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First published online June 26, 2008; 10.1104/pp.108.122614 Plant Physiology 147:2164-2178 (2008) © 2008 American Society of Plant Biologists Apoplastic Polyamine Oxidation Plays Different Roles in Local Responses of Tobacco to Infection by the Necrotrophic Fungus Sclerotinia sclerotiorum and the Biotrophic Bacterium Pseudomonas viridiflava1,[W]Unidad de Biotecnología 1, Instituto Tecnológico de Chascomús/Universidad Nacional de General San Martín-Consejo Nacional de Investigaciones Científicas y Técnicas, B7130IWA Chascomus, Argentina
The role of polyamine (PA) metabolism in tobacco (Nicotiana tabacum) defense against pathogens with contrasting pathogenic strategies was evaluated. Infection by the necrotrophic fungus Sclerotinia sclerotiorum resulted in increased arginine decarboxylase expression and activity in host tissues, as well as putrescine and spermine accumulation in leaf apoplast. Enhancement of leaf PA levels, either by using transgenic plants or infiltration with exogenous PAs, led to increased necrosis due to infection by S. sclerotiorum. Specific inhibition of diamine and PA oxidases attenuated the PA-induced enhancement of leaf necrosis during fungal infection. When tobacco responses to infection by the biotrophic bacterium Pseudomonas viridiflava were investigated, an increase of apoplastic spermine levels was detected. Enhancement of host PA levels by the above-described experimental approaches strongly decreased in planta bacterial growth, an effect that was blocked by a PA oxidase inhibitor. It can be concluded that accumulation and further oxidation of free PAs in the leaf apoplast of tobacco plants occurs in a similar, although not identical way during tobacco defense against infection by microorganisms with contrasting pathogenesis strategies. This response affects the pathogen's ability to colonize host tissues and results are detrimental for plant defense against necrotrophic pathogens that feed on necrotic tissue; on the contrary, this response plays a beneficial role in defense against biotrophic pathogens that depend on living tissue for successful host colonization. Thus, apoplastic PAs play important roles in plant-pathogen interactions, and modulation of host PA levels, particularly in the leaf apoplast, may lead to significant changes in host susceptibility to different kinds of pathogens.
Polyamines (PAs) are natural aliphatic polycations ubiquitous in prokaryotic and eukaryotic cells and are essential for cell growth, proliferation, and differentiation (Cohen, 1998  ). Although the mechanism of PA action in many of these processes is not completely clear, it is well known that they are positively charged at cellular pH, and therefore bind to negatively charged molecules such as nucleic acids, acidic phospholipids, and proteins (Igarashi et al., 1982). In this way, PAs modulate DNA-protein (Shah et al., 1999) interactions, protein-protein (Thomas et al., 1999) interactions, and RNA structure (Igarashi and Kashiwagi, 2000). In addition to their free forms, PAs also exist as conjugates, covalently bound to small molecules and proteins. The most abundant PAs, namely the diamine putrescine, the triamine spermidine, and the tetraamine spermine are synthesized from the amino acids Orn and Arg by two alternative pathways. One of these pathways is shared by almost all organisms and involves the decarboxylation of Orn by the enzyme Orn decarboxylase (ODC; EC 4.1.1.17) to form putrescine. This diamine is then successively aminopropylated to produce spermidine and spermine by spermidine synthase (EC 2.5.1.16) and spermine synthase (EC 2.5.1.22), respectively. The aminopropyl groups are donated by decarboxylated S-adenosyl-Met, a compound synthesized by S-adenosyl-Met decarboxylase (AdoMetDC; EC 4.1.1.50) from S-adenosyl-Met. An alterative pathway occurs in plants and bacteria, where Arg is decarboxylated to agmatine by Arg decarboxylase (ADC; EC 4.1.1.19), and agmatine is then transformed into putrescine via N-carbamoyl-putrescine.
In addition to the above-mentioned biosynthetic pathways, oxidative deamination mediated by amine oxidases plays an important role in the regulation of cellular PA levels (Tiburcio et al., 1997
PA metabolism undergoes significant modifications during plant responses to various abiotic and biotic stresses (for review, see Flores, 1990
The bulk of the work performed in relation with plant PAs during the response to pathogen attack has pointed to metabolic changes occurring at the organ or tissue level, without focusing on intercellular spaces. Most phytopathogenic bacteria multiply in the intercellular space, also known as the apoplast, and frequently, this is the first site of pathogenic fungi invasion (Glazebrook, 2005
To our knowledge, information about the role of apoplastic PAs in plant defense against pathogens is restricted to the tobacco-TMV pathosystem, the consequences of fungal or bacterial infection on apoplastic PA metabolism having not been studied so far. Moreover, a comparison of the changes in PA metabolism elicited in a given plant host by pathogenic organisms with contrasting (necrotrophic and biotrophic) pathogenic strategies was not performed so far. Therefore, this work aimed to determine the role of plant PAs as part of local defense responses to plant infection by pathogenic microorganisms with contrasting pathogenesis strategies, with emphasis on the consequences of apoplastic PA oxidation on the ability of plant pathogens to colonize host tissues. For this purpose, experiments were carried out using tobacco as a plant host, and two different pathogens. One of them was the cosmopolitan and polifagous ascomycete Sclerotinia sclerotiorum, which infects more than 400 plant species. This fungus is considered a necrotroph because it kills host cells during the infection process and causes extensive tissue damage (Bolton et al., 2006
Accumulation of Free PAs in Leaf Extracts and Apoplastic Fluids of Tobacco Plants Infected by S. sclerotiorum Tobacco ‘Xanthi-nc’ leaves developed typical brownish necrotic lesions 24 h after inoculation (HAI) with S. sclerotiorum mycelium, and lesion size further increased as a function of time. Samples for free PA determination in leaf extracts and apoplastic fluids were taken from noninfected tissues adjacent to the lesions 48 HAI, and were found to be free of fungal mycelium by observation under a binocular microscope. In leaf extracts, putrescine and spermidine were the most abundant free PAs, their levels being not affected by fungal infection (Fig. 1A). A small but significant decrease of spermine level was detected in leaf extracts of inoculated plants, as compared with controls (Fig. 1A). When apoplastic PA levels were evaluated, an increase of free putrescine, spermidine, and spermine was detected in leaves infected by S. sclerotiorum, although the increase in spermidine levels lacked statistical significance (Fig. 1B). Thus, the levels of free putrescine in intercellular washing fluids (IWFs) of infected leaves increased by 7-fold, as compared with noninoculated controls. Apoplastic spermine level was below the detection limit in control leaves, and reached 0.013 nmol mL–1 in infected ones (Fig. 1B).
Leaf Infection by S. sclerotiorum Increases ADC Activity and Induces ADC Transcript Accumulation To determine the biochemical basis of the above-described changes in PA levels elicited by S. sclerotiorum infection, the activities of several enzymes involved in PA biosynthesis were determined. For this purpose, leaf samples were taken from noninfected tissues adjacent to the lesions 48 HAI, as described in the preceding section. ADC activity was 4-fold higher in extracts obtained from leaves infected by S. sclerotiorum than in controls (mock-inoculated) 48 HAI. (Fig. 2A). On the contrary, ODC and AdoMetDc activities were not affected by fungal infection (Fig. 2A). ADC, ODC, and AdoMetDc activities were also evaluated in IWFs and found to be extremely low, being <0.5% of the activities detected in leaf extracts (data not shown), thus suggesting that the apoplastic compartment makes no significant contribution to the activity of these enzymes detected in leaf extracts.
Given that ADC was the sole enzymatic activity found to be modified as a consequence of fungal infection, the steady-state levels of ADC mRNA were analyzed by semiquantitative reverse transcription (RT)-PCR and found to be higher in infected than in mock-inoculated leaves (Fig. 2B).
As demonstrated in previous sections, infection of tobacco leaves by S. sclerotiorum resulted in the induction of changes in free PA levels, both in leaf extracts and apoplastic fluids. To understand the consequences of changes in PA levels on the local response to fungal infection and further disease development, tobacco leaf discs with enhanced PA levels were used as a target for inoculation with S. sclerotiorum and subsequent determination of the percentage of necrotic tissue, as an indicator of disease severity. For this purpose, two alternative approaches were employed. The first approach aimed to modify cellular PA levels, and was based in the use of transgenic tobacco ‘Wisconsin W38’ TetR/Oat ADC plants that express an oat (Avena sativa) ADC gene under the control of a tetracycline-inducible promoter (Masgrau et al., 1997
The second approach for the modification of PA levels prior to inoculation with S. sclerotiorum aimed toward the modification of apoplastic PAs, and consisted of vacuum infiltration of leaf discs of wild-type Xanthi-nc tobacco plants with 5 µM solutions of putrescine, spermidine, and spermine in water. Infiltration with these PAs increased lesion size, respectively, by 12-, 18-, and 15-fold, as compared with control discs infiltrated with water (Fig. 4).
Activity and Inhibition of Amine-Oxidizing Enzymes in Tobacco Leaf Apoplast
DAO and PAO activities were detected in vitro using IWFs as a source of enzyme and PAs as substrates (Fig. 5, A and B), these activities being not affected by S. sclerotiorum infection (Supplemental Fig. S1A). A temporal analysis of DAO and PAO activities throughout the life cycle of tobacco plants was also performed, both activities being found to decline as plant age increased (Supplemental Fig. S2). To evaluate the participation of DAO and PAO in lesion development as a consequence of PA accumulation triggered by S. sclerotiorum infection, the ability of specific inhibitors to interfere with the activity of these enzymes was tested. Two of the inhibitors used were N,N'-diaminoguanidine, a compound reported to act as a competitive inhibitor of soybean (Glycine max) DAO (Nikolov et al., 1990
Given that no information was available regarding the ability of 1,19-bis(ethylamino)-5,10,15-triazanonadecane to inhibit PA oxidation in vivo, the effect of this compound on spermine oxidation was also evaluated in vivo, by infiltration of leaf discs and subsequent determination of apoplastic spermine levels, as well as those of 1,3-diaminopropane, one of the products of spermine oxidation. As expected, infiltration of leaf discs with spermine led to a significant increase in the apoplastic concentration of this tetraamine 4 h after infiltration, as compared with controls, in which spermine was undetectable (Fig. 6A). Disc infiltration with 1,19-bis(ethylamino)-5,10,15-triazanonadecane in combination with spermine resulted in higher apoplastic levels of this tetraamine, its concentration being 3-fold higher than in discs infiltrated only with spermine (Fig. 6A). Apoplastic spermine was undetectable in discs infiltrated only with 1,19-bis(ethylamino)-5,10,15-triazanonadecane, as occurred with controls (Fig. 6A). At this time point, 1,3-diaminopropane was not detected in IWFs obtained from leaf discs subjected to any infiltration treatment (data not shown).
When apoplastic PAs were evaluated 24 h after infiltration with spermine, this tetraamine decreased to undetectable levels (data not shown), while the level of 1,3-diaminopropane raised to 0.1 nmol mL–1 in control discs (Fig. 6B). At this time after infiltration, the level of apoplastic of 1,3-diaminopropane was 2-fold higher in discs infiltrated with spermine than in controls (Fig. 6B). Coinfiltration of spermine along with 1,19-bis(ethylamino)-5,10,15-triazanonadecane decreased apoplastic 1,3-diaminopropane concentration to levels similar to those of controls (Fig. 6 B). In discs infiltrated only with 1,19-bis(ethylamino)-5,10,15-triazanonadecane, apoplastic 1,3-diaminopropane levels were similar to those of controls (Fig. 6B). In summary, results obtained in this section demonstrated that N,N'-diaminoguanidine, 1,19-bis(ethylamino)-5,10,15-triazanonadecane, and guazatine are able to inhibit apoplastic PA oxidation in tobacco leaves.
Results presented in the previous section suggested that the use of amine oxidase inhibitors could contribute to determine whether apoplastic PA oxidation plays a role during plant defense against S. sclerotiorum infection. Taking into account that PA accumulation in the leaf apoplast led to enhanced severity of leaf rot caused by S. sclerotiorum, it was decided to evaluate whether this effect is mediated by PA oxidation. For this purpose, the compounds demonstrated in previous sections to act as tobacco amine oxidase inhibitors were used to evaluate their effect on leaf rot severity. Previously, the effect of these compounds on mycelial growth of S. sclerotiorum was tested in vitro. Concentrations up to 100 µM of N,N'-diaminoguanidine (data not shown) and 1,19-bis(ethylamino)-5,10,15-triazanonadecane (S. Maiale, unpublished data) proved to exert no effect on mycelial growth when added to the growth medium. Guazatine, on the contrary, strongly inhibited mycelial growth in concentrations as low as 5 µM (S. Maiale, unpublished data), which is in good agreement with the previously reported fungicidal effect of this compound (Mackintosh and Walters, 2003 As demonstrated in previous sections, infiltration with putrescine, spermidine, and spermine led to an increase in the percentage of leaf necrosis, as compared with controls infiltrated with no PAs (Fig. 7, A and B). In this way, infiltration with putrescine increased leaf necrosis by 2-fold. However, when putrescine was coinfiltrated with the DAO inhibitor N,N'-diaminoguanidine, the percentage of leaf necrosis was similar to that of control discs. Moreover, lesion size was reduced by 69% in discs infiltrated only with N,N'-diaminoguanidine, as compared with controls infiltrated with water (Fig. 7A). Infiltration with spermidine and spermine increased lesion size by 2- and 3-fold, respectively, as compared with controls. Spermidine and spermine-mediated enhancement of leaf necrosis was blocked by coinfiltration of these PAs along with the PAO inhibitor 1,19-bis(ethylamino)-5,10,15-triazanonadecane, being reduced to values as low as 45% and 35% of controls, respectively (Fig. 7B). When discs were infiltrated only with 1,19-bis(ethylamino)-5,10,15-triazanonadecane, necrosis decreased to 41% of controls (Fig. 7B).
Oxidation of Apoplastic PAs and Its Relation with the Generation of ROS Besides 1,3-diaminopropane, PA oxidation also leads to hydrogen peroxide formation. Taking into account the diversity and importance of the roles played by hydrogen peroxide and other reactive oxygen species (ROS) in plant defense against pathogen attack, it was decided to evaluate whether oxidation of apoplastic PAs contributes to ROS accumulation in the apoplastic compartment of tobacco leaves. In this way, hydrogen peroxide production by leaf discs infiltrated with 5 µM putrescine, spermidine, and spermine was evaluated using 3,3'-diaminobenzidine, no accumulation of this active oxygen species being detected (data not shown). In addition, superoxide generation in leaf discs infiltrated with putrescine and spermine was evaluated, using nitro blue tetrazolium (NBT). Infiltration with either 5 µM putrescine or spermine led to superoxide accumulation in the intercellular spaces, as evidenced by the formation of a blue formazan precipitate. In this way, superoxide levels in discs infiltrated with these PAs were 2-fold higher than in water-infiltrated controls, in which superoxide levels were very low (Fig. 8). Coinfiltration of discs with 5 µM putrescine and 20 µM N,N'-diaminoguanidine reduced superoxide formation to levels similar to those of controls (Fig. 8). Similarly, coinfiltration of 5 µM spermine with 20 µM 1,19-bis(ethylamino)-5,10,15-triazanonadecane or 20 µM guazatine also reduced superoxide generation to levels similar to those of controls (Fig. 8).
Changes in Apoplastic PA Levels and Amine-Oxidizing Enzymes in Response to Infection by the Biotrophic Bacterial Pathogen P. viridiflava As a whole, results obtained in previous sections demonstrated that infection of tobacco leaves by the necrotrophic fungus S. sclerotiorum induces PA accumulation in the apoplastic compartment, which results in PA oxidation and thus increases leaf rot severity. This prompted us to evaluate whether similar responses operate during infection of tobacco leaves by biotrophic pathogens. For this purpose, tobacco plants were infiltrated with the bacterium P. viridiflava. Samples for the determination of free PAs and PAO and DAO activities in leaf extracts and apoplastic fluids were taken from tissues adjacent to the infiltration zone, 20 and 48 HAI. Prior to using IWFs for the above-mentioned determinations, bacteria were eliminated by centrifugation to minimize contamination of this fraction with metabolites from bacterial origin. The IWFs obtained in this way were free of bacterial cells, as demonstrated by the absence of bacterial colonies after inoculation of 20 µL aliquots onto solid King's B medium. No changes in PA levels were evident in leaf extracts of inoculated plants at both HAI times, as compared with controls (data not shown). On the contrary, changes in apoplastic spermine levels were evident in leaves infected by P. viridiflava, the levels of this tetraamine being 4- and 10-fold higher in leaves infected by P. viridiflava than in controls, when evaluated 20 (Fig. 9A) and 48 (Fig. 9B) HAI, respectively. As occurred after leaf infection by S. sclerotiorum, DAO and PAO activities were not affected by P. viridiflava infection (Supplemental Fig. S1B).
PA Accumulation and Oxidation Decreases Leaf Colonization by P. viridiflava The effect of increased PA levels on P. viridiflava multiplication in planta was evaluated by two alternative approaches similar to those previously described for the determination of leaf rot severity caused by S. sclerotiorum. The first approach consisted of tetracycline-mediated induction of heterologous ADC gene expression in leaf discs of transgenic tobacco W38 Tet/Oat ADC plants, which results in 2- and 3-fold increases in putrescine and spermidine levels, respectively, as described in previous sections. Bacterial growth was strongly reduced in tetracycline-induced discs of W38 Tet/Oat ADC plants, as compared with noninduced discs of the same transgenic line, in which 3.3 x 109 colony-forming units (CFU) mL–1 were detected (Fig. 10). Incubation in tetracycline of leaf discs obtained from the transgenic W38 TetR line, which expresses no heterologous ADC gene, had no effect on bacterial growth, as compared with discs of the same line incubated in the absence of the antibiotic (Fig. 10). In this way, the transgenic W38 TetR line served to verify that tetracycline itself does not inhibit bacterial growth in planta.
The second approach for the modification of tobacco PA levels prior to inoculation with P. viridiflava was based on vacuum infiltration of leaf discs obtained from wild-type Xanthi-nc plants with a 5 µM solution of spermine in water. Infiltration with this tetraamine strongly decreased bacterial growth in inoculated discs, as compared with controls infiltrated with water (Fig. 11). Inclusion of both 1,19-bis(ethylamino)-5,10,15-triazanonadecane and spermine in the infiltration solution partly reverted the inhibitory effect of spermine on bacterial growth in planta, this parameter reaching an intermediate value between that of discs infiltrated only with spermine and controls (Fig. 11). Moreover, infiltration of discs with 1,19-bis(ethylamino)-5,10,15-triazanonadecane and no spermine resulted in higher bacterial growth than in controls infiltrated with water, this treatment leading to the highest bacterial growth in planta, as compared to all the treatments evaluated (Fig. 11).
Infection by pathogenic microorganisms has been reported to induce modifications in PA metabolism in different plant hosts (for review, see Walters, 2003 ), which in many cases consisted of PA accumulation, either in their free or conjugated forms. Although the understanding of the physiological significance of pathogen-induced changes in PA metabolism of plants is far from being complete, PAs have been shown to play several roles in plant defense against pathogen attack. Conjugated PAs exert antimicrobial effects (Peng and Kuc, 1992; Walters et al., 2001), and the oxidation of free PAs has been proposed to contribute to cell-death processes occurring during the HR of tobacco plants to TMV (Marini et al., 2001). In addition, the tetraamine spermine acts as an inducer of pathogenesis-related proteins (Yamakawa et al., 1998) and also participates in signaling cascades that activate HR-specific genes in tobacco (Takahashi et al., 2003, 2004; Kusano et al., 2007). In this work, the consequences of apoplastic PA oxidation on host tissue colonization by microorganisms with contrasting pathogenic strategies, and its relation with plant tolerance to these kinds of pathogens, were investigated.
Studies conducted so far in relation with the role of PAs in plant-pathogen interactions have focused mainly on viruses (Negrel et al., 1984
To evaluate the consequences of changes in host PA levels on tolerance to pathogen infection, tobacco leaf discs with enhanced PA levels were infected with S. sclerotiorum. Accumulation of free putrescine and spermidine in leaf discs of transgenic tobacco plants as a consequence of tetracycline-induced overexpression of an oat ADC gene, led to a strong increment in the necrosis caused by S. sclerotiorum infection. Similarly, enhancement of apoplastic putrescine, spermidine, or spermine levels by means of infiltration also caused a strong increase of leaf disc necrosis caused by this fungus. Thus, the two above-mentioned alternative approaches demonstrated that enhanced PA levels increase the severity of S. sclerotiorum infection. The question then rose about the mechanism by which PA accumulation in plant tissues increases disease severity. Yoda et al. (2003)
The DAO inhibitor N,N'-diaminoguanidine was found to attenuate the development of necrotic lesions caused by S. sclerotiorum infection, both in leaf discs infiltrated with putrescine and in control discs infiltrated with water. Similarly, the PAO inhibitor 1,19-bis(ethylamino)-5,10,15-triazanonadecane diminished S. sclerotiorum rot in leaf discs infiltrated with spermidine and spermine, as well as in control discs infiltrated with water. As a whole, these results demonstrate that DAO and PAO-mediated PA oxidation occurs during infection of tobacco tissues by S. sclerotiorum and, far from being beneficial to the host, contributes to disease development. This observation is in good agreement with previous works that found oxidative responses to increase the severity of infections caused by necrotrophic pathogens in plant hosts (Govrin and Levine, 2000
Hydrogen peroxide is known to play several roles in plant defense against pathogen attack. In addition to the proposed direct antimicrobial effect (Mehdy et al., 1996 As opposed to hydrogen peroxide, superoxide radical was found to be accumulated when leaf discs were infiltrated with PA concentrations capable of increasing leaf necrosis caused by S. sclerotiorum. PA-induced accumulation of superoxide was decreased by amine oxidase inhibitors, thus demonstrating that production of this active oxygen species was the result of PA oxidation. In this way, superoxide produced after PA accumulation and oxidation in the apoplast probably contributes to the development of leaf necrosis during infection by S. sclerotiorum, as a consequence of the toxicity of this active oxygen species on host tissues.
The way in which superoxide was produced after PA oxidation remains to be explained. A possible explanation is that hydrogen peroxide is converted into superoxide by the nonenzymatic reactions of Fenton and Haber-Weiss (Fenton, 1894
Results obtained in this work demonstrate that tobacco plants respond to S. sclerotiorum infection by accumulating PAs in the leaf apoplast and also that PA oxidation leads to enhanced cell death. Thus, apoplastic PA accumulation and oxidation, far from being beneficial to the host, contributed to increased disease severity. It is not surprising that such a response enhances the development of lesions caused by a necrotrophic fungus, given that these kinds of pathogens feed on necrotic tissue. To our knowledge, this is the first report about PA accumulation and oxidation playing a detrimental role in plant responses to pathogen infection. The question then arose whether responses of tobacco PA metabolism to infection by a biotrophic pathogen are similar to those developed in response to a necrotrophic one. Thus, to address this question, experiments similar to those performed with S. sclerotiorum were done using P. viridiflava as a pathogen. No previous information was available regarding the role of host PA metabolism in response to biotrophic bacteria. As do many other biotrophic bacteria, P. viridiflava propagates in the apoplastic space after penetration into the host. In this work, this bacterium was found to induce spermine accumulation in apoplastic fluids, as evaluated 20 and 48 HAI. It is worth pointing out that spermine was not detected in P. viridiflava extracts (data not shown), which suggests that the accumulation of this tetraamine in infected tissues resulted from the host's metabolic machinery, not from bacterial origin. Moreover, IWFs were freed from bacteria by centrifugation prior to the determination of PA levels, thus minimizing the risk of sample contamination with bacterial metabolites. Deliberate modification of leaf PA levels prior to infection either by using leaf discs from the transgenic plants previously described or by PA infiltration, demonstrated that enhanced host PA levels result in a significant decrease of bacterial growth in planta. More specifically, it was demonstrated that spermine infiltration inhibited bacterial growth, this effect being due to PAO-mediated oxidation of this tetraamine. This hypothesis is supported by the observation that the PAO inhibitor 1,19-bis(ethylamino)-5,10,15-triazanonadecane reverted the inhibition of bacterial growth caused by spermine infiltration. Moreover, 1,19-bis(ethylamino)-5,10,15-triazanonadecane decreased bacterial growth in discs not infiltrated with exogenous spermine. As a whole, these results demonstrate that PA accumulation and further oxidation contribute to restrict the propagation of P. viridiflava in tobacco tissues.
Taking into account the previously discussed roles of PA oxidation in the generation of hydrogen peroxide and superoxide radical, it is tempting to speculate that active oxygen species derived from spermine oxidation are involved in the reduction of P. viridiflava growth in tobacco leaves. However, it cannot be ruled out that spermine plays additional roles during the response of tobacco plants to P. viridiflava infection, as well as in response to S. sclerotiorum. Spermine has been shown to act as a systemic inducer of PR gene expression (Yamakawa et al., 1998
PA metabolism of tobacco plants undergoes similar, although not identical, changes in response to infection by two pathogens with opposing pathogenic strategies. These changes lead to the accumulation of apoplastic free putrescine and spermine in response to infection by the necrotrophic fungus S. sclerotiorum, and free spermine in response to the biotrophic bacterium P. viridiflava, these PAs acting as substrates of amine oxidases. PA accumulation and oxidation play a role that is beneficial for the host during local responses to infection by the biotrophic bacterium P. viridiflava, by restricting bacterial growth in planta. On the contrary these responses are detrimental in local defense to infection by the necrotrophic fungus S. sclerotiorum, and lead to increased disease severity. It still remains to be determined whether reactions similar to those found in this work are of general occurrence in other plant hosts infected by necrotrophic and biotrophic pathogens. However, the findings of this work are in good agreement with the fact that host responses leading to cell death contribute to restricting infection by biotrophic pathogens, but play a negative role in local responses to necrotrophic organisms. As a summary, it can be concluded that apoplastic free PAs play an important role in local responses to pathogen infection by affecting the ability of pathogenic microorganisms to colonize host tissues. This work also demonstrates that modification of plant PA metabolism by means of a transgenic approach based on the use of PA biosynthetic genes under the control of an inducible promoter can contribute to modifying plant responses to pathogen infection.
Fungal and Bacterial Strains
An isolate of Sclerotinia sclerotiorum from the IIB-INTECH Fungal Culture Collection (IFCC 458/02), originally isolated from sunflower (Helianthus annuus) capitula with head rot symptoms, was used for all the experiments. The fungus was routinely maintained in potato-dextrose agar slants at 4°C. Prior to inoculation on tobacco (Nicotiana tabacum), mycelium was grown in solid Czapek-Dox medium (50 g L–1 Glc, 2 g L–1 NaNO3, 1 g L–1 KH2PO4, 0.5 g L–1 MgSO4.7H2O, 0.5 g L–1 KCl, 0.05 g L–1 FeSO4.7H2O, 20 g L–1 agar, pH 5.5–6.0) at 24°C. Pseudomonas viridiflava strain Pvalb8 (Alippi et al., 2003
Tobacco (Wisconsin W38 and Xanthi-nc) seeds were disinfected with 70% (v/v) ethanol for 2 min, 5% (w/v) sodium hypochlorite, and 0.1% (w/v) SDS for 15 min and thoroughly rinsed with sterile distilled water. Disinfected seeds were dispensed in petri dishes containing Murashigue and Skoog agar medium (Murashigue and Skoog, 1962
Inoculation with S. sclerotiorum
Inoculation with P. viridiflava
Standard chemicals and guazatine acetate of the highest purity available were purchased from Sigma Chemical. 1,19-Bis(ethylamino)-5,10,15-triazanonadecane was kindly gifted by Dr. Benjamin Frydman (SLIL Biomedical Corporation). N,N'-Diaminoguanidine monohydrochloride was purchased from ICN Biomedicals. L-[1-14C]Orn was purchased from NEN Life Sciences Products. L-[1-14C]Arg and S-adenosyl-L-[carboxyl-14C]Met were purchased from Amersham Pharmacia Biotech. Inhibitors and PAs were dissolved in distilled water and stored at –20°C until used.
Superoxide (O2·–) accumulation in tobacco tissues was determined with NBT, which reacts with O2·– producing a blue formazan precipitate. Leaf discs were infiltrated with 5 µM putrescine and 5 µM N,N'-diaminoguanidine monohydrochloride, either separately or in combination. Another set of leaf discs was incubated with 5 µM spermine, either separately or in combination with 5 µM each of the amine oxidase inhibitors 1,19-bis(ethylamino)-5,10,15-triazanonadecane and 5 µM guazatine. Discs infiltrated with water were used as controls of O2·– accumulation in the absence of chemical treatments. NBT was added to all the infiltration solutions at a final concentration of 0.01% (w/v). After infiltration, discs were incubated in the infiltration solution for 3 h in the dark at 30°C with gentle shaking. Stained leaf discs were mounted on glass slides and scanned. Images thus obtained were inverted to render a negative prior to transformation into black and white 8-bit images. Formazan color intensity (in a negative corresponding to the lighter tones of gray) was determined by using the Image-ProPlus V 4.1 software (Media Cybernetics). Discs infiltrated with 10 mM MnCl2, a highly effective O2·– dismutating catalyst agent, developed no blue formazan precipitate, thus confirming the specificity of the method for the detection of O2·–.
IWFs were collected from tobacco leaves as described by De Wit and Spikman (1982)
To determine free PA levels in leaf extracts, plant material (300 mg) was ground in liquid nitrogen, extracted in 600 µL 5% (v/v) perchloric acid, and incubated overnight at 4°C. After centrifugation at 10,000g for 15 min, 10 µL of 100 µM 1,7-heptanediamine (ICN Biomedicals) was added as internal standard to 200 µL aliquots of leaf extracts. For PA determinations in the apoplastic compartment, IWFs obtained as described in the previous section were mixed with perchloric acid to a final concentration of 5% (v/v). Then, 200 µL of saturated Na2CO3 and 400 µL of dansyl chloride (10 mg mL–1 acetone) were added and the mixture was incubated overnight in the dark at room temperature. Reaction was stopped by adding 100 µL of Pro (100 mg mL–1) and dansylated PAs were extracted in 500 µL of toluene. Organic phase was vacuum evaporated and dansylated PAs were dissolved in 200 µL of acetonitrile and analyzed by reversed phase HPLC as described previously (Marcé et al., 1995
ADC, ODC, and AdoMetDC were extracted by homogenization of leaf tissue (500 mg fresh weight) in 2 volumes of 100 mM, pH 7.5, phosphate buffer containing 0.5 mM EDTA, 10 mM dithiothreitol, 1 mM pyridoxal phosphate, and 20 mM sodium ascorbate. Crude extracts thus obtained were clarified by centrifugation at 10,000g for 10 min. All the above-described procedures were carried out at 4°C. Protein concentration in the supernatants was determined as described by Bradford (1976) For the determination of DAO activity, 190 µL aliquots of IWFs obtained as described previously were mixed with 10 µL of a substrate solution containing 10 mM nonradioactive putrescine and 5 nCi µL–1 [1,4-14C]putrescine. A similar procedure was used for the determination of PAO activity, by using a substrate solution consisting of 10 mM nonradioactive spermine and 5 nCi µL–1 [1,4-14C]spermine. For inhibition of amine oxidases, guazatine acetate, 1,19-bis(ethylamino)-5,10,15-triazanonadecane, and N,N'-diaminoguanidine monohydrochloride were added to the reaction mixture in a final concentration of 0.5 mM.
Total RNA was extracted from frozen leaf tissue using TRI reagent (Sigma) according to the manufacturer's instructions. First strand of cDNA was obtained by using the following mixture: 2 µg of total RNA, 0.5 mM dNTPs, 1 µL of moloney murine leukemia virus RT (200 units µL–1; Promega), 5 µL 5x reaction buffer (250 mM Tris-HCl, 375 mM KCl, 15 mM MgCl2, 50 mM DTT, pH 8.3), 10 pmol of oligo(dT) primer (Biodynamics SRL), and distilled water up to a total volume of 25 µL. The reaction mixture was incubated at 37°C for 1 h. PCR amplification was done with 2 µL of RT reaction as template, 4 µL of 10x Taq polymerase buffer (500 mM KCl, 100 mM Tris-HCl, 15 mM MgCl2, pH 9.0), 0.05 mM dNTPs, 0.2 µL Taq polymerase (5 units µL–1; Promega), 12.5 pmol of primers 5'-TGGCCTTCAGGAGTATGCATC-3' (sense), and 5'-CCACGAATAGCAGCAGCAGA-3' (antisense) corresponding to NtADC1 (GenBank accession no. AF127240), or 5'-GGATTCTGGTGATGGTGTTAG-3' (sense) and 5'-ACTTCCTCTCAGGTGGAGCTAC-3' (antisense) corresponding to tobacco actin (GenBank accession no. X63603), in a total volume of 40 µL. Amplification conditions for both genes were 20, 24, 27, and 30 cycles (94°C, 1 min; 58°C, 1 min; and 72°C, 1 min) and a final elongation at 72°C for 7 min. Amplification products were visualized by staining gels with ethidium bromide and equal loading of RNAs was confirmed by monitoring the levels of actin gene. Bands were analyzed by densitometry using the Gel Pro Analyzer 3.0 software (Media Cybernetics).
PAs (putrescine, spermidine, and spermine), guazatine acetate, 1,19-bis(ethylamino)-5,10,15-triazanonadecane, and N,N'-diaminoguanidine monohydrochloride were infiltrated into the apoplastic space of leaf discs (18-mm diameter) using a vacuum pump (2 cycles, 667 mbar, 20 min). All these chemicals were employed at a concentration of 5 µM. After infiltration, discs were placed in petri dishes containing water-agar and incubated in the plant growth chamber during the time periods indicated for each experiment.
Leaf discs obtained from transgenic tobacco Wisconsin W38 TetR/Oat ADC plants, which express an oat (Avena sativa) ADC gene under the control of the tetracycline-inducible promoter TetR (Masgrau et al., 1997
Treatments consisted of three to five replicates, and each experiment was conducted at least two times with similar results. Results from representative experiments are shown as means ± SD. Data were analyzed by appropriate Student's t test or ANOVA followed by posthoc comparisons by Bonferroni or Dunnet's test.
The following materials are available in the online version of this article.
F.L.P. and O.A.R. are members of Comisión de Investigaciones Científicas de la Provincia de Buenos Aires (CIC) and Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), respectively. M.M. and S.J.M. are postdoctoral fellows of CONICET and Agencia Nacional de Promoción Científica y Tecnológica, respectively. The authors are very grateful to Antonio F. Tiburcio and Teresa Altabella (Facultad de Farmacia, Universidad de Barcelona) for kindly providing transgenic tobacco Wisconsin W38 TetR/Oat ADC and tobacco Wisconsin W38 TetR plants, as well as for helpful advice. The gift of 1,19-bis(ethylamino)-5,10,15-triazanonadecane by Dr. Benjamin Frydman (SLIL Biomedical Corporation) is greatly appreciated. Received May 21, 2008; accepted June 15, 2008; published June 26, 2008.
1 This work was supported by grants from Consejo Nacional de Investigaciones Científicas y Técnicas (PIP 5740–CONICET, Argentina) and Agencia Nacional de Promoción Científica y Tecnológica (PICT 04–26517, ANPCYT, Argentina). 
2 Present address: Laboratory of Gene Regulation and Development, National Institutes of Health, Building 6A, Room B1A13, Bethesda, MD 20892–2716. The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Fernando Luis Pieckenstain (pieckenstain{at}intech.gov.ar).
[W] The online version of this article contains Web-only data. www.plantphysiol.org/cgi/doi/10.1104/pp.108.122614 * Corresponding author; e-mail pieckenstain{at}intech.gov.ar.
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ebela et al., 2001
0.05; ***, P 
Pyrroline and 1,5-diazabicyclononane are products of DAO and PAO activities, respectively. The inhibitors N,N'-diaminoguanidine (AG), 1,19-bis(ethylamino)-5,10,15-triazanonadecane (SL-11061), and guazatine (G) were added to the reaction mixture at a final concentration of 0.5 mM. Different letters indicate statistically significant differences (P 
50) were vacuum infiltrated (two cycles, 667 mbar, 20 min) with distilled water, while 0.1 M, pH 8.0, Tris-HCl buffer with 0.5 mM phenylmethanesulfonyl fluoride was used for the infiltration of discs to be used for the determination of DAO and PAO. After gently drying on filter paper, leaf discs were introduced into a 10-mL syringe and IWFs were recovered by centrifugation for 20 min at 2,000g (4°C) in a 50-mL Nalgene centrifuge tube. Chlorophyll and cytosolic nonphosphorylating glyceraldehyde-3-phosphate dehydrogenase activity were determined in IWFs as described by Lichtenthaler (1987)
B by polyamines in breast cancer cells. Biochemistry 38: 14763–14774
-aminobutyric acid induced by salt stress through stimulating the activity of diamine oxidases in Glycine max (L.) Merr. roots. Plant Physiol Biochem 45: 560–566
