checked every day microscopically for sporulating lesions. The earliest sporulating lesions were visible only 4 DAI in all cases. Percentage diseased leaf area was assessed on days 4 and 5 (after day 5, the controls showed 100% sporulation).

Trial I: screening of 13 accessions

Thirteen tomato accessions (Table 1) were selected based on their variation in inducibility and susceptibility in the preliminary trial (Sharma et al., 2009). Lateral leaflets from leaves grown in the week following BABA treatment (termed first leaves) were included in the test.

Table 1.   Origins and codes of tomato accessions used in two trials
Common nameCodeTrialaSource
  1. aFor description of trials I and II see Materials and methods.

MatinaT3IBingenheim, Germany
HarzfeuerT9IBingenheim, Germany
Balkonzauber GST10I, IIErfurter Samen, Germany
QuadroT11IBingenheim, Germany
Berner RoseT54I, IIBingenheim, Germany
MarmandeT61I, IIBingenheim, Germany
ZuckertraubeT72I, IIBingenheim, Germany
C1131T74I, IIKathmandu, Nepal
LYC2524T110IIPK, Gatersleben, Germany
SupermarmandeT121I, IIThompson & Morgan Ltd, UK
VollendungT125IIPK, Gatersleben, Germany
PieralboT128IINRA, France
Bonny BestT124bIIPK, Gatersleben, Germany

Phytophthora infestans isolates 75 and 108 were used and the experiment was repeated three times on 12, 14 and 15 November 2007. Each experiment was replicated six times.

To determine sporulation capacity (SC) immediately after the final disease assessments, the leaflets were frozen at −20°C in the Petri plates. For counting, the plates were thawed and kept at room temperature for about 20 min. Each leaf disc was placed in a test tube to which 1 mL sterile distilled water was added, vortexed strongly for 30 s and sporangia were counted with a haemocytometer. Data for SC were obtained for four replications from the first experiment only.

Trial II: effects of isolate and leaf age on inducibility

Six tomato accessions (Table 1) were used to determine isolate and leaf-age effects. Isolates 19, 66, 75, 85, 101 and 108 were used and leaf-age effects were tested on the first leaf as described in trial I. In addition, leaflets from leaves directly treated with BABA (termed second leaves) and also from the leaf below the second, i.e. the third leaves, were included in the tests. The entire experiment was performed with leaf discs as described for trial I. Six leaf discs per accession per treatment per leaf age were prepared. In total, there were 36 leaf discs from six plants per accession per treatment per leaf age. Again, the experiment was repeated three times on 21, 22 and 23 November 2007 with six replications per experiment, except for isolate 101, which was included only in the first two experiments. SC was determined as described above for four replications of the first experiment.

Data analysis

Lesion area (in cm2) was calculated from percentage diseased leaf disc. From this, area under the disease progress curve (AUDPC) (Campbell & Madden, 1990) and the sporulation capacity per cm2 lesion (SC) were calculated. The infection efficiency (IE) on the non-induced controls was always 100%. For the induced treatments, IE was calculated as the proportion of inoculations that developed into sporulating lesions.

Data were either log (x + 1) or square-root transformed when necessary to improve the normality and homogeneity of variance. Combined data from the repeated trials were analysed with the experimental date as a factor to determine effects or interactions resulting from experiment. As there were neither effects of experimental date nor significant interactions between date and other factors, the analyses were performed across experiments resulting in 18 replications per treatment.

All experiments were analysed with the glm procedure and proc mixed of the Statistical Analysis System version 9·1 (SAS Institute, Inc.) as a factorial design with the factors treatment, accession and isolate in trial I and, in addition, leaf age in trial II. Mean separations were generally based on the Tukey–Kramer test. Linear contrasts were used to determine significant differences between control and induced treatments.

Results

Trial I: screening of 13 accessions

Whilst disease on induced plants was significantly reduced on all accessions tested, the level of induction achieved varied and was not related to the degree of susceptibility of an accession (Table 2). Also, the degree of induction depended on the isolate used (significant accession × treatment interactions) (= 16·86, < 0·0001).

Table 2.   Aggressiveness parametersa of Phytophthora infestans isolates 75 and 108 on 13 tomato accessions as affected by resistance induction with dl-3-amino butyric acid (BABA). Plants were sprayed to near runoff with a solution of 1 g BABA L−1 in demineralized water 7 days before inoculation; control plants were sprayed with demineralized water
Tomato accessionIsolate 75Isolate 108
AUDPCSC × 1000IEAUDPCSC × 1000IE
ControlbInducedbFcControlbInducedbFcInducedbControlbInducedbFcControlbInducedbFcInducedb
  1. aAUDPC: area under the disease progress curve (data were log-transformed for analysis, back-transformed data are shown); SC: sporulation capacity per cm2 lesion on day 5; IE: infection efficiency for induced treatments, control treatments all sporulated. Data for AUDPC and IE are the means of three experiments with six replications each; data for SC are from four replications of the first experiment only. Leaf discs were prepared from the youngest fully emerged leaves 1 week after treatment with BABA (first leaf).

  2. bNumbers within columns followed by different letters are statistically different at < 0·05 (SAS proc mixed, Tukey–Kramer).

  3. c***, ** and *: differences between control and induced treatments significant at < 0·001, < 0·01and < 0·05, respectively. Non-significant effects indicated by ns (> 0·05) (linear contrasts).

  4. dIE of 0·83 has SC of 0 because data for SC are from four replications of the first experiment only. See Materials and methods section for details.

T30·36ab0·02ab***16·0a–d0·0a***0·56b0·69ef0·18de***23·1b17·0c*0·94c
T90·41a–c0·05b***13·8a–c4·9ab***1·00c0·54a–c0·07bc***16·0a3·1ab***1·00c
T100·38a–c0·21d***17·6b–e10·4ab***1·00c0·51ab0·07c***22·1b5·5ab***1·00c
T110·34a0·00a***11·1a0·0a***0·00a0·44a0·00a***19·4ab0·0a***0·00a
T540·65d0·18d***19·1c–e7·4ab***1·00c0·69ef0·24e***22·1b16·4c*1·00c
T610·62d0·14c***18·1b–e12·4b**1·00c0·65d–f0·31f***19·7ab14·5bcns1·00c
T720·71e0·14c***19·8de5·8ab***1·00c0·66d–f0·00a–c***18·0ab5·5ab***1·00c
T740·44c0·00a***12·6ab0·0a***0·11a0·55b–d0·00a***22·0b0·0a***0·00a
T1100·42bc0·00a***19·2c–e0·0a***0·11a0·62c–e0·17d***19·2ab0·6a***1·00c
T1210·40a–c0·05b***15·5a–d7·0ab***1·00c0·59b–e0·01ab***21·3ab0·0a***0·83dbc
T124b0·37a–c0·00a***16·5a–d0·0a***0·06a0·56b–d0·09c***30·2c10·9ab***1·00c
T1250·68de0·14c***20·4de2·8a***1·00c0·49ab0·10c***21·4ab3·7ab***0·56b
T1280·43c0·00a***22·1e0·0a***0·06a0·75f0·37f***19·3ab9·5ab**1·00c

BABA significantly reduced SC on infected leaves with the degree of reduction depending on accession and isolate. Whilst the reduction patterns for AUDPC and SC were more or less similar for isolate 75 on most accessions tested, for isolate 108 there were some distinct deviations. For example, with isolate 108 accession T128 had the highest AUDPC (0·75) among the 13 accessions and produced about 19 × 103 sporangia cm−2. In contrast, AUDPC on accession T124b was significantly lower (0·56), whilst the sporulation capacity was significantly higher (30 × 103 sporangia cm−2). After induction, SC of the two accessions were no more significantly different (T128: SC = 9·5 × 103, T124b: SC = 11 × 103) (Table 2).

Reductions in infection efficiency (IE) through induction varied among accessions and isolates. Whilst induction reduced IE of isolate 75 on six accessions, for isolate 108 it was only reduced on three accessions (Table 2).

Trial II: effects of isolate and leaf age on inducibility

Both pathogen isolate and leaf age affected the inducibility of resistance through BABA, depending on host genotype. The susceptibility, degree of induction and effects on IE and SC in trials I and II were very similar for isolates 75 and 108. As in trial I, there was a strong interactive effect of isolate and accession on inducibility in trial II (= 52·07, < 0·0001).

Overall, isolates 85 and 66 were the most aggressive, with mean AUDPC on the controls of 0·83 and 0·92, respectively, and isolate 19 the least aggressive (mean AUDPC = 0·33) and isolates interacted significantly with resistance induction by BABA (= 743·49, < 0·0001). Disease reduction on the first leaves through BABA was significant in all combinations tested. Reductions relative to the control treatment were highest for isolate 19, ranging between 80% and 100% on the youngest leaves, and lowest for isolate 85 (43–97%).

Again, level of induction was not related to the degree of susceptibility of an accession to the isolate used (Fig. 1). For example, tomato accession T72 was similarly susceptible to isolates 75, 85, 101 and 108, but the level of induction achieved by BABA was variable (Fig. 1d). T74 appeared to be the most readily inducible of the six tomato accessions independent of isolate used (Fig. 1e). There were also some cases where the level of induction decreased with increased susceptibility of the accession (e.g. T54, T61 and T121, Fig. 1b,c and f, respectively). In contrast, on T10 greater disease reductions through resistance induction by BABA were achieved for the more aggressive isolates 85 and 66 than for the less aggressive isolate 75 (Fig. 1a).

Figure 1.

 Area under the disease progress curve (AUDPC) on leaf discs of the first, second and third leaf of tomato accessions (a) T10 (b) T54, (c) T61, (d) T72, (e) T74 and (f) T121, either induced with dl-3-amino butyric acid (BABA) (open bars) or not induced (black bars) (plants were sprayed to near runoff with a solution of 1 g BABA L−1 in demineralized water 7 days before inoculation; control plants were sprayed with demineralized water). Challenge inoculations were performed separately with six isolates of Phytophthora infestans. Leaf age and induction interacted significantly in all cases. Different lower case letters above the bars indicate significant differences within each accession × isolate combination (Tukey–Kramer test, > 0·05). Bars represent ±SD. Data on the figures are the means of three experiments with six replications each. Data were log-transformed for analysis and back-transformed data are presented.

Whilst AUDPC in the controls was similar across leaf ages (Fig. 1) the protection achieved by induction with BABA with respect to AUDPC was significantly higher on the first leaf (mean reduction: 72%; range: 43–100%) than on the second (mean: 48%; range: 22–100%) or third leaf (mean: 32%; range: 15–100%) across all tomato accession × pathogen isolate combinations (Fig. 1). Some reductions were not significant for the second and third leaves, whilst they had been significant on the first leaves (Fig. 1). Overall, the correlation between susceptibility and inducibility was visibly higher in the second (Pearson correlation coefficient = 0·77, < 0·001) and third (Pearson correlation coefficient = 0·83, < 0·001) leaves than the first (Pearson correlation coefficient = 0·60, < 0·001).

BABA significantly reduced SC of most of the infected leaves (= 1914·72). SC was also significantly affected by isolates (= 280·94) and tomato accessions (= 143·03). SC of the isolates decreased in the order 85 > 66 > 101 > 108 > 75 > 19 on accessions T72, T10, T54, T121, T61 and T74, respectively. The isolate × accession × treatment interaction was highly significant (= 9·09, < 0·0001), but the F-value was an order of magnitude smaller than those of the main effects. Overall, SC was somewhat higher on the older leaves (21·3 × 103 sporangia cm−2 on first leaves versus 25·5 × 103 sporangia cm−2 on third leaves) when not induced and the mean reduction decreased from more than two-thirds on first leaves (8·7 × 103 sporangia cm−2) to roughly one-third on third leaves (15·6 × 103 sporangia cm−2) when induced (Fig. 2). There were also some cases where effects of BABA on SC were insignificant; for example, isolate 101 on accession T54 on all three leaves (Fig. 2b) or only significant on older leaves (T10, isolate 85, Fig. 2a; T61, isolate 75, Fig. 2c).

Figure 2.

 SC (sporulation capacity cm−2 × 1000) of six isolates of Phytophthora infestans on leaf discs of the first, second and third leaf of tomato accessions (a) T10 (b) T54, (c) T61, (d) T72, (e) T74 and (f) T121 induced with dl-3-amino butyric acid (BABA) (open bars) or not induced (black bars) (plants were sprayed to near runoff with a solution of 1 g BABA L−1 in demineralized water 7 days before inoculation; control plants were sprayed with demineralized water). Challenge inoculations were performed separately with each P. infestans isolate. In cases where leaf age and induction interacted significantly, different lower-case letters above the bars indicate significant differences within each accession–isolate combination (Tukey–Kramer test, > 0·05). Where the interactions were not significant, different leaf-age effects are indicated by upper-case letters (Tukey–Kramer test, > 0·05). Effects of BABA treatment were usually significant (linear contrast, P < 0·01). Bars represent ±SD. Data for SC are from four replications of the first experiment only. Data were log-transformed for analysis and back-transformed data are presented.

As for AUDPC and SC, effects of BABA on IE were affected by host and pathogen genotype, but only on first leaves. Thus, IE of isolate 19 was reduced on T10, T74 and T121; IE of isolates 75 and 101 on T74, and that of isolate 108 on T74 and T121. IE of isolates 66 and 85 was unaffected and IE of isolate 19 was most affected by BABA treatment. There were no reductions of IE on second and third leaves, except for isolate 19 on T74, where BABA treatment completely suppressed infection (Fig. 1e).

Discussion

The results presented in this paper confirm that tomato accessions vary considerably in inducibility of resistance against P. infestans and the degree of induction generally decreases with increasing leaf age. The level of induction is not always related to the resistance level of the tomato accessions and it is significantly affected by the pathogen isolate used for challenge inoculation. Similar interactions apply to leaf age effects. Whilst AUDPC, SC and IE were all affected by treatment with BABA, effects were greatest on AUDPC and least on IE, which was only affected on the youngest leaves, except in one case.

The leaf-disc assays used produced highly repeatable results. The advantage of leaf discs is that conditions are more reproducible, the duration of the test is shorter and space requirements are greatly reduced. Thus, multiple isolate × host interactions can be assessed simultaneously and precise data can be obtained. Laboratory assays with leaf discs therefore appear to be a good method for studying particular aspects of resistance induction and for eliminating confounding influences of whole-leaf architecture.

Inoculations were performed with inoculum droplets containing 1000 sporangia which had been given the chance to hatch zoospores before inoculation by cooling down the inoculum for 2 h only. In a later trial leaf discs of accessions T10 and T121 that were BABA-induced and controls were inoculated with isolates 75, 101 and 108 and destained with KOH before staining with aniline blue. No germinated zoospores were found and all infections were established by directly germinated sporangia. It could be the case that the 2 h of cold storage were not enough to allow zoospores to hatch. Thus, potential differences among the isolates in the number of zoospores per sporangium were unlikely to play a role in this study. However, it is possible that at lower inoculum densities effects on infection efficiency might have been more visible.

IE was affected almost exclusively in the youngest leaves. Whilst this could indicate that systemic induced resistance is mostly responsible for inhibition of infection, this would have to be tested with much lower inoculum densities or by spray inoculation and counting successful infections under the microscope. The complete suppression of infections on all leaf ages of accession T74 through BABA when challenged by isolate 19 therefore deserves closer investigation. Whilst isolate 19 was the least aggressive, it still showed 100% IE in the controls and most other induced accessions, even on the youngest leaves. Whilst induction by BABA made no difference to the percentage of germinated sporangia on T10 and T121, depending on isolate and host the successful establishment of infections increased, decreased or remained unaffected by BABA treatment (unpublished data).

The effect of induced resistance in reducing diseased leaf area could be direct, in that the growth of young mycelia might be restricted, so that they are killed or otherwise fail to form large visible lesions. Induced resistance with BABA was shown to be effective for up to 12 days after infection by P. infestans under the conditions used by Cohen (1994). Thus, it is possible that the compounds which reduce the diseased leaf area persist in the leaf, reducing the size or vigour of colonies and thereby causing decreased sporulation capacity.

Higher resistance induction by BABA against P. infestans on younger leaves than on older leaves was reported before (Cohen, 1994; Cohen & Gisi, 1994) and can be explained by an acropetal systemic effect of BABA (Cohen & Gisi, 1994). The fact that induction by BABA was highest on young leaves might be the result of a combination of local and systemic induction of resistance, since the young leaves were not fully developed when BABA was sprayed. However, in some cases induction by BABA was almost the same on old leaves, suggesting that local induction levels may be more important in these accessions. It is therefore essential for reliable comparison of different tomato accessions and treatments that leaves of the same age are tested for resistance induction.

The variation in inducibility observed was within the ranges reported in earlier studies (e.g. Enkerli et al., 1993; Cohen, 1994). Little is known about the tomato genotypes and the type of resistance reactions induced on them, making speculations about the resistance mechanisms that may be activated and important in the containment of infections difficult. IR through BABA can be the result of the triggering of a variety of mechanisms, including physical barriers and biochemical changes leading to resistance (Jakab et al., 2001; Cohen, 2002). As well callose as lignin formation have been found in BABA treated tomato leaves challenged with P. infestans (Jeun et al., 2000; Cohen, 2002). In potatoes differing in race-non-specific resistances, Olivieri et al. (2009), found significantly higher levels of phenol, phytoalexin and aspartyl protease StAp1 accumulation after treatment with BABA in a more resistant potato cultivar than in a less resistant one. They concluded that BABA treatment increases the resistance of potatoes but that the degree of increase depends on the original level of resistance present in each cultivar.

The lack of a clear relationship between the level of resistance and the level of resistance induction does not fit the results from other host–pathogen systems. For example, induction of resistance in soybean to Sclerotinia sclerotiorum with INA or acibenzolar-S-methyl (ASM) was greatest in susceptible accessions (Dann et al., 1998). In contrast, resistance induction to Blumeria graminis was strongest in the most resistant and weakest in the most susceptible barley genotype (Martinelli et al., 1993). The differential resistance induction could be the result of defence mechanisms and responses to BABA specific to the accessions used which work differentially against isolates possessing different virulence mechanisms. Specific effects of specific genes on inducibility of resistance were also found in Arabidopsis thaliana. There, the plant growth-promoting rhizobacterium (PGPR) Pseudomonas fluorescens WCS417r induced systemic resistance to Pseudomonas syringae pv. tomato and Fusarium oxysporum f.sp. raphani in two out of three ecotypes (van Wees et al., 1997). Subsequent studies showed that a recessive trait in the non-inducible ecotype affected IR by disrupting ethylene signalling (Ton et al., 1999, 2001).

In addition to host-specific differences in inducibility and type of resistance reaction, the virulence mechanisms of pathogens are also variable. Different resistance suppressors (glucans) from different races of P. infestans affected defence responses of potato in a different way (Andreu et al., 1998). It could be that the resistance suppressors of the six P. infestans isolates used in the present study interacted differentially with the tomato accessions and their induced resistance mechanisms. This is supported by the fact that host–pathogen interactions were accession- and isolate-specific (Table 2; Fig. 1).

It has been claimed that unlike race-specific (vertical) resistance or pesticides, induced resistance does not appear to apply selective pressure to pathogen or parasite populations on the basis of any single genetic determinant or specific mode of action, but rather is quantitative because of the cumulative effects of numerous plant defence mechanisms (Sticher et al., 1997; van Loon et al., 1998). Whilst the race-specific effects found in the present study appear to contradict these claims, they may not be as important in the field, where crops are normally exposed to varying soil conditions and to pathogen populations made up of different genotypes. It is therefore likely that a certain level of resistance is normally induced in the field (Walters, 2009) and that different resistance mechanisms act simultaneously, reducing isolate-specific effects. Nevertheless, because of its similarity to horizontal resistance, the effectiveness of induced resistance has the potential to erode over time as the pathogen or parasite population evolves (McDonald & Linde, 2002; Vallad & Goodman, 2004). There is a need for research to evaluate the effects of IR on pathogen or parasite populations.

There are many examples where resistance was induced successfully in the field (Reglinski et al., 1994; Calonnec et al., 1996; Görlach et al., 1996; Morris et al., 1998; Zehnder et al., 2001; Silva et al., 2004). The usefulness of induced resistance in practice and especially for breeding will depend on the one hand on plant performance under the highly variable conditions found in agricultural practice and on the other hand on how easy it will be to select for inducibility. There is a need to determine if there are environmental conditions that enhance induced resistance to ease selection for this trait. The details concerning effects of single isolates are of great interest and may help to further elucidate the mechanisms of induced resistance.

Acknowledgements

This work was in part made possible through a grant provided to KS by the University of Kassel graduate student fund. Technical support was given by Mrs E. Geithe, Mrs R. Shresta and Mrs C. Aguilar.

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