Methyl jasmonate-elicited herbivore resistance: does MeJA function as a signal without being hydrolyzed to JA?
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Treatment with methyl jasmonate (MeJA) elicits herbivore resistance in many plant species and over-expression of JA carboxyl methyltransferase (JMT) constitutively increases JA-induced responses in Arabidopsis. When wild-type (WT) Nicotiana attenuata plants are treated with MeJA, a rapid transient endogenous JA burst is elicited, which in turn increases levels of nicotine and trypsin proteinase inhibitors (TPIs) and resistance to larvae of the specialist herbivore, Manduca sexta. All of these responses are impaired in plants silenced in lipoxygenase 3 expression (asLOX3) but are restored to WT levels by MeJA treatment. Whether these MeJA-induced responses are directly elicited by MeJA or by its cleavage product, JA, is unknown. Using virus-induced gene silencing (VIGS), we silenced MeJA-esterase (NaMJE) expression and found this gene responsible for most of the MeJA-cleaving activity in N. attenuata protein extracts. Silencing NaMJE in asLOX3, but not in WT plants, significantly reduced MeJA-induced nicotine levels and resistance to M. sexta, but not TPI levels. MeJA-induced transcript levels of threonine deaminase (NaTD) and phenylalanine ammonia lyase (NaPAL1) were also decreased in VIGS MJE (asLOX3) plants. Finally the performance of M. sexta larvae that fed on plants treated with JA or MeJA demonstrated that silencing NaMJE inhibited MeJA-induced but not JA-induced resistance in asLOX3 plants. From these results, we conclude that the resistance elicited by MeJA treatment is directly elicited not by MeJA but by its de-methylated product, JA.
KeywordsMeJA esterase (NaMJE) Methyl jasmonate (MeJA) Jasmonate (JA) Nicotiana attenuata Manduca sexta
Methyl jasmonate (MeJA), jasmonic acid (JA) and its amino acid conjugates, collectively referred to as jasmonates, are important cellular regulators mediating diverse developmental processes including root growth, pollen production, and plant resistance to insects and pathogens (Creelman and Mullet 1997; Kessler and Baldwin 2002). Jasmonates are synthesized in plants via the octadecanoid pathway (Creelman and Mullet 1997). Briefly, linolenic acid is oxygenated by lipoxygenase (LOX) and then converted to 12-oxo-phytodienoic acid (12-oxo-PDA) by allene oxide synthase (AOS) and allene oxide cyclase (AOC). JA is synthesized from 12-oxo-PDA through reduction and three steps of β-oxidation, and then catabolized further to form its volatile counterpart, MeJA (Seo et al. 2001), and numerous conjugates including JA-isoleucine (JA-Ile, Staswick and Tiryaki 2004; Kang et al. 2006; Wang et al. 2007a, b). MeJA is one of the JA metabolites proposed to play an important role in inter- and intra-plant signaling (Farmer and Ryan 1990; Seo et al. 2001; Karban et al. 2000; Kessler et al. 2006; Baldwin et al. 2006). When plants are exposed to volatile MeJA, they quickly elicit a series of JA mediated defense responses and MeJA treatment is the most commonly used means of eliciting herbivore resistance in many different plant species (McConn et al. 1997; Baldwin 1998; Li et al. 2002). However, it is still not known how plants elicit herbivore resistance traits in response to MeJA exposure.
When Nicotiana attenuata is attacked by herbivores, it produces both volatiles to recruit the herbivores’ natural enemies (Kessler and Baldwin 2001; Mattiacci et al. 1995) and secondary metabolites that function as direct defenses, such as the neurotoxin nicotine (Baldwin 1999; Steppuhn et al. 2004), and trypsin proteinase inhibitors (TPIs, Zavala et al. 2004). JA signaling plays a central role in these responses. Silencing a key gene involved in supplying fatty acid hydroperoxides for JA biosynthesis, lipoxygenase 3 (NaLOX3), reduces the wound- and herbivore-induced accumulation of JA, but not the constitutive levels of JA. This JA deficiency inhibits the elicitation of direct (TPIs and nicotine) and indirect (volatiles) defenses and reduces N. attenuata’s resistance to attack by larvae of the specialist herbivore of N. attenuata, Manduca sexta (Halitschke and Baldwin 2003), and makes the plants susceptible to two new herbivores, the leaf-chewing beetle Diabrotica undecimpunctata and the piercing–sucking leafhopper Empoasca spp. (Kessler et al. 2004). Interestingly, when NaLOX3-silenced plants are treated with MeJA, their ability to produce nicotine and herbivore-resistance traits is fully restored (Halitschke and Baldwin 2003), suggesting that the exogenous MeJA treatment is sufficient to elicit most JA responses. However, these results raise an important question: does the exogenously supplied MeJA function directly as a signal or must it first be hydrolyzed to JA? Both JA and MeJA are elicitors of defense responses when applied exogenously, and they induce almost the same set of genes (Taki et al. 2005), but the nature of the endogenous signal remains unclear as both are rapidly interchangeable. Over-expression of JA carboxyl methyltransferase (JMT) in Arabidopsis increases endogenous MeJA levels 3-fold without altering JA levels and results in the constitutive expression of JA-responsive genes, including VSP and PDF1.2 (Seo et al. 2001). These results suggest that MeJA rather than JA elicits systemically transmitted defense responses. Similarly, a long-standing debate about whether salicylic acid (SA) or its methyl ester, MeSA, was the elicitor of systemically acquired resistance to pathogen attack was recently resolved by the ingenious idea of grafting together combinations of plants altered in their expression of either the MeSA-esterase and SA-methyl transferases. MeSA was confirmed to be the critical mobile signal for SAR (Park et al. 2007).
The discovery of MeJA-esterase (MJE, Stuhlfelder et al. 2002, 2004), which hydrolyzes MeJA to JA, provides a means of determining whether MeJA is the elicitor of MeJA-elicited herbivore resistance. Since MeJA and JA treatment of plants is known to elicit endogenous JA production (Ziegler et al. 2001; Miersch and Wasternack 2000; Pluskota et al. 2007), the hypothesis is best tested in plants reduced in their endogenous JA production. We tested the hypothesis that MeJA-elicited herbivore resistance is actually elicited after de-esterification to JA. We used virus-induced gene silencing (VIGS) to silence MeJA esterase (NaMJE) transcripts in wild-type (WT) and asLOX3N. attenuata plants (Halitschke and Baldwin 2003) with a tobacco rattle virus-based system that had been optimized for N. attenuata (Saedler and Baldwin 2004) and measured defense responses, transcripts and herbivore performance in plants elicited with MeJA or JA treatments.
Materials and methods
We used seeds of the 21st generation of an inbred line of N. attenuata Torr. Ex Watts (synonymous with Nicotiana torreyana: Solanaceae) for transformation. Seed germination and plant growth were conducted as described by Krügel et al. (2002). In brief, seeds were sterilized and germinated on agar with Gamborg B5 (Duchefa http://www.duchefa.com) after soaking in a 1:50 (v/v) diluted liquid smoke (House of Herbs, Passaic, NY, USA) and 1 mM of gibberellic acid (GA3). After 10 days, seedlings were planted into soil in Teku pots. Once established, plants were transferred to 1 L pots in soil and grown in a growth chamber at 22°C, under 16 h of light supplemented with Philips Sun-T Agro 400 Na lights (Philips, http://www.philips.com) or not.
A 312 bp fragment of the NaMJE cDNA sequence was cloned with primers (MJE30_for: 5′-GCTAGTTCATGGAGCTTGTC-3′ and MJE341_rev: 5′-TTAGGACCAGGCATGAAAGC-3′), the design of which was based on the sequence similarity of EST (DB679695) and LeMJE (AY455313). After the first round of PCR with AP primer (5′-GCCACGCGTCGACTAGTACTTTTTTTTTTTTTTTTT-3′) and primer MJE30_for, 3 terminal cDNA was amplified with primer MJE 49_for (5′-CACGGTGCATGGTGTTGGTA-3′) and primer NAP (5′-GCCACGCGTCGACTAGTAC-3′). The 5 terminal cDNA was obtained with primer MJE0_for (5′-AGATGACATGGAAAAGGGT-3′) and primer MJE341_rev. All cDNA fragments were cloned into a pGEM-T EASY vector (Promega, http://www.promega.com) and sequenced.
Generation of VIGS plants
A 312 bp fragment of the NaMJE cDNA sequence, which was amplified by primers MJE30_for and MJE341_rev, was cloned into the pTV00. The pTV00 vector is a 5.5-kb plasmid with an origin of replication for Escherichia coli and A. tumefaciens and a gene for kanamycin resistance (Ratcliff et al. 2001). The A. tumefaciens (strain GV3101)-mediated transformation procedure was described previously (Saedler and Baldwin 2004). To monitor the progress of VIGS, we silenced phytoene desaturase, a gene that oxidizes and cyclizes phytoene to α- and β-carotene. These compounds are subsequently converted into the xanthophylls of the antenna pigments of the photosystems of plants, resulting in the visible bleaching of green tissues (Saedler and Baldwin 2004). When the leaves of phytoene desaturase-silenced plants began to bleach (5 weeks after germination), leaves of NaMJE-silenced (VIGS MJE) and empty vector-inoculated (EV) plants from both WT and asLOX3 plants were used.
Eggs of M. sexta were acquired from North Carolina State University (http://www.ncsu.edu) and kept in a growth chamber (Snijders Scientific, http://www.snijders-tilburg.nl) at 26°C 16 h light, 24°C 8 h darkness, until the larvae hatched. Freshly hatched neonates were placed directly on the source–sink transition leaves 3 days after MeJA treatment. In each treatment, 20 larvae were weighed after the indicated times.
Analysis of nicotine levels and TPI activity
Nicotine was extracted and quantified by HPLC as described in (Keinanen et al. 2001). Trypsin proteinase (TPI) activity was analyzed by radial diffusion activity as described in (van Dam et al. 2001).
JA and JA-Ile measurements
JA and JA-Ile were extracted and quantified by LC/MS as described in (Wang et al. 2007a). In brief, about 200 mg of leaf tissues from each sample was homogenized on a FastPrep homogenizer (http://www.thermo.com) with 1 mL of ethyl acetate spiked with 200 ng of 1,2-13C-JA, D4-SA and p-coumaric acid (PCA) in FastPrep tubes. After being centrifuged, the supernatants were transferred to fresh 2 mL tubes and evaporated on a vacuum concentrator. The residue was resuspended in 0.5 mL of 70% methanol (v/v) and centrifuged at maximum speed for 5 min. The supernatants were analyzed for JA, JA-Ile, and SA with a 1200L LC/MS/MS system (Varian, http://www.varianinc.com).
SYBR green real-time PCR assay (qPCR)
- NaMJE forward primer:
- NaMJE reverse primer:
- NaTPI forward primer:
- NaTPI reverse primer:
- NaTD forward primer:
- NaTD reverse primer:
- NaPAL1 forward primer:
- NaPAL1 reverse primer:
- NaActin forward primer:
- NaActin reverse primer:
MeJA esterase activity assay
Around 200 mg leaf sample was ground in liquid nitrogen, and total proteins were extracted with buffer (100 mM Tris, 5% polyvinyl polypyrrolidone, 0.2% phenylthio urea, and 0.5% diethyl dithio carbamate, pH 7.6). A standard enzyme incubation mixture (total volume 50 μL) contained 10 μL total protein (2 μg), 2 μL MeJA (2.2 μg in 10% ethanol), and 38 μL 100 mM Tris (pH 7.6). Reactions were performed at 40°C as suggested as the optimum temperature by Stuhlfelder et al. (2002) for 0, 20, and 60 min and terminated by chilling on ice. One milliliter of ethyl acetate (spiked with internal JA standard 13C2-JA 200 ng) was added quickly. The product JA from cleaved MeJA was quantified by LC/MS/MS as described in (Wang et al. 2007a). From the concentration of JA produced, we obtained the amount of cleaved MeJA. We also performed reactions for negative controls with only 10 μL total proteins of EV and VIGS MJE plants individually or only MeJA in Tris buffer at the same condition for 60 min, the results showed that the amount of JA produced by these reactions is under the detection limit of our LC/MS/MS.
Silencing NaMJE dramatically reduces the hydrolysis of MeJA in protein extracts
To determine if the NaMJE we silenced was responsible for hydrolyzing MeJA to free JA, we measured the MeJA-cleaving activity of proteins extracted from leaves of EV and VIGS MJE plants (Fig. 1b). In proteins extracted from EV asLOX3 plants, 45 μg MeJA was cleaved to free JA by 1 mg EV proteins after 20 min under standard conditions, while only 9 μg MeJA was cleaved by 1 mg of proteins extracted from VIGS MJE asLOX3 plants (unpaired t test, P = 0.0002); after 60 min, the MeJA cleaved by VIGS MJE was only 16% of the amount that EV proteins had cleaved (unpaired t test, P = 0.0003). Dramatically reduced MeJA-cleaving activity was also found in VIGS MJE proteins in WT background (Fig. 1b). These results demonstrate that NaMJE is largely responsible for the MeJA-cleaving activity in N. attenuata leaves.
Silencing NaMJE inhibits MeJA-induced resistance to M. sexta in asLOX3 plants but not in WT plants
Silencing NaMJE impairs MeJA-induced nicotine, NaPAL1 and TD transcripts but not TPI responses in asLOX3 plants
MeJA treatment elicited the same amount of TPI activity in EV and VIGS MJE plants in both WT and asLOX3 backgrounds (Fig. 3b), suggesting that silencing NaMJE had no effect on the elicitation of TPIs by MeJA. Measurements of TPI transcripts in asLOX3 plants 8 h after MeJA treatment also revealed no significant differences between EV and VIGS MJE plants (unpaired t test, P = 0.5).
Threonine deaminase (TD) catalyzes the conversion of Thr to a-keto butyrate in Ile biosynthesis, and is strongly elicited in MeJA-elicited leaves in N. attenuata where it supplies the Ile required for JA-Ile production (Kang et al. 2006). MeJA treatments dramatically increased NaTD transcripts in EV plants; however, in VIGS MJE plants, NaTD transcript levels were only 9% of those in EV plants 8 h after MeJA treatment (Fig. 4b; unpaired t test, P = 0.03).
Silencing NaMJE inhibits MeJA-induced but not JA-induced resistance to M. sexta in asLOX3 plants
Staswick’s pioneering work on MeJA-insensitive jar1 mutant in Arabidopsis suggests that exogenously applied MeJA is first demethylated and then conjugated to Ile before it becomes an active inhibitor of root growth (Staswick et al. 1992; Staswick and Tiryaki 2004). Indeed, MeJA hydrolyzing enzyme activity occurs in all the plant species that have been examined to date (Stuhlfelder et al. 2002). It is also reported that MeJA was rapidly hydrolyzed to JA and further metabolized like JA when tobacco BY-2 cells were treated with MeJA (Swiatek et al. 2004). In N. attenuata, a model plant with extensively studied herbivore-induced responses, we also detected high levels of MeJA-cleaving activity (Fig. 1b). Using the sequence similarity with tomato LeMJE (Stuhlfelder et al. 2004), we cloned NaMJE, a MeJA esterase gene that shared very high sequence similarity with LeMJE (Fig. S1). By silencing the expression of this gene, we demonstrated that NaMJE is largely responsible for the MeJA-cleaving activity of N. attenuata leaves (Fig. 1b).
MeJA treatment is the most commonly used means of eliciting herbivore resistance in many different plant species (McConn et al. 1997; Baldwin 1998; Li et al. 2002). However, it is still not known how herbivore resistance traits are elicited by MeJA treatment. We determined whether MeJA functions as a signal prior to being hydrolyzed to JA by investigating MeJA-induced defense responses in both WT and JA-deficient asLOX3 plants (Figs. S2, S3) with reduced MeJA-cleaving activity. Silencing NaMJE transcripts was sufficient to block most MeJA-induced responses in asLOX3 but not WT plants, including the production of nicotine (Fig. 3), transcripts of NaPAL1 and NaTD (Fig. 4). Importantly, the dramatic decreases in mass that are usually observed in larvae that feed on MeJA-treated EV plants disappeared when NaMJE transcripts were silenced (Fig. 2). Furthermore, silencing NaMJE inhibited MeJA-induced but not JA-induced resistance in asLOX3 plants (Fig. 5). Why is silencing NaMJE sufficient to block most MeJA-induced responses in asLOX plants but not in WT plants? MeJA treatment of plants is known to elicit endogenous JA production (Ziegler et al. 2001; Miersch and Wasternack 2000). Without any treatments, asLOX3 plants have the same level of endogenous JA and JA-Ile as WT plants (Figs. S2a, S3). However, after MeJA treatments, asLOX3 plants accumulate lower levels of JA than do WT plants (Fig. S2b). These results are consistent with the reports in Halitschke and Baldwin (2003) that asLOX3 plants have the same basal levels of JA as WT plants do but accumulate half of the JA levels of WT plants after wounding, suggesting that the JA burst arises in part from de novo biosynthesis and in part from JA that is released from some unknown storage pool. Since NaMJE mRNA levels and MeJA cleaving activity are the same in EV (WT) and EV (asLOX3) plants, the higher levels of JA (Fig. S2b) elicited by MeJA in EV (WT) plants compared to EV (asLOX3) plants are likely due to the larger JA storage pool in WT plants. In light of these considerations, we conclude that most exogenous MeJA-induced herbivore responses are actually elicited after MeJA is demethylated to JA.
Trypsin proteinase inhibitors (TPIs) play a role as a direct defensive against herbivores in N. attenuata (Zavala et al. 2004). The activities and transcripts of TPIs can be dramatically induced by MeJA treatments. Preston et al. (2004) showed that the application of 5 μg MeJA was sufficient to elicit a significant increase in TPI activity, which suggested that TPIs are very sensitive to MeJA treatment, which may explain why even a reduction of 84% of the MeJA-cleaving activity was not sufficient to inhibit the MeJA-induced TPI response in asLOX3 plants. Therefore from these results, we cannot determine whether the accumulations of TPIs were elicited by MeJA directly or after demethylation to JA.
Silencing NaMJE completely blocked the MeJA-induced resistance to M. sexta larvae in asLOX3 plants (Fig. 3). Although the levels of MeJA-induced nicotine were significantly lower in VIGS MJE plants than in EV plants, the reduction in the nicotine response was not likely sufficiently large to account for the increase in herbivore performance (Steppuhn et al. 2004). Two additional possible explanations are suggested by the results: (1) the reduction in NaPAL1 transcripts could reflect reductions in an unknown phenolic-based defense; (2) reduced NaTD transcripts may reflect decreased TD activity, which may function post-ingestively as an antinutritive defense that limits the supply of Thr needed for herbivore growth (Chen et al. 2005).
In summary, our results demonstrate that most herbivore-resistant responses are elicited by JA when plants were treated with exogenously applied MeJA.
We thank Dr. K. Gase, Thomas Hahn, Susan Kutschbach, Antje Wissgott for sequencing NaMJE and making the VIGS constructs; Jianqiang Wu for help in the laboratory and discussion; Eva Rothe and Dr. M. Schöttner for help in the analysis of JA and JA-Ile; Emily Wheeler for improving the manuscript. We thank the funding from the Virtual Institute Biotic Interaction (ViBi) by the Helmholtz Association for JW’s research stipend, and the Max Planck Society for funding all other aspects of the research.
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