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Keywords:

  • Hordeum vulgare;
  • 11C;
  • barley;
  • carbon-11;
  • phloem loading;
  • root herbivory;
  • split-roots;
  • tolerance

ABSTRACT

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIAL AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES

It is known that shoot application of jasmonic acid (JA) leads to an increased carbon export from leaves to stem and roots, and that root treatment with JA inhibits root growth. Using the radioisotope 11C, we measured JA effects on carbon partitioning in sterile, split-root, barley plants. JA applied to one root half reduced carbon partitioning to the JA-treated tissue within minutes, whereas the untreated side showed a corresponding – but slower – increase. This response was not observed when instead of applying JA, the sink strength of one root half was reduced by cooling it: there was no enhanced partitioning to the untreated roots. The slower response in the JA-untreated roots, and the difference between the effect of JA and temperature, suggest that root JA treatment caused transduction of a signal from the treated roots to the shoot, leading to an increase in carbon allocation from the leaves to the untreated root tissue, as was indeed observed 10 min after the shoot application of JA. This supports the hypothesis that the response of some plant species to both leaf and root herbivores may be the diversion of resources to safer locations.


INTRODUCTION

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIAL AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES

Plants can allocate recently fixed carbon (C) to, for example, reproduction, growth, storage or the synthesis of defence compounds. When recently fixed carbon is allocated to growth, it can be invested into new photosynthetic or reproductive tissue, or allocated to root growth that occurs particularly in the elongation zone of the root tips to increase the uptake of nutrients and water (Pritchard et al. 2004). However, roots may also store photoassimilates or may synthesize defence compounds. It has been suggested that, in response to herbivory on leaves, plants may very rapidly alter carbon partitioning patterns in favour of the lower stem or into roots for storage to be used for later regrowth or reproduction (Strauss & Agrawal 1999; Babst et al. 2005; Schwachtje et al. 2006). The diverted resources may also protect the existing tissue through lignification or thickening of the stem (Hudgins, Christiansen & Franceschi 2004; Babst et al. 2005). While resistance traits, like the synthesis of defence compounds, prevent plants from being consumed, or reduce the extent of feeding by herbivores, tolerance traits reduce the fitness impact of herbivores on the infested plant. Tolerance behaviour in response to herbivory is thought to be associated with a change in carbon partitioning (Babst et al. 2005; Schwachtje et al. 2006). In response to herbivory, signalling cascades are activated in the plant, which lead to the formation of jasmonic acid (JA), a signalling compound involved in a multitude of plant responses. Therefore, JA is frequently applied to plants in an attempt to mimic herbivory, as are caterpillar oral secretions (Baldwin 1996; Thaler et al. 1996; Röse & Tumlinson 2005). Both treatments have been shown to mimic the induction of plant resistance by herbivores when applied to the leaves (McCloud & Baldwin 1997; Halitschke et al. 2001; Roda et al. 2004). When leaves of the woody perennial Populus tremuloides (Babst et al. 2005) were treated with JA, and leaves of the annual Nicotiana attenuata (Schwachtje et al. 2006) were treated with caterpillar oral secretions, the distribution of photoassimilates changed immediately in favour of the roots. However, it appears that methyl jasmonate, the methyl ester of JA, is not responsible for the changes in carbon partitioning in N. attenuata. An antisense genotype with a silenced β subunit of SnRK1 protein kinase was shown to be involved in the regulation of assimilate transport to the roots, and was not regulated by methyl jasmonate (Schwachtje et al. 2006).

Given that localized treatment of roots with high concentrations of 1–10 µm JA is known to inhibit root growth, for example, in tomato root cultures (Tung et al. 1996), Arabidopsis (Staswick, Su & Howell 1992), potato (Ravnikar, Vilhar & Gogala 1992) and in spruce (Regvar & Gogala 1996), whereas much lower concentrations of 10−3 µm can promote the frequency of lateral root initiation and elongation (Tung et al. 1996), we expect changes in sink strength that may also affect carbon partitioning. However, depending on the developmental stage of the plant, the concentration of JA applied and the duration of the experiment, the effect of JA on root biomass may vary (van Dam, Witjes & Svatos 2004).

Our aim was to investigate the effect of the shoot application of JA on shoot/root partitioning of recently fixed carbon compared with the effect of root application. Therefore, we compared the carbon partitioning response to JA treatment of shoots with that of partial roots. We also examined the effect of lowered root temperature, hypothesizing that cooling parts of the root system would reduce root metabolism and thus the sink strength of the root: this would provide a reference for the effect of JA treatment that may include a signalling function throughout the plant. Changes in partitioning of recent photoassimilates were determined by applying the radioisotope 11C as 11CO2 to source leaves. The transport of 11C in the plant can be measured in vivo by scintillation detectors because of the high energy of its decay products. Repeated experiments with a single plant at high time resolution were possible because there is no build-up of tracer with repeated applications because of the short half-life of 20 min for 11C. To allow the separation of a sterile root system in two fractions, we developed a tightly sealed split-root chamber design in combination with an 11C detection system to determine short-term changes in carbon distribution within split-root barley plants.

MATERIAL AND METHODS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIAL AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES

Plant material

Barley (Hordeum vulgare L.) plants of the variety ‘Barcke’ (Irnich Inc., Frechen, Germany) were grown from seeds harvested in 2003. To avoid microbial contamination that may affect carbon partitioning within the plant, the seeds were sterilized prior to planting. Approximately 200 seeds were incubated in 50 mL 50% H2SO4 for 1 h, shaken to remove the glumes and then rinsed three times with bi-distilled water to remove the sulphuric acid. The seeds were subsequently sterilized with a freshly prepared 2% AgNO3 solution for 20 min on a shaker at 200 r.p.m. Then in a sterile laminar-flow cabinet, the seeds were washed once with sterile 1% NaCl solution, once with Aquabidest, again with NaCl, and five times with Aquabidest to remove the AgNO3 completely.

To verify sterility, the seeds were germinated on agar plates (20 on each) with 50 mL 1/10 NB-NMAS (Page 1976) 0.8% agar in darkness for 4 d at 20 °C. The germinated seeds were checked visually for contamination with microorganisms: only sterile seedlings were transferred singly into individual autoclaved glass tubes (length 135 mm, 25 mm diameter) containing 50 mL sterile 50% Hoagland solution. A silicon closed-cell foam rubber stopper (with a longitudinal slit for the plant stem; VWR, Darmstadt, Germany) sealed the roots, seed and shoot base into the glass tube to exclude microorganisms from the roots. The plants were grown at 60% RH with a 16 h day (100 µE m−2 s−1, 25 °C) and 8 h night (20 °C).

After 7 d, each plant was transferred into its autoclaved two-chamber rhizotron. The roots were divided in two roughly equal parts and were inserted into the Y-junction leading to the two chambers of the rhizotron (Fig. 1). Sterile silicon grease (Baysilone, Bayer, Leverkusen, Germany) was used to isolate both sides. The rhizotrons were filled on each side with 300 mL 50% Hoagland solution, buffered with 5 mm MES adjusted to pH 5.8 with KOH. Fresh Hoagland solution was added as necessary through a sterilizing filter. The plants were taken for radiotracer experiments after 5–7 d when there were three mature leaves. Light intensity was 350 µE m−2 s−1 at the load leaf, and 300 µE m−2 s−1 at the rest of the shoot.

image

Figure 1. Effect of application of 50 µm jasmonic acid (JA) to part of a barley root system on root elongation. The root system is shown in a rhizotron (a) before treatment and (b) 48 h after treatment. Root chambers: JA(+) treated, JA(−) untreated. Black lines mark the positions of some root tips at the time of treatment; white bars the position of growing tips after 48 h. Aeration was interrupted briefly while taking the photographs.

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Split-root rhizotrons

To allow plant cultivation under sterile conditions, the split-root rhizotrons were constructed from autoclavable materials and were autoclaved for 20 min at 120 °C before use. The main body (330 mm height, 235 mm width, 18 mm depth) was built from polypropylene, the transparent cover plate out of 6-mm-thick polycarbonate (Fig. 1). To avoid anoxia during the experiments, both root halves were aerated with sterile filtered air.

11C labelling

Two to three days prior to a 11C-labelling experiment, a plant was transferred to the climatic chamber for adaptation. It was connected to the 11C-labelling system at least 16 h before the start of the measurement, to ensure that it had fully recovered from a mechanical disturbance before it was labelled. The second leaf was sealed with two-pot silicone rubber (Xantopren VL, Heraeus Kulzer, Hanau, Germany) in a cylindrical Plexiglas chamber (70 mm length, 18 mm diameter), and was labelled three times with about 100 MBq 11CO2 in air at 5, 7.5 and 10.5 h into the light phase. Plants were treated during the second application of 11C when tracer activity in the root detectors was maximal (i.e. equal rates of decay and arrival), ∼60–70 min after the start of labelling, giving a good measure of transport changes. The 11CO2 was produced with a baby cyclotron in the Forschungszentrum Jülich.

11C detection and analysis

Scintillation detectors were positioned within radiation shielding to be uniformly sensitive to well-defined parts of the plant. The counts were corrected for background, dead-time and their different sensitivities to equal amounts of tracer. The measured plant parts were the shoot excluding the load leaf, and both root portions. Strips of clear 4-mm-thick Plexiglas were placed around the shoot of the plant to ensure that β+ radiation escaping from the plants was annihilated near its source (Minchin et al. 2002).

The data were analysed by the ‘input-output’ method for the analysis of carbon-11 tracer profiles (Minchin & Troughton 1980). The method estimates the transfer function for movement of tracer through a pathway in the plant (Minchin & Thorpe 1989, 2003), and by accounting for radioisotope decay, the analysis quantifies the transport of ‘recently assimilated carbon’. In the analysis, the input was the total mobilized tracer (the sum of the three plant parts), and the output was the tracer entering a specific sink (either or both root portions). The steady-state gain of the transfer function is the fraction of the mobilized photosynthate that is partitioning into that sink. To allow comparison between plants, treatment responses were normalized to the value observed at the time of treatment.

JA treatment

To investigate how external application of JA affects the C partitioning between the shoot and root, and within the root system, we treated barley plants with this phytohormone either on the shoots or on the roots. For the root treatment, 5 mL of a JA stock solution containing 3 mm JA (Sigma, Steinheim, Germany) and 0.6% EtOH (v/v) was added to the hydroponic solution in one-half of the rhizotron through a sterile filter to give a final concentration of 50 µm JA. One root half was exposed to this solution throughout the subsequent measurement time. Both root halves were aerated with sterile filtered air taken from outside the plant cabinet and vented outside. Completely untreated plants were used as control plants. To account for possible effects of EtOH or Triton-X 100 on C partitioning, both compounds were applied in the same concentration and manner to a second set of barley plants as controls. For the JA shoot experiments, 200 µL of a solution containing 1 mm JA in 1% EtOH and 0.125% (v/v) Triton-X 100 was applied with a pipette to form small droplets on the leaf surface (similar to Arnold & Schultz 2002).

Root cooling

For root-cooling experiments, the hydroponic solution of one root chamber was cooled with a stainless steel tube (350 mm length, 6 mm diameter) as a heat exchanger receiving water from a temperature-regulated water bath (Type: F32-MC, Julabo, Seelbach, Germany) set at 14 °C. With the pump set to a maximum speed, a chamber temperature of 15 °C was reached within 20 min (Fig. 4). The non-cooled chamber maintained a constant temperature of 26 °C, because of the low thermal conductance of the plastic walls of the rhizotrons; even after 120 min of cooling, no temperature change in the solution of the non-cooled root chamber was detectable. The plants where both roots were not cooled were used as controls.

image

Figure 4. Effect of cooling part of the root system of split-root grown barley plants on the fraction of 11C-tracer that was transported to each root portion. (a) Temperature of the two hydroponic solutions. (b) Relative 11C partitioning of each root portion. The data were normalized at time of treatment. Ctrl shows the mean of the two root portions of untreated control plants. The two root portions of the split-root plants are designated cool+, cool–, for the treated plants. (Mean + SE, n = 12 for control plants, n = 7 for root cooled plants). Asterisks indicate significant changes in time compared to the control (i.e. different slopes). ((*)P < 0.1, *P < 0.05, **P < 0.01, ***P < 0.001).

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Root elongation

An image of the split-root system was taken through the transparent front plate of the rhizotron every 5 min using a CCD camera (Sony XC-ST75; Sony, Köln, Germany) with a chip resolution of 700 × 480 pixels. JA root treatment was administered to one root half as described previously. The root elongation measurements were performed for 48 h with three independent plants.

Statistical analyses

Normalized partitioning measurements were analysed using repeated measurement analysis of variance (rm-anova) with time as the repeated factor. The normalized partitioning to individual root portions was analysed independently, by comparing each root half separately with the mean of control root halves. The data of the control root halves were tested for homogeneity of variance (Levene test). Statistical analyses were carried out using SAS 9.1 (Cary, FL, USA).

RESULTS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIAL AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES

Effect of JA on root growth

Application of 50 µm JA to one root chamber completely stopped the elongation of the treated barley roots (Fig. 1). No increase in root length of primary or lateral roots was detectable within 48 h after JA application. In contrast, the untreated roots continued to grow.

Effect of JA root treatment on C partitioning

Our barley plants invested on average 51% of the 11C mobilized from the load leaf into their root system (data not shown), which is comparable to other 11C tracer studies with barley seedlings (Minchin, Farrar & Thorpe 1994). There was a slight reduction in C partitioning over the time of measurements (Fig. 2), with both root portions of the untreated control plants showing a similar reduction. Because there were no statistically detectable differences between the two portions, they were combined for all controls. Treatment of one root portion with 50 µm JA led to a rapid reduction of its 11C partitioning compared with the root portion in control plants (Fig. 2; Table 1); the partitioning to the untreated portion of the same plants increased but with a longer delay. The magnitudes of decrease and increase were similar: 120 min after treatment, the treated roots received 11% less than in control plants (F1,17 = 13.07, P = 0.002), and the untreated roots received 11% more (F1,17 = 12.67, P = 0.0024). However, the first significant differences were detected for the treated roots (compared with controls) after a delay of 30 min, whereas the first significant differences for the untreated roots were only detected after 50 min (Fig. 2). Because the magnitudes of increase and decrease of root partitioning were indistinguishable, we expected no change in 11C partitioning to the entire root. There was no change in the fraction of mobilized 11C partitioned to the entire root system after JA treatment of one root half, compared with control plants (see section on root cooling). This lack of response occurred even though the treated fraction of the root system varied from 38 to 62%.

image

Figure 2. Effect of application of 50 µm jasmonic acid (JA) to one part of the barley split-root system on carbon partitioning between both root halves, showing the normalized fraction of recent photoassimilate (mobilized from the 11CO2 labelled leaf) that was delivered to each root portion. The data were normalized by setting the relative root partitioning to 1 at the time of treatment and adjusting all values by dividing each measurement by its value at the time of treatment t = 0. Ctrl shows the mean of the two root portions of untreated control plants. The two portions of the split-root plants are designated JA(+), JA(−) for the JA-treated plants. (Mean + SE, n = 12 for control plants, n = 7 for JA treated plants). Asterisks indicate significant changes in time compared to the control (i.e. different slopes). ((*)P < 0.1, *P < 0.05, **P < 0.01, ***P < 0.001).

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Table 1.  Repeated measurement anova table of F values of the effect of jasmonic acid (JA) treatment on barley shoots (JAs1, JAs2) or roots (JAr+, JAr−), and F values of the effect of root cooling (cool−, cool+) on the allocation of 11C to root portions in three split-root experiments
Compared root portionsSource of variationdFFP
JAr− versus CtrlJA−1.1711.08<0.001
Time11.1871.620.0971
JA × time11.18711.68<0.001
JAr+ versus CtrlJA+1.1719.7<0.001
Time11.18719.32<0.001
JA × time11.1876.28<0.001
Cool− versus CtrlCool−1.180.090.7660
Time11.19831.12<0.001
Cool− × time11.1982.860.0016
Cool+ versus CtrlCool+1.1849.44<0.001
Time11.198136.90<0.001
Cool+ × time11.19847.77<0.001
JAs1 versus CtrlJAs11.1647.52<0.001
Time11.1764.08<0.001
JAs1 × time11.17632.98<0.001
JAs2 versus CtrlJAs21.1625.66<0.001
Time11.1764.77<0.001
JAs2 × time11.17627.52<0.001

Effect of JA shoot treatment on C partitioning

Application of 1 mm JA to the shoot induced a very rapid increase in 11C allocation to the roots with no difference in allocation between the two root portions (Fig. 3, Table 1). Within 10 min after JA application, root partitioning had significantly increased compared with that in the untreated plants (P = 0.007): after 120 min, the C partitioning was increased by 14%. Control experiments with the application of Triton-X and EtOH (no JA) to the shoot showed no significant effect on 11C partitioning.

image

Figure 3. Effect of 1 mm jasmonic acid (JA) application to the shoot on the fraction of mobilized 11C-tracer that was transported to roots in split-root barley plants. The data were normalized at time of treatment. Ctrl shows the mean of the two root portions of untreated control plants. The two root portions of the split-root plants are designated JAs1 and JAs2 for the JA-shoot-treated plants. (Mean + SE, n = 12 for control plants, n = 6 for JA treated plants). Asterisks indicate significant changes in time compared to the control (i.e. different slopes). ((*)P < 0.1, *P < 0.05, **P < 0.01, ***P < 0.001).

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Effect of root cooling on C partitioning

The cooling treatment reduced the temperature of the cooled roots by 10 °C within 10 min, while the temperature of the untreated roots remained unaffected (Fig. 4a, Table 1). Correlating with the decline in temperature, partitioning of 11C to the cooled roots declined quickly (Fig. 4b) and was significantly reduced (P ≤ 0.001) within the first 10 min after the onset of cooling. After 120 min, the 11C partitioning to the cooled roots was reduced by 19% compared with that in control plants. Despite the reduction in 11C partitioning to the cooled roots, no change in 11C partitioning to the untreated roots was detectable compared with the roots of control plants, and therefore the total partitioning to the roots was significantly reduced (Fig. 5).

image

Figure 5. A comparison of shoot treatment by jasmonic acid (JA) with part-root treatment (by cooling or by JA) on the fraction of 11C-tracer transported to the entire root system of split-root barley plants. The data were normalized at time of treatment. ctrl: control plants; JAr: 50 µm JA treatment of one root half; JAs: JA treatment of the shoot; cool: plants where one root portion was cooled. (Mean + SE: n = 12 for control plants, n = 7 for JA root, n = 6 for JA shoot; n = 7 for root cooled plants). Asterisks indicate significant changes in time compared to the control (i.e. different slopes). ((*)P < 0.1, *P < 0.05, **P < 0.01, ***P < 0.001).

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DISCUSSION

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIAL AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES

Our aim was to compare the effect of exogenously applied JA to leaves or roots on the partitioning of recently fixed carbon. It has been shown for leaf treatment with JA that carbon export is enhanced, for example, in poplar and tobacco (Babst et al. 2005; Thorpe et al. 2007), and partitioning of the exported carbon changes in favour of roots (Babst et al. 2005; Schwachtje et al. 2006). To our knowledge, there is no corresponding information on carbon partitioning in response to root treatment, even though local treatment of roots inhibits root growth, so that ‘inhibition of root growth became the most prominent assay for screening of mutants affected in respect of JA signalling’ (Wasternack 2007). Further, it has recently been reported that in very young N. attenuata seedlings, the shoot application of JA or caterpillar oral secretions is followed rapidly by a significant but transient reduction in root elongation rate (Hummel and colleagues, unpublished results) and, within a day, an increase in defence compounds such as nicotine (Hummel et al. 2007). In addition, amino acid composition and sugar content are affected by JA application within 7 d (van Dam & Oomen 2008). Our method employing the short-lived isotope 11C quantifies the fate of ‘mobilized tracer’ (the tracer carbon that has exited a labelled leaf), giving the fraction of that mobilized tracer that eventually enters each root portion (the balance of the mobilized tracer remains within the shoot) (Minchin & Thorpe 1989, 2003). The analysis accounts for isotope decay, but because the isotope has a half-life of only 20 min, the tracer reflects only recent photoassimilate and not all the carbon that is moving. Our measurements thus give ‘partitioning of recent photoassimilate’ that mainly consists of sucrose. Nevertheless, in a split-root barley plant, it can be assumed that changes in partitioning of recent assimilate in treated roots, relative to the untreated roots, reflect changes in the partitioning of total carbon, because we can expect both old and new carbon arriving from the shoot into the crown of graminaceous plants to be distributed in the same ratio between all parts of the root system (Williams, Minchin & Farrar 1991), unlike many dicotyledons where the vascular architecture is sectorial (Orians 2005). This combined use of short-lived isotopes, which allows repeated and non-invasive monitoring of plant function, with the facility to maintain sterile hydroponic split-root systems, is expected to form a valuable platform for the study of biotic interactions.

In our experiment, root elongation was inhibited locally by 50 µm JA, which is in accordance with observations on the entire root systems of Arabidopsis and tomato plant roots treated with JA at similarly high concentrations (Staswick et al. 1992; Tung et al. 1996; Creelman & Mullet 1997). Associated with this inhibition of root elongation, there was a highly significant reduction in carbon partitioning to the JA-treated part of a root system, within the first 30 min of the treatment. After a further time delay of 30 min, partitioning to the untreated roots increased significantly: as a consequence of this compensatory interaction between the two root systems there was no significant overall change in root/shoot partitioning. Such short-term changes in carbon distribution, where one sink benefits at the expense of another, have been reported frequently (Pickard, Minchin & Troughton 1979; Thorpe & Lang 1983), and are commonly utilized in horticulture. However, Minchin et al. (1994) reported that cooling one part of a barley split-root system caused no compensatory increase in carbon partitioning into the untreated roots, despite a strong reduction of carbon partitioning to the cooled roots. Because plant growth conditions can affect source–sink interactions by altering carbon status by altering the relative priority of sinks (Minchin & Thorpe 1996), and by affecting the cellular pathway for radial exchange with phloem (Hayes, Patrick & Offler 1987), we investigated whether this difference in response was related to the nature of the treatment, or to the physiological state of our barley plants that were grown with sterile roots and under about 50% lower irradiance compared with those of Minchin et al. (1994). Because our plants showed no compensatory response between root portions when one portion was cooled (Fig. 4), the differences between JA and cooling responses cannot be ascribed to growing conditions, rather to the nature of the treatment.

The lack of a compensating response in the untreated roots after cooling one root portion of split-root barley was explained by a model of osmotically driven Münch phloem transport through water-impermeable tubes with saturable carbohydrate (and water) unloading at sinks (Vmax) (Minchin, Thorpe & Farrar 1993). In their split-root plant model, with sinks near saturation, and with cooling merely reducing Vmax, no compensating increase in partitioning is expected in the non-cooled root until some time later when, for example, gene expression responds to sugar levels (Chiou & Bush 1998). In this model, carbon partitioning responds to mass action and fluid dynamics, without the need for a chemical signal.

Because this compensatory response of sinks does occur after partial JA root treatment, but not in response to cooling, we conclude that JA caused more widespread physiological changes than mere cooling. We suggest that JA or a related signal moves to the shoot in response to root application where it promotes both export and root partitioning, so root partitioning can be expected to change in response to leaf export alone, as has been seen in response to shading (Minchin & Thorpe 2003); however, the inhibition of local root elongation by the treatment makes for a lower carbon demand. Several facts are relevant. Firstly, the response time after JA treatment was much slower in the non-treated root, suggesting a delay in signal transport to the shoot, and for the subsequent partitioning response. The additional time needed for a response (30 min) corresponded roughly to the response time after shoot treatment, suggesting that increased root partitioning was induced by a signal in the shoot. Secondly, it is likely that some JA or a conjugate will move to the shoot in the xylem (Babst et al. 2005, 2008; Schwachtje et al. 2006; Thorpe et al. 2007). Finally, leaf treatment with jasmonate or caterpillar oral secretion increases the export of recent photoassimilate to the roots (Babst et al. 2005; Schwachtje et al. 2006), and jasmonates or caterpillar oral secretion promotes partitioning of photosynthate towards stem and roots (Babst et al. 2005, 2008; Schwachtje et al. 2006). We attribute the lower carbon partitioning to JA-treated roots to the total inhibition of their elongation rate. Incidentally, we noted that the treated roots quickly took on a yellow colouration, quite unlike senescent roots, suggesting the presence of phenolic compounds, like flavonoids, as we have observed in response to biotic stress (Lanoue et al., unpublished results), which may correspond to the synthesis of defence compounds in response to shoot treatments, as observed in Nicotiana (Hummel et al. 2007). Assuming that JA treatment partially mimics both herbivore attack (Baldwin 1998; Walls et al. 2005; Hare & Walling 2006) and root attack (van Dam et al. 2004; van Dam & Raaijmakers 2006), it seems possible that root treatment may simulate an attack by root herbivores, giving rise to a tolerance response in which resources are diverted to other roots that have escaped attack, a response analogous to that after leaf herbivory, where resources are diverted to a less vulnerable location than the shoot (Babst et al. 2005; Schwachtje et al. 2006). Mimicked herbivory on the leaves led to an up-regulation of invertase activity in the roots within 5 h, suggesting that an increase in sugar-cleaving activity leads to an increased sink strength of roots (Schwachtje et al. 2006). Recent results demonstrated that the allocation of recently fixed carbon in response to partial root pathogen infection in barley also led to an enhanced allocation of carbon to the untreated roots without changes in shoot/root partitioning (Henkes et al., unpublished results), suggesting that barley plants can adjust carbon partitioning within the root system in response to biotic stress.

We have demonstrated the benefit of a combined use of short-lived isotopes, which allows for a non-invasive, repeated monitoring of plant function with high time resolution in combination with the facility to maintain sterile hydroponic split-root systems for the study of biotic and abiotic stresses and sundry signalling compounds. In a split-root system, JA treatment of one root portion resulted in a shift in carbon partitioning in favour of the other roots; this effect was not able to be achieved with cooling. Despite complete cessation of elongation of the treated roots, their carbon import continued (at a reduced rate). We suggest that the diversion of resources to root systems is a tolerance behaviour exhibited following the simulation of both herbivory and root attack.

ACKNOWLEDGMENTS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIAL AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES

We thank Marco Dautzenberg for his help in the preparation of 11C tracer and plant labelling; and Gregoire Hummel and Arnaud Lanoue for their assistance and helpful discussions. This work emerged out of the Virtual Institute for Biotic Interactions (ViBi), which is financially funded by the Helmholtz Association of German Research Centres.

REFERENCES

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIAL AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES
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  • Babst B.A., Ferrieri R.A., Thorpe M.R. & Orians C.M. (2008) Lymantria dispar (gypsy moth) herbivory induces rapid changes in carbon transport and partitioning in Populus. Entomologia Experimentalis et Applicata. doi: 10.1111/j.1570-7458.2008. 00698.x
  • Baldwin I.T. (1996) Methyl jasmonate-induced nicotine production in Nicotiana attenuata: inducing defenses in the field without wounding. Entomologia Experimentalis et Applicata 80, 213220.
  • Baldwin I.T. (1998) Jasmonate-induced responses are costly but benefit plants under attack in native populations. Proceedings of the National Academy of Sciences of the United States of America 95, 81138118.
  • Chiou T.J. & Bush D.R. (1998) Sucrose is a signal molecule in assimilate partitioning. Proceedings of the National Academy of Sciences of the United States of America 95, 47844788.
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