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- Materials and Methods
- Supporting Information
Plants acclimate to their dynamic environment, and this requires both the ability to sense the environment and a signalling network via which to respond to these environmental changes. When plants grow in dense stands, as is the case in most agricultural and productive natural fields, they will have to deal with the close proximity of neighbouring plants that compete for resources. Aboveground, plants can sense neighbours through alterations in the light intensity and quality. Plants typically absorb red and blue light for photosynthesis, whereas far-red is reflected and transmitted through neighbouring leaves. Changes in the ratio between red and far-red light (the R : FR ratio) are the first light cues associated with imminent shading, and plants can detect these changes using their phytochrome photoreceptors (reviewed by Franklin, 2008). The light spectrum of horizontally reflected light is already FR-enriched before true shading occurs, as a result of FR reflection by neighbouring plants, which is sensed as an early neighbour detection signal (Ballaré et al., 1990). At the physiological level, plants respond to a lowering of the R : FR ratio with a broad range of responses, including enhanced stem and petiole elongation, apical dominance and early flowering, which constitute the so-called shade avoidance syndrome (reviewed in Ballaré, 1999; Franklin, 2008; Martinez-Garcia et al., 2010; Keuskamp et al., 2011). These responses help plants to consolidate a favourable position in dense stands to support light capture and sustain their growth. In addition to light signals, volatile organic compounds (VOCs) have also been hypothesized to be involved in plant neighbour detection during competition (Kegge & Pierik, 2010), but this has been experimentally shown only for the volatile plant hormone ethylene (Pierik et al., 2003, 2004). The production of ethylene is generally stimulated by low R : FR conditions (Finlayson et al., 1999; Pierik et al., 2004, 2009; Foo et al., 2006; Kurepin et al., 2007). It remains to be demonstrated whether and how emissions of VOCs other than ethylene are affected by the light conditions in stands with high plant densities.
Competing neighbours are not the only organisms that can threaten a plant's potential for growth and reproduction in dense stands. Particularly at high densities, plants are at risk of being attacked by herbivorous insects, and plants have evolved various mechanisms to fend off herbivores. Studies on simultaneous exposure to competing neighbours and herbivore attack have led to the hypothesis of a trade-off between shade-induced growth responses and plant defences (Cipollini, 2004). This conflict between growth and defence has also been coined ‘the dilemma of plants’ (Herms & Mattson, 1992; Ballaré, 2009). Indeed, mechanistic studies focussing on shade avoidance and defence showed that low R : FR conditions lead to a severe suppression of inducible plant defences in Nicotiana longiflora and Arabidopsis thaliana against herbivores (Izaguirre et al., 2006; Moreno et al., 2009), as well as against pathogens (Cerrudo et al., 2012; reviewed in Ballaré et al., 2012). Defences against herbivore attack are induced upon insect feeding through increased production of jasmonic acid (JA; Baldwin et al., 1997; Dicke et al., 1999). Accordingly, treatment of plants with exogenous JA induces various defence responses (Pieterse et al., 2009), and the JA receptor mutant coi1-1 (coronatin insensitive protein1-1) displays an enhanced susceptibility to herbivorous insects (Reymond et al., 2004; Bodenhausen & Reymond, 2007; Van Oosten et al., 2008). JA is not only involved in direct defence against herbivores, but also plays a significant role in the production of herbivore-induced plant volatiles (HIPVs) in response to herbivore attack (Dicke et al., 1999; Snoeren et al., 2009). Exogenous application of JA induces the emission of different volatile classes, such as green leaf volatiles (GLVs), phenylpropanoids/benzenoids and mono-, di- and sesquiterpenes (Snoeren et al., 2009). Moreover, methyl jasmonate (MeJA), the methylated and volatile form of JA, is effective in inducing direct defences in plant leaves (Farmer & Ryan, 1990; Miksch & Boland, 1996) and inducing volatile emissions (Chen et al., 2003; Faldt et al., 2003; Bruce et al., 2008; Herde et al., 2008).
The emission of HIPVs is considered to serve as an indirect defence mechanism because HIPVs can attract natural enemies of herbivores (Vet & Dicke, 1992; Dicke & Baldwin, 2010). However, herbivores and plants can also exploit HIPVs: herbivores may avoid oviposition on plants that already contain eggs or feeding herbivores (Dicke, 2000), while specialist herbivores may be attracted by HIPVs (Bolter et al., 1997) and plants grown near damaged neighbours may become more resistant to herbivory, as was shown for tobacco (Nicotiana tabacum) grown near damaged sagebrush (Artemisia tridentata) (Karban et al., 2003). JA-deficient mutants display a reduced induction of many volatiles upon herbivore attack, especially terpenoids and GLVs (Thaler et al., 2002; Snoeren et al., 2009). Accordingly, JA-deficient tomato (Lycopersicon esculentum) mutants receive more oviposition from herbivorous insects than do wild-type plants (Sanchez-Hernandez et al., 2006). These findings indicate that JA controls VOC-mediated interactions between plants and herbivores.
Because interplant distances in dense stands are small, the likelihood of VOCs emitted by a plant reaching physiologically meaningful concentrations at its neighbours, and this would favour eavesdropping on neighbouring plants to detect, for example, upcoming herbivore attack. However, the emission of many VOCs relies on JA signalling, and the low R : FR conditions in a dense stand reduce JA-mediated defences. We therefore investigated in Arabidopsis thaliana whether (1) low R : FR, green shade (mimicking light conditions at early and late stages of canopy development, respectively) and dense vegetation affect constitutive VOC emissions; (2) low R : FR affects MeJA-induced VOC emissions; and (3) low R : FR affects localization of the food plant by naïve newly hatched caterpillars (neonates) of Pieris brassicae, a specialist herbivore of brassicaceous plants such as A. thaliana. We demonstrate that the emission of several constitutively emitted VOCs is reduced under low R : FR conditions, green shade and dense canopy conditions. Furthermore, we show that the emission of MeJA-induced VOCs is also reduced when plants are grown in low R : FR and that plant preference of P. brassicae based on VOCs is impeded. We conclude that light quality can have a strong impact on the emission of plant VOCs and their role in biotic interactions.