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Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

Plant responses to mechanical stress (e.g. wind or touch) involve a suite of physiologic and developmental changes, collectively known as thigmomorphogenesis, including reductions in height increment, Young’s modulus of stems, shoot growth, and seed production, and increased stem girth and root growth. A role of the phytohormone ethylene in thigmomorphogenesis has been proposed but the extent of this involvement is not entirely clear. To address this issue, wild-type (WT) and ethylene-insensitive transgenic (Tetr) tobacco (Nicotianum tabacum) plants were subjected to three levels of mechanical stress: 0, 25 and 75 daily flexures. Flexed plants produced shorter, thicker stems with a lower Young’s modulus than non-flexed ones, and these responses occurred independently of genotype. This suggests that ethylene does not play a role in thigmomorphogenesis-related changes in stem characteristics in tobacco. The effect of mechanical stress on dry mass increment (growth), on the other hand, differed between the genotypes: in the WT plants, shoot growth but not root growth was reduced under mechanical stress, resulting in reduced total growth and increased root mass fractions. In the Tetr plants, neither shoot nor root growth were affected. This suggests that ethylene is involved in the inhibition of tobacco shoot growth under mechanical stress.


Abbreviations –
E

Young’s modulus

LAR

leaf area per unit mass

LMR

leaf mass ratio

NAR

growth per unit leaf area

RGR

growth per unit mass

SLA

specific leaf area

σb

break stress

Tetr

ethylene-insensitive transgenic

WT

wild type

Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

When plants are exposed to mechanical stimuli such as wind, touching, rubbing or flexing, a suite of physiologic and developmental changes occurs that are collectively known as thigmomorphogenesis (Jaffe 1973). These include reductions in height increment (e.g. Jones et al. 1990; Holbrook and Putz 1989; Henry and Thomas 2002; Anten et al. 2005), dry mass increment (Jones et al. 1990; Niklas 1998), seed production (Niklas 1998), and leaf area (Jones et al. 1990), and increases in stem girth (Boyer et al. 1986; Holbrook and Putz 1989; Mitchell 2003; Anten et al. 2005), root allocation, stem flexibility and root strength (Crook and Ennos 1996; Goodman and Ennos 1996; Niklas 1998). These responses generally make plants more resistant to mechanical failure (Jaffe and Forbes 1993; Niklas 1992).

The signal transduction pathway leading to thigmomorphogenesis is not clearly understood (Braam 2005), although a number of touch genes (TCH) have been implicated in mechanical stress responses in Arabidopsis thaliana(Braam and Davies 1990; Braam et al. 1996). For a long time there have been indications that the phytohormone ethylene plays a prominent role in thigmomorphogenesis. Mechanical stimulation results in a rapid increase in ethylene evolution (e.g. Poovaiah 1974; Eisinger 1983; Emery et al. 1994), and in an increase in the activity of ACC (1-amino-cyclopropane-carbxylic acid) synthase, a key enzyme in ethylene synthesis (Biro and Jaffe 1984). Exogenous application of ethylene has been shown to induce growth responses in plants that are very similar to those associated with thigmomorphogenesis, such as reduced stem elongation (Erner and Jaffe 1982; de Jaegher et al. 1987; Emery et al. 1994). In the perennial Stellaria longipes, application of an ethylene inhibitor reversed the negative effects of mechanical stress on stem elongation (Emery et al. 1994). In addition, it appears that ethylene can induce TCH3 expression in the absence of mechanical stimulation (Sistrunk et al. 1994; Wright et al. 2002).

A study with ethylene-insensitive mutants of A. thaliana(Johnson et al. 1998) seems to contradict the notion that ethylene is important in thigmomorphogenesis. The etr1–3 and ein2–1 mutants, which have been shown to be very insensitive to ethylene, responded to wind exposure with reductions in height growth, and changes in flowering time and flower number that were similar to the responses of wild types (WTs). These mutants also exhibited a similar degree of upregulation of TCH gene expression in response to touch stimulation.

Johnson et al. (1998) only investigated a few of the many thigmomorphogenesis-related responses, and did not study the involvement of ethylene sensitivity in, for example, changes in stem diameter and flexibility, root characteristics, or growth and biomass allocation. A. thaliana does not exhibit some of these responses (e.g. changes in stem diameter), since, as a rosette species, it forms no stems during its vegetative growth. The stem is part of the inflorescence, whereas in many other plants (i.e. stem-bearing plants), it usually develops soon after germination and serves to support the leaves. It has been found in stem-bearing plants that ethylene biosynthesis/action inhibitors can block stem diameter enhancement but not reductions in stem elongation in mechanically disturbed plants (Biro and Jaffe 1984; Boyer et al. 1979; 1983; 1986; Braam 2005). Thus, it could be that ethylene is involved in some aspects of thigmomorphogenesis but not in others (Braam 2005). It could further be that results obtained for the rosette plant A. thaliana are not generally applicable to plants in which the stem has an ontogenetically different function.

Here we seek to determine the involvement of ethylene in the changes in morphology, growth, biomass allocation and mechanical properties of stem tissue associated with thigmomorphogenesis in tobacco. To this end, we compared the mechanical stress effects on WT tobacco to those on ethylene-insensitive transgenic plants. Tobacco was chosen because of the availability of ethylene-insensitive plants, and because this species appears to display a large variety of thigmomorphogenic responses to mechanical stress, including reductions in stem elongation, biomass increment and elastic modulus of the stem, and increases in stem girth and root allocation (Anten et al. 2005). It also has a very simple growth form, bearing its leaves along a single vertical stem, making it particularly useful for performing and interpreting mechanical analyses.

Materials and methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

Plant material

For this experiment, we used WT tobacco (Nicotianum tabacum cv. ‘Samsun NN’) and the ethylene-insensitve transgenic, Tetr. Tetr was derived from ‘Samsun NN’ through the introduction of an A. thaliana ETR1 allele with the dominant activating mutation etr1–1(Knoester et al. 1998; Pierik et al. 2003). Insensitivity to ethylene of the transgenics was clearly documented by Pierik et al. (2003).

Mechanical stress experiment

The experiment was carried out in a greenhouse of Utrecht University, in The Netherlands. On 3 November 2003, seeds were sown in trays in a mixture of sand and potting soil and grown at 30% of natural daylight created by neutral density shade cloth and shading by the greenhouse roof. On 3 December, seedlings were selected for equal size, both within and between genotype, and transplanted into pots (volume 6 L) on a mixture of sand and sieved potting soil (half of each) and without shade cloth at about 60% of daylight. Additional lighting was supplied with HPI Quick 400-W lamps for 16 h, and the mean daytime photon flux density during the experiment was 187 umol m−2 s−1, measured with a quantum sensor (LI190SA, LiCor, Lincoln NE) connected to a datalogger (LI1000). At this time, 3 g of slow-release fertilizer (Osmocote 10% N + 10% P + 10% K + 3% Mg + trace elements, release time 3 months) was added to each pot. Plants were watered daily throughout the experiment.

On 17 December, plant height from soil level to the highest leaf node, stem diameter just above the first true leaf, leaf number and the length of the biggest leaf were measured. For the experiment, we then chose 48 plants of each genotype, which were of intermediate height: the shortest and the tallest plants were excluded from the experiment. On the next day, plants were assigned to one of three mechanical disturbance treatments: 0, 25 or 75 flexures each day (the control, m25 and m75 treatments, hereafter) or to a separate group used for the initial harvest (see below). There were 12 replicate plants per treatment genotype combination, and these were placed in four blocks.

The flexing treatment was carried out by gently grasping the stem at about four-fifths of its height and bending it back and forth no further than 45° from the vertical. Flexing lasted about 30 s for the m25 treatment and 90 s for the m75 treatment. We chose this type of flexing because it simulates the mechanical effect of wind on plants (compressive and tensile forces on stems and roots) without affecting their microclimate.

The first harvest was conducted on 18 December, before initiation of treatments, to determine the baseline biomass distribution at the onset of the experiment. The 12 selected plants from each genotype were cut at ground level and divided into stems and leaves, and the fresh weights of both parts were determined. Leaf area was determined with a leaf area meter (LI 3100). Root systems were carefully washed. Dry weights of all plant parts were determined after oven-drying for at least 72 h at 70°C to constant weight.

The final harvest was conducted on 13 January 2004, 26 days after the first harvest, to determine growth rates and patterns of biomass allocation. Plants were cut at ground level and their height was measured from the base to the top meristem. Basal diameter and the diameters at one-third and two-thirds of stem length were measured with a digital caliper to the closest 0.1 mm. To determine the vertical distribution of fresh leaf and stem mass, plants were divided into three equal-height segments. Leaves of each segment were removed with a razor blade and weighed, and their area was determined as described above. After the mechanical measurements on stems were completed (see below), they were clipped into the three height segments and also weighed. Dry masses of each clipping segment, stems and leaves, and root mass were measured as described above.

Growth per unit mass (relative growth rate, RGR), growth per unit leaf area (net assimilation rate, NAR) and the mean leaf area per unit plant mass (mean leaf area ratio, LAR) were determined following classic growth analysis, following Equations 3.3, 3.6 and 3.8 from Beadle (1993). Leaf losses during the experimental period were minimal and were not considered (see Anten and Ackerly 2001). Plants from the two harvests were randomly paired.

The mechanical stress experiment was independently repeated between October and December 2005. The observed responses were very similar to those found for the first experiment (supplementary material), although at the final harvest plants were smaller than in the first experiment, as they had been harvested at an earlier stage (16 instead of 26 days after the initial harvest, which was 61 instead of 71 days after sowing). The data presented in Results, therefore, refer to the experiment described in the above paragraphs.

Mechanical measurements

Two material properties were measured on the stems: the elastic Young’s modulus (E, N m−2) and the breaking stress (σb, N m−2), following Anten et al. (2005). E indicates the rigidity and σb the resistance to breaking of a given tissue. Both measurements were completed no longer than 15 min after a stem had been cut from its pot and its leaves removed. Stems were fixed at both ends between small clamps that were coated with a layer of rubber foam to prevent the tissue from being crushed. To measure E, we placed an empty water bottle exactly halfway along the stem. Small loads of water (P) were weighed and then added to the bottle, and the vertical displacement (δ) was measured to the nearest millimetre. The advantage of this arrangement is that the force is held perpendicular to the stem even as the stem bends. The Young’s modulus was calculated from the linear regression of δ against P from the equation of a fixed end beam (Equations 10–19 in Gere and Timoshenko 1999):

  • image(1)

where L is the length of the stem segment and I the second moment of area (I = 1/4πR4, where R is the stem radius). The breaking stress (σb) was measured by increasing the loads in small steps to the point where the stem failed and was calculated as:

  • image(2)

with

  • image(3)

where M is the bending moment and P, in this case, is the load that was supported just before failure occurred (Gere and Timoshenko 1999).

Ethylene experiment

WT and Tetr plants were grown in a growth chamber in pots with 16 h of light at 220 umol m−2 s−1(Phillips HDS 600W, Eindhoven, The Netherlands), a temperature of 20°C, and 70% relative humidity. After 5 weeks, 48 plants were transplanted to closed chambers (volume 35 dm3), which were flushed with either of two ethylene concentrations (0 or 1 ul l−1, Praxair; Oevel Belgium) in air (see Pierik et al. 2003 for details). There were 12 replicate plants for each treatment–genotype combination. Plants were harvested after 7 days, and their shoot dry mass was determined as described for the mechanical stress experiment.

Statistical analysis

The experiment was laid out in a randomized block design. In the mechanical stress experiment, a three-way analysis of variance (anova) was used to test for differences in response parameters with mechanical treatment (df = 2), genotype (df = 1) and block (df = 3) as fixed factors. The ethylene experiment was analyzed with a two-way anova with ethylene concentration (df = 1) and genotype (df = 1) as fixed factors. Some parameters were log transformed based on Levene’s test for equality of variance.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

Stems of flexed plants were significantly shorter and thicker than those of the control plants, whereas WT plants had taller and thinner stems than the ethylene-insensitive transgenics (Tetr)(Fig. 1; Table 1). Reductions in height increase under mechanical stress were similar, 27–36%, for the two genotypes, and there was no significant mechanical stress × genotype interactive effect on this characteristic (Table 1). Enhancement in diameter due to flexing appeared to be greater in the Tetr (7–11%) than in the WT plants (3–6%)(Fig. 1), but this difference was not significant (Table 1).

image

Figure 1. Height, basal diameter, Young’s elastic modulus (E) and breaking stress (σb) of stems of WT and ethylene-insensitive transgenic (Tetr) tobacco plants subjected to either 0, 25 or 75 daily flexures. Bars indicate sem values (n = 12).

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Table 1.  Results (P-values) of analysis of variance for the mechanical stress experiment, with genotype (df = 1) and mechanical stress (df = 2) as factors (for the genotype × mechanical stress interaction df = 2).
 GenotypeMechanical stressGenotype × mechanical stress
  • a

    Data were log transformed. Numbers in bold indicate significant effects (P < 0.05).

Stem properties
Heighta<0.00010.00010.211
Diametera<0.0001<0.00010.153
Internode number0.0010.0820.589
Internode length<0.0001<0.00010.182
Elastic modulusa<0.00010.0270.860
Breaking stressa0.8160.1510.713
Growth and mass allocation
Total growth0.6170.2630.046
Shoot growth0.2960.1810.014
Leaf growth<0.00010.7680.033
Stem growth<0.0050.0230.006
Root growtha0.1380.8320.695
Root allocationa0.0260.6460.019
Root mass ratioa0.0110.2040.015
RGR<0.00010.4110.104
NAR0.6940.1490.123
LAR0.0060.0680.325
SLA0.3370.2560.304

The Young’s modulus (E) was lower in the flexed than in the control plants, and lower in the Tetr than in the WT plants (Fig. 1), where a lower E value indicates that stems were made of more flexible tissue. The effect of flexure on E was independent of genotype (Table 1). The value of the breaking stress (σb) was not affected by either mechanical stress or genotype (Fig. 1; Table 1), with σb indicating the resistance of stem tissue to rupture.

The level of mechanical stress, 25 or 75 flexures a day, had relatively little effect on the properties of the stems; there was no significant difference between these two treatments for any of the four characteristics (Scheffe’s post hoc test P > 0.05).

There were clear interactive effects of genotype and mechanical stress on leaf, stem and whole plant growth rates (measured in terms of dry mass increment)(Table 1; Fig. 2). Among the WT plants, all three of these were significantly lower for the flexed plants than for the control plants (P = 0.012, 0.0001 and 0.002, respectively), but among the Tetr plants this was not the case (P = 0.432, 0.651 and 0.633), based on a two-way anova with data limited to either genotype. Root growth, on the other hand, was unaffected by either genotype or mechanical stress (Table 1; Fig. 2). Consequently, we found that WT plants had significantly larger root mass fractions than control WT plants, but that this was not the case for Tetr plants (Fig 2; Tables 1 and 2). Compared to WT plants, Tetr plants allocated a larger fraction of their biomass to leaves and less to stems (Tables 1 and 2).

image

Figure 2. Total growth, growth of shoots, leaves, stems and roots and the mean fraction of new growth that is allocated to roots of WT and ethylene-insensitive transgenic (Tetr) tobacco plants subjected to either 0, 25 or 75 daily flexures. Bars indicate sem values (n = 12).

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Table 2.  Mean values of stem properties, growth and mass allocation for WT and ethylene-insensitive transgenic (Tetr) tobacco plants subjected to either 0 (control), 25 (m25) or 75 (m75) daily flexures. Data in parentheses are sem values (n = 12).
 WT controlm25m75Tetr controlm25m75
Stem properties
Node number27.3 (1.09)25.8 (0.43)26.7 (0.76)24.6 (0.78)23.2 (0.49)26.7 (0.63)
Node length (cm)2.70 (0.11)1.96 (0.06)1.76 (0.07)2.14 (0.09)1.58 (0.09)1.54 (0.10)
Growth and allocation
Leaf allocation (g g−1)0.490 (0.009)0.511 (0.010)0.521 (0.012)0.566 (0.010)0.590 (0.010)0.577 (0.007)
Stem allocation (g days−1)0.360 (0.007)0.316 (0.013)0.305 (0.013)0.280 (0.009)0.268 (0.010)0.282 (0.012)
Root/shoot ratio0.164 (0.010)0.199 (0.009)0.225 (0.009)0.178 (0.008)0.160 (0.007)0.164 (0.011)
RGR (mg g−1 days−1)117.3 (3.0)111.8 (3.0)107.8 (3.1)130.0 (2.7)128.1 (3.0)130.3 (2.9)
NAR (g m−2 days−1)5.48 (0.23)4.63 (0.25)5.18 (0.19)4.77 (0.19)4.91 (0.29)5.24 (0.16)
LAR (m2 kg−1)22.3 (0.9)25.4 (1.3)22.7 (0.9)27.0 (1.3)26.9 (1.9)25.2 (0.7)
SLA (m2 kg−1)31.9 (2.1)36.8 (2.6)30.4 (1.7)36.1 (2.1)34.7 (3.1)32.5 (1.3)

The RGR was not significantly affected by mechanical treatment. Tetr plants had higher RGR values than WT plants (Tables 1 and 2). This resulted from the fact that at the onset of the experiment, Tetr plants had a smaller mass (data not shown), but at the end of the experiment they had achieved the same mass as the WT plants (see above). RGR is the product of NAR and LAR. Tetr plants had higher LAR values than WT plants, but this ratio was not affected by flexing. The higher LAR values of Tetr plants resulted from them having higher leaf mass ratios (LMRs)(data not shown) and not from them having a higher specific leaf area (SLA)(LAR = SLA × LMR), which was unaffected by either genotype or flexing (Table 1). NAR was also not significantly affected by either flexing or genotype.

Exogenous application of ethylene resulted in a 40% reduction in shoot mass in WT plants, whereas in the Tetr plants ethylene application had no effect (Fig. 3). There was also a significant interactive effect of genotype and ethylene level on shoot mass (P = 0.018, two-way anova).

image

Figure 3. Shoot dry mass of WT and ethylene-insensitive transgenic (Tetr) tobacco flushed for 7 days with ethylene-free air (Air) or air containing 1 ul l−1 ethylene (Ethylene). Bars indicate SEM values (n = 12). Letters indicate significant differences (P < 0.05, Tukey hsd).

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Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

Stem properties

Mechanical stress through flexing resulted in plants having shorter, thicker stems with a lower Young’s modulus (E, i.e. less rigid; Fig. 1; Table 2), which is in accordance with the results of most other studies (e.g. Jones et al. 1990; Holbrook and Putz 1989; Mitchell 2003; Anten et al. 2005). These responses are probably adaptations to windy conditions, because short, thick stems that are made of more elastic tissue have a lower probability of stem rupture, and convert a smaller bending moment to the anchorage root system when exposed to lateral forces (Niklas 1992; 1998; Anten et al. 2005). For example, using Equation 3 from Anten et al. (2005), we estimated the maximum lateral force needed to cause stem rupture at the base of the stem to be between 50% and 100% higher in the flexed than in the control plants (calculations not shown).

Changes in Young’s elastic modulus (E) were probably associated with changes in the stem anatomy. For example, lower E values of stems of flexed Capsella bursa-pastoris plants were associated with a relative increase in parenchyma cells and a reduction in phloem fibers (Niklas, personal communication), with the former having a much lower E than the latter (Niklas 1993).

The two genotypes exhibited comparable changes in stem properties under mechanical stress (Fig. 1; Tables 1 and 2). This suggests that in tobacco, ethylene does not play an important role in any of the changes in stem properties that occur during thigmomorphogenesis: reduction in stem elongation and increases in stem diameter and tissue flexibility (reduced Young’s elastic modulus, E). Johnson et al. (1998) obtained similar results when two ethylene-insensitive A. thaliana mutants were compared to a WT. They also found that vibration treatment elicited a very similar degree of upregulation in TCH gene expression in mutants as in WTs. It was suggested that ethylene is not important in the signal transduction pathway leading to TCH gene expression and changes in stem properties under mechanical stress, and that this pathway involves other factors, including the transmembrane proteins (He et al. 1996) or stretch-activated channels (Falke et al. 1988) as mechanosensors, and Ca2+ as second messenger (Knight et al. 1991). The suggestion that ethylene is not involved in thigmomorphogenesis-related changes in stem characteristics, however, contradicts Biro and Jaffe (1984), whose results suggest that ethylene plays a role in changes in stem girth (see Introduction).

It is possible that our results and those of Johnson et al. (1998) apply to some but not all plants. The ability to synthesize ethylene in response to environmental stimuli and the types of response differ between species and between different genotypes of the same species (Fiorani et al. 2002). Emery et al. (1994) found that an alpine ecotype of Stellaria longipes, which occurs in exposed, windy habitats, exhibited a strong increase in ethylene production in response to wind exposure, whereas application of ethylene inhibitors reversed the negative effect of wind on height growth. By contrast, a prairie ecotype of this species, commonly growing in dense vegetation, showed no change in ethylene production under wind exposure, and exhibited increased stem elongation in response to low levels of applied ethylene. Similar results, stimulation of stem elongation by low levels of ethylene, have been obtained in several other species (see Voesenek et al. 2003). These findings are consistent with our observation that WT plants produced taller, thinner stems than Tetr plants (Fig. 1). Thus the role of ethylene in the changes in stem properties induced by mechanical stress might depend on the growth environment in which plants have evolved. For plants typically occurring in dense vegetation, an inhibition of stem elongation in response to ethylene produced in the canopy might be detrimental, as it causes them to be overgrown by neighbors, and a stimulatory effect is probably more adaptive (Pierik et al. 2004). In open habitats, where wind exposure is a more serious factor, this inhibition of stem elongation could be adaptive (Emery et al. 1994).

Growth and biomass allocation

In the WT plants, flexing significantly reduced growth (i.e. biomass accumulation), and this reduction could be entirely attributed to lower shoot growth, with root growth remaining unchanged (Fig. 2; Tables 1 and 2). These changes were reflected in a higher root-to-mass fraction of the flexed WT plants. By contrast, in the Tetr plants, flexing did not have an effect on either shoot or root growth. These results were replicated in an independent second experiment, and suggest that in tobacco ethylene is involved in the inhibition of shoot growth caused by mechanical stress. This is confirmed by the fact that exogenous ethylene also inhibited shoot growth in this species (Fig. 3). As expected, shoot growth of Tetr plants was entirely unaffected by exogenous ethylene, confirming this line’s insensitivity to ethylene in many plant traits, such as stem growth and leaf movement (Pierik et al, 2003, 2004), the classic triple response of seedlings (Knoester et al, 1998) and thus also biomass accumulation (Fig. 3).

A reduction in shoot growth in favor of root growth is probably adaptive in windy environments, as smaller shoots have less wind-exposed area and are thus subjected to smaller drag forces, whereas roots provide anchorage (Vogel 1994). Previous studies found that mechanically stressed plants increased root growth and reduced shoot growth compared to undisturbed plants (e.g. Goodman and Ennos 1996; Niklas 1998). These changes in allocation in the stressed plants were associated with the production of thicker roots made of more rigid (higher elastic modulus) and stronger (higher breaking stress) material, and probably as a consequence the anchorage strength of these root systems is enhanced (Goodman and Ennos 1996).

No study known to us has investigated the potential role of ethylene in the changes in root vs shoot growth that occur in response to stem bending or flexing, i.e. as they would occur when plants are exposed to wind. However, a potential role of ethylene has been implicated in growth responses to another type of mechanical stress, namely in situations where root growth is impeded by soil compaction. Roots growing in compacted soil exhibit increased radial growth, and this response was associated with an increase in ethylene evolution and greater ACC activity (Sarquis et al. 1991; He et al. 1996).

Hussain et al. (1999) compared growth responses to soil compaction between WT tomato plants and a transgenic genotype with a strongly reduced capacity to produce ethylene, and obtained results that were surprisingly similar to ours. In the WT, shoot growth was reduced and root growth remained unchanged. This response was associated with increased ethylene production, whereas treatment with an ethylene inhibitor helped restore shoot growth to levels similar to those in uncompacted soil. In contrast to the WT, shoot growth of the transgenic plants was not affected by soil compaction. Hussain et al. (1999) proposed that ACC, an ethylene precursor, has a role as a root-sourced signal, easily transported in the transpiration stream that is converted to ethylene in the shoot, where it acts to inhibit growth. Thus, ethylene seems to be involved in the inhibition of shoot growth in situations where roots are mechanically impeded by compacted soil. Our results suggest that in tobacco, ethylene plays a similar role when plants are mechanically stressed in windy conditions.

Edited by D. Van Der Straeten

Acknowledgments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

Acknowledgements –  We thank Marinus Werger for valuable comments on the manuscript and Diego Aguilar Sanchez, Sander van Halst, Sonja Huggers, Henri Noordman, Fred Siesling, Máximo Vaquero Llamas and Betty Verduyn for technical assistance. This work was partly supported by a post-doctoral grant from the Graduate School of Plant Ecology and Recource Conservation, Wageningen UR to NPRA and an Erasmus exchange-student fellowship to RCG.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References