Ozone-induced ethylene production is dependent on salicylic acid, and both salicylic acid and ethylene act in concert to regulate ozone-induced cell death

Authors

  • Mulpuri V. Rao,

    1. Paradigm Genetics, Inc., 108 Alexander Drive, Research Triangle Park, NC 27709, and
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  • Hyung-il Lee,

    1. Biotechnology Center, Cook College-Foran Hall, Rutgers University, New Brunswick, NJ 08901, USA
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    • Present address: Sunol Molecular Corporation, 2810 North Commerce Parkway, Mirama, Florida 33025, USA.

  • Keith R. Davis

    Corresponding author
    1. Paradigm Genetics, Inc., 108 Alexander Drive, Research Triangle Park, NC 27709, and
      * For correspondence (fax +919 485 8789; e-mail kdavis@paragen.com).
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* For correspondence (fax +919 485 8789; e-mail kdavis@paragen.com).

Summary

Ethylene is known to influence plant defense responses including cell death in response to both biotic and abiotic stress factors. However, whether ethylene acts alone or in conjunction with other signaling pathways is not clearly understood. Ethylene overproducer mutants, eto1 and eto3, produced high levels of ethylene and developed necrotic lesions in response to an acute O3 exposure that does not induce lesions in O3-tolerant wild-type Col-0 plants. Treatment of plants with ethylene inhibitors completely blocked O3-induced ethylene production and partially attenuated O3-induced cell death. Analyses of the responses of molecular markers of specific signaling pathways indicated a relationship between salicylic acid (SA)- and ethylene-signaling pathways and O3 sensitivity. Both eto1 and eto3 plants constitutively accumulated threefold higher levels of total SA and exhibited a rapid increase in free SA and ethylene levels prior to lesion formation in response to O3 exposure. SA pre-treatments increased O3 sensitivity of Col-0, suggesting that constitutive high SA levels prime leaf tissue to exhibit increased magnitude of O3-induced cell death. NahG and npr1 plants compromised in SA signaling failed to produce ethylene in response to O3 and other stress factors suggesting that SA is required for stress-induced ethylene production. Furthermore, NahG expression in the dominant eto3 mutant attenuated ethylene-dependent PR4 expression and rescued the O3-induced HR (hypersensitive response) cell death phenotype exhibited by eto3 plants. Our results suggest that both SA and ethylene act in concert to influence cell death in O3-sensitive genotypes, and that O3-induced ethylene production is dependent on SA.

Introduction

Rapid changes in urbanization and human activities during the 21st century have substantially increased tropospheric O3 concentrations to levels that threaten cultivated crop and forest species (Heath and Taylor, 1997). Numerous laboratory and field studies have demonstrated that acute O3 exposure can cause significant reductions in plant growth and induce necrotic lesions, reminiscent of lesions activated during plant–pathogen interactions (reviewed in Rao et al. 2000a). Historically, O3 is believed to affect plant metabolism by oxidizing cellular membranes (Heath and Taylor, 1997; Mudd, 1997; Rao et al. 2000a); however, recent studies suggest that O3 acts by inducing a hypersensitive response (HR) similar to that induced by pathogens and other stress factors (Rao et al., 2000a; Sandermann et al., 1998; Sharma et al., 1996). Activation of an oxidative burst is one of the earliest responses of plants to pathogen infection and is an essential component of pathogen-induced HR cell death (Jabs, 1999; Lamb and Dixon, 1997). Similar to plant pathogens, O3 has been shown to activate an oxidative burst and the concomitant accumulation of active oxygen species (AOS) such as superoxide radical (O2) and hydrogen peroxide (H2O2) (Overmyer et al., 2000; Pellinen et al., 1999; Rao and Davis, 1999; Rao et al. 2000b; Schraudner et al., 1998).

O3-induced AOS are believed to activate distinct signaling pathways dependent on salicylic acid (SA), jasmonic acid (JA), and ethylene, and induce a wide array of defense reactions including HR cell death (Overmyer et al., 2000; Rao et al., 2000a,b; Sandermann et al., 1998; Sharma et al., 1996). Although SA has been shown to be a major regulator of plant defense reactions including cell death, other studies suggest that antagonistic and/or synergistic interactions between SA-, JA-, and/or ethylene-signaling pathways influence plant cell death (Asai et al., 2000; Overmyer et al., 2000; Rao et al., 2000b). Plants produce ethylene in response to several biotic and abiotic stress factors and the involvement of ethylene in wide range of stress responses led to the speculation that ethylene is a major regulator of plant defense responses including cell death (Johnson and Ecker, 1998; Kieber, 1997). Ethylene has been shown to modulate plant cell death induced by pathogens (Moore et al., 1999; O'Donnell et al., 2001), O3 exposure (Bae et al., 1996; Overmyer et al., 2000; Tuomainen et al., 1997), hypoxia (He et al., 1996), and organ senescence (Quirino et al., 2000). O3 exposure induces genes involved in ethylene biosynthesis (Miller et al., 1999; Overmyer et al., 2000; Tuomainen et al., 1997; Vahala et al., 1998) and the aminocyclopropane-1-carboxylic acid synthase-mediated conversion of S-adenosyl-l-methionine to 1-aminocyclopropane-1-carboxylic acid is the rate limiting step of ethylene biosynthesis in O3-exposed plants (Tuomainen et al., 1997). Although several studies support a casual relationship between ethylene levels and plant sensitivity to O3 (Mehlhorn and Wellburn, 1987; Wellburn and Wellburn, 1996), it is not clear whether ethylene acts alone or in conjunction with other signaling pathways.

In plants, two major forms of cell death have been characterized in detail, HR cell death and senescence (Quirino et al., 2000). Because both HR and senescence are genetically defined cell death programs and bear several similarities, it has been speculated that common steps might exist for the induction and/or execution of these two processes (Pontier et al., 1999; Quirino et al., 2000). Since O3 induces both ethylene and SA biosynthesis (Miller et al., 1999; Overmyer et al., 2000; Rao et al., 2000a; Vahala et al., 1998), we speculated that a synergistic interaction between SA and ethylene may modulate the magnitude of lesion formation in response to O3 exposure. Therefore, we have sought to investigate the relationship between SA- and ethylene-signaling pathways in Arabidopsis plants exposed to O3. Here, we report that functional SA-signaling pathways are required for O3-induced ethylene biosynthesis and that both SA and ethylene act in concert to modulate the magnitude of O3-induced HR-like cell death. Furthermore, we also demonstrate that the previously described ethylene overproducer (eto) mutants have an altered stress-ethylene phenotype at postseedling stages of development.

Results and discussion

Over the past several decades, studies have documented a casual relationship between high levels of ethylene and plant sensitivity to O3 (Mehlhorn and Wellburn, 1987; Wellburn and Wellburn, 1996). To further investigate the role of ethylene and plant sensitivity to O3, we used Arabidopsis mutants, eto1 and eto3. Etiolated seedlings of these mutants have been shown to overproduce ethylene (Guzman and Ecker, 1990; Kieber et al., 1993). As shown in Figure 1(a), eto1 and eto3 plants exposed to 300 ppb O3 for 6 h rapidly developed necrotic lesions within 24 h after the initiation of O3 exposure. No major changes were observed in wild-type Col-0 plants exposed to O3.

Figure 1.

Ozone-induced necrotic lesions (a) and ethylene production (b) in Arabidopsis genotypes.

(a) All experiments were conducted three times with identical results and a representative example of each genotype from the same experiment is shown. Pictures were taken 24 h after initiation of O3 exposure.

(b) Ethylene measurements were performed by removing plants from O3 chamber at the indicated time intervals and incubating in air-tight glass containers for 2 h. About 1 ml head space was removed in duplicate and used for ethylene measurements. Plants maintained in O3-free air served as controls. All experiments were repeated with two replicates and the data from one representative experiment is presented. Error bars indicate SD.

The observations that untreated soil-grown eto1 and eto3 plants produce wild-type levels of ethylene (Woeste et al., 1999) prompted us to investigate whether the O3 sensitivity exhibited by treated eto plants is related to increased ethylene biosynthesis. Control (untreated) plants of Col-0, eto1, eto3 produced similar levels of ethylene. However, compared to untreated control plants, ethylene levels in Col-0 plants exposed to O3 for 3 h increased by ninefold, whereas the ethylene levels in eto1 and eto3 increased by 26- and 27-fold, respectively (Figure 1b). These findings demonstrate a strong correlation between high levels of ethylene production and plant sensitivity to O3. In addition, these results demonstrate, for the first time, that the eto1 and eto3 mutants grown in soil under light conditions overproduce ethylene in response to stress conditions.

Treatment of plants with inhibitors of ethylene perception or biosynthesis attenuated the magnitude of O3-induced lesions in various plant species (Bae et al., 1996; Tuomainen et al., 1997). Therefore, we used two inhibitors of ethylene biosynthesis, aminoethoxyvinyl-glycine (AVG) and cobalt chloride (CoCl2) to verify whether high levels of ethylene produced by eto1 and eto3 plants in response to O3 exposure are important for O3-induced cell death. Both AVG and CoCl2 have been shown to inhibit ethylene biosynthesis by inactivating the activities of ACC synthase and ACC oxidase, respectively (Kende, 1993; Yang and Hoffman, 1984). A previously described conductivity assay was used to obtain a more quantitative measure of the decrease in cell death exhibited by plants treated with the inhibitors of ethylene biosynthesis (Rao and Davis, 1999). Pre-treatment of plants with 1 mm AVG or 1 mm CoCl2, 20 h before exposure to O3, attenuated O3-induced ethylene production by >90% in both eto1 and eto3 plants compared to untreated plants exposed to O3 (Figure 2). O3-induced ion leakage of eto1 and eto3 plants pre-treated with ethylene inhibitors was reduced by 25–30% compared to untreated plants exposed to O3. No major changes were observed in the ion leakage of wild-type Col-0 plants irrespective of the treatment (Figure 2). These results demonstrate that increased ethylene production is at least partially responsible for O3-induced cell death.

Figure 2.

The effect of ethylene inhibitors on O3-induced ethylene production and cell death in Arabidopsis genotypes. Ethylene measurements were performed on plants exposed to O3 for 3 h as described in Figure 1(b). Ion leakage was estimated by punching leaf discs from plants that were allowed to recover for 2 h in O3-free air after the completion of a 6-h O3 exposure. A total of six leaf discs, 1 cm in diameter, were excised from third and fourth rosette leaves of six different plants and incubated in 5 ml of water for 6 h at room temperature, prior to measuring the conductivity. Mean values marked with asterisk are significantly different compared to their respective control plants (P < 0.05).

Earlier studies demonstrated that the antagonistic action of SA- and JA- signaling pathways regulate the magnitude of O3-induced HR cell death (Rao and Davis, 1999; Rao et al. 2000b), while other studies showed that JA and ethylene act synergistically in inducing plant defense responses (Maleck and Dietrich, 1999; Pieterse and Van Loon, 1999; Thomma et al., 1998). Therefore, we investigated whether perturbations in SA- and/or JA-signaling pathways may have pre-disposed eto1 and eto3 plants to produce high levels of ethylene production by monitoring the responses of molecular markers of SA-, ethylene-, and JA-signaling pathways. No major changes were observed in the steady-state levels of SA-dependent PR1 (Sharma et al., 1996), ethylene-dependent PR4 (Lawton et al., 1994), and JA-dependent PDF1.2 mRNA transcripts (Penninckx et al., 1996) between control plants of all genotypes. However, a 6-h O3 exposure induced an increase the transcript levels of PR1 and PR4 in eto1 and eto3 plants to a greater magnitude, compared to wild-type Col-0 plants exposed to O3 (Figure 3a). In contrast, exposure of Col-0, eto1, and eto3 plants to O3 induced the expression of JA-dependent PDF1.2 transcripts (Figure 3a) to similar levels in all the genotypes. Although O3 exposure caused an increase in JA biosynthesis, the magnitude of induction was similar in Col-0, eto1, and eto3 plants (15.8-, 16.5-, and 17.2-fold, respectively, compared to control plants; data not shown), suggesting that O3-induced changes in JA-dependent molecular marker (PDF1.2) is reflective of changes in JA signaling. These results suggest that the O3 sensitivity of eto1 and eto3 plants is associated with the hyperinduction of both SA- and ethylene-signaling pathways, and independent of changes in JA signaling (Figure 3a).

Figure 3.

Ozone-induced changes in defense gene expression (a) and SA (b) in Arabidopsis genotypes.

(a) Rosette leaves harvested and the RNA blots were hybridized with the probes indicated on the left of the figure. Hybridized filters were exposed to Phosphorimager screens (Molecular Dynamics, Sunnyvale, CA, USA) and the signal intensity was quantified using Molecular Dynamics ImagQuant Software. The data shown is corrected for RNA loading differences determined by re-probing the blots with a control 28S ribosomal gene (Rao and Davis, 1999). The experiments were repeated with similar results and the relative counts from a representative experiment are presented.

(b) Whole rosette leaves were harvested and used for free and total SA measurements. The data shown are the averages of two independent experiments with two replicates in each and represent the mean ±SD. Mean values with same letter are not significantly different (P < 0.05).

Since ethylene has been shown to increase plant sensitivity to SA (Lawton et al., 1994), we tested whether the significant induction of PR1 transcripts observed in eto1 and eto3 plants exposed to O3 is related to increased SA biosynthesis and/or to increased plant sensitivity to SA. Control plants of both eto1 and eto3 mutants accumulated free and total SA levels that are greater by 50% and threefold, respectively, compared to wild-type Col-0 (Figure 3b). Further, a 6-h O3 exposure increased free SA levels of Col-0, eto1, and eto3 plants by 2.5-, 5-, and 6.4-fold, respectively, compared to control plants. Although the magnitude of O3-induced increase in total SA content was almost identical (∼threefold) in all three genotypes, the absolute amounts of total SA in eto1 and eto3 plants was significantly greater than the levels detected in Col-0 plants (Figure 3b). These findings suggest that the hyperinduction of PR1 transcripts observed in eto mutants exposed to O3 is related to increased SA biosynthesis. This conclusion is supported by observations that pre-treatment of plants with 1 mm AVG did not significantly alter the magnitude of O3-induced PR1 transcripts in Col-0, eto1, and eto3 plants (data not shown).

Measurements of SA levels presented indicates a relationship between high levels of SA and increased sensitivity of eto1 and eto3 mutants to O3 (Figure 3b). Studies with various cell death mutants have demonstrated that constitutive high SA levels induce HR-related defense responses including lesions (Dangl et al., 1996; Greenberg et al., 2000; Rate et al., 1999). Further, pre-existing SA levels have also been shown to potentiate plant defense responses and cell death in response to elicitor or pathogen treatments (Draper, 1997; Shirasu et al., 1997). Therefore, we explored whether (i) high levels of SA detected in eto1 and eto3 plants may have contributed to the increased cell death in these plants and (ii) pre-treatment of O3-tolerant wild-type Col-0 plants with SA prior to O3 exposure would increase their sensitivity. Leaves of Col-0, eto1, and eto3 plants grown under standard conditions stained with trypan blue: a dye that stain dead cells not visible to the naked eye (Weigel and Glazebrook, 2002) did not reveal evidence for dead cells (data not shown). These results suggest that SA levels detected in eto mutants are not sufficient to induce cell death under non-stress conditions. However, treatment of Col-0 plants with SA prior to O3 exposure increased the magnitude of O3-induced cell death in a dose-dependent manner (Figure 4). Pre-treatment of Col-0 plants with 0.1 and 0.25 mm SA increased O3-induced ion leakage by 82 and 166% compared to a 22% induction observed in plants exposed to O3 alone. In contrast, no major changes were observed in the magnitude of O3-induced ion leakage of NahG plants pre-treated with SA compared to plants exposed to O3 alone (Figure 4). These findings support the hypothesis that pre-existing SA levels potentiate the magnitude of O3-induced cell death.

Figure 4.

SA pre-treatment increases the magnitude of O3-induced cell death. Plants were treated with 0.1 and 0.25 mm SA 24 h prior to O3 exposure and exposed to a single dose of 300 ppb O3 for 6 h. Ion leakage was estimated as described in the legend of Figure 2. Mean values marked with asterisk are significantly different compared to their respective control plants (P < 0.05).

The discovery that both high levels of SA and ethylene are associated with O3-sensitive phenotype exhibited by eto1 and eto3 led to the question of whether O3-induced ethylene is dependent on SA or vice versa. Treatment of plants with ethylene alone failed to induce SA-dependent PR1 transcripts (Lawton et al., 1994), while control plants of both eto1 and eto3 accumulated higher levels of SA and produced high levels of ethylene in response to a O3 exposure (Figures 1–3). These observations suggest that O3-induced ethylene production may be SA-dependent. To test this possibility, we used transgenic Col-0 expressing the bacterial salicylate hydroxylase gene (NahG) that converts SA to catechol and an Arabidopsis npr1 mutant blocked in SA-dependent systemic acquired resistance. In contrast to Col-0 plants, both NahG and npr1 plants failed to exhibit increased ethylene production in response to an O3 exposure (Figure 5a). Compared to untreated plants, a 3-h O3 exposure increased ethylene levels of Col-0 by eightfold, while no significant changes were observed in NahG plants. Exposure of npr1 plants to O3 for 3 h increased ethylene levels by 86% compared to control plants. These results suggest that O3-induced ethylene production is dependent on a functional SA-signaling pathway.

Figure 5.

SA is required for stress-induced ethylene production in Arabidopsis exposed to O3 (a), drought and paraquat (b).

(a) At indicated intervals, plants were removed from O3 chamber and ethylene levels were measured as described in Figure 1(b).

(b) Plants exposed to drought and paraquat were used for ethylene measurements as described for Figure 1(b). All experiments were repeated with two replicates and the data from one representative experiment is presented. Error bars indicate SD.

Several plant species have been shown to produce ethylene in response to various stress factors (Johnson and Ecker, 1998; Kieber, 1997). Hence, we tested whether there is a SA requirement for ethylene production induced by other stress factors such as drought and paraquat treatments. Neither treatment induced visible damage. Exposure of plants to water deficits for 6 days induced ethylene levels in Col-0 and eto3 by 2.6- and 5.5-fold, respectively, while paraquat treatments for 24 h induced ethylene levels in Col-0 and eto3 plants by 2.1- and 6.4-fold, respectively, compared to control plants. Neither treatment induced ethylene production in NahG plants (Figure 5b) suggesting that SA is required for increased ethylene production by these two stresses.

The observations that SA pre-treatment increased O3 sensitivity of Col-0 plants (Figure 4) and that SA is required for stress-induced ethylene production (Figure 5) led us to test whether the modestly higher SA levels detected in control eto3 plants may have primed leaf tissue to hyperinduce ethylene in response to a O3 exposure. Double mutants were generated by crossing eto3 plants with pollen from Col-0:NahG plants. Since both the eto3 allele and NahG transgene act as dominant loci, F1 progeny were used for most of these experiments (Figure 6). As shown in Figure 6(b), NahG expression in Col-0 and eto3 plants decreased the O3-induced increases in ethylene-dependent PR4 transcripts by 75–81% compared with the levels detected in Col-0 and eto3 plants exposed to O3. The success of the genetic cross was confirmed by monitoring the strong suppression in O3-induced, SA-dependent PR1 transcripts (Figure 6a) and the expression of NahG transcripts (data not shown). Further, expression of NahG in a background containing dominant mutant allele of eto3 strongly attenuated O3-induced HR lesions (data not shown). Expression of NahG in the dominant eto3 mutant background rendered a Col-0:NahG phenotype in response to O3 exposure that is well characterized by the absence of HR-related defense gene expression and the development of small, slower developing lesions that are distinct from HR lesions (Rao and Davis, 1999). These results were further confirmed with F2 plants homozygous for both the NahG transgene and the eto3 mutant allele (data not shown), suggesting that accumulation of SA is epistatic to stress-induced ethylene production in Arabidopsis.

Figure 6.

NahG expression attenuates O3-induced expression of SA-dependent PR1 transcripts (a) and ethylene-dependent PR4 transcripts (b) in Arabidopsis. Rosette leaves harvested and the RNA blots were hybridized with the probes indicated on the left of the figure as described in Figure 3(a). The experiment was repeated twice with similar results and the relative counts from a representative experiment are presented.

Over the past several decades, ethylene has been shown to regulate many of the plant developmental and stress responses including cell death (He et al., 1996; Kieber, 1997; Moore et al., 1999; Pell et al., 1997; Wellburn and Wellburn, 1996). Although plants produce ethylene in response to O3 exposure, its precise role in regulating O3-induced responses is not clearly understood. Historically, ethylene is believed to react with gaseous O3 to form cell-damaging AOS and aldehydes (Elstner et al., 1985); however, recent studies suggest a regulatory role for ethylene in modulating O3-induced lesions. Firstly, a recent survey of various plant species indicated a relationship between O3 sensitivity and O3-induced ethylene production (Wellburn and Wellburn, 1996). Secondly, inhibition of ethylene production attenuated the magnitude of O3-induced cell death in a wide variety of plant species (Bae et al., 1996; Overmyer et al., 2000; Tuomainen et al., 1997). Thirdly, ethylene-insensitive (ein2) mutant plants exhibited reduced damage in response to O3 exposure even under those conditions that induced cell death in O3-tolerant Col-0 (Overmyer et al., 2000).

Studies involving various cell death mutants indicated that three distinct processes regulate cell death in plants: lesion initiation, propagation, and containment (Dangl et al., 1996; Greenberg et al., 2000; Maleck and Dietrich, 1999; Rate et al., 1999; Yu et al., 1998). Several signaling molecules have been proposed to act either synergistically or antagonistically to regulate the kinetics of lesion initiation, propagation, and containment. Among them, SA, JA, and ethylene have received wide attention as global regulators of plant defense responses including cell death (Dong, 1998; Maleck and Dietrich, 1999). Studies with mutants defective in SA signaling demonstrated a central role for SA in initiating lesions in response to O3 exposure (Rao and Davis, 1999; Rao et al., 2000b), while studies with rcd1 and ein2 plants exposed to O3 have demonstrated a specific role for ethylene in lesion propagation by promoting superoxide radical production (Overmyer et al., 2000). Expression of ethylene insensitivity (ein2) attenuated lesion propagation in rcd1 mutant without altering lesion initiation kinetics, suggesting that both lesion initiation and propagation are two distinct processes.

Exposure of jar1, a JA-insensitive mutant, and fad3/7/8, a mutant blocked in JA biosynthesis, to acute O3 rapidly developed lesions (Rao et al., 2000b). Further, treatment of plants with Me-JA attenuated O3-induced, SA-dependent cell death in an O3-sensitive ecotype Cvi-0 (Rao et al. 2000b), and O3-induced, ethylene-dependent lesion propagation in an O3-sensitive rcd1 mutant (Overmyer et al., 2000) suggest that JA-signaling pathways play an important role in lesion containment processes. Therefore, it appears that many O3-induced defense reactions including cell death are dependent on the plant's ability to respond to SA, JA, and ethylene and the nature and the final response depend on the extent of cross-talk that exists between these molecules (Overmyer et al., 2000; Rao et al., 2000b).

In the present report, we demonstrated a role for SA in not only initiating but also in propagating lesions by controlling ethylene production, thus defining an increased level of complexity in this cell death pathway. Control plants of eto1 and eto3 accumulated moderate levels of SA, hyperinduced ethylene production, and defense gene expression and developed lesions in response to acute O3 exposure (Figures 1 and 3). SA pre-treatments increased O3 sensitivity of Col-0 plants suggesting that constitutive high SA levels potentiate O3-induced plant defense responses including cell death (Figure 4). Although ethylene inhibition partially reduced the magnitude of O3-induced lesions (Figure 2), compromising SA accumulation completely reduced ethylene production in plants exposed to O3, drought, and paraquat (Figure 5). Further, NahG expression attenuated the expression of ethylene-dependent molecular markers (Figure 6) and lesions in eto3 mutants suggesting that SA is required for stress-induced ethylene production and that the synergistic action of SA and ethylene fine-tune the kinetics and magnitude of lesion formation in O3-sensitive plants.

Our novel observations on a SA requirement for stress-induced ethylene production are also supported by studies of double mutants of accelerated cell death 5 (acd5) and ethylene insensitive 2 (ein2) (Greenberg et al., 2000). The ein2 mutation partially attenuated cell death phenotypes exhibited by single acd5 mutant alone, while NahG expression completely rescued all the phenotypes associated with acd5 mutant, suggesting a synergistic interaction between SA and ethylene on cell death (Greenberg et al., 2000). SA is required for the induction of senescence-associated gene SAG12, which has been shown to be ethylene-responsive (Quirino et al., 2000). Similarly, studies with various Arabidopsis mutants defective in SA signaling also revealed a role for SA in regulating senescence (Moris et al., 2000). Further, Arimura et al. (2002) have recently demonstrated that SA-signaling pathways regulate ethylene biosynthesis in Tetranychus urticae-infested lima bean plants. In addition, Pontier et al. (1999) have documented senescent cells immediately at the periphery of primary HR lesions and concluded that signals for senescence emerge from cells undergoing HR. This notion is supported by studies that have localized ethylene biosynthetic enzymes such as ACS and ACO to the chlorotic tissue surrounding primary HR lesions (de Laat and van Loon, 1983).

Based on the data presented above, we propose a schematic model to illustrate that O3-induced, HR cell death in Arabidopsis is the net result of extensive cross-talk between multiple interacting signaling pathways that converge to modulate the type and the magnitude of O3-induced defense responses (Figure 7). Upon entering the leaf tissue via stomata, O3 generates excess AOS resulting in increased biosynthesis of signaling molecules such as SA, which in turn, potentiates the feedback amplification loop of runaway cell death cycle that induces the biosynthesis of signaling molecules such as ethylene. Ethylene has been shown to induce lipases known to promote senescence, a slow form of cell death (Hong et al., 2000). As yet, we do not know whether SA alone is sufficient to induce ethylene. Exogenous treatment of plants with 1 mm SA induced ethylene levels by fivefold within 6 h of treatment; however, we were not able to obtain a clear dose dependency (data not shown). These findings suggest a possibility that SA may require an additional stress-induced component(s) to maximize ethylene production. On the other hand, O3, either by reacting directly with membrane lipids (Mudd, 1997) and/or by generating excess AOS, induces the biosynthesis of JA or methyl jasmonate, which has been shown to reduce O3-induced lesions both by attenuating SA-dependent lesion initiation (Rao et al., 2000b) and ethylene-dependent lesion propagation processes (Overmyer et al., 2000; Figure 7, depicted by dotted lines).

Figure 7.

A schematic model illustrating the synergistic and antagonistic interactions between SA-, ethylene-, and JA-signaling pathways on O3-induced cell death.

Several factors such as genotype, physiological state of the plant, presence of other stress factors, and any specific interactions that might occur between the activated signaling pathways govern the outcome of plant responses to stress factors. The results presented here indicate that O3-induced HR cell death is dependent on the concerted action of both SA and ethylene-signaling pathways. Interaction between these two pathways appears to fine-tune lesion initiation and propagation. On the other hand, O3-induced JA has been shown to attenuate both SA- and ethylene-mediated cell death (Overmyer et al., 2000; Rao et al., 2000b). Thus, the nature of the balance and interaction between the signaling pathways of SA, ethylene, and JA modulate the kinetics and the magnitude of lesion formation. However, how and where different stimuli converge to result in different responses remains yet to be answered. One possibility is that plants contain regulatory switches such as SSI1, MPK4 to control the temporal expression and/or the amplitude of multiple pathways (Petersen et al., 2000; Shah et al., 1999) and provide flexibility for plants to specifically tailor their responses to multiple environmental and developmental cues. Ongoing detailed analyses of double mutants will help to further clarify the regulatory circuits that control the cross-talk between various signaling pathways and their influence on plant defense responses and cell death.

Although our results clearly demonstrate a SA requirement for stress-induced ethylene during certain treatments in Arabidopsis, it is important to note that this interaction may not hold true for all stresses in all plants. Different plant species have been shown to have different responses and requirements with regard to SA and ethylene signaling during stress responses, particularly during plant–pathogen interactions. For example, O'Donnell et al. (2001) have shown that SA accumulation during a susceptible interaction in tomato is dependent on ethylene production and that SA apparently does not have a role in resistance. Similarly, SA accumulates at a later stage during the interaction of tomato with Xanthomonas campestris pv. Vesicatoria, while Arabidopsis accumulates SA within hours in response to pathogen infection (Zhou et al., 1998). Further, inhibition of O3-induced ethylene production completely attenuated O3-induced cell death in tomato (Bae et al., 1996; Tuomainen et al., 1997), while ethylene inhibition in Arabidopsis partially attenuated O3-induced cell death (Overmyer et al., 2000; Figure 2). These observations are in direct contrast to the role of SA the resistance response in Arabidopsis. Given that SA and ethylene apparently play different roles in different plant species during responses to biotic and abiotic stresses, it is likely that it will be impossible to completely understand the interactions between signaling pathways by analysis in a single model system such as Arabidopsis.

Experimental procedures

Plant growth conditions

Arabidopsis thaliana, accession Col-0 (Columbia), the NahG transgenic line expressing salicylate hydroxylase and the npr1 mutant line were the same as those described earlier (Rao and Davis, 1999; Sharma et al., 1996). Seeds of both eto1 and eto3 were obtained from the Arabidopsis Biological Resource Center (ABRC, Ohio State University, Columbus, OH, USA), and all the mutant lines used in this study are in Col-0 background. Plant growth conditions were as described earlier (Rao and Davis, 1999).

Stress and chemical treatments

Unless indicated otherwise, O3 treatments were performed by exposing 18-day-old plants to a single dose of 300 ± 50 ppb O3 for 6 h in an O3 chamber as described previously (Rao and Davis, 1999). Plants maintained in ambient air served as controls. O3-induced ethylene production was inhibited by spraying plants, until runoff, with 1 mm AVG and 1 mm CoCl2, 20 h prior to O3 exposure. Drought conditions were imposed on 16-day-old plants by withholding water for 6 days. Plants that received water regularly served as controls. Paraquat and SA treatments were performed by spraying plants with 10 µm methyl viologen (Sigma, St. Louis, MO, USA) and with 0.1 and 0.25 mm SA until runoff 24 h prior to sampling. Plants sprayed with water served as controls.

Cell death quantification

O3-induced changes in cell death were quantified by measuring ion leakage with a conductivity meter (model 1051; Amber Science, San Diego, CA, USA) as described elsewhere (Rao and Davis, 1999).

Ethylene and SA measurements

For ethylene measurements, plants were grown in plastic pots of 15-cm diameter. At the indicated intervals, during and after O3 treatments, three pots containing two plants each were placed in an air tight glass container. Duplicate containers were left in control growth chambers for 2 h prior to removal of 1 ml head space with a syringe and used for ethylene measurements (Knee et al., 2000). Whole rosettes of 3–5 plants were pooled for the estimation of both free and total SA following the methods described in (Rao et al., 2000b). All data were corrected for recovery by including internal controls.

RNA isolation and analysis

Total RNA was isolated using a SDS–phenol extraction method and subjected to RNA gel blot hybridization analysis as described previously (Rao and Davis, 1999). PR4 and PDF1.2 gene products were obtained by amplifying the genomic sequences by PCR using the primer sets 5′-AATGGATCCACAATGCGGTCGTCAAGG-3′/5′-AATGAAT TCTTCTGGAATCAGGCTGCC-3′ and 5′-GAGTCTGGTCATGGCACAAGTTC-3′/5′-CTT GGCACATTGTTCCGACGCTC-3′, respectively. PCR was run for 40 cycles with annealing temperatures of 56°C and the 600 and 400 bp products of PR4 and PDF1.2, respectively, were cloned in pGEM-T easy vector (Promega). The PCR products were sequenced on both strands before use. The probe for PR1 gene was as described (Rao and Davis, 1999).

Double mutant generation

Double mutants of eto3:NahG were generated by performing reciprocal crosses with eto3 and Col-0:NahG plants following standard protocols. For comparative purposes, reciprocal crosses were also made between eto3 and wild-type Col-0 plants.

Acknowledgements

We thank Arabidopsis Biological Resource Center at the Ohio State University, Drs J. Ryals (formerly at Novartis) and X. Dong (Duke University), for providing the seed stock used in this study. Drs I. Raskin (Rutgers University) and M. Knee (Ohio State University) are thanked for providing facilities for SA and ethylene measurements, respectively. Drs J. Koch and J. Edmonds for assisting in ethylene measurements and Charles Bettini for assisting in trypan blue staining. The studies described in the paper were supported by USDA Co-operative State Research Service Grant (#96-35100-3214) to KRD.

Ancillary

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