Probenazole

Probenazole (3-allyloxy-1,2-benziothiazole-1,1-oxide) was developed by Meiji Seika Kaisha Ltd. This resistance inducer was first introduced in 1975 for the control of rice blast disease (Pyricularia oryzae) and bacterial blight (Xanthomonas oryzae). Probenazole is widely used in Asia, where it is applied as a granular treatment either to paddy fields or as a seedling box treatment. After its application, the compound is absorbed by the roots, then systemically transferred to the rest of the plant and can control rice blast disease for 40–70 days post application (Iwata et al. Reference Iwata2001). However, despite continuous use since its introduction, there have been no reports of pathogen insensitivity to probenazole and indeed, it still accounted for 0·53 of the chemicals used for seedling box treatments on rice in 2005 (Ishii Reference Ishii, Dehne, Deising, Gisi, Kuck, Russell and Lyr2008). It is believed this is because the compound is only weakly toxic to fungi and activates disease defence systems in rice (Watanabe Reference Watanabe1977; Watanabe et al. Reference Watanabe, Igarashi, Matsumoto, Seki, Mase and Sekizawa1977; Iwata et al. Reference Iwata, Suzuki, Watanabe, Mase and Sekizawa1980). These are thought to be the result of the activation of a signal transduction pathway, which in turn alters the balance of the plant–pathogen relationship in favour of the plant.

ASM

Since the introduction of ASM more than 10 years ago, a sizeable body of data has accumulated on its efficacy against a range of diseases under field conditions (Vallad & Goodman Reference Vallad and Goodman2004; Walters et al. Reference Walters, Walsh, Newton and Lyon2005). Most studies report disease control, although the level of control ranges from 0·04 to 0·99. Particularly high levels of disease control were achieved on tobacco, where infection by P. syringae pv. tabaci, Cercospora nicotianae and Alternaria alternata was reduced by 99, 91 and 89%, respectively (Cole Reference Cole1999; Perez et al. Reference Perez, Rodriguez, Rodriguez and Roson2003). In wheat, the crop that ASM was originally aimed at, disease control was not so impressive, ranging from 0·35 for Puccinia recondita and Septoria spp. to 0·77 for Blumeria graminis f. sp. hordei (Gorlach et al. Reference Gorlach, Volrath, Knauf-Beiter, Hengy, Beckhove, Kogel, Oostendorp, Staub, Ward, Kessmann and Ryals1996; Stadnik & Buchenauer Reference Stadnik and Buchenauer1999). ASM even increased disease levels in peanut, where infection by Cercosporidium personatum was greater than untreated controls by 52% (Zhang et al. Reference Zhang, Reddy, Kokalis-Burelle, Wells, Nightengale and Kloepper2001). Working on oilseed rape, Liu et al. (Reference Liu, Liu, Fitt, Evans, Foster, Huang, Latunde-Dada and Lucas2006) found that pre-treatment with ASM in October/November decreased the number of leaf lesions caused by the Phoma stem canker pathogen Leptosphaeria maculans in the autumn/winter, as well as the severity of stem canker in the subsequent spring/summer. Liu et al. (Reference Liu, Liu, Fitt, Evans, Foster, Huang, Latunde-Dada and Lucas2006) found that reductions in numbers of leaves with lesions were between 25 and 55%. In some more recent work, ASM was shown to reduce infection of barley by the leaf scald pathogen Rhynchosporium secalis by 45% (Paterson et al. Reference Paterson, Walsh and Walters2008).

Chitosan/Elexa®

Chitosan is a de-acetylated form of N-acetylchito-oligosaccharide containing poly-D-glucosamine and is a common polymer in shells of crustaceans, exoskeletons of insects and cell walls of fungi (Hadwiger Reference Hadwiger, Jolles and Muzzarelli1999). A commercial formulation of chitosan, Elexa®, contains 0·04 chitosan as its active ingredient and has been shown to protect a range of crops against pathogens. For example, when used as a seed treatment, it reduced downy mildew severity on pearl millet by 58% and when used as a foliar spray, it reduced infection by 75% (Sharathchandra et al. Reference Sharathchandra, Niranjan Raj, Shetty, Amruthesh and Shekar Shetty2004). When used on grapevines, eight applications of Elexa® applied over the season, reduced the incidence of downy mildew by 50% and powdery mildew by 75% compared with untreated controls (Schilder et al. Reference Schilder, Gillet, Sysak and Wise2002).

Harpin/Messenger®

Several commercially available products are based on microbial proteins. One of these is Messenger®, which is based on the protein harpin obtained from Erwinia amylovora (Wei et al. Reference Wei, Laby, Zumoff, Bauer, He, Collmer and Beer1992). Used as a crop protectant, Messenger® has had mixed success. For example, although it possessed good efficacy against blue mould in apples (de Capdeville et al. Reference de Capdeville, Beer, Watkins, Wilson, Tedeschi and Aist2003), its efficacy against grey mould in strawberry (Meszka & Bielenin Reference Meszka and Bielenin2004) and target spot of tomato (Pernezny et al. Reference Pernezny, Stoffella, Collins, Carroll and Beaney2002) was poor. In some interesting recent work, Chen et al. (Reference Chen, Qian, Qu, Long, Yin, Zhang, Wu, Sun, Wu, Hayes, Beer and Dong2008a) generated specific fragments of HpaG_Xooc, a harpin from X. oryzae pv. oryzicola, and found that one of these fragments, HpaG_10–42, stimulated growth of rice plants and provided enhanced resistance to X. oryzae pv. oryzae and Magnaporthe grisea. HpaG_10–42 was also shown to control bacterial blight, rice blast and sheath blight, and to increase grain yields, under field conditions (Chen et al. Reference Chen, Zhang, Zhang, Qu, Ren, Long, Yin, Qian, Sun, Zhang, Wang, Wu, Wu, Zhang, Cheng, Hayes, Beer and Dong2008b). Here the level of disease control depended on the cultivar, with greater control obtained with indica compared with japonica cultivars.

Milsana®

Milsana® is an ethanolic extract of giant knotweed (R. sachalinensis) and is registered as a plant activator for use on glasshouse-grown ornamental plants in the USA. It has been shown to control fungal pathogens on various crops, including strawberry (Carlen et al. Reference Carlen, Faby, Karjalainen, Pommier and Steffek2004) and cucumber (Daayf et al. Reference Daayf, Schmitt and Belanger1995; Fofana et al. Reference Fofana, McNally, Labbe, Boulanger, Benhamou, Seguin and Belanger2002). Used on grapes, Milsana® applied every 7–10 days provided similar levels of control of powdery mildew and bunch rot on grape berries to those obtained using a commercial fungicide (Schmitt et al. Reference Schmitt, Kunz, Nandi, Seddon and Ernst2002). More recently, Milsana® was found to reduce powdery mildew (Leveillula taurica) infection of tomato by 42–64%, with efficacy depending on application rate and disease pressure (Konstantinidou-Doltsinis et al. Reference Konstantinidou-Doltsinis, Markellou, Kasselaki, Fanouraki, Koumakis, Schmitt, Liopa-Tsakalidis and Malathrakis2006). Milsana® also controlled powdery mildew (Uncinula necator) on grape under field conditions (Konstantinidou-Doltsinis et al. Reference Konstantinidou-Doltsinis, Markellou, Kasselaki, Siranidou, Kalamarakis, Tzembelikou, Schmitt, Koumakis and Malathrakis2007).

BABA

The non-protein amino acid BABA has been shown to induce resistance in a range of crops (Jakab et al. Reference Jakab, Cottier, Toquin, Rigoli, Zimmerli, Metraux and Mauch-Mani2001). In field trials with grapevines, BABA reduced infection by the downy mildew fungus Plasmopara viticola by 57% on cv. Chardonnay and by 98% on cv. Cabernet Sauvignon (Reuveni et al. Reference Reuveni, Zahavi and Cohen2001). BABA has also been shown to provide protection against the late blight pathogen Phytophthora infestans on potato. For example, BABA-protected potato plants against P. infestans, especially when applied early in crop development and also provided some protection in tubers against late blight (Andreu et al. Reference Andreu, Guevara, Wolski, Daleo and Caldiz2006). Similar results were obtained by Altamiranda et al. (Reference Altamiranda, Andreu, Daleo and Olivieri2008), who obtained 0·20–0·60 protection of potato plants against late blight, with efficacy dependent upon cultivar and time of application.

ISR

PGPR-mediated ISR was first shown to be effective under field conditions in the mid-1990s. Thus, application of PGPR as a seed treatment followed by soil drench application led to a reduction in severity of bacterial wilt (Wei et al. Reference Wei, Yao, Zehnder, Tuzun and Kloepper1995), and control of bacterial angular leaf spot and anthracnose (Wei et al. Reference Wei, Kloepper and Tuzun1996). Later work by Raupach & Kloepper (Reference Raupach and Kloepper2000) demonstrated that treatment of cucumber seed with PGPR led to increased plant growth and control of angular leaf spot and anthracnose. Field experiments in Thailand in 2001 and 2002 studied the effects of PGPR, used alone or as mixtures, on control of southern blight of tomato caused by Sclerotium rolfsii, anthracnose of long cayenne pepper caused by Colletotrichum gloeosporoides and mosaic disease of cucumber caused by cucumber mosaic virus (CMV) (Jetiyanon et al. Reference Jetiyanon, Fowler and Kloepper2003). Mixtures of PGPR (all Bacillus spp.) were found to suppress disease more consistently than the PGPR strain Bacillus pumilus IN937b, used alone. In more recent work, Harish et al. (Reference Harish, Kavino, Kumar, Saravanakumar, Soorianathasundaram and Samiyappan2008) found that certain strains of PGPR and bacterial endophytes induced ISR in banana against banana bunchy top virus, while the PGPR strain Pseudomonas aeruginosa LY-11, applied as an alginate seed coating, reduced infection of lettuce by Rhizoctonia solani by 70–85% (Heo et al. Reference Heo, Lee, Lee, Jung, Lee and Moon2008).

Host genotype and the efficacy of induced resistance

It has been known for some time that the expression of induced resistance can be influenced by host genotype (Fig. 2). Twenty years ago, Steiner et al. (Reference Steiner, Oerke and Schonbeck1988) found that control of powdery mildew on barley using a Bacillus subtilis culture filtrate was dependent on the cultivar used, while later work by Martenelli et al. (Reference Martenelli, Brown and Wolfe1993) showed that induction of resistance in barley by prior inoculation with an avirulent isolate of B. graminis f. sp. hordei, differed in lines carrying different race-specific resistance genes. Clearly therefore, expression of induced resistance is cultivar-dependent, although the influence of the resistance rating of the cultivar on induced resistance is less clear. For example, resistance in cucumber to the powdery mildew pathogen, Sphaerotheca fuliginea, induced by treatment with 2,6-dichloroisonicotinic acid (INA), was cultivar-dependent, with highest levels of resistance expressed in a partially resistant cultivar and lower levels of resistance expressed in susceptible cultivars (Hijwegen & Verhaar Reference Hijwegen and Verhaar1994). In contrast, resistance in soybean to Sclerotinia sclerotiorum, induced with INA or ASM, was greatest in susceptible cultivars (Dann et al. Reference Dann, Diers, Byrum and Hammerschmidt1998). In some interesting work, Resende et al. (Reference Resende, Nojosa, Cavalcanti, Aguilar, Silva, Perez, Andrade, Carvalho and Castro2002) found that ASM provided 55 and 85% control of Verticillium dahliae and Crinipellis perniciosa, respectively, on cocoa seedlings, although the defences activated differed depending on the host cultivar used.

PGPR-mediated ISR has also been shown to be influenced by genotypic effects. The PGPR strain Pseudomonas fluorescens WCS417r elicited ISR in all ecotypes of Arabidopsis thaliana examined, apart from the ecotypes RLD and Wassilewskija (Van Wees et al. Reference Van Wees, Pieterse, Trijssenaar, Van't Westende, Hartog and Van Loon1997; Ton et al. Reference Ton, Pieterse and Van Loon1999). Further work revealed the presence of a locus (ISR1) involved in the ET signalling pathway, and it appeared therefore that the ecotypes RLD and Wassilewskija carried a recessive trait that affected ISR by perturbing ET signalling, although the plants could still express SAR (Ton et al. Reference Ton, Davison, Van Wees, Van Loon and Pieterse2001). This is clear evidence that in ecotypes of A. thaliana, allelic variability exists in genes that exert an influence on ISR pathways. Unfortunately, it is not known whether allelic variability exists for regulatory genes of SAR.

Costs and trade-offs associated with induced resistance

Plants are a source of food for a great many pathogens and herbivores. Survival in an environment teeming with consumers requires good defences. Plants possess a remarkable array of defence mechanisms to protect themselves from pathogens and pests and it would seem sensible for such defences to be continually present, i.e. to be constitutively expressed. However, plant defence is a costly business, requiring energy and resources that would otherwise be used for growth and development. In this context, constitutive resistance would appear to be a costly option. Indeed, it is argued that induced resistance, where defences are only activated following pathogen attack, represents a selective advantage over constitutive resistance (Walters & Heil Reference Walters and Heil2007). One explanation for this selective advantage lies with fitness costs, where resistant plants would have decreased reproductive success than non-resistant plants under pathogen free conditions (Heil & Baldwin Reference Heil and Baldwin2002). These costs include: (i) allocation costs arising from the diversion of metabolites and energy away from fitness-relevant processes such as growth and reproduction towards defence, (ii) ecological costs that result when the expression of a defence trait negatively interacts with one of the other ecological interactions that the plant has with the environment, e.g. mycorrhizal associations and (iii) genetic or pleiotropic costs that arise when resistance genes negatively affect fitness-relevant traits.

The study of costs of induced resistance to insect herbivory is well established and there is now much evidence to support the existence of such costs (e.g. Zavala et al. Reference Zavala, Patankar, Gase, Hui and Baldwin2004). In contrast, costs and trade-offs in relation to induced resistance to pathogens have received considerably less attention. In the early 1980s, it was shown that the expression of resistance in barley to powdery mildew was associated with a 7% reduction in grain yield and a 4% reduction in grain size and protein content (Smedegaard-Petersen & Stolen Reference Smedegaard-Petersen and Stolen1981). This pioneering work received little attention at the time and in contrast, later work found either no effects of induced resistance on yield or increased yield associated with induced resistance in barley (Oerke et al. Reference Oerke, Steiner and Schonbeck1989; Reglinski et al. Reference Reglinski, Lyon and Newton1994). Subsequent studies examined the effects of chemically induced resistance on costs in the absence of pathogen pressure. For example, Heil et al. (Reference Heil, Hilpert, Kaiser and Linsenmair2000) applied ASM to wheat in the absence of pathogens and found that treated plants had reduced biomass and reduced numbers of ears and grains, with most marked effects under nitrogen-limiting conditions. Work on other systems produced similar results. Thus, ASM reduced shoot fresh weight in sunflower (Prats et al. Reference Prats, Rubiales and Jorrin2002), suppressed growth of tobacco and cauliflower (Csinos et al. Reference Csinos, Pappu, McPherson and Stephenson2001; Ziadi et al. Reference Ziadi, Barbedette, Godard, Monot, Le Corre and Silue2001) and reduced shoot growth and leaf enlargement in cowpea (Latunde-Dada & Lucas Reference Latunde-Dada and Lucas2001).

These data suggest that use of ASM incurs allocations costs and supports the ‘growth-differentiation balance’ hypothesis, which assumes a metabolic competition between processes involved in plant growth and those necessary for plant differentiation, such as the synthesis of chemicals for plant defence (Herms & Mattson Reference Herms and Mattson1992). Interestingly however, not all work on ASM yielded the same results. Thus, in the work on bean treated with ASM, no evidence could be found for the existence of allocation costs (Iriti & Faoro Reference Iriti and Faoro2003). That work, together with that of Oerke et al. (Reference Oerke, Steiner and Schonbeck1989) and Reglinski et al. (Reference Reglinski, Lyon and Newton1994), showing either no effect on yield or even yield increases in induced plants suggests either that the plants possessed sufficient resources to finance both growth and defence, or they compensated for the resources diverted from growth to defence. Compensation might take the form of increased photosynthetic rates and, interestingly, Murray & Walters (Reference Murray and Walters1992), working on broad bean, found that in plants induced by prior inoculation with rust, systemically protected leaves also exhibited increased rates of photosynthesis. Unfortunately, little information exists on rates of photosynthesis in plants expressing SAR or ISR.

From the above discussion, it seems reasonable to assume that plants growing under resource-limiting conditions will experience greater costs associated with resistance expression or that their ability to express resistance should be compromised. Indeed, competition has been shown to reduce peroxidize activity in SA-treated Arabidopsis plants and reduced seed set compared with non-competing or untreated controls (Cipollini Reference Cipollini2002), a dependency of activities of two defence-related enzymes on CO₂ concentration was observed by Plessl et al. (Reference Plessl, Heller, Payer, Elstner, Habermeyer and Heiser2005) and mycorrhizal colonization and establishment might be necessary for successful resistance induction by ASM (Sonnemann et al. Reference Sonnemann, Streicher and Wolters2005). Further, whether costs were incurred as a result of resistance induction in wheat and Arabidopsis was dependent on nitrogen supply (Heil et al. Reference Heil, Hilpert, Kaiser and Linsenmair2000; Dietrich et al. Reference Dietrich, Ploss and Heil2005).

In the studies on costs of induced resistance described above, it is likely that defences were directly activated by the inducing agent. Direct induction of defences is likely to be wasteful in the absence of disease, in contrast to priming, where defences are activated upon pathogen challenge. Work carried out by van Hulten et al. (Reference van Hulten, Pelser, Van Loon, Pieterse and Ton2006) found that priming involved fewer costs than direct induction of defences and, indeed, was beneficial in terms of the plant growth rate and fitness under disease pressure. Priming appears therefore to have clear ecological benefits and would also represent a promising approach for crop protection.

Induced resistance can also lead to trade-offs with other defence responses, e.g. with defence against insect pests. JA is known to be important in regulating induced resistance to insect attack (Bostock Reference Bostock2005) and there are several reports of negative crosstalk between the JA pathway for defence against insects and the SA pathway for defence against pathogens. For example, activation of SA-dependent SAR has been shown to suppress JA signalling, thereby compromising induced defence responses to insect attack (Bostock Reference Bostock2005; Stout et al. Reference Stout, Fidantsef, Duffey and Bostock1999; Thaler et al. Reference Thaler, Fidantsef, Duffey and Bostock1999, Reference Thaler, Fidantsef and Bostock2002). Although there are fewer reports of negative crosstalk in the opposite direction, JA has been shown to suppress SA-induced responses (Niki et al. Reference Niki, Mitsuhara, Seo, Ohtsubo and Ohashi1998; Glazebrook et al. Reference Glazebrook, Chen, Estes, Chang, Nawrath, Metraux, Zhu and Katagiri2003). It should be noted, however, that not all interactions between pathogen and insect resistance are negative, with some workers finding no effect (Ajlan & Potter Reference Ajlan and Potter1992; Inbar et al. Reference Inbar, Doostdar, Sonoda, Leibee and Mayer1998) and others reporting a positive effect (Stout et al. Reference Stout, Fidantsef, Duffey and Bostock1999; Hatcher & Paul Reference Hatcher and Paul2000; Walters et al. Reference Walters, Cowley and Weber2006). These examples of negative and positive crosstalk between the JA and SA signalling pathways highlight the complexity of signalling in pest and disease resistance. Interestingly, it seems that although JA, SA and ET play a primary role in orchestrating plant defence, the final defence response is shaped by other regulatory mechanisms, such as crosstalk between different signalling pathways and other attacker-induced signals (De Vos et al. Reference De Vos, Van Oosten, Van Poecke, Van Pelt, Pozo, Mueller, Buchala, Metraux, Van Loon, Dicke and Pieterse2005).

Since induced resistance is a broad-spectrum resistance against micro-organisms, it is not unreasonable to suggest that it might interfere with plant-microbe mutualisms. Unfortunately, however, little work has been carried out in this area. Nevertheless, several studies have shown that application of SA to the rooting substrate had a negative effect on nodule formation and/or function (Martínez-Abarca et al. Reference Martínez-Abarca, Herrera-Cervera, Bueno, Sanjuan, Bisseling and Olivares1998; Ramanujam et al. Reference Ramanujam, Abdul Jaleel and Kumara Velu1998; Lian et al. Reference Lian, Zhou, Miransari and Smith2000), while in ASM-induced SAR in broad bean, treated plants were found to develop fewer and smaller nodules than untreated controls (Heil Reference Heil2001). Even less effort has been spent on the impact of induced resistance on mycorrhizal infection, colonization and establishment, although a field study by Sonnemann et al. (Reference Sonnemann, Finkhaeuser and Wolters2002) found no effect of ASM on mycorrhizal infection of barley roots.

Influence of the abiotic environment on induced resistance

As indicated in the previous section, allocation costs can be incurred if plants expressing induced resistance are grown under resource-limiting conditions. Indeed, nitrogen supply was shown to exert a marked effect on the expression of both constitutive and induced resistance (Dietrich et al. Reference Dietrich, Ploss and Heil2004). Here, constitutive levels of defence-related enzymes, as well as levels in plants induced by ASM treatment, were significantly lower under limiting nitrogen supply. Dietrich et al. (Reference Dietrich, Ploss and Heil2004) also found significantly reduced levels of total soluble protein during the first 12 h following ASM treatment. This observation agrees with the other work demonstrating reductions in expression of genes relating to primary metabolism following elicitation of resistance (Somssich & Hahlbrock Reference Somssich and Hahlbrock1998) and suggests that such a down-regulation of primary metabolism might be necessary in order to make available the substrates and metabolites required for the synthesis of defence compounds (Heil Reference Heil2002). This concept is supported by work showing that plants treated with ASM exhibited a growth reduction in the week following treatment, but recovered thereafter (Dietrich et al. Reference Dietrich, Ploss and Heil2005). These workers also found that costs, no costs or even higher seed production could be obtained in ASM-induced plants depending on the combination of environmental factors to which the plants were exposed.

Expression of induced resistance can also be influenced by abiotic stress. For example, Wiese et al. (Reference Wiese, Kranz and Schubert2004) showed that osmotic stress and proton stress led to the induction of active defences against powdery mildew in barley. This suggests that under field conditions, the effectiveness of induced resistance will be greatly influenced by both the biotic and abiotic environment.

Are plants in the field already induced?

Current understanding of induced resistance is based on early research using pathogens to induce resistance. Thus, Cruickshank & Mandryk (Reference Cruickshank and Mandryk1960) showed that resistance to infection by Peronospora tabacina in tobacco could be induced by prior injection of sporangia of the same pathogen into stems, while Ross (Reference Ross1961a, Reference Rossb) demonstrated local and systemic resistance in tobacco to infection by the tobacco mosaic virus (TMV). Since then, there have been many reports of resistance induction using prior inoculation with pathogens (see Hammerschmidt Reference Hammerschmidt, Walters, Newton and Lyon2007). Resistance can also be induced by mycorrhizal infection and colonization and it appears to be effective against necrotrophic pathogens and generalist chewing insects, but not against biotrophic pathogens (Pozo & Azcón-Aguilar Reference Pozo and Azcón-Aguilar2007). Fungal and bacterial endophytes have been shown to induce resistance (Waller et al. Reference Waller, Achatz, Baltruschat, Fodor, Becker, Fischer, Heier, Huckelhoven, Neumann, von Wettstein, Franken and Kogel2005; Kang et al. Reference Kang, Cho, Cheong, Ryu, Kim and Park2007), and resistance can also be induced by avirulent nematode species (Ogallo & McClure, Reference Ogallo and McClure1996; Kosaka et al. Reference Kosaka, Aikawa, Ogura, Tabata and Kiyohara2001). In addition, and as already mentioned above, insect attack can also induce resistance (Bostock Reference Bostock2005).

It is clear, therefore, that in the field, plants will be at least partly induced through interaction with both the biotic and abiotic environment. Indeed, in a study of defence gene expression, Pasquer et al. (Reference Pasquer, Isidore, Zarn and Keller2005) found that when ASM was applied to wheat in the field, no differences in gene expression could be detected, because gene expression was already high in the untreated plants. In more recent work, Herman et al. (Reference Herman, Restrepo and Smart2007) examined defence gene expression in three tomato cultivars treated with ASM under field conditions. They found that some defence genes were already expressed prior to treatment, although gene expression was increased further following ASM treatment. Both the baseline levels of gene expression and the magnitude of the increase in gene expression following ASM treatment was cultivar dependent (Herman et al. Reference Herman, Restrepo and Smart2007). The studies of both Pasquer et al. (Reference Pasquer, Isidore, Zarn and Keller2005) and Herman et al. (Reference Herman, Restrepo and Smart2007) examined gene expression, but work on spring barley has shown that activities of defence-related enzymes (peroxidise, cinnamyl alcohol dehydrogenase, chitinase and glucanase) are already induced in untreated plants under field conditions (Walters et al. unpublished results). Interestingly, Heil & Ploss (Reference Heil and Ploss2006) found that a range of wild (non-cultivated) plants exhibited markedly high constitutive activities of a number of defence-related enzymes; they suggested that these high constitutive enzyme activities might be the result of prior, natural infections.

The studies outlined above suggest that plants in the field are already induced, but does this compromise the ability of plants to induce resistance further? The work of Heil & Ploss (Reference Heil and Ploss2006) on wild plants showed that despite exhibiting high constitutive activities of defence enzymes, some species were clearly able to respond to ASM treatment by further induction of defence enzymes. However, the ability to induce resistance further was dependent on plant life history. Thus, species that flowered early in the spring exhibited low inducibility of resistance, while larger perennials that flowered in late spring or summer were able to induce resistance to much higher levels following ASM treatment (Heil & Ploss Reference Heil and Ploss2006). In their work on tomato, Herman et al. (Reference Herman, Restrepo and Smart2007) found that while ASM-induced defence gene expression following the first application, a much greater level of gene expression was observed following the second ASM application. This suggests that prior induction of resistance does not compromise the ability of the plant to respond to subsequent inductions. Understanding of this phenomenon is still rudimentary, with little information available on the factors controlling the ability of already induced plants to induce resistance further. For example, whether a plant that is already induced can be induced further will depend on other factors e.g. genotype (Walters et al. Reference Walters, Walsh, Newton and Lyon2005; Herman et al. Reference Herman, Restrepo and Smart2007), but information in this area remains inadequate.