Fig. 11.7 Influence of different stresses on plant metabolism. The activation of systemic acquired resistance results in the accumulation of numerous defence compounds that may imbalance the equilibrium between primary and secondary metabolism, thus resulting in fitness costs for the plant. On the other end, the synthesis of secondary metabolites due to a stress may protect plant from other different stresses same induced by pathogens, as well as the increased activity of key enzymes of the phenylpropanoid pathway have been reported in several plant systems (Figs. 11.3, 11.4, and 11.7). In Arabidopsis (Arabidopsis thaliana), PAL mRNA is rapidly and transiently induced within 3h of ozone treatment (300 nLL-1 daily for 6h), reaching a 3-fold higher levels than control plants (Sharma and Davis 1994). A similar trend has been reported in parsley (Petroselinum crispum) plants, in which ozone treatment (200 nLL-1 for 10 h) induced an early 3-fold and 1.2-fold increase of PAL and CHS activity, respectively, followed by a 2-fold increase of total leaf fura-nocoumarins and flavone glycosides (Eckey-Kaltenbach et al. 1994). The content of psoralen, bergapten and other furanocoumarins in celery (Apium graveolens) dropped 24 h after ozone (0.2 nLL-1 for 2h) fumigation, but levels of these chemicals, in treated leaves, increased rapidly at 120 h (Derckset al. 1990). Further studies have shown an overlap between patterns of genes induced by ozone exposure and pathogen infection, probably due to the role of ROS as effector molecules involved in transduction pathways activated either by pathogens and ozone. The enhancement of SA content in plant tissues due to ozone treatment is well documented (Rao and Davis 2001). Intriguingly, in tobacco (Nicotiana tabacum) plants, a pulse ozone treatment (120-170nLL-1 for 5h) enhanced the emission of methyl sali-cylate, a volatile SA derivative, to a greater extent in sensitive cv. Bel W3 compared to tolerant cv. Bel B (Heiden et al. 1999). In Arabidopsis, SA accumulation, necessary for the expression of hypersensitive response to pathogens (HR) and SAR, is also required for the accumulation of some ozone-induced mRNAs, particularly PAL and pathogenesis related protein 1 (PR1) transcripts, although a SA-independent signal transduction pathway is activated by the pollutant (Sharma et al. 1996). In fact, in transformed Arabidopsis plants unable to accumulate these transcripts, ozone exposure induces resistance against the bacterial pathogen Pseudomonas syringe, a phenomenon known as cross induction (Sharma et al. 1996). HR is a type of programmed cell death (PCD) triggered at the attempted pathogen penetration site, frequently at the onset of systemic immunity (SAR) (Langebartels et al. 2002; Iriti and Faoro 2007), whereas PR proteins are enzymes induced in plants by pathogen infection as well as by abiotic and environmental stresses including ozone (Schraudner et al. 1992; Thalmair et al. 1996; Paakkonen et al. 1998). In sensitive bean (Phaseolus vulgaris) cv. Pinto, ozone exposure (120 nLL-1 for 4h) causes a stimulation of phenylpropanoid route and flavonoid branch, as shown by the increased mRNA accumulation of PAL, CHS and chalcone isomerase (CHI), the latter involved in isoflavonoid biosynthesis (Paolacci et al. 2001). In grapevine (Vitis vinifera), STS, the first enzyme of the stilbene branch involved in the synthesis of resveratrol and other stilbenic compounds, is considered the most sensitive ozone-induced biomarker (Schubert et al. 1997). In grapevine callus, either PAL or STS activity increase, after ozone fumigation (0.3 ^molmol-1 for 2h) unlike CHS (Sgarbi et al. 2003). This could be due to the STS and CHS competition for the same substrate (Fig. 11.3), during the veraison (the berry ripening), when a metabolic switch occurs between the two branches of the same pathways. At this phenological stage, declined STS activity and resveratrol concentration in berries may cause grey mould (Botrytis cinerea) infection, whereas enhanced CHS activity and anthocyanin accumulation are required for berry colouring (Jeandet et al. 1995). Furthermore, a general drop in the amount of some assayed phenylpropanoids (coumaric acid, fer-ulic acid, gallic acid and catechin) has been reported unlike caffeic acid, whose level raised only in one cell line (Sgarbi et al. 2003). In 20 soybean (Glyicine max) cul-tivars, ozone tolerance was associated with the presence of kaempferol glycosides, a powerful antioxidant flavonol, as well as tolerance to manganese (Mn) toxicity in one soybean line (Foy et al. 1995). In European silver birch (Betula pendula) chronically exposed to ozone, it has been reported a 16.2% increase in total phenyl-propanoids and a corresponding 9.9% increase of 10 compounds, among simple phenols and flavonoids, such as chlorogenic acid and catechin, respectively (Saleem et al. 2001). Interestingly, the combined action of CO2 and ozone greatly enhances the synthesis of total and polymeric PA, in Betula sp. leaf tissues, suggesting an additive effect of these environmental pollutants on phenylpropanoid biosynthesis (Karonen et al. 2006).
Shikimate dehydrogenase (SKDH), a key enzyme of shikimate pathway, PAL and cinnamyl alcohol dehydrogenase (CAD), a key enzyme of lignin biosynthesis which forms monolignols (Fig. 11.4), have been investigated in poplar (Populus tremula x alba) leaves. Under ozone exposure (60-120 nLL-1, during the 14 h light period, for 1 month), either CAD activity and transcript levels were rapidly and strongly stimulated, increasing up to 15-fold and 23-fold the control values, respectively. In contrast, SKDH and PAL activities raised only in old and middle-aged leaves, but not in the youngest ones. Interestingly, the increased activity of these enzymes was associated with a higher lignin content in ozone-exposed leaves and additionally, the newly synthesized lignin structurally differed from the control lignin. Particularly, stress lignin appeared more condensed, i.e. enriched in carbon-carbon interunit linkages, in p-hydroxyphenyl (H) units and in terminal units with free phenolic groups (Fig. 11.4) (Cabane et al. 2004). The same enzymes have been studied in two genotypes of ozone-treated (150 nLL-1 for 3 h) tomato (Lycopersicon esculentum) plants (Guidi et al. 2005). However, while SKDH and PAL activity augmented significantly only in one line, CAD activity diminished in both the genotypes in contrast to the results reported by other authors (Galliano et al. 1993; Cabane et al. 2004). An explanation could reside in the different response of herbaceous and woody plants and in the different acute or chronic ozone dose employed in tomato and poplar, respectively.
To conclude, the importance of phenylpropanoids in plant tolerance against ozone injury is related to their different properties (Iriti and Faoro 2004). These compounds include an array of molecules with a plethora of biological activities, besides being precursors of structural biopolymer, such as lignin. Particularly, their protective role is mainly inherent to their antioxidant power, i.e. the ability to trap free radicals (ROS) functioning as electron donors. Nonetheless, increased synthesis of lignin, as well as structural modifications of lignin itself, represent another important defence mechanism in order to protect plasmalemma from the ROS injury, thus preventing membrane damages due to lipid peroxidation.
Strictly speaking, not all the isoprenoids are secondary metabolites, as some primary metabolites, such as sterols, arise from isoprenoid pathway as well. The effects of the pollutant on either the sterol concentration and composition have been early reported in several plants (Tomlinson and Rich 1971; Trevathan et al. 1979; Grunwald and Endress 1985). Sterols are important component of cell membranes, involved in their stabilization. Generally, exposure to high ozone concentrations (>12 ^LL-1) results in a decrease of free sterols (FS) and an increase in bound sterols (BS). Conversely, at low ozone concentration (<12 ^LL-1), an accumulation of FS occurs, resulting in a decrease of the FS:BS ratio. In this case, the enhancement of FS synthesis is followed by a shift towards sterols with a more bulky C-17 side chain, i.e. sitosterol and stigmasterol vs. campesterol. As FS have a much greater capacity to stabilize membranes than BS, the FS:BS ratio has a more determinant effect on membrane permeability than the composition of FS fraction itself. Accordingly, a decrease in FS:BS ratio with high ozone concentration results in cell injury and visible damages, whereas modification of FS composition, occurring at lower ozone levels, results neither in permanent membrane injury nor in visible damages (Grunwald 1971; Evans and Ting 1973; Grunwald 1974). In tobacco leaves, ozone fumigation enhances total lipid concentrations, but it decreases levels of FS and triglycerides (Trevathan et al. 1979), whereas a higher amount of total phytosterols was reported in fumigated plants by other researchers (Menser etal. 1977).
In ozone treated (300 nL L-1 for 8 h) Scots pine (Pinus sylvestris), it was reported a transient increase of a transcript corresponding to the cytosolic/endoplasmic B-Hydroxy-B-methyl-glutaryl-CoA (HMG-CoA) synthase, a key enzyme of isoprenoid biosynthesis (Wegener et al. 1997). The C5 precursor of isoprenoids, IPP, arises either from mevalonate pathway, in the cytosol, as previously described, or from plastidial precursors (Cheng et al. 2007). In ozone-treated pine seedlings (250 nLL-1, 12 h day-1 for 4 days), the biosynthesis of plastidial IPP was inhibited, differently from the mevalonate synthesis in the cytosol, due to the resource allocation between the two IPP synthesis pathways (Shamay et al. 2001).
Biogenic Volatile Organic Compounds (BVOC) comprise mainly isoprenoids (particularly hemi-, mono- and sesquiterpenes) emitted from plants during cell growth and in response to several kinds of stresses, they are having various eco-physiological functions and mediating plant-arthropod interactions (Fig. 11.8) (Kesselmeier and Staudt 1999). BVOC can act as attractants for pollinators or repellents for noxious insects, besides being involved in tritrophic signalling, i.e. the relationship between plant, herbivorous and carnivorous arthropods. In particular, phytophagus feeding can induce BVOC emission from plants, which can function as foraging cues for the recruitment of the natural enemies of herbivores (Dicke 2000). Besides wounding and trithrophic interactions, several environmental factors can affect the BVOC emission from plants, such as light intensity, temperature, water supply and pollutants (Fig. 11.8) (Penuelas and Llusia 2001).
The relationships between ozone and BVOC is somewhat complex, both exerting a mutual influence (Llusia et al. 2002). On one hand, the release of either BVOC or volatile organic compounds (VOC) from anthropogenic origin into the atmosphere can constitute a significant input of photochemical oxidant precursors, thus contributing to the regional-scale air pollution, on the other, BVOC emission can be triggered by the exposure to high ozone concentration (Roselle 1994; Heiden et al. 1999). Additionally, chronic ozone exposure can modify the composition of the plant BVOC emissions, thus not only affecting the tritrophic interactions, but also directly weakening plant defence responses against arthropods (Alstad et al. 1982). Isoprenoids can be synthesized and emitted in order to tolerate the ozone injury. Isoprene, a hemiterpene (Fig. 11.5), has been reported to reduce ozone damages in leaves, because of its antioxidant activity. This gas protects the photosynthetic apparatus, quenches ozone byproducts and radical species responsible for lipid peroxidation of cell membranes and cell death (Loreto and Velikova 2001; Loreto et al. 2001; Velikova et al. 2005). Besides, monoterpenes may exert an isoprene-like antioxidant activity, too (Loreto et al. 2004).
The effect of ozone on alkaloid biosynthetic pathways has not been extensively investigated, although a generalized influence of the pollutant on the nitrogen metabolism has been ascertained (Menser and Chaplin 1975; Aycock 1975; Jackson et al. 2000). Generally, ozone exposure reduces the amount of total alkaloids in tobacco plants (Menser and Chaplin 1969; Aycock 1975), and lower levels of nicotine, a pyridine alkaloid, in ozone exposed plants were related to increased survival, growth and development of hornworm (Manduca sexta) larvae (Jackson et al. 2000). Several studies reported increased preferences or enhanced survival and fitness of different insect species on ozonated plants, thus pointing out again the detrimental effect of the pollutant on the plant chemical defences (Jeffords and Endress 1984; Endress and Post 1985; Trumble et al. 1987; Chappelka et al. 1988; Heagleetal. 1994).
The role of polyamines, important alkaloid precursors (Fig. 11.9), has been correlated with ozone tolerance. Polyamines are polycationic nitrogenous compounds of low molecular weight ubiquitous in all living organisms. In plants, they function as growth regulators involved in an array of physiological processes, being involved in embryogenesis, cell division, morphogenesis, development, flowering and senescence (Martin-Tanguy 2001). In addition, they serve as an integral component of plant response to both biotic and abiotic stresses (Walters 2000;
Navakoudis et al. 2003; Liu et al. 2007). The most important polyamines are the diamine putrescine (Put), the triamine spermidine (Spd) and the tetraamine sper-mine (Spm), arising directly from the free non-proteinogenic amino acid ornithine, by ornithine decarboxylase (ODC), or indirectly from arginine by arginine decar-boxylase (ADC). Further steps include Put conversion into Spd, via spermidine synthase (SPDS), and the similar synthesis of Spm from Spd via spermine synthase (SPMS). Either of two enzymes employed the aminopropyl moiety provided by S-adenosylmethionine (SAM) (Bagni and Tassoni 2001). In plants, ADC activity is enhanced by ozone exposure, whereas ODC remains unchanged (Rowland-Bamford et al. 1989; Langebartels et al. 1991). Free and conjugated polyamines improve ozone tolerance with two different mechanisms: (i) by inhibiting the ethy-lene biosynthesis and (ii) by direct ROS scavenging (Ormrod and Beckerson 1986; Bors et al. 1989). Ethylene and polyamines share the same biosynthetic precursor, SAM and thereby, they mutually inhibit their own biosynthesis. In particular, aminocyclopropane carboxylic acid (ACC), a precursor of ethylene, arises from SAM via ACC synthase, a rate-limiting step in ethylene production. Therefore, this metabolic shift to ethylene or polyamine biosynthesis can enhance ozone susceptibility or tolerance, respectively, due to the correlation between the stress ethylene production and visible ozone injury (Langebartels et al. 1991).
Additionally, apoplastic polyamines can form conjugates with hydroxycinna-mates and phenolic acid derivatives, effective in ROS detoxification (Bouchereau et al. 1999). As reported above, polyamine are precursors of alkaloids and apart from it, they play a role in ozone tolerance. Higher amounts of these compounds, as a consequence of ozone exposure, could induce an increase of pyrrolidine, tropane and pyrrolizidin alkaloids, deriving from ornithine via Put.
In the global climate change scenario, the relation between the anthropogenic emissions of greenhouse gases, responsible for climate warming and rainfall alteration, and troposheric ozone is quite complex, specially in the agroecosystem context. Crop potential yield depends on defining factors (CO2, radiation, temperature and crop traits), limiting factors (water, nitrogen and phosphorous) and reducing factors (pests, pathogens, weeds and pollutants) (Goudrian and Zadoks 1995). A deep knowledge of the role played by these factors under altered climatic conditions is necessary to evaluate the responses at crop and agroecosystem level (Fuhrer 2003).
It is well known that increasing troposheric ozone concentration at ambient CO2, causes a decline in the yield of many crop species, and this negative effect is reduced in a CO2-enriched atmosphere, probably due to the decrease of stomatal conductance and ozone flux, or to the increase in the activity of anti-oxidant enzymes (Heagle et al. 1998,1999,2000). Thus, the detrimental effect of enhanced CO2 concentration on Heart temperature is in some way, compensate by its protective role in plants under ozone stress, as assessed by biomass and yield stimulation studies in these conditions (McKee et al. 1995,1997).
Fig. 11.10 Protective effect of ozone against Uromyces appendiculatus, causal agent of bean rust (Iriti and Faoro, unpublished). The leaf in (b) has been fumigated with ozone, 48 h before inoculation, and does not show any disease symptoms, while the control leaf in (a), only inoculated with the fungus, has developed typical rust pustules (arrow)
Fig. 11.10 Protective effect of ozone against Uromyces appendiculatus, causal agent of bean rust (Iriti and Faoro, unpublished). The leaf in (b) has been fumigated with ozone, 48 h before inoculation, and does not show any disease symptoms, while the control leaf in (a), only inoculated with the fungus, has developed typical rust pustules (arrow)
The influence of global climate change in plant-pathogen interactions is quite complex too (Violini 1995; Manning and von Tiedemann 1995). On one hand, ozone may induce the same sequel of events involved in plant immunity (Iriti and Faoro 2007) i.e. oxidative burst and hypersensitive response at the onset of systemic acquired resistance (SAR) (Fig. 11.10) (Langebartels et al. 2002), on the other, plants weakened by ozone stress may be particularly susceptible to infections (Manning and von Tiedemann 1995). Interestingly, wheat rust (Puccinia recondita f. sp. tritici) is strongly inhibited by ozone, but unaffected by elevated CO2, both in presence or absence of ozone stress (von Tiedemann and Firsching 2000). Vice versa, a protective effect of rust (Uromyces fabae) infection in broad bean (Vicia faba) was reported against ozone, sulphur dioxide either alone or combined (Lorenzini et al. 1994).
Finally, regardless of the specific ecophysiological meaning, plants cope with the plethora of stressful abiotic and biotic conditions by modifying their secondary metabolic pathways. In this view, during the evolution, the chemical diversity improved the fitness of plant organisms, thus ensuring their evolutionary radiation. Phytochemicals, with their broad spectrum activities, are primarily involved in plant tolerance against environmental pollutants and worsening climatic conditions, as well as in resistance against pests and pathogens. The metabolic processes activated in these defence responses may be tightly separated or overlapping, according to the stress factor and the plant cultivar, as a result of negative (trade off) or positive (cross resistance) cross talk, that is to say the communication between molecular signals and transduction pathways involved in different plant defence responses. The most important consequence of this cross talk is the alert of defence mechanisms against an abiotic stress in consequence of priming with a biotic elicitor and vice versa.
Acknowledgments This work was funded by the Regione Lombardia, Piano per la ricerca e lo sviluppo 2003, d.g.r. 13077/2003 and ERSAF Lombardia.
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