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Keywords:

  • Arabidopsis thaliana;
  • competition;
  • costs of resistance;
  • herbivory;
  • jasmonic acid;
  • jasmonic acid carboxyl methyl transferase (JMT);
  • methyl jasmonate (MeJA);
  • tolerance

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  • • 
    Accumulation of methyl jasmonate (MeJA) after herbivore attack in plants is associated with the induction of defenses that can benefit fitness, but are costly to express; effects often explored using exogenous application of jasmonates.
  • • 
    Here I explored the consequences of the overexpression of MeJA on seed production, tolerance to defoliation and competitive effect and response, using a genotype of Arabidopsis thaliana that overexpresses jasmonic acid carboxyl methyltransferase (JMT) and contains threefold higher levels of MeJA than wild-type plants.
  • • 
    Without competition, JMT plants produced 37–40% less total seed mass than vector controls or wild-type plants, and had reduced seed germination. Defoliation reduced height more strongly in wild-type than in JMT plants, but reduced total seed production equally. In a competition experiment, the presence of a neighbor reduced fitness more strongly in wild-type than in JMT plants, but JMT plants exhibited dampened opportunity costs and benefits of induction with jasmonic acid of itself or its neighbor. This may have related to the higher constitutive expression but reduced inducibility of jasmonate-mediated defenses, including trypsin inhibitors, exhibited by JMT plants.
  • • 
    In natural plant populations, overexpression of MeJA-mediated responses should be beneficial to resistance to herbivores, pathogens and competitors, but is directly costly to fitness and probably constrains plasticity in response to changing environmental conditions.

Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Methyl jasmonate (MeJA) is a volatile signaling compound in plants that, in addition to numerous developmental roles, mediates many important ecological interactions. Accumulation of MeJA after herbivore or pathogen attack is involved in the induction of direct defenses (Farmer et al., 1992) and in the production of volatiles that attract parasitoids and predatory arthropods to plants (Kessler & Baldwin, 2001). In addition, MeJA has been proposed as an airborne signal mediating interplant communication (Farmer et al., 1992; Karban et al., 2000). MeJA is formed when the enzyme S-adenosyl-l-methionine : jasmonic acid carboxyl methyltransferase (JMT) specifically methylates jasmonic acid (JA) (Seo et al., 2001; Schaller et al., 2005). In Arabidopsis thaliana this enzyme is inducible by wounding or exogenous MeJA treatment, which leads to accumulation of MeJA and activation of MeJA-responsive genes. Transgenic A. thaliana plants overexpressing JMT constitutively produce threefold higher MeJA and twofold higher total jasmonate levels in their leaves than wild-type plants, with no differences in JA levels (Seo et al., 2001). In turn, JMT plants exhibit constitutively elevated MeJA-responsive genes in their leaves, including defensins, various PR proteins and oxidative stress-related genes, and are more resistant than wild-type plants to attack by the fungus Botrytis cinerea (Seo et al., 2001) and the bacterium Pseudomonas syringae pv. tomato (Jung et al., 2003). Fitness benefits to the plant of the overproduction of MeJA may be inferred from such results, although they have not been quantified. Fitness costs of the overproduction of MeJA have never been examined in A. thaliana or in any other plant.

Costs of responses induced by MeJA or other signals in plants can be realized in several ways. Direct physiological costs of induced responses are seen independently of the environment, and presumably result from internal allocation trade-offs between growth and defense production, and other pleiotropic effects (Herms & Mattson, 1992; Heil & Baldwin, 2002; Cipollini et al., 2003). Ecological costs of induced responses are seen only under particular ecological conditions (Heil, 2001, 2002; Cipollini et al., 2003), and can include increased susceptibility to alternate attackers (Agrawal et al., 1999; Cipollini et al., 2004); trade-offs between resistance and tolerance to herbivory (Strauss & Agrawal, 1999); and trade-offs between resistance and competitive ability (van Dam & Baldwin, 1998; Dietrich et al., 2005). Although there is growing experimental support for physiological and ecological costs of induced responses (Heil, 2001, 2002; Heil & Baldwin, 2002; Cipollini et al., 2003), conditions under which they are expressed and their influence on the ecology and evolution of plant defenses are still widely debated (Cipollini, 2002; Zangerl, 2003; Koricheva et al., 2004; Dietrich et al., 2005).

Since the identification of important defense hormones such as JA/MeJA, many ecologists have used purified forms of these hormones to manipulate plant defenses in order to address costs of induced responses (Baldwin, 1998; Agrawal et al., 1999; Cipollini, 2002). While these studies have greatly advanced our understanding of the effects of these hormones, pharmacological studies have been criticized for use of sometimes unnatural levels or placement of hormones; use of inappropriate isomers; or producing unwanted side-effects that may be partly responsible for some of the observed effects (Purrington, 2000; Cipollini et al., 2003; Jung et al., 2003). The use of MeJA-overproducing JMT plants provides an opportunity to examine the costs and benefits of the constitutive production of elevated levels of this important hormone in the absence of at least some of these confounding factors. In particular, JMT plants have been engineered to manufacture and accumulate high levels of their own MeJA from their own endogenous JA, circumventing problems associated with uptake of exogenous sprays, and ensuring the presence of appropriate isomers of MeJA. Although JMT plants exhibit high levels of MeJA as a result of genetic manipulation, some highly defended plants occurring in natural plant populations may be the result of constitutive activation of normally inducible defenses. The JMT plant serves as an appropriate model of the fitness costs and benefits incurred by such plants. In this study, I explored the physiological and ecological consequences of the overexpression of MeJA on seed production, tolerance to defoliation and competitive effect and response, by comparing the responses of JMT plants to vector controls and their wild-type Columbia parent ecotype in a series of experiments.

Materials and Methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Plant material

The transgenic JMT genotype contains a full-length JMT cDNA isolated from Arabidopsis thaliana (L.) Heynh. (ecotype Columbia-0) that was inserted into vector pBI121 (Clontech, Clontech Laboratories, Inc., Mountain View, CA, USA) containing the CaMV35S promoter, and introduced into ecotype Col-0 using Agrobacterium-mediated transformation (Seo et al., 2001; Jung et al., 2003). JMT plants contain the pCaM–JMT construct (copy number unknown) along with the endogenous copy of the JMT gene, while the wild-type Columbia parent contains only the endogenous gene. JMT plants also constitutively express kanamycin resistance, due to the expression of the neomycin phosphotransferase II (nptII) gene driven by the nopaline synthase promoter. Five independently transformed JMT lines were used in the first experiment, which Seo et al. (2001) state had similar JMT mRNA expression, MeJA contents and responses to pathogen attack. Subsequent studies focused on the JMT1 line, described in detail by Seo et al. (2001). To control for the potential effects of the insertion of vector T-DNA, three independently transformed lines of A. thaliana (ecotype Col-0) containing a single insert of vector T-DNA from pBIN19 (Clontech), introduced using Agrobacterium-mediated transformation, were used in the first experiment. Purrington & Bergelson (1997) had shown previously that these vector control lines did not differ significantly in total seed production from null segregants lacking vector T-DNA. The pBIN19 vector is very similar to the pBI121 vector used to transform JMT plants (including carrying the nptII gene), except that it does not contain the CaMV35S promoter. Seeds of the JMT, pBIN vector controls, and wild-type genotypes used in this study were obtained from self-fertilized plants grown in Pro Mix BX potting medium under identical conditions in the glasshouse at Wright State University. Seeds of the JMT genotypes were kindly donated by Dr Yang Do Choi, Seoul National University; seeds of the pBIN19 vector controls were donated by Dr Joy Bergelson, University of Chicago. Seeds of the Columbia parent ecotype (Col-0) were originally obtained from the Arabidopsis Biological Resource Center.

Experiment 1 – Costs to seed production of the overexpression of JMT

This experiment was designed to examine variation in seed production in five independently transformed JMT lines (JMT1, JMT2, JMT4, JMT5, JMT6), each containing the pCaM–JMT construct and a kanamycin-resistance gene, in comparison with three independently transformed lines each containing a kanamycin-resistance gene from the vector pBIN19, but lacking the JMT gene and CaMV35S promoter. Plants of the JMT and vector controls were grown from seed singly in 150-ml pots in Pro-Mix BX potting medium in the glasshouse, as described by Cipollini et al. (2004). Plants were watered with distilled water as needed and were not fertilized beyond the starter nutrients found in the growth medium. Height of the primary inflorescence on each plant was measured at cessation of flowering. Siliques were collected as they matured and seeds were separated from silique walls, dried and weighed. Total seed mass measured in this way correlates highly with total seed number in A. thaliana (unpublished data). There were eight to 18 (average 13) replicate plants grown of each line. Height and total seed mass were compared among JMT and vector control lines using nested anova on sas ver. 8.2 (SAS Institute, Cary, NC, USA), with genotype (JMT or vector control), and replicate nested with genotype as factors.

Experiment 2 – Tolerance to defoliation

This experiment was designed to examine both direct physiological costs of overproduction of MeJA and its ecological effects on tolerance to defoliation. Only the JMT1 genotype was used in comparison with wild-type control plants in this and all subsequent experiments. Plants of the JMT1 and wild-type genotypes were grown as above. When plants reached the initiation of the bolting stage (approx. 21 d), plants of each genotype were randomly assigned to control or defoliation treatments. On defoliated plants, the total number of rosette leaves was counted on each plant and the numerical half of the rosette leaves closest to the apex (the younger half) removed at the base of the petiole with scissors, as described by Barto & Cipollini (2005). This defoliation treatment at the initiation of bolting removes approx. 50% of the total leaf area and leaf mass present on the plant, and results in greater fitness impacts than either removing young leaves of plants in the vegetative or flowering stages, or removing old leaves at any growth stage (Barto & Cipollini, 2005). Height and total seed mass were measured as above. In addition, germination of seeds collected from plants grown in this experiment was assessed by placing 10 seeds from each plant on moistened filter paper in Petri dishes in an incubator set at 22°C on a 16 : 8 light : dark cycle, and counting the number of seeds that had germinated after 7 d. Seeds of the Columbia ecotype require no cold stratification in order to germinate. There were 17–18 replicate plants in each treatment combination. Height, total seed mass, and number of seeds germinated were compared among genotypes (Col wild type or JMT1) and defoliation treatments (+/–) using two-way anova on sas ver. 8.2.

Experiment 3 – Competitive effect and response

This experiment was designed to examine the effect of overproduction of MeJA on the ability of a plant to affect and respond to a competitor in the pots. In addition, the induction status of the target and neighboring plant was manipulated to examine whether the opportunity costs and benefits of induction (sensu van Dam & Baldwin, 1998) were affected by overproduction of MeJA. Plant-growing conditions were as in Experiment 1, except that one JMT1 and one wild-type individual were grown from seed spaced approx. 2 cm apart in the center of each pot. Some mutual shading occurs in this experimental design, but competition is probably primarily for soil resources. One plant in each pot was randomly designated the target and the other the neighbor. In the first treatment combination (con–con), the target and neighbor were treated with an aqueous control solution. In the second treatment combination (con–JA), the neighbor was treated with a foliar spray of 0.45 mm jasmonic acid on day 21 of the experiment, as described by Cipollini et al. (2004), while the target was treated with control solution. In the third treatment combination (JA–con), the target was treated with JA as above, while the neighbor was treated with control solution. In the fourth treatment combination (JA–JA), both target and neighbor were treated with JA as above. The design of this experiment is shown in Table 1. Seeds were collected from target plants and measured as in Experiment 1. For each genotype there were eight to 12 replicate pots of each treatment combination with that genotype as the target. Total seed mass was compared among target plants using three-way anova on sas with target plant genotype (Col wild type or JMT1); target plant induction (+/–), and neighbor plant induction (+/–) as main effects, including all interactions.

Table 1.  Design of the experiment to compare opportunity costs and benefits of induction by jasmonic acid (JA) on the Columbia wild-type and jasmonic acid carboxyl methyltransferase (JMT)1 genotypes of Arabidopsis thaliana, using a target-neighbor design
Target genotypeTarget treatmentNeighbor treatment
Wild typeControlControl
Wild typeControlJA-induced
Wild typeJA-inducedControl
Wild typeJA-inducedJA-induced
JMT1ControlControl
JMT1ControlJA-induced
JMT1JA-inducedControl
JMT1JA-inducedJA-induced

Experiment 4 – Constitutive and jasmonate-inducible trypsin inhibitor activity

This experiment was designed to examine variation in constitutive and jasmonate-inducible expression of trypsin inhibitors in JMT1 and wild-type plants. Trypsin inhibitors are antinutritive defense proteins, expression of which is associated with both resistance to insect herbivores and significant fitness costs in A. thaliana and other plants (Cipollini, 2002; Cipollini et al., 2004; Zavala & Baldwin, 2004; Zavala et al., 2004). Plants of each genotype were grown from seed singly in pots, as in Experiment 1. When plants were 21 d old, eight plants of each genotype were treated with a foliar spray of JA and eight plants were treated with an aqueous control solution, as in Experiment 3. Three days after treatment the entire above-ground portion of each plant was harvested, flash frozen and held at −20°C until analysis. Trypsin inhibitors were analysed in soluble protein extracts of each rosette using a radial diffusion assay, as described by Cipollini et al. (2004). Trypsin inhibitor activities were expressed as µg g−1 FW. Trypsin inhibitor activities were compared among genotypes (Col wild type or JMT1) and JA treatments (+/–) using two-way anova on sas, including the interaction of these factors.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Experiment 1 – Direct costs of overexpression of JMT

Total seed mass of the JMT genotypes was 37% lower on average than the vector control genotypes (Genotype: F1,100 = 25.77, P < 0.0001; Fig. 1). There was significant variation in seed production among replicate lines within the JMT and vector controls [Replicate (Genotype): F6,100 = 2.79, P = 0.015; Fig. 1], but no individual JMT line produced a higher total seed mass than any vector-only control line. The JMT1 line used in subsequent experiments was one of the higher performing JMT lines in this experiment, while JMT4 plants produced the least total seed mass. The third vector control line was notable in its lower fitness relative to the other two vector controls.

image

Figure 1. Total seed mass of replicated jasmonic acid carboxyl methyltransferase (JMT) and vector control lines of Arabidopsis thaliana (all in the Columbia background). Each bar represents mean (±1 SE) of eight to 18 replicate plants.

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Experiment 2 – Tolerance to defoliation

Removal of 50% of the young leaf area at the bolting stage reduced the height of both JMT1 and wild-type genotypes (Defoliation: F1,66 = 21.9, P < 0.0001; Fig. 2a). Although undamaged JMT1 plants were slightly shorter than undamaged wild-type plants, defoliation resulted in greater decreases in final height in the wild type than in JMT1 plants (Genotype × Defoliation: F1,66 = 3.81, P = 0.05). Undamaged JMT1 plants produced approx. 40% less total seed mass than undamaged wild-type plants (Genotype: F1,66 = 6.62, P = 0.012; Fig. 2b). However, defoliation resulted in similar decreases in total seed mass in both genotypes (Defoliation: F1,66 = 3.82, P = 0.05), although wild-type plants tended to be more affected by defoliation. Germination of seeds produced by plants in this experiment was 15% lower in JMT1 than in wild-type plants (Genotype: F1,66 = 4.694, P = 0.046; Fig. 1c).

image

Figure 2. Growth and fitness responses of Columbia wild-type and jasmonic acid carboxyl methyltransferase (JMT)1 genotypes of Arabidopsis thaliana to defoliation. (a) Final height of primary inflorescence; (b) total seed mass; (c) number of seeds germinated. Closed bars, control; open bars, defoliated. Each bar represents mean (±1 SE) of 17–18 replicate plants.

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Experiment 3 – Competitive effect and response

Induction of the target plant (Target induction: F1,75 = 18.07, P < 0.0001); the interaction between the target plant genotype and induction of its neighbor (Genotype × Neighbor induction: F1,75 = 4.21, P = 0.0437); and the interaction of induction of the target and induction of its neighbor (Target induction × Neighbor induction: F1,75 = 6.60, P = 0.0122) all significantly influenced the total seed mass produced by the target plant. The interaction between target plant genotype and its induction status approached significance (Genotype × Target induction: F1,75 = 3.27, P = 0.075). Total seed mass was similar in the wild type and the JMT1 genotype when neither the target nor the neighbor was induced (Fig. 3). Likewise, total seed mass was reduced to similarly low levels in both wild type and JMT1 when both target and neighbor were induced with JA. However, total seed mass of target wild-type plants increased greatly when their JMT1 neighbors were induced with JA and they were not, and decreased greatly when they were induced with JA and their JMT1 neighbors were not. In contrast, target JMT1 plants failed to show either the marked increase in total seed mass when their wild-type neighbors were induced and they were not, or the marked decrease in seed production when they were induced and their wild-type neighbors were not. This differential pattern in responses of JMT1 and wild-type plants to a neighbor and to its induction status was also found in experiments where the target and neighbor were the same genotype (unpublished data). Across both experiments 2 and 3, it can also be seen that total seed mass was reduced much more in wild-type than in JMT1 plants simply by the presence of a neighbor in the pot (Figs 2c, 3).

image

Figure 3. Total seed mass produced by target wild-type and jasmonic acid carboxyl methyltransferase (JMT)1 Arabidopsis thaliana plants grown in pots with a neighbor, in which target and neighbor were factorially treated with jasmonic acid (JA). Con–con, Target uninduced, neighbor uninduced; con–JA, target uninduced, neighbor induced; JA–con, target induced, neighbor uninduced; JA–JA, target induced, neighbor induced. Closed circles, Columbia wild-type target plants; open circles, JMT target plants. Each point represents mean (±1 SE) of eight to 12 replicate plants.

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Experiment 4 – Constitutive and jasmonate-inducible trypsin inhibitor activity

Both JA treatment (F1,28 = 9.219, P = 0.005) and the genotype–JA treatment interaction (F1,28 = 3.99, P = 0.05) significantly affected trypsin inhibitor activities (Fig. 4). Jasmonic acid treatment increased trypsin inhibitor activities by 82% in wild-type plants over control levels. In contrast, constitutive trypsin inhibitor activities were 47% higher in JMT1 than in wild-type plants, but were only insignificantly increased by JA treatment.

image

Figure 4. Constitutive and jasmonic acid (JA)-induced levels of trypsin inhibitor in whole-rosette extracts of jasmonic acid carboxyl methyltransferase (JMT)1 and Columbia wild-type Arabidopsis thaliana plants. Plants were treated when 21 d old and sampled 3 d later. Closed bars, control; open bars, JA-treated. Each bar represents mean (±1 SE) of eight replicate plants.

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Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Costs of the overproduction of MeJA and associated plant responses in A. thaliana have been examined here for the first time. Production of threefold higher levels of MeJA in transgenic A. thaliana plants overexpressing JMT, and constitutive upregulation of MeJA-responsive genes (apparently including those encoding trypsin inhibitors), was associated with a significant decline in total seed production and reduced seed germination compared with vector controls or wild-type plants when grown alone in pots. Some portion of the costs observed in JMT plants may be caused by the random insertion of JMT construct(s) in the A. thaliana genome or expression of the kanamycin-resistance gene (Strauss et al., 2002; Jackson et al., 2004), but at least some of these effects were controlled by comparing fitness in replicated JMT lines with several replicated vector control lines. Indeed, although variation in total seed production was evident among replicate lines, JMT lines universally produced lower total seed mass than vector control lines containing the kanamycin-resistance gene. In two other carefully controlled studies of costs of resistance using genetically engineered plants, fitness of several vector-only control lines (the same ones as used here) did not differ significantly from untransformed wild-type plants in a study of herbicide resistance in A. thaliana (Purrington & Bergelson, 1997), and in a study of trypsin inhibitor expression in Nicotiana attenuata (see supplementary material in Zavala et al. (2004)). This suggests that side-effects of the transformation process or expression of the kanamycin-resistance gene probably contribute in only minor ways to the variation in fitness among JMT and wild-type plants seen in this study. Reduced total seed mass in JMT plants could also have extended from a direct reduction in pollen fertility or embryo development caused by elevated MeJA levels. Both a lack of JA production in the fad3-1 fad7-2 fad8 mutant (McConn & Browse, 1996) and the lack of JA sensitivity in the coi1 mutant (Xie et al., 1998) lead to male sterility. However, JMT plants contain normal levels of JA and a functional COI1 gene, and there is as yet no evidence that elevated MeJA or altered MeJA : JA ratios can directly affect male fertility.

The substantial reduction in seed production in JMT plants seen here is better explained by resource allocation trade-offs between the production of high levels of MeJA and alterations in associated defense responses and growth processes (Herms & Mattson, 1992), or other pleiotropic effects of elevated MeJA levels. In A. thaliana, MeJA treatment upregulates numerous defense genes and some growth and maintenance-related genes, but downregulates important photosynthetic genes such as RuBisCO (Schenk et al., 2000). A similar pattern of response to MeJA occurs in N. attenuata (Hermsmeier et al., 2001), which is particularly costly when plants are growing with neighbors (van Dam & Baldwin, 1998; Baldwin & Hamilton, 2000). JMT plants constitutively express a variety of defense-related genes and exhibit downregulated expression of growth-related genes, such as the chlorophyll a/b-binding protein (Jung et al., 2003), which may combine to constrain fitness of these plants. Physiological costs of responses induced by MeJA in A. thaliana have not been reported previously, but Cipollini (2002) showed that JA treatment of A. thaliana three times during the growing season led to an 18% decline in total seed production relative to uninduced plants. Dietrich et al. (2005) showed that A. thaliana has the capacity to compensate for growth depressions caused by a single elicitation of defenses with the salicylate mimic, BION, given the time and soil resources. As expected, the constitutive expression of MeJA and associated responses studied here was much more costly than the upregulation and subsequent relaxation of responses associated with repeated JA application. This finding supports the view that inducibility of defenses is a cost-saving mechanism over constitutive expression.

Although overproduction of MeJA was directly costly to seed production and seed germination, additional ecological costs to tolerance of the removal of a particularly sensitive leaf class were not observed in this study. A lack of a negative genetic correlation between tolerance and resistance to both defoliation (Mauricio et al., 1997) and apical meristem removal (Weinig et al., 2003) has been reported for A. thaliana, but I am aware of no studies that have examined whether induction of jasmonate-mediated defenses is costly to plant tolerance. Although studies have shown negative relationships between the expression of single classes of compound or biological resistance and tolerance (Stowe, 1998), potential benefits of the MeJA-induced upregulation of genes putatively associated with plant tolerance (Strauss & Agrawal, 1999; Schenk et al., 2000) may have counterbalanced any additional costs predicted to accrue in defoliated JMT plants. Identification of genes specifically associated with tolerance would help guide investigations of this mechanism.

Several aspects of plant response to, and effect on, competitors were altered by overproduction of MeJA. JMT plants were generally less negatively affected by a competitor in the pot than wild-type plants, regardless of the identity of the neighbor and its induction status. This relates, to some extent, to the greater fitness that wild-type plants attained in the absence of competition, making reductions in the presence of competition more apparent in wild-type than in lower-yielding JMT plants. The size of the jasmonate-overproducing cev1 mutant of A. thaliana is reduced less by MeJA treatment than wild-type plants, partly because cev1 plants have a much lower growth potential than wild-type plants (Ellis & Turner, 2001). Likewise, costs of responses induced by MeJA in N. attenuata were more apparent in well fertilized, high-yielding plants than in nutrient-deprived, low-yielding plants (van Dam & Baldwin, 1998). MeJA has also been suggested to be allelopathic to growth or germination of competing species (Preston et al., 2002), which may contribute to the ‘resistance’ of JMT plants to a competitor in the pot. If JMT plants emitted greater amounts of MeJA into the shared airspace or soil solution than wild-type plants, then JMT plants may have had a greater allelopathic effect on neighboring wild-type plants than wild-type plants had on JMT, which may have complemented effects mediated by resource competition. Reciprocal allelopathic effects, combined with resource competition, could also explain why fitness of JMT plants did decline in pots where both the target and its wild-type neighbor were induced with JA.

Opportunity costs and benefits of induction were also different in JMT than in wild-type plants. Wild-type plants incurred an opportunity cost of induction with JA when grown with an uninduced neighbor, a pattern also shown by Dietrich et al. (2005) in their study of A. thaliana plants treated with BION. In contrast, JMT plants did not exhibit an opportunity cost of induction. This could relate to the differential degree of response that JA induces in JMT plants (which already express jasmonate-mediated defenses at high levels) relative to ‘naive’ wild-type plants that are more highly inducible. The jasmonate-lacking fad3-2 fad7-2 fad8 mutant of A. thaliana is much more inducible by JA than wild-type A. thaliana (Cipollini et al., 2004), suggesting that the strength of the response to exogenous jasmonate treatment depends on endogenous jasmonate levels. Here I have shown that JMT plants expressed higher constitutive activity of trypsin inhibitor than wild-type plants, as expected for a jasmonate-mediated defense (Cipollini et al., 2004). But trypsin inhibitor activities were increased by JA treatment much more in wild-type than in JMT plants. In addition to dampened opportunity costs of induction, JMT plants did not exhibit an opportunity benefit in response to induction of their neighbors as the wild-type did, suggesting that JMT plants were less able than wild-type plants to take advantage of a resource opportunity created by induction of their neighbor. Costs of defense production to growth mechanisms responsible for rapid plastic responses to resource opportunities probably explained this pattern. The patterns observed in wild-type A. thaliana here almost exactly match the patterns observed in wild-type N. attenuata (van Dam & Baldwin, 1998), suggesting that opportunity costs and benefits of induction in the presence of neighboring plants are a general phenomenon in plants, a pattern from which JMT plants deviate.

Together, these results suggest that overproduction of MeJA is directly costly to total seed production and seed germination rates, and constrains plasticity in response to alterations in the competitive status of neighboring plants. However, tolerance of defoliation was unaffected by overproduction of MeJA, and in some scenarios JMT plants were more resistant than wild-type plants to competitors. As suggested by Cipollini (2004), JMT plants appear to represent a phenotype that can defend against both herbivores and competitors, but at a cost to phenotypic plasticity. In natural plant populations, overexpression of MeJA-mediated responses should be beneficial to resistance to herbivores, pathogens and competitors, but is directly costly to fitness and probably constrains plasticity in response to changing environmental conditions, including resource opportunities created by manipulation of neighboring plants. Such ecologically dependent and independent costs and benefits of expression of MeJA and associated resistance mechanisms may help maintain polymorphisms in resistance in natural plant populations.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

I thank Jeanne Mbagwu, Cherissa Rainey and Stephanie Enright for technical assistance, Dr Yang Do Choi for donation of seeds of the JMT genotype and Dr Joy Bergelson for seeds of the pBIN vector lines. Financial assistance was received from the NSF UMEB program and Wright State University. Comments by three anonymous reviewers substantially improved this manuscript.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
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