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Biogenic volatile organic compounds and plant competition
Opinion
Special Issue: Induced biogenic volatile organic compounds from plants
Biogenic volatile organic compounds and plant competition Wouter Kegge and Ronald Pierik Plant Ecophysiology, Institute of Environmental Biology, Utrecht University, Padualaan 8, 3584 CH Utrecht, the Netherlands
One of the most important factors to shape plant communities is competition between plants, which impacts on the availability of environmental factors such as light, nutrients and water. In response to these environmental parameters, plants adjust the emission of many different biogenic volatile organic compounds (BVOCs). BVOCs can also elicit responses in neighbouring plants, thus constituting a platform for plant–plant interactions. Here, we review the relationship between BVOC emissions and competition among neighbouring plants. Recent progress indicates that BVOCs can act both as allelochemicals and as neighbour detection signals. It is suggested that BVOCs provide information about neighbouring competitors, such as their identity or growth rate, that classic neighbour detection signals cannot provide. Plant competition: battling with neighbours Competition with neighbouring plants for resources such as light, water and nutrients, is one of the most common stresses that a plant has to deal with. Competitive interactions among plants shape vegetation composition and control biodiversity [1,2], making it one of the most important processes for vegetation patterns on a global scale. To compete successfully, plants exploit a range of phenotypically plastic responses that help to enhance resource capture and thus increase their fitness during competition [3– 5]. Given the strong selection pressure during competition, rapid detection of neighbours, followed by adaptive responses, is key to competitive success. Such plastic responses to neighbours encompass defensive strategies that help avoid competitive interactions (e.g. shade avoidance) as well as offensive strategies that inhibit the performance of proximate competitors (e.g. allelopathy) [4,6]. Depending on which resources plants compete for, they will allocate more carbon to roots (competition for nutrients or water) or shoots (competition for light). During competition for light, this carbon is invested in so-called ‘shade avoidance’ responses, which include upward movement of leaves, enhanced elongation of internodes and petioles, and increased apical dominance, which all function to position the photosynthesizing leaves in the well-lit, upper parts of the vegetation [7,8]. When competing belowground, root proliferation into resource-rich patches is Corresponding author: Pierik, R. ([email protected]).
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thought to be one trait that could contribute to competitive ability [9], but there is also evidence that this is not always the case [10,11] and the complete set of adaptive traits to enhance belowground competitive vigour is yet to be unveiled. The above-mentioned competitive behaviours are activated upon sensing of neighbour detection cues, such as the classic reduction in the red:far-red light ratio (R:FR) of the light reflected by, or transmitted through, plants. This R:FR reduction is created by selective absorption of R light for photosynthesis and reflectance of FR light. The low R:FR is sensed by the phytochrome family of photoreceptors, a process that precedes actual shading and thus competition for light [7], and initiates shade avoidance responses. In addition to a reduced R:FR, the reduction in blue light photon fluence rates based on photon absorbance for photosynthesis, can also act as a neighbour detection signal (Figure 1) [7,8]. In addition to these aboveground signals, plant neighbour detection also involves various belowground signals, such as depletions of water and/or nutrients as a consequence of efficient uptake by neighbouring root systems [12,13] and organic exudates excreted by neighbouring plants [14]. The past five years have seen major advances in the research area of plant neighbour detection during competition. It is believed that the volatile hormone ethylene could serve as a neighbour detection cue [15], because it is released by most plant species and accumulates in the atmosphere inside dense stands of greenhouse-grown plants, such as tobacco (Nicotiana tabacum) [15]. This provided the first evidence for competition-mediated regulation of emissions of biogenic (plant-produced) volatile organic compounds (BVOCs) and their biological function in plant neighbour detection. More recently, reports emerged showing that the emissions of various other BVOCs, such as mono- and sesquiterpenes, are affected by competition, as well as by abiotic stressfactors, such as deficiencies in water or light, that typically occur during competition. Here, we review and discuss the effects of competition on BVOC emissions and how these BVOC emissions might affect the growth of neighbouring competitors. We suggest that the combination of classic neighbour detection signals and species-specific BVOC blends provides information about competing neighbours with unprecedented detail and reliability.
1360-1385/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.tplants.2009.11.007 Available online 28 December 2009
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Figure 1. Detection of neighbouring competitors by plants in dense vegetation. Blue and R light are selectively absorbed by chlorophyll for photosynthesis, whereas FR light is mostly reflected by the leaves. Plants sense reduced blue light photon fluence rates using the cryptochrome and phototropin families of photoreceptors, whereas R:FR is monitored using the phytochrome family of photoreceptors [7,8]. Ethylene can serve as a volatile neighbour detection signal in a canopy [64], as might other BVOCs. The reduced wind exposure inside dense stands might also act functionally as an indicator of dense vegetation [65]. Although mechanical stress can elicit responses in plants [65–67], we are unaware of any studies indicating that touch interactions between individuals would affect the outcome of competition. Belowground neighbour detection can occur through neighbour-induced changes in resource availability, such as local water and nutrient depletions [12], through root exudates [14] and can even be affected by soil microorganisms [68]. Plants can also negatively affect neighbours by exuding allelopathic compounds that inhibit growth or seed germination [14].
Competition impacts on BVOC emissions Competition is a multi-facetted stress: water and nutrient availability decreases belowground and there is a canopy density-dependent drop in light quantity and quality aboveground. These changes in the abiotic environment, caused by the presence of competitors, can affect volatile production and emission. Resource limitations affect BVOC emissions Nutrients become depleted in soils that contain high numbers of competing plants. Maize plants (Zea mays) that are grown on nutrient-depleted soil produce lower amounts of total BVOCs upon herbivore induction than do maize plants growing on fertilized soil [16]. However, although the total sum of BVOCs in the maize example is reduced owing to nutrient deficiency, particular compounds, such as linalool, a monoterpenoid, can be increased. Another study in maize plants showed enhanced emissions of volicitin (a BVOC elicitor)-induced sesquiterpenes when grown on soil with reduced nitrogen content [17]. A study of leaf terpene concentrations found that the leaf terpene concentration of Aleppo pine (Pinus halepensis) increased with an increasing amount of nitrogen or phosphorus in the soil, whereas soil nitrogen and phosphorus content did not affect leaf terpene concentration of rock rose (Cistus albidus) and rosemary (Rosmarinus officinalis) [18]. Although the extent to which leaf terpene concentrations affect volatile terpene emission in these two studies remains to be investigated, the results indicate a strong variability of terpene production between species under varying soil nutrient contents.
In addition to nutrients, light intensity is strongly reduced in dense canopies and BVOC emissions are known to be tightly correlated with light availability. For example, BVOC emission of lima bean (Phaseolus lunatus), induced by mechanical damage, occurs mainly during the light period, even when plants were damaged during the dark period [19], indicating a light dependency of BVOC release. In maize, total BVOC emissions of healthy, undamaged plants are only mildly affected by light intensity, whereas BVOC emissions of herbivore-attacked plants are strongly light dependent [16]. Perhaps more importantly, different BVOCs show different emission patterns in response to light, leading to a correlation between the composition of the BVOC blend and a change in light intensity [16]. The light dependency of BVOC emissions might be related to a variety of factors, such as stomatal conductance, evaporation rates of the compounds [20–22] and rates of photosynthesis [23,24]. Contrary to control through light intensity, little is known about BVOC emission under different qualities of light, except for the volatile hormone ethylene. Studies of ethylene emissions have shown that, in a variety of unrelated species, reduced R:FR, mimicking proximate neighbours, stimulate foliar ethylene emissions (Box 1). This is thought to contribute to the low R:FRinduced shoot elongation response. Blue light depletion can also stimulate shoot elongation, but in Arabidopsis (Arabidopsis thaliana), this does not coincide with increased ethylene emissions [25]. In addition to light and nutrients, proximate plants also compete for water. Literature concerning BVOC emission in relation to water availability is ambiguous. A general 127
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Box 1. Ethylene: a gas for competition Ethylene is a well-studied volatile plant hormone that has long been known to control fruit ripening [69] and flower senescence [70,71]. It also has a major role in disease resistance [72,73] and growth regulation [64,74]. Recent studies on tobacco showed that ethylene can be key to the appropriate timing of shade avoidance responses to proximate neighbours. Its levels in the canopy atmosphere of dense, greenhouse-grown tobacco stands were found to be elevated approximately fourfold compared with ambient levels. This is the combined result of low R:FR-induced stimulation of ethylene emissions through increased ethylene biosynthesis, and entrapment in the still canopy air [15]. The ethylene levels recorded were applied to standard grown tobacco plants and were found to induce shade avoidance responses, such as upward leaf movement and stem elongation. Furthermore, transgenic plants that expressed the mutant etr1-1 receptor allele from the Arabidopsis etr1-1 mutant, thus rendering them insensitive to ethylene, displayed significantly delayed shade avoidance responses to neighbours [40]. These transgenics were outcompeted by wild-type neighbours that did display appropriately timed shade avoidance responses. The competitive ability of these ethylene-insensitive plants could be restored by an end-of-day FR light pre-treatment that induced a mild shade avoidance phenotype before the onset of competition, thus showing that the ethylene insensitive plants performed poorly because of their poor shade avoidance reactions, rather than because of putative uncharacterized pleiotropic effects [40]. It has also been shown recently that, by using transgenic genotypes of Nicotiana attennuata that either were insensitive to ethylene, or produced less ethylene, root elongation can be inhibited by ethylene emissions from neighbour plants [39]. Ethylene is produced in roots and inhibits root elongation in most of the plant species studied so far [64,75], but it is unknown if ethylene emitted by the roots of one plant can affect root growth in another. Ethylene emission appears to be a general response to low R:FR for several species from different families (Table I), suggesting that ethylene involvement in plant competition in dense plant communities is a general phenomenon.
Table I. Ethylene emissions in various species as controlled by low (representative of canopy shade) versus high R:FR conditions Species
Plant tissue or organ
Arabidopsis thaliana Brassica napus Helianthus annuus
Whole shoot Leaves Leaves Internodes Whole shoot Whole shoot Seedling Shoot tip
Nicotiana tabacum Pisum sativum Sorghum bicolour Stellaria longipes
Effect low R:FR on ethylene emission a ++ + + ++ ++ + =
Refs
[25] [76] [77] [78] [79] [80] [81] [82]
b
a
++ strong stimulation; + mild stimulation; mild inhibition; = no effect. Phytochrome mutant used to mimic low R:FR treatment.
b
increase was found for the total amount of volatiles emitted by maize plants with decreasing soil water content [16]. However, similar to the effects of light, different BVOCs showed different responses; indole or the sesquiterpene (E)-b-farnesene showed a clear induction as a result of water deficit, whereas linalool or b-caryophyllene emissions decreased with decreasing soil humidity and yet other BVOCs are not affected at all. In a study of four different plant species, progressing soil water deficiency enhanced monoterpene emissions in P. halepensis and C. albidus, whereas no overall effects were seen in R. officinalis and Kermes oak (Quercus coccifera) [26]. By contrast, 128
overall emissions of sesquiterpenes were reduced by water deficiency in all four species studied, indicating that different types of BVOC respond differently to water stress. Additionally, a study on cork oak (Quercus suber) showed that mild water stress stimulated monoterpene emissions, whereas severe water deficiency suppressed monoterpene emissions [27], indicating that the effects of water deficiency on BVOC emissions depend on the severity of the stress. Competition affects BVOC emissions Terpene emissions were upregulated for several Mediterranean species that grow in monocultures under natural environmental conditions [28]. On calcareous soil, P. halepensis, C. albidus and Q. coccifera showed upregulation of mono- or sesquiterpene emission under intraspecific competition. However, emission patterns for specific volatiles differed between species. For example, a-pinene emission was upregulated in Q. coccifera and P. halepensis, but not in C. albidus. Interestingly, when these species were not competing with conspecific neighbours but with plants from other species, terpene emissions were generally reduced [29]. These observations indicate that terpene emissions are affected by competition, but the details of this regulation depend on the plant species and identity of the compound. Similar to the aerial plant parts, roots emit BVOCs in a regulated manner [30]. In holm oak (Quercus ilex), the most abundant belowground volatiles are limonene and a-pinene [31] and a-pinene can have allelopathic effects, such as inhibition of seed germination [32]. In maize and cotton (Gossypium hirsutum), root-based BVOC emissions can be induced by herbivore attack [33], but we are not aware of studies that have investigated the effect of competition or its related abiotic stresses on belowground BVOC emissions. Anticipating neighbours: BVOCs can affect plant growth and competition As indicated in the previous section, BVOC emissions differ for competing plants and non-competing individuals, in both quantitative and qualitative aspects. Because plants can perceive variations in BVOC emissions, at least upon herbivory [34], this opens up a novel form of interactions between competing plants. In dense vegetation where plants grow closer together than in open stands, chances are higher for BVOCs to affect neighbouring plants as they are more likely to reach physiologically relevant concentrations, as has been shown for ethylene in dense tobacco stands [15]. BVOCs can affect neighbouring plants in two ways: they can (i) cause allelopathic effects (e.g. inhibit growth or developmental programs); and (ii) be exploited by neighbouring plants as a cue for the presence of proximate competitors, thus inducing or priming responses that increase the competitive power of the ‘eavesdropping’ neighbour. There are several studies that show evidence for BVOC-mediated control of neighbour plant growth. However, only a few of these studies have been performed under natural field conditions. Therefore, for studies that were performed under laboratory conditions, it remains unknown whether the reported BVOC
Opinion effects would occur in neighbouring plants under natural field conditions, for example because concentrations will not always be high enough for an effect to occur [35]. BVOCs as allelochemicals Allelopathic effects of root-emitted BVOCs, such as the monoterpenoid a-pinene, have been shown in several plant species. For example, holm oak [31] and purple sage (Salvia leucophylla) [36] emit a-pinene belowground. In a study of five different species, a-pinene application appeared to inhibit seed germination of three species. Early root growth was inhibited in all five species studied and was accompanied by increased levels of oxidative stress in the roots [32]. In addition, a range of other S. leucophylla rootemitted monoterpenoid BVOCs, such as camphor, camphene, 1,8-cineole and b-pinene, inhibited germination and growth of rapeseed (Brassica campestris), when applied externally. Those effects were restricted to the root apical meristem and did not affect the mitotic index in the shoot apical region, indicating tissue specificity for the allelopathic effects of these volatile compounds [36]. The authors suggest that these allelopathic effects of BVOCs emitted by this Salvia species prevent other species from entering Salvia vegetation, thus avoiding interspecific competition [36]. Interestingly, the seed germination of potential future competitors is not only inhibited by root-emitted BVOCs, but also by shoot-emitted BVOCs. For example, volatile compounds that are emitted by snapdragon (Antirrhinum majus) flowers inhibit root growth in neighbouring Arabidopsis seedlings under laboratory conditions [37]. This might be caused by the compound methyl benzoate, a component of the floral BVOC blend of A. majus, which appears to affect expression of genes associated with various classes of growth regulating hormones, such as auxin and cytokinin [37]. A study of sagebrush (Artemisia tridentata) showed that, upon clipping of foliage, the germination of neighbouring seeds of different species is inhibited. Conditions under which air contact was avoided, but transmittance through the soil was permitted, prevented these inhibitory effects on germination [38]. These data indicate that, upon foliage clipping, sagebrush emits BVOCs that inhibit germination of neighbouring seeds, thus preventing future competition. Formally, it cannot be excluded that the reduced germination is, rather than an allelopathic effect of the BVOC emitter, an adaptive strategy of the receiving seeds, which by delaying germination upon detection of herbivoreinduced BVOCs might prevent direct challenges with herbivores in the early seedling stage. In a Petri-dish experiment with coyote tobacco (Nicotiana attenuata), neighbour-emitted ethylene exerted allelopathic effects in roots of neighbouring plants, as root length of receiver plants was reduced by the presence of ethyleneemitting neighbours. However, transgenic neighbours that produced less ethylene did not cause these root growth effects [39]. Eavesdropping on BVOCs for neighbour detection BVOCs can also act as a neighbour detection signal and induce growth responses that are adaptive (i.e. confer a fitness advantage) in neighbouring plants (Box 1). For
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example, ethylene concentrations in the protected atmosphere of a greenhouse-grown dense tobacco canopy could reach levels that induce shade avoidance responses, such as enhanced stem elongation [15]. Transgenic plants that could not respond to ethylene showed reduced shade avoidance responses and were consequently outcompeted by ethylene-sensitive wild-type neighbours [40]. Similar to the classic neighbour detection signals, such as reduced R:FR, ethylene emissions hold little information about the competitive vigour of neighbouring competitors since most plants produce and emit this volatile. A volatile that could give more information about the growth rate of a neighbouring competitor might be volatile methanol. Methanol is produced in most plants by the action of pectin methylesterases, which are involved in vegetative growth by demethylating pectins in the cell wall [41,42]. As a result, daytime methanol emissions are correlated with the growth rates of leaves, at least in cotton and beech (Fagus sylvatica) [43]. Therefore, it could be hypothesized that volatile methanol emitted by neighbours would be an indicator of the growth rate and thus competitive strength of neighbouring plants, and there is evidence that methanol can also be sensed in neighbour plants [44]. However, because herbivore attack will also induce methanol emissions [44], the reliability of this possible signal for competition remains uncertain. Different from ethylene and methanol, other (combinations of) BVOCs might hold potential for more sophisticated neighbour detection and even discrimination between different neighbour identities, because different plant species typically produce different bouquets or blends of BVOCs. Although such neighbour recognition would require sophisticated BVOC perception systems, there is some evidence that such intricate signalling of, and responsiveness to, neighbours does exist. An example comes from a greenhouse study on two barley (Hordeum vulgare) cultivars, Alva and Kara [45]. The Kara cultivar allocates more biomass to its roots when it is exposed to volatiles from the barley cultivar Alva, compared with plants exposed to clear air or to Kara volatiles. Total biomass did not differ between the three treatments, but Kara plants exposed to Alva volatiles had a higher specific leaf area (m2 leaf kg 1 leaf), a response that also occurs upon low light exposure, such as occurs in shaded leaves in a canopy. Thus, exposure to BVOCs from the same cultivar had no effect, whereas exposure to the BVOC blend of a different cultivar did result in phenotypic adjustments. This suggests there are differences between the two volatile blends that can be detected by plants, although this remains to be tested. Another example of BVOCmediated discrimination of neighbour identity comes from a study of host localization by golden dodder (Cuscuta pentagona), a parasitic plant [46]. It was shown that seedlings of this species find their host through volatile cues emitted by the host plant, rather than through other signals, such as light. Even more strikingly, seedlings could discriminate between different neighbouring species, again based on the relative BVOC blends from the different species, thus ensuring growth towards a host (tomato; Lycopersicon esculentum) rather than a non-host (wheat; Triticum aestivum, in these experiments). Discrimination 129
Opinion between the host and non-host BVOC blends might be based on a combination of positive responses to some volatiles from tomato [e.g. b-phellandrene, b-myrcene (also produced by wheat) and a-pinene] and repellent effects of a wheat-produced volatile [(Z)-3-hexenyl acetate] [46]. Taken together, a multitude of different signals exists that can hold information about neighbours. Some, such as low R:FR, are reliable indicators of proximate neighbours but hold little information about, for example, growth rate. Other signals, such as the variety of BVOC blends, might contain detailed information about neighbour identity, but might be less reliable indicators of competition because they are sensitive to disturbing factors, such as wind. We suggest that exploiting the combination of different signals from neighbours is the most reliable way for plants to evaluate the threat of competition. Conflict of interest: interaction between competition and defense Various recent reports have shown that BVOCs emitted upon wounding or attack by herbivores can function as within-plant signals to induce defense responses in remote plant parts, such as branches, that are not yet attacked [47– 50]. When plants are growing in dense stands, the likelihood of these volatiles reaching biologically meaningful concentrations in the air between proximate neighbours is relatively high because inter-plant distances are small and wind velocities are low. Therefore, under such dense conditions, herbivory-induced BVOCs might be relatively likely to be sensed by eavesdropping neighbours, a phenomenon that can occur among conspecifics as well as among individuals from different species [51,52]. Therefore, the potential for BVOC-induced defense against herbivores is relatively high in dense stands with strong competitive interactions between individuals. A second implication might be that herbivory-induced BVOC accumulations could conflict with competitive strategies that involve growth investments, because some of these volatiles, such as methyl-jasmonate and methyl-salicylate, can inhibit growth [53–55], or induce the production of allelopathic compounds [55,56]. Recently, Ballare´ and co-workers studied the interplay between competition and defense against herbivores in wild tobacco (Nicotiana longiflora) and Arabidopsis. Phytochrome-induced shade avoidance responses appeared to dominate over responses that enhance defense against herbivores [57,58]. Importantly these studies did not account for potential effects of low R:FR on volatile emission and/or consequences for BVOC signaling between individuals. Given the finding that low R:FR seemed to desensitize plants for jasmonate (JA) [58], it is possible that JA-induced BVOC emissions [59] would be reduced by low R:FR. It would be of interest to study the effects of R:FR on herbivore-induced BVOC emissions. If the tradeoff between competition and defense [57,58] also occurs at the level of BVOCs, growth-inhibiting effects of herbivoreinduced volatiles might be less likely to occur. During simultaneous exposure to competition and herbivory, the multitude of different signal combinations indicating herbivory and competition provides a more subtle, complete and reliable picture of the competitive environment of the plant. 130
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Box 2. Outstanding questions Are BVOCs, other than ethylene, involved in early neighbour detection during competition? Do BVOCs function in plant neighbour detection primarily through direct induction of phenotypically plastic responses (e.g. shade avoidance) or do BVOCs mostly prime plants for low R:FR-induced shade avoidance responses? Are BVOC emissions affected by neighbour-induced changes in the quality of the incident light?
Conclusions and perspectives In conclusion, there is evidence for BVOC emissions being altered upon the presence of nearby neighbours or upon the environmental changes that might be caused by the impact of neighbours on resource availability. However, strong indications for functional effects of BVOCs on interactions between competing plants under natural conditions are still scarce. Nevertheless, there is good evidence for phenotypic consequences (e.g. root/shoot allocation, internodal elongation, leaf anatomy, root growth and seed germination) of BVOC signaling that can affect competitive interactions. It has been demonstrated that BVOCs emitted by herbivore-attacked plants can induce a so-called ‘primed state’ in non-attacked, but BVOC-exposed neighbours. This means that these non-attacked, BVOC-exposed neighbours of herbivore attacked plants show a more rapid and/or more pronounced adaptive response once attacked by the herbivore [48,60,61]. Although it has not been studied so far, it is possible that adaptive responses to enhance competitive power are also more pronounced upon simultaneous or subsequent exposure to neighbouremitted BVOCs and classic cues, such as low R:FR (Box 2). The composition of BVOC blends is species specific and affected by external conditions, such as resource availability, thus providing a layer of information that other neighbour detection signals, such as light quality, do not hold. To shed more light on the role of BVOCs during plant competition, future studies could pair biochemical analysis of BVOC blends in dense vegetations with experimental studies on genotypes that have defects in biosynthesis or detection of particular BVOCs. Studying the competitive behaviour of these genotypes under high canopy density conditions, combined with exposures of individual plants to candidate BVOCs, will help to unravel the exact functions of BVOCs during plant competition. A multidisciplinary approach to this theme, combining ecology, biochemistry, and molecular physiology [62,63], would have great potential to forward this field and unravel the extent to which BVOC-mediated plant–plant interactions contribute to the fundamental process of plant competition. Acknowledgements We thank Caroline von Dahl, Marcel Dicke, Rens Voesenek and four anonymous reviewers for valuable comments on drafts of this article. The authors are funded by research grants from the Netherlands organization for scientific research (NWO, grant nr. 818.01.003 to W.K. and VENI grant nr 863.06.001 to R.P.).
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30 Hiltpold, I. and Turlings, T.C.J. (2008) Belowground chemical signaling in maize: when simplicity rhymes with efficiency. J. Chem. Ecol. 34, 628–635 31 Asensio, D. et al. (2007) On-line screening of soil vocs exchange responses to moisture, temperature and root presence. Plant Soil 291, 249–261 32 Singh, H.P. et al. (2006) Alpha-pinene inhibits growth and induces oxidative stress in roots. Ann. Bot. 98, 1261–1269 33 Rasmann, S. and Turlings, T.C.J. (2008) First insights into specificity of belowground tritrophic interactions. Oikos 117, 362–369 34 Baldwin, I.T. et al. (2006) Volatile signaling in plant-plant interactions: ‘Talking trees’ In the genomics era. Science 311, 812–815 35 Barney, J.N. et al. (2009) Biogenic volatile organic compounds from an invasive species: impacts on plant–plant interactions. Plant Ecol. 203, 195–205 36 Nishida, N. et al. (2005) Allelopathic effects of volatile monoterpenoids produced by Salvia leucophylla: inhibition of cell proliferation and DNA synthesis in the root apical meristem of Brassica campestris seedlings. J. Chem. Ecol. 31, 1187–1203 37 Horiuchi, J. et al. (2007) The floral volatile, methyl benzoate, from snapdragon (Antirrhinum majus) triggers phytotoxic effects in Arabidopsis thaliana. Planta 226, 1–10 38 Karban, R. (2007) Experimental clipping of sagebrush inhibits seed germination of neighbours. Ecol. Lett. 10, 791–797 39 Inderjit et al. (2009) Use of silenced plants in allelopathy bioassays: a novel approach. Planta 229, 569–575 40 Pierik, R. et al. (2003) Ethylene is required in tobacco to successfully compete with proximate neighbours. Plant Cell Environ. 26, 1229–1234 41 Fall, R. and Benson, A.A. (1996) Leaf methanol – the simplest natural product from plants. Trends Plant Sci. 1, 296–301 42 Pelloux, J. et al. (2007) New insights into pectin methylesterase structure and function. Trends Plant Sci. 12, 267–277 43 Huve, K. et al. (2007) Simultaneous growth and emission measurements demonstrate an interactive control of methanol release by leaf expansion and stomata. J. Exp. Bot. 58, 1783–1793 44 von Dahl, C.C. et al. (2006) Caterpillar-elicited methanol emission: a new signal in plant-herbivore interactions? Plant J. 46, 948–960 45 Ninkovic, V. (2003) Volatile communication between barley plants affects biomass allocation. J. Exp. Bot. 54, 1931–1939 46 Runyon, J.B. et al. (2006) Volatile chemical cues guide host location and host selection by parasitic plants. Science 313, 1964–1967 47 Frost, C.J. et al. (2007) Within-plant signalling via volatiles overcomes vascular constraints on systemic signalling and primes responses against herbivores. Ecol. Lett. 10, 490–498 48 Frost, C.J. et al. (2008) Priming defense genes and metabolites in hybrid poplar by the green leaf volatile cis-3-hexenyl acetate. New Phytol. 180, 722–733 49 Heil, M. and Silva Bueno, J.C. (2007) Within-plant signaling by volatiles leads to induction and priming of an indirect plant defense in nature. Proc. Natl. Acad. Sci. U.S.A. 104, 5467–5472 50 Karban, R. and Shiojiri, K. (2009) Self-recognition affects plant communication and defense. Ecol. Lett. 12, 502–506 51 Karban, R. (2001) Communication between sagebrush and wild tobacco in the field. Biochem. Syst. Ecol. 29, 995–1005 52 Karban, R. et al. (2004) The specificity of eavesdropping on sagebrush by other plants. Ecology 85, 1846–1852 53 Staswick, P.E. et al. (1992) Methyl jasmonate inhibition of root-growth and induction of a leaf protein are decreased in an Arabidopsis thaliana mutant. Proc. Natl. Acad. Sci. U. S. A. 89, 6837–6840 54 Hummel, G.M. et al. (2009) Herbivore-induced jasmonic acid bursts in leaves of Nicotiana attenuata mediate short-term reductions in root growth. Plant Cell Environ. 32, 134–143 55 Bi, H.H. et al. (2007) Rice allelopathy induced by methyl jasmonate and methyl salicylate. J. Chem. Ecol. 33, 1089–1103 56 Fang, C.X. et al. (2009) Analysis of gene expressions associated with increased allelopathy in rice (Oryza sativa l.) induced by exogenous salicylic acid. Plant Growth Regul. 57, 163–172 57 Izaguirre, M.M. et al. (2006) Remote sensing of future competitors: impacts on plant defenses. Proc. Natl. Acad. Sci. U. S. A. 103, 7170– 7174 58 Moreno, J.E. et al. (2009) Ecological modulation of plant defense via phytochrome control of jasmonate sensitivity. Proc. Natl. Acad. Sci. U. S. A. 106, 4935–4940
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Opinion 59 Snoeren, T.A. et al. (2009) Multidisciplinary approach to unravelling the relative contribution of different oxylipins in indirect defense of Arabidopsis thaliana. J. Chem. Ecol. 35 (9), 1021–1031 60 Heil, M. and Kost, C. (2006) Priming of indirect defences. Ecol. Lett. 9, 813–817 61 Choudhary, D.K. et al. (2008) Volatiles as priming agents that initiate plant growth and defence responses. Curr. Sci. 94, 595–604 62 Sultan, S.E. (2007) Development in context: the timely emergence of eco-devo. Trends Ecol. Evol. 22, 575–582 63 Sultan, S.E. (2010) Plant developmental responses to the environment: eco-devo insights. Curr. Opin. Plant Biol. 13, 96–101 64 Pierik, R. et al. (2007) Growth control by ethylene: adjusting phenotypes to the environment. J. Plant Growth Regul. 26, 188–200 65 Anten, N.P.R. et al. (2005) Effects of mechanical stress and plant density on mechanical characteristics, growth, and lifetime reproduction of tobacco plants. Am. Nat. 166, 650–660 66 Anten, N.P.R. et al. (2006) Ethylene sensitivity affects changes in growth patterns, but not stem properties, in response to mechanical stress in tobacco. Physiol. Plant. 128, 274–282 67 Braam, J. (2005) In touch: plant responses to mechanical stimuli. New Phytol. 165, 373–389 68 Raaijmakers, J.M. et al. (2009) The rhizosphere: a playground and battlefield for soilborne pathogens and beneficial microorganisms. Plant Soil 321, 341–361 69 Barry, C.S. and Giovannoni, J.J. (2007) Ethylene and fruit ripening. J. Plant Growth Regul. 26, 143–159 70 Jones, M.L. (2008) Ethylene signaling is required for pollinationaccelerated corolla senescence in petunias. Plant Sci. 175, 190–196 71 Jones, M.L. et al. (2005) Ethylene-sensitivity regulates proteolytic activity and cysteine protease gene expression in petunia corollas. J. Exp. Bot. 56, 2733–2744
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Trends in Plant Science Vol.15 No.3 72 van Loon, L.C. et al. (2006) Ethylene as a modulator of disease resistance in plants. Trends Plant Sci. 11, 184–191 73 von Dahl, C.C. et al. (2007) Tuning the herbivore-induced ethylene burst: the role of transcript accumulation and ethylene perception in Nicotiana attenuata. Plant J. 51, 293–307 74 Pierik, R. et al. (2006) The Janus face of ethylene: growth inhibition and stimulation. Trends Plant Sci. 11, 176–183 75 Visser, E.J.W. and Pierik, R. (2007) Inhibition of root elongation by ethylene in wetland and non-wetland plant species and the impact of longitudinal ventilation. Plant Cell Environ. 30, 31–38 76 Kurepin, L.V. et al. (2007) Light quality regulation of endogenous levels of auxin, abscisic acid and ethylene production in petioles and leaves of wild type and acc deaminase transgenic Brassica napus seedlings. Plant Growth Regul. 52, 53–60 77 Kurepin, L.V. et al. (2007) Uncoupling light quality from light irradiance effects in Helianthus annulus shoots: putative roles for plant hormones in leaf and internode growth. J. Exp. Bot. 58, 2145–2157 78 Kurepin, L.V. et al. (2007) Interaction of red to far red light ratio and ethylene in regulating stem elongation of Helianthus annuus. Plant Growth Regul. 51, 53–61 79 Pierik, R. et al. (2004) Interactions between ethylene and gibberellins in phytochrome-mediated shade avoidance responses in tobacco. Plant Physiol. 136, 2928–2936 80 Foo, E. et al. (2006) A role for ethylene in the phytochrome-mediated control of vegetative development. Plant J. 46, 911–921 81 Finlayson, S.A. et al. (2007) Phyb-1 sorghum maintains responsiveness to simulated shade, irradiance and red light: far-red light. Plant Cell Environ. 30, 952–962 82 Kurepin, L.V. et al. (2006) Possible roles for ethylene and gibberellin in the phenotypic plasticity of an alpine population of Stellaria longipes. Can. J. Bot. 84, 1101–1109
Special Issue: Induced biogenic volatile organic compounds from plants
Biogenic volatile organic compounds and plant competition Wouter Kegge and Ronald Pierik Plant Ecophysiology, Institute of Environmental Biology, Utrecht University, Padualaan 8, 3584 CH Utrecht, the Netherlands
One of the most important factors to shape plant communities is competition between plants, which impacts on the availability of environmental factors such as light, nutrients and water. In response to these environmental parameters, plants adjust the emission of many different biogenic volatile organic compounds (BVOCs). BVOCs can also elicit responses in neighbouring plants, thus constituting a platform for plant–plant interactions. Here, we review the relationship between BVOC emissions and competition among neighbouring plants. Recent progress indicates that BVOCs can act both as allelochemicals and as neighbour detection signals. It is suggested that BVOCs provide information about neighbouring competitors, such as their identity or growth rate, that classic neighbour detection signals cannot provide. Plant competition: battling with neighbours Competition with neighbouring plants for resources such as light, water and nutrients, is one of the most common stresses that a plant has to deal with. Competitive interactions among plants shape vegetation composition and control biodiversity [1,2], making it one of the most important processes for vegetation patterns on a global scale. To compete successfully, plants exploit a range of phenotypically plastic responses that help to enhance resource capture and thus increase their fitness during competition [3– 5]. Given the strong selection pressure during competition, rapid detection of neighbours, followed by adaptive responses, is key to competitive success. Such plastic responses to neighbours encompass defensive strategies that help avoid competitive interactions (e.g. shade avoidance) as well as offensive strategies that inhibit the performance of proximate competitors (e.g. allelopathy) [4,6]. Depending on which resources plants compete for, they will allocate more carbon to roots (competition for nutrients or water) or shoots (competition for light). During competition for light, this carbon is invested in so-called ‘shade avoidance’ responses, which include upward movement of leaves, enhanced elongation of internodes and petioles, and increased apical dominance, which all function to position the photosynthesizing leaves in the well-lit, upper parts of the vegetation [7,8]. When competing belowground, root proliferation into resource-rich patches is Corresponding author: Pierik, R. ([email protected]).
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thought to be one trait that could contribute to competitive ability [9], but there is also evidence that this is not always the case [10,11] and the complete set of adaptive traits to enhance belowground competitive vigour is yet to be unveiled. The above-mentioned competitive behaviours are activated upon sensing of neighbour detection cues, such as the classic reduction in the red:far-red light ratio (R:FR) of the light reflected by, or transmitted through, plants. This R:FR reduction is created by selective absorption of R light for photosynthesis and reflectance of FR light. The low R:FR is sensed by the phytochrome family of photoreceptors, a process that precedes actual shading and thus competition for light [7], and initiates shade avoidance responses. In addition to a reduced R:FR, the reduction in blue light photon fluence rates based on photon absorbance for photosynthesis, can also act as a neighbour detection signal (Figure 1) [7,8]. In addition to these aboveground signals, plant neighbour detection also involves various belowground signals, such as depletions of water and/or nutrients as a consequence of efficient uptake by neighbouring root systems [12,13] and organic exudates excreted by neighbouring plants [14]. The past five years have seen major advances in the research area of plant neighbour detection during competition. It is believed that the volatile hormone ethylene could serve as a neighbour detection cue [15], because it is released by most plant species and accumulates in the atmosphere inside dense stands of greenhouse-grown plants, such as tobacco (Nicotiana tabacum) [15]. This provided the first evidence for competition-mediated regulation of emissions of biogenic (plant-produced) volatile organic compounds (BVOCs) and their biological function in plant neighbour detection. More recently, reports emerged showing that the emissions of various other BVOCs, such as mono- and sesquiterpenes, are affected by competition, as well as by abiotic stressfactors, such as deficiencies in water or light, that typically occur during competition. Here, we review and discuss the effects of competition on BVOC emissions and how these BVOC emissions might affect the growth of neighbouring competitors. We suggest that the combination of classic neighbour detection signals and species-specific BVOC blends provides information about competing neighbours with unprecedented detail and reliability.
1360-1385/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.tplants.2009.11.007 Available online 28 December 2009
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Figure 1. Detection of neighbouring competitors by plants in dense vegetation. Blue and R light are selectively absorbed by chlorophyll for photosynthesis, whereas FR light is mostly reflected by the leaves. Plants sense reduced blue light photon fluence rates using the cryptochrome and phototropin families of photoreceptors, whereas R:FR is monitored using the phytochrome family of photoreceptors [7,8]. Ethylene can serve as a volatile neighbour detection signal in a canopy [64], as might other BVOCs. The reduced wind exposure inside dense stands might also act functionally as an indicator of dense vegetation [65]. Although mechanical stress can elicit responses in plants [65–67], we are unaware of any studies indicating that touch interactions between individuals would affect the outcome of competition. Belowground neighbour detection can occur through neighbour-induced changes in resource availability, such as local water and nutrient depletions [12], through root exudates [14] and can even be affected by soil microorganisms [68]. Plants can also negatively affect neighbours by exuding allelopathic compounds that inhibit growth or seed germination [14].
Competition impacts on BVOC emissions Competition is a multi-facetted stress: water and nutrient availability decreases belowground and there is a canopy density-dependent drop in light quantity and quality aboveground. These changes in the abiotic environment, caused by the presence of competitors, can affect volatile production and emission. Resource limitations affect BVOC emissions Nutrients become depleted in soils that contain high numbers of competing plants. Maize plants (Zea mays) that are grown on nutrient-depleted soil produce lower amounts of total BVOCs upon herbivore induction than do maize plants growing on fertilized soil [16]. However, although the total sum of BVOCs in the maize example is reduced owing to nutrient deficiency, particular compounds, such as linalool, a monoterpenoid, can be increased. Another study in maize plants showed enhanced emissions of volicitin (a BVOC elicitor)-induced sesquiterpenes when grown on soil with reduced nitrogen content [17]. A study of leaf terpene concentrations found that the leaf terpene concentration of Aleppo pine (Pinus halepensis) increased with an increasing amount of nitrogen or phosphorus in the soil, whereas soil nitrogen and phosphorus content did not affect leaf terpene concentration of rock rose (Cistus albidus) and rosemary (Rosmarinus officinalis) [18]. Although the extent to which leaf terpene concentrations affect volatile terpene emission in these two studies remains to be investigated, the results indicate a strong variability of terpene production between species under varying soil nutrient contents.
In addition to nutrients, light intensity is strongly reduced in dense canopies and BVOC emissions are known to be tightly correlated with light availability. For example, BVOC emission of lima bean (Phaseolus lunatus), induced by mechanical damage, occurs mainly during the light period, even when plants were damaged during the dark period [19], indicating a light dependency of BVOC release. In maize, total BVOC emissions of healthy, undamaged plants are only mildly affected by light intensity, whereas BVOC emissions of herbivore-attacked plants are strongly light dependent [16]. Perhaps more importantly, different BVOCs show different emission patterns in response to light, leading to a correlation between the composition of the BVOC blend and a change in light intensity [16]. The light dependency of BVOC emissions might be related to a variety of factors, such as stomatal conductance, evaporation rates of the compounds [20–22] and rates of photosynthesis [23,24]. Contrary to control through light intensity, little is known about BVOC emission under different qualities of light, except for the volatile hormone ethylene. Studies of ethylene emissions have shown that, in a variety of unrelated species, reduced R:FR, mimicking proximate neighbours, stimulate foliar ethylene emissions (Box 1). This is thought to contribute to the low R:FRinduced shoot elongation response. Blue light depletion can also stimulate shoot elongation, but in Arabidopsis (Arabidopsis thaliana), this does not coincide with increased ethylene emissions [25]. In addition to light and nutrients, proximate plants also compete for water. Literature concerning BVOC emission in relation to water availability is ambiguous. A general 127
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Box 1. Ethylene: a gas for competition Ethylene is a well-studied volatile plant hormone that has long been known to control fruit ripening [69] and flower senescence [70,71]. It also has a major role in disease resistance [72,73] and growth regulation [64,74]. Recent studies on tobacco showed that ethylene can be key to the appropriate timing of shade avoidance responses to proximate neighbours. Its levels in the canopy atmosphere of dense, greenhouse-grown tobacco stands were found to be elevated approximately fourfold compared with ambient levels. This is the combined result of low R:FR-induced stimulation of ethylene emissions through increased ethylene biosynthesis, and entrapment in the still canopy air [15]. The ethylene levels recorded were applied to standard grown tobacco plants and were found to induce shade avoidance responses, such as upward leaf movement and stem elongation. Furthermore, transgenic plants that expressed the mutant etr1-1 receptor allele from the Arabidopsis etr1-1 mutant, thus rendering them insensitive to ethylene, displayed significantly delayed shade avoidance responses to neighbours [40]. These transgenics were outcompeted by wild-type neighbours that did display appropriately timed shade avoidance responses. The competitive ability of these ethylene-insensitive plants could be restored by an end-of-day FR light pre-treatment that induced a mild shade avoidance phenotype before the onset of competition, thus showing that the ethylene insensitive plants performed poorly because of their poor shade avoidance reactions, rather than because of putative uncharacterized pleiotropic effects [40]. It has also been shown recently that, by using transgenic genotypes of Nicotiana attennuata that either were insensitive to ethylene, or produced less ethylene, root elongation can be inhibited by ethylene emissions from neighbour plants [39]. Ethylene is produced in roots and inhibits root elongation in most of the plant species studied so far [64,75], but it is unknown if ethylene emitted by the roots of one plant can affect root growth in another. Ethylene emission appears to be a general response to low R:FR for several species from different families (Table I), suggesting that ethylene involvement in plant competition in dense plant communities is a general phenomenon.
Table I. Ethylene emissions in various species as controlled by low (representative of canopy shade) versus high R:FR conditions Species
Plant tissue or organ
Arabidopsis thaliana Brassica napus Helianthus annuus
Whole shoot Leaves Leaves Internodes Whole shoot Whole shoot Seedling Shoot tip
Nicotiana tabacum Pisum sativum Sorghum bicolour Stellaria longipes
Effect low R:FR on ethylene emission a ++ + + ++ ++ + =
Refs
[25] [76] [77] [78] [79] [80] [81] [82]
b
a
++ strong stimulation; + mild stimulation; mild inhibition; = no effect. Phytochrome mutant used to mimic low R:FR treatment.
b
increase was found for the total amount of volatiles emitted by maize plants with decreasing soil water content [16]. However, similar to the effects of light, different BVOCs showed different responses; indole or the sesquiterpene (E)-b-farnesene showed a clear induction as a result of water deficit, whereas linalool or b-caryophyllene emissions decreased with decreasing soil humidity and yet other BVOCs are not affected at all. In a study of four different plant species, progressing soil water deficiency enhanced monoterpene emissions in P. halepensis and C. albidus, whereas no overall effects were seen in R. officinalis and Kermes oak (Quercus coccifera) [26]. By contrast, 128
overall emissions of sesquiterpenes were reduced by water deficiency in all four species studied, indicating that different types of BVOC respond differently to water stress. Additionally, a study on cork oak (Quercus suber) showed that mild water stress stimulated monoterpene emissions, whereas severe water deficiency suppressed monoterpene emissions [27], indicating that the effects of water deficiency on BVOC emissions depend on the severity of the stress. Competition affects BVOC emissions Terpene emissions were upregulated for several Mediterranean species that grow in monocultures under natural environmental conditions [28]. On calcareous soil, P. halepensis, C. albidus and Q. coccifera showed upregulation of mono- or sesquiterpene emission under intraspecific competition. However, emission patterns for specific volatiles differed between species. For example, a-pinene emission was upregulated in Q. coccifera and P. halepensis, but not in C. albidus. Interestingly, when these species were not competing with conspecific neighbours but with plants from other species, terpene emissions were generally reduced [29]. These observations indicate that terpene emissions are affected by competition, but the details of this regulation depend on the plant species and identity of the compound. Similar to the aerial plant parts, roots emit BVOCs in a regulated manner [30]. In holm oak (Quercus ilex), the most abundant belowground volatiles are limonene and a-pinene [31] and a-pinene can have allelopathic effects, such as inhibition of seed germination [32]. In maize and cotton (Gossypium hirsutum), root-based BVOC emissions can be induced by herbivore attack [33], but we are not aware of studies that have investigated the effect of competition or its related abiotic stresses on belowground BVOC emissions. Anticipating neighbours: BVOCs can affect plant growth and competition As indicated in the previous section, BVOC emissions differ for competing plants and non-competing individuals, in both quantitative and qualitative aspects. Because plants can perceive variations in BVOC emissions, at least upon herbivory [34], this opens up a novel form of interactions between competing plants. In dense vegetation where plants grow closer together than in open stands, chances are higher for BVOCs to affect neighbouring plants as they are more likely to reach physiologically relevant concentrations, as has been shown for ethylene in dense tobacco stands [15]. BVOCs can affect neighbouring plants in two ways: they can (i) cause allelopathic effects (e.g. inhibit growth or developmental programs); and (ii) be exploited by neighbouring plants as a cue for the presence of proximate competitors, thus inducing or priming responses that increase the competitive power of the ‘eavesdropping’ neighbour. There are several studies that show evidence for BVOC-mediated control of neighbour plant growth. However, only a few of these studies have been performed under natural field conditions. Therefore, for studies that were performed under laboratory conditions, it remains unknown whether the reported BVOC
Opinion effects would occur in neighbouring plants under natural field conditions, for example because concentrations will not always be high enough for an effect to occur [35]. BVOCs as allelochemicals Allelopathic effects of root-emitted BVOCs, such as the monoterpenoid a-pinene, have been shown in several plant species. For example, holm oak [31] and purple sage (Salvia leucophylla) [36] emit a-pinene belowground. In a study of five different species, a-pinene application appeared to inhibit seed germination of three species. Early root growth was inhibited in all five species studied and was accompanied by increased levels of oxidative stress in the roots [32]. In addition, a range of other S. leucophylla rootemitted monoterpenoid BVOCs, such as camphor, camphene, 1,8-cineole and b-pinene, inhibited germination and growth of rapeseed (Brassica campestris), when applied externally. Those effects were restricted to the root apical meristem and did not affect the mitotic index in the shoot apical region, indicating tissue specificity for the allelopathic effects of these volatile compounds [36]. The authors suggest that these allelopathic effects of BVOCs emitted by this Salvia species prevent other species from entering Salvia vegetation, thus avoiding interspecific competition [36]. Interestingly, the seed germination of potential future competitors is not only inhibited by root-emitted BVOCs, but also by shoot-emitted BVOCs. For example, volatile compounds that are emitted by snapdragon (Antirrhinum majus) flowers inhibit root growth in neighbouring Arabidopsis seedlings under laboratory conditions [37]. This might be caused by the compound methyl benzoate, a component of the floral BVOC blend of A. majus, which appears to affect expression of genes associated with various classes of growth regulating hormones, such as auxin and cytokinin [37]. A study of sagebrush (Artemisia tridentata) showed that, upon clipping of foliage, the germination of neighbouring seeds of different species is inhibited. Conditions under which air contact was avoided, but transmittance through the soil was permitted, prevented these inhibitory effects on germination [38]. These data indicate that, upon foliage clipping, sagebrush emits BVOCs that inhibit germination of neighbouring seeds, thus preventing future competition. Formally, it cannot be excluded that the reduced germination is, rather than an allelopathic effect of the BVOC emitter, an adaptive strategy of the receiving seeds, which by delaying germination upon detection of herbivoreinduced BVOCs might prevent direct challenges with herbivores in the early seedling stage. In a Petri-dish experiment with coyote tobacco (Nicotiana attenuata), neighbour-emitted ethylene exerted allelopathic effects in roots of neighbouring plants, as root length of receiver plants was reduced by the presence of ethyleneemitting neighbours. However, transgenic neighbours that produced less ethylene did not cause these root growth effects [39]. Eavesdropping on BVOCs for neighbour detection BVOCs can also act as a neighbour detection signal and induce growth responses that are adaptive (i.e. confer a fitness advantage) in neighbouring plants (Box 1). For
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example, ethylene concentrations in the protected atmosphere of a greenhouse-grown dense tobacco canopy could reach levels that induce shade avoidance responses, such as enhanced stem elongation [15]. Transgenic plants that could not respond to ethylene showed reduced shade avoidance responses and were consequently outcompeted by ethylene-sensitive wild-type neighbours [40]. Similar to the classic neighbour detection signals, such as reduced R:FR, ethylene emissions hold little information about the competitive vigour of neighbouring competitors since most plants produce and emit this volatile. A volatile that could give more information about the growth rate of a neighbouring competitor might be volatile methanol. Methanol is produced in most plants by the action of pectin methylesterases, which are involved in vegetative growth by demethylating pectins in the cell wall [41,42]. As a result, daytime methanol emissions are correlated with the growth rates of leaves, at least in cotton and beech (Fagus sylvatica) [43]. Therefore, it could be hypothesized that volatile methanol emitted by neighbours would be an indicator of the growth rate and thus competitive strength of neighbouring plants, and there is evidence that methanol can also be sensed in neighbour plants [44]. However, because herbivore attack will also induce methanol emissions [44], the reliability of this possible signal for competition remains uncertain. Different from ethylene and methanol, other (combinations of) BVOCs might hold potential for more sophisticated neighbour detection and even discrimination between different neighbour identities, because different plant species typically produce different bouquets or blends of BVOCs. Although such neighbour recognition would require sophisticated BVOC perception systems, there is some evidence that such intricate signalling of, and responsiveness to, neighbours does exist. An example comes from a greenhouse study on two barley (Hordeum vulgare) cultivars, Alva and Kara [45]. The Kara cultivar allocates more biomass to its roots when it is exposed to volatiles from the barley cultivar Alva, compared with plants exposed to clear air or to Kara volatiles. Total biomass did not differ between the three treatments, but Kara plants exposed to Alva volatiles had a higher specific leaf area (m2 leaf kg 1 leaf), a response that also occurs upon low light exposure, such as occurs in shaded leaves in a canopy. Thus, exposure to BVOCs from the same cultivar had no effect, whereas exposure to the BVOC blend of a different cultivar did result in phenotypic adjustments. This suggests there are differences between the two volatile blends that can be detected by plants, although this remains to be tested. Another example of BVOCmediated discrimination of neighbour identity comes from a study of host localization by golden dodder (Cuscuta pentagona), a parasitic plant [46]. It was shown that seedlings of this species find their host through volatile cues emitted by the host plant, rather than through other signals, such as light. Even more strikingly, seedlings could discriminate between different neighbouring species, again based on the relative BVOC blends from the different species, thus ensuring growth towards a host (tomato; Lycopersicon esculentum) rather than a non-host (wheat; Triticum aestivum, in these experiments). Discrimination 129
Opinion between the host and non-host BVOC blends might be based on a combination of positive responses to some volatiles from tomato [e.g. b-phellandrene, b-myrcene (also produced by wheat) and a-pinene] and repellent effects of a wheat-produced volatile [(Z)-3-hexenyl acetate] [46]. Taken together, a multitude of different signals exists that can hold information about neighbours. Some, such as low R:FR, are reliable indicators of proximate neighbours but hold little information about, for example, growth rate. Other signals, such as the variety of BVOC blends, might contain detailed information about neighbour identity, but might be less reliable indicators of competition because they are sensitive to disturbing factors, such as wind. We suggest that exploiting the combination of different signals from neighbours is the most reliable way for plants to evaluate the threat of competition. Conflict of interest: interaction between competition and defense Various recent reports have shown that BVOCs emitted upon wounding or attack by herbivores can function as within-plant signals to induce defense responses in remote plant parts, such as branches, that are not yet attacked [47– 50]. When plants are growing in dense stands, the likelihood of these volatiles reaching biologically meaningful concentrations in the air between proximate neighbours is relatively high because inter-plant distances are small and wind velocities are low. Therefore, under such dense conditions, herbivory-induced BVOCs might be relatively likely to be sensed by eavesdropping neighbours, a phenomenon that can occur among conspecifics as well as among individuals from different species [51,52]. Therefore, the potential for BVOC-induced defense against herbivores is relatively high in dense stands with strong competitive interactions between individuals. A second implication might be that herbivory-induced BVOC accumulations could conflict with competitive strategies that involve growth investments, because some of these volatiles, such as methyl-jasmonate and methyl-salicylate, can inhibit growth [53–55], or induce the production of allelopathic compounds [55,56]. Recently, Ballare´ and co-workers studied the interplay between competition and defense against herbivores in wild tobacco (Nicotiana longiflora) and Arabidopsis. Phytochrome-induced shade avoidance responses appeared to dominate over responses that enhance defense against herbivores [57,58]. Importantly these studies did not account for potential effects of low R:FR on volatile emission and/or consequences for BVOC signaling between individuals. Given the finding that low R:FR seemed to desensitize plants for jasmonate (JA) [58], it is possible that JA-induced BVOC emissions [59] would be reduced by low R:FR. It would be of interest to study the effects of R:FR on herbivore-induced BVOC emissions. If the tradeoff between competition and defense [57,58] also occurs at the level of BVOCs, growth-inhibiting effects of herbivoreinduced volatiles might be less likely to occur. During simultaneous exposure to competition and herbivory, the multitude of different signal combinations indicating herbivory and competition provides a more subtle, complete and reliable picture of the competitive environment of the plant. 130
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Box 2. Outstanding questions Are BVOCs, other than ethylene, involved in early neighbour detection during competition? Do BVOCs function in plant neighbour detection primarily through direct induction of phenotypically plastic responses (e.g. shade avoidance) or do BVOCs mostly prime plants for low R:FR-induced shade avoidance responses? Are BVOC emissions affected by neighbour-induced changes in the quality of the incident light?
Conclusions and perspectives In conclusion, there is evidence for BVOC emissions being altered upon the presence of nearby neighbours or upon the environmental changes that might be caused by the impact of neighbours on resource availability. However, strong indications for functional effects of BVOCs on interactions between competing plants under natural conditions are still scarce. Nevertheless, there is good evidence for phenotypic consequences (e.g. root/shoot allocation, internodal elongation, leaf anatomy, root growth and seed germination) of BVOC signaling that can affect competitive interactions. It has been demonstrated that BVOCs emitted by herbivore-attacked plants can induce a so-called ‘primed state’ in non-attacked, but BVOC-exposed neighbours. This means that these non-attacked, BVOC-exposed neighbours of herbivore attacked plants show a more rapid and/or more pronounced adaptive response once attacked by the herbivore [48,60,61]. Although it has not been studied so far, it is possible that adaptive responses to enhance competitive power are also more pronounced upon simultaneous or subsequent exposure to neighbouremitted BVOCs and classic cues, such as low R:FR (Box 2). The composition of BVOC blends is species specific and affected by external conditions, such as resource availability, thus providing a layer of information that other neighbour detection signals, such as light quality, do not hold. To shed more light on the role of BVOCs during plant competition, future studies could pair biochemical analysis of BVOC blends in dense vegetations with experimental studies on genotypes that have defects in biosynthesis or detection of particular BVOCs. Studying the competitive behaviour of these genotypes under high canopy density conditions, combined with exposures of individual plants to candidate BVOCs, will help to unravel the exact functions of BVOCs during plant competition. A multidisciplinary approach to this theme, combining ecology, biochemistry, and molecular physiology [62,63], would have great potential to forward this field and unravel the extent to which BVOC-mediated plant–plant interactions contribute to the fundamental process of plant competition. Acknowledgements We thank Caroline von Dahl, Marcel Dicke, Rens Voesenek and four anonymous reviewers for valuable comments on drafts of this article. The authors are funded by research grants from the Netherlands organization for scientific research (NWO, grant nr. 818.01.003 to W.K. and VENI grant nr 863.06.001 to R.P.).
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