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Plants growing in dense vegetations compete with their neighbors for resources such as water, nutrients and light. The competition for light has been particularly well studied, both for its fitness consequences as well as the adaptive behaviors that plants display to win the battle for light interception. Aboveground, plants detect their competitors through photosensory cues, notably the red:far-red light ratio (R:FR). The R:FR is a very reliable indicator of future competition as it decreases in a plant-specific manner through red light absorption for photosynthesis and is sensed with the phytochrome photoreceptors. In addition, also blue light depletion is perceived for neighbor detection. As a response to these light signals plants display a suite of phenotypic traits defined as the shade avoidance syndrome (SAS). The SAS helps to position the photosynthesizing leaves in the higher zones of a canopy where light conditions are more favorable. In this review we will discuss the physiological control mechanisms through which the photosensory signals are transduced into the adaptive phenotypic responses that make up the SAS. Using this mechanistic knowledge as a starting point, we will discuss how the SAS functions in the context of the complex multi-facetted environments, which plants usually grow in.
Plants usually grow in dynamic environments with oftentimes severe competition over limited resources with surrounding neighbors. To deal with the limitations in resources such as water, nutrients and light, plants display phenotypic responses to neighboring plants in order to maximize resource capture in dense canopies. To do so plants have to sense the vicinity of neighbors. Perception of neighbors and/or the abiotic stresses such as altered light levels that come with them, may lead to different types of behavior: shade tolerance, shade avoidance or confrontation (e.g., allelopathy in roots).1 The ability of plants to develop different phenotypes in response to environmental cues of (future) selective conditions is an important determinant of plant performance and ultimately plant fitness. To respond appropriately to neighbors, plants use reliable external signals to detect the presence of neighbors and internal receptor systems to perceive and process these signals. Perception of such neighbor detection signals can induce rapid changes in gene expression and physiological processes, which regulate the phenotypic plasticity required to competitively acquire resources.
Phenotypic plasticity is thought to be an essential feature of plants in response to their environment, which is usually dynamic for various aspects simultaneously. In summary, plants use a wide variety of external cues and internal perception mechanisms, which are subsequently integrated through cross-talk at the signal transduction level, leading to an integral phenotypic outcome. Examples of environmental signals that can be used as input signals for the presence of neighboring competitors are light quality,2,3 plant produced volatile organic compounds,4,5 nutrients6,7 and root exudates.8 The mechanistic regulation and functional importance of plant neighbor detection through light quality signals is probably the best studied example of plant phenotypic plasticity. Here we document the current knowledge on what is perhaps the most important and wide-spread behavior of plants under competition: the Shade Avoidance Syndrome (SAS). We will review recent progress on molecular and physiological regulation of the SAS, how cell wall modifying proteins play a role in the SAS essential cell elongation and the current understanding of its adaptive significance for plant competition.
Light is the main source of energy for plants. When plants grow side by side, leaves will eventually overlap and shade each other, leading to competition for light. This competition for light is for example an important factor determining biodiversity of dense plant communities.9
Besides being a source of energy, light is also a source of information that plants can respond to. Most plants are able to react to the direction, intensity, composition and duration of light. These light components regulate such features as seed germination, photomorphogenesis, flowering time and the SAS.10–12 The SAS encompasses various phenotypic traits (Fig. 1), including elongation of internodes, petioles and hypocotyls, apical dominance, early flowering and upward leaf movement (hyponasty).3 These growth and developmental responses help plants to outgrow shade imposed by neighbors, thus allowing them to position the young (photosynthesizing) leaves in the upper, better lit parts of the vegetation. Phenotypic traits that are also observed in shaded plants, but that are not part of the SAS, include an increase of Specific Leaf Area (SLA) and a decrease of the chlorophyll a/b ratio.13 These latter two responses are thought to maximize light harvesting under shaded conditions and constitute shade tolerance, rather than shade avoidance.
Plants display SAS upon detection of certain cues that indicate crowding or the initiation of crowding. In a canopy there is not only a reduction in the light availability, but also a dramatic change in light composition. Plants typically do not absorb far-red (FR) light (λ = 700–800 nm) whilst strongly absorbing red (R: λ = 600–700 nm) and blue (B: λ = 400–500 nm) light for photosynthesis.3 The photon fluence rate of B light is a reliable indicator of light intensity, whereas the red to far-red ratio (R:FR) declines as more R light is absorbed by leaves. Plants have the capacity to respond to both these light signals. A long-standing paradigm in plant biology predicts that the SAS is induced by detection of a reduced R:FR, indicative of proximate vegetation.3 However, studies on a variety of species have shown that also reduction of B light photon fluence rates in the incident light can induce pronounced shade avoidance responses.4,14–17
Leaves even reflect FR light, and the subsequent lowering of the R:FR is therefore an accurate and early indicator of neighbor proximity even in stages of vegetation development where leaf overlap and shading are not yet occurring.14 A low R:FR is, therefore, considered an early warning signal for upcoming competition for light. It is likely that a simultaneous occurrence of low R:FR and low B is used to evaluate actual shade, and thus competition, by neighbors.17
All higher plant species studied so far have photoreceptors tailored to detecting B, R and FR light cues. The model plant A. thaliana for example has three families of photoreceptors. The cryptochrome and phototropin families of photoreceptors are sensitive to B light fluence rates, whereas phytochromes are mostly sensitive to R and FR light.18–20
Cryptochromes are the major blue light receptors involved in stimulating hypocotyl elongation of light-grown seedlings exposed to B light depletion,21 but they also regulate de-etiolation of seedlings and entrainment of the circadian clock.22,23 In addition to B light, cryptochromes can also perceive ultraviolet-A (320–390 nm) light.23 It has been shown that A. thaliana cryptochromes 1 (CRY1) and 2 (CRY2), are phosphorylated upon blue light exposure and this autophosporylation affects both their activity and stability.23 CRY2 is localized to the nucleus, but the exposure of A. thaliana seedlings to B light leads to a rapid degradation of CRY2, whereas CRY1 is much more light-stable and acts at higher fluence rates.23 In a microarray study on de-etiolation in A. thaliana,24 it was observed that most of the genes that were regulated in wild-type plants upon B light exposure were not differentially regulated anymore in the cry1 cry2 double mutant under the same light conditions. In both the single mutants there were still pronounced B-induced changes in gene expression indicating that the two cryptochromes are partially redundant but are both responsible for B light-mediated de-etiolation. Pierik et al.21 showed that light-grown A. thaliana seedlings display increased hypocotyl elongation upon B light depletion of the incident light, and that similar to de-etiolation, this response is abolished only in the cry1 cry2 double mutant. This implies that for B light-mediated shade avoidance the two cryptochromes are the main photoreceptors that modify the shade avoidance response.
Upon activation by B light CRYs have been shown to be able to bind to the downstream factor, CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1).25 COP1 acts as an E3 Ubiquitin ligase and is a key repressor of photomorphogenesis. COP1 is located in the nucleus where it can interact with the transcription factor LONG HYPOCOTYLS 5 (HY5), and this interaction will lead to the ubiquitination and subsequent degradation of HY5.26 In this way CRY can regulate light responses through HY5 abundance. In addition, cryptochromes can also directly interact with the phytochromes.27
Phototropins are photoreceptors which are sensitive to blue and ultraviolet-A light. But they are involved in a different set of responses to optimize light harvesting and growth promotion, like phototropism, chloroplast movements, B light-induced inhibition of hypocotyl elongation, cotyledon expansion, leaf expansion and light-stimulated leaf movement.28 Of these responses, phototropism is a particularly important phenomenon for optimization of light capture in dense stands, as it will guide plant organs to grow differentially towards better lit conditions, for example at the border of a dense field.
Little is known about the targets of phototropin, but upon B light radiation PHOTOTROPIN1 (PHOT1) and PHOT2 are rapidly internalized or transported to the golgi apparatus respectively. An example of a known phototropin-interacting protein is NONPHOTOTROPIC HYPOCOTYLS 3 (NPH3), which is involved in auxin distribution during phototropism and NPH3 function depends on B light induced phosphorylation of phototropin.29 More recently, studies demonstrated that the phytochrome signaling component PHYTOCHROME KINASE SUBSTRATE 1 (PKS1) protein is required for hypocotyl phototropism in A. thaliana and PKS1 can form a complex with PHOT1 and NPH3.30 Since phytochromes (and cryptochromes) influence phototropic curvature in A. thaliana as well,31 PKS proteins may constitute a link between the photoreceptor families.
As mentioned above, phytochromes are sensitive to R and FR light. Phytochromes exist in two different conformation states, the inactive Pr form and the active Pfr form, with somewhat different light absorption characteristics.19 The biologically inactive form of phytochrome (Pr) has an absorption peak at 666 nm and is therefore sensitive for R light. When R light is absorbed this will lead to a photoconversion of Pr into Pfr, which is the active form of phytochrome. The Pfr has an absorption peak at 730 nm (FR light) and upon absorbing FR light it will photo-convert back into the inactive Pr form. In this way the R:FR of light reaching the plant will determine the equilibrium between Pr and Pfr forms of phytochrome in plants. Relative phytochrome activity (Pfr:Pr) thus is a direct function of the R:FR. Upon activation, Pfr translocates to the nucleus where it can interact with its molecular regulatory partners. In A. thaliana five different phytochromes have been characterized, which are named PHYA-E. Of these, PHYB is the main regulator for low R:FR-mediated shade avoidance, although PHYD and PHYE are also involved.3,32 In various species, such as A. thaliana, Brassica rapa (turnip mustard) and Lycopersicon esculentum (tomato), mutants which are deficient in phyB display a constitutive shade avoidance phenotype.32–35 An important phytochromemediated regulatory pathway is through interaction with PIF (PHYTOCHROME INTERACTING FACTORS) proteins. PIF and PIF-like (PIL) proteins are a subfamily of the bHLH (BASIC HELIX-LOOP-HELIX) family of transcription factors that bind DNA to regulate gene transcription as part of the phytochrome signal transduction.36 At least PIF3, 4, 5 and 7 are important regulators of phytochrome-mediated light responses.37–40 A subset of these are even specific to shade avoidance responses, such as shown for PIL1 and HFR1 (LONG HYPOCOTYL IN FAR-RED 1).41–43 In addition to the interaction with PIFs, PHYB can also bind directly to CRY2,27 possibly mediating cross-talk with the B light signaling pathway.
PIFs seem to be important players in plant responses to the environment and the hormonal regulation of these responses. They play a key role in modulating developmental responses to both light and temperature.44,45 As indicated earlier, phytochromes migrate to the nucleus when activated by R light, where PHYB can interact with PIFs.46 PIF4 for example controls genes mediating cell elongation and is targeted for ubiquitination and subsequent degradation by binding of PHYB.47 It has recently been shown that PIFs can also interact with DELLA proteins.48,49 The DELLA proteins are growth-repressing proteins and a subfamily of the GRAS domain family of transcriptional regulators.50 The regulation of these DELLA proteins appear to be key to photomorphogenic responses, including shade avoidance responses.16,51 DELLA abundance is downregulated during shade avoidance in low R:FR and in dense stands, which is essential to prevent DELLAs from inhibiting the SAS.16 Interestingly, DELLA stability is primarily controlled by the plant hormone gibberellin (GA), thus connecting phytochrome signaling to hormone action. However, DELLA stability is also affected by other hormones, such as auxin and ethylene21,52,53 and these hormones are all essential regulators of the SAS as well (Fig. 2).4,21,54 This suggests that DELLA proteins not only play a key role in integrating the regulatory effect of PIFs and GA, but are also an integrator of several hormonal signal transduction pathways.
Gibberellin (GA) is a key regulator of cell elongation, and for that alone it would be a good candidate for regulation of the SAS. Transgenic tomato plants expressing high levels of oat phytochrome A, which results in an inhibition of the SAS phenotype,55 are remarkably similar in phenotype to tomato mutants defective in GA biosynthesis.56 Both plant lines display a shortened stature, curled leaves, and increased leaf and fruit pigmentation. A study with tobacco also overexpressing oat PHYA, confirmed the strong inhibition of the SAS,57 including inhibition of internode and petiole elongation, but also showed that the SAS could be restored by external GA application.58 In A. thaliana, low R:FR promotes the expression of GA-related genes59 and the constitutively elongated phenotype of phyB mutants is suppressed by GA deficiency and GA insensitivity.60 A direct link between GA and the SAS has been shown for A. thaliana, since low R:FR treatment appears to enhance both GA biosynthesis61 and responsiveness.62 In addition, GA-related mutants are less responsive to low B and low R:FR treatment to induce the SAS.16
When GA is present, it will bind to its receptor GID1 and will facilitate direct interaction with DELLA proteins, which are subsequently ubiquitinated and targeted for proteasome-mediated degradation.63 As mentioned above, DELLA protein abundance is reduced in plants grown in dense stands,16 confirming earlier data on R:FR controlled GA-levels and DELLA stability. In the absence of GA, DELLA proteins will accumulate to higher levels and interact with PIF3 and PIF4 and prevent these PIFs from regulating gene expression associated with cell elongation.48,49
As mentioned above, not only GA-related genes are under the control of R:FR and B light, but auxin-related genes are as well.21,59,64 Auxin is associated with several processes like embryogenesis, stem cell niche, cell division and cell elongation65 and a number of studies propose that auxin plays a vital role in the SAS.21,54,59,64,66–68 Tao et al.68 showed a rapid upregulation of auxin biosynthesis in A. thaliana seedlings through a dedicated auxin biosynthesis route under the control of the TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS1 (TAA1) gene. This enhanced auxin production is needed for the hypocotyl to respond to a FR-enrichment treatment to lower the R:FR. Another way for light to regulate auxin biosynthesis is through RED ELONGATED 1 (RED1), which acts downstream of phyB, and is involved in auxin homeostasis.69 Morelli and Ruberti54 proposed ten years ago that polar auxin transport (PAT) could play an important role in the redistribution of auxin and thereby induce elongation responses, as part of the SAS. This was partly confirmed by Devlin et al.59 who showed that not only auxin-related genes like several AUX/IAAs are regulated but also PIN-FORMED3 (PIN3) and PIN7 are upregulated upon far-red light enrichment. These PINs are facilitators of auxin efflux and can thereby determine the direction of PAT.65 It was observed that phototropism in response to B light induces intracellular lateral relocalization of PIN1 and PIN3 proteins, thus producing a differential auxin gradient which induces differential cell elongation.70,71 As a result, there will be hypocotyl bending towards the light. The importance of this redistribution of auxin has also been shown for unidirectional shoot elongation as part of the SAS in hypocotyls and petioles of A. thaliana. Studying the auxin activity reporter line pIAA19::GUS, showed an increased auxin activity throughout the elongated hypocotyls and petioles upon low B or low R:FR exposure.21 When these plants where treated with the auxin efflux inhibitor 1-naphthylphthalamic acid (NPA), not only was the pIAA19::GUS pattern abolished, but also the shade-induced elongation response.21 These data indicate that not only auxin biosynthesis plays an important role in the SAS but also an intact PAT is needed for A. thaliana to respond properly to shade.
Auxin and brassinosteroids (BR) are linked to many of the same growth processes, including vascular differentiation, flower and fruit development, root growth and elongation. Furthermore auxin and BR show a large overlap in the genes they regulate.72–74 This suggests crosstalk between BR and IAA and also indicates a possible role for BR in the SAS. Additionally it has been suggested that BR may fine tune phytochrome-mediated responses.75 Although this has not been studied yet in a SAS context, light is involved in BR biosynthesis76 and/or BR inactivation in a phytochrome- and cryptochrome-dependent manner.77 In addition, BR-related mutants are dark green, slow-growing dwarfs with epinastic leaves and short stems and petioles,77 which is the opposite of a SAS phenotype. There are also indications that BR is involved in auxin and ethylene responses in A. thaliana.78 It is suggested that BR might affect auxin transport in response to light,78 which is consistent with the fact that BR can induce the expression of some of the PINs.79 The role of auxin in the SAS is clearer than the role of BR, but there is a considerable overlap in the regulatory pathway between these two hormones72–74,80 and PIF3 expression is at least partly under the control of BR81 indicating that a role for BR in the SAS is at least possible.
In several species, ethylene production is stimulated by low R:FR.5,82,83 Ethylene can accumulate within dense stands of cultivated tobacco in a greenhouse to three-fold the ambient concentration and can therefore be a signal for neighbor detection.4 The elevated ethylene levels reached up to 20 ppb (parts per billion) which was sufficiently high to induce stem elongation and hyponastic leaf growth, two SAS components, in wild-type plants. Ethylene-insensitive transgenic tobacco plants showed a reduced and delayed response to neighboring plants and were therefore out competed by wild-type neighbors.84 Although the ethylene-insensitive tobacco plants did show a reasonable response to low R:FR, the response to a reduction in B light fluence rates was entirely absent in these plants.4 In contrast, in A. thaliana the response to low R:FR light is ethylene dependent, but the response of seedlings to B light depletion appears to be independent of ethylene.21
Ethylene can stimulate auxin production and transport in A. thaliana.85 Consistently, for ethylene to stimulate hypocotyl elongation in A. thaliana, intact auxin signaling is required.21 Interestingly, auxin can also enhance ethylene production by stimulating the activity of ACC synthase, a precursor of ethylene biosynthesis.86
The rapid shoot elongation occurring during SAS is driven primarily by cellular expansion. Cellular expansion requires the stretching of the normally rigid cell wall in response to turgor pressure. To facilitate this, the properties of the cell wall need to be altered in order to increase its extensibility in a process termed wall loosening.87 Wall loosening involves a modification of the structure and organization of cell wall matrix polymers via the action of certain cell wall proteins.88 Cell wall modifying proteins that increase cell wall extensibility and thereby facilitate cellular expansion are therefore potential targets downstream of the photoreceptors during shade-induced extension growth. This is supported by several A. thaliana microarray studies analyzing global changes in gene expression in response to shade conditions. Seedling elongation responses in wild type and phyB mutants upon low R:FR treatment corresponded with the regulation of several cell wall related genes. Amongst these were the cell wall enzymes, pectinesterases and pectin lyases both of which are associated with wall loosening.59 Changes in light quality and light/dark transitions strongly regulated XTH 15 expression in correlation with hypocotyl elongation.24,89 Blue light was found to downregulate two XTHs in Arabidopsis seedlings corresponding with decreased hypocotyl elongation.64 A more functional study was performed with two ecotypes of Stellaria longipes that show contrasting shade avoidance responses.17 The prairie ecotype from competitive prairie grasslands showed strong internodal elongation in response to low R:FR in contrast to the non responsive alpine ecotype from a sparsely vegetated habitat (Fig. 3). In both ecotypes, growth patterns correlated with corresponding changes in expansin gene expression and activity. Simulated canopy shade induced elongation responses and the upregulation of specific expansin genes and activity in both ecotypes. Thus the shade avoiding responses of the two ecotypes could be correlated to their ability to express cell wall modifying proteins.17 Cell wall modifying proteins such as expansins and XTHs comprise large gene families90–92 and individual members are known to be regulated by the hormones GA, ethylene, auxin and BR.93–96 Auxin plays a role in the acidification of the apoplast, where auxin can lower the apoplastic pH within minutes.97 This rapid acidification probably sets the optimal pH for cell wall modifying proteins such as expansins98 and XTHs.99 These genes are thus likely activated during SAS as the downstream targets of the interacting network of hormones mentioned above.
The actual adaptive value of the SAS can be derived from research where it was shown that elongated plants (thus showing the SAS) have increased fitness compared to non-elongated plants growing at a high density, but a reduced fitness at low densities.100,101 At the same time, from an agronomic viewpoint, inhibition of elongation responses to neighbors in crop monocultures may actually enhance the harvest index since more carbon will be allocated to harvestable organs, rather than to be invested in non-harvestable stems.102
Not all species show a similarly strong SAS. A relatively steep vertical light gradient in a canopy makes it more likely that a plant can benefit from enhanced light interception by elongation. It will therefore be more effective to show this adaptive behavior in for example dense grasslands than underneath an overstory canopy, where the reduction in light occurs at a greater height and the SAS cannot enable understory plants to escape from these shaded conditions. This is consistent with the observation that in general forest understory plants do not show strong shade avoidance responses to low R:FR.100,103,104 In addition to between-species variation, there are also examples of variation for shade avoidance responses between ecotypes of the same species.
The previously mentioned study on two ecotypes of S. longipes, constitutes an interesting example of ecotypic variation for the SAS. A prairie ecotype is naturally subjected to crowding in its native dense grasslands, whereas a dwarfed alpine ecotype grows in alpine regions of the Rocky Mountains with very little vegetation in which above-ground competition is almost absent. It was shown17,105 that there is a clear variation in response to different light signals, between the two ecotypes (Fig. 3). Alpine plants show no response to low R:FR, consistent with the lack of competing neighbors in its native habitat, whereas the prairie ecotype displays the classic SAS in low R:FR. When these plants were exposed to true canopy shade, both ecotypes showed a clear internodal elongation response, in the alpine ecotype probably representing a response to grow away from the deep shade created by rocky surfaces from which the shoots need to grow into the light (Pierik R and Sasidharan R, personal observation). The adaptiveness of the SAS is, thus, contingent upon local conditions of competition. As mentioned in the previous section, consistent with the differential induction of low R:FR-induced SAS, it was also the prairie ecotype, but not the alpine, that shows a clear induction of expansin genes and activity to allow for enhanced shoot elongation.
Plants in their natural environments usually encounter, in addition to neighboring competitors, a vast array of other stresses which may affect the adaptiveness of SAS. An example of such a (biotic) stress is the one imposed by pathogens and herbivores. Interestingly there seems to be a tradeoff between resistance against attackers and the SAS. Growth of pathogens, such as the biotrophic bacteria Pseudomonas syringae is enhanced in constitutively shade avoiding phytochrome mutants of A. thaliana106,107 and plants are less resistant against herbivory when grown in crowded stands or when exposed to low R:FR conditions.108–110 It is thought that for herbivore resistance in Arabidopsis, defense is reduced in low R:FR conditions due to a possible desensitization to jasmonic acid (JA).110
Thus, for the SAS to be beneficial to the plant it should be induced only by light signals that reliably signal crowding. It should result in a more favorable place within a canopy leading to higher rates of photosynthesis and SAS-inducing signals should also be integrated with other stress signals. Plant hormones can play an important role in this signal integration as many of the hormones mentioned above, see for example ethylene,111 are involved in responses to stress, like drought,112 temperature,113 submergence,114 herbivory and pathogen infection.115
Plants perceive the threat of competing neighbors through various signals. Aboveground the most important one seems to be the reduced R:FR, which signals upcoming competition well before actual shading occurs. Plants carry sophisticated photoreceptor systems to signal this and subsequently activate an interacting network of various hormones and transcriptional regulators. The complete signal detection and signal transduction network together defines the SAS to be expressed. This complicated network of interacting molecular and physiological regulators allows for fine-tuned modifications of the response by additional inputs, such as signals coming from defense pathways. The relative advantage of expressing the SAS during competition will thus depend on the presence of additional threats to plant performance, such as herbivory. The current understanding of the physiological and molecular regulation of the SAS is instrumental to understanding how plant behavior in dense, competitive vegetations will be adjusted to additional biotic or abiotic stress factors.
We thank Rens Voesenek and Mieke de Wit for helpful comments on a draft of this manuscript. The authors are financed by the Netherlands Organisation for Scientific Research (Veni grant 863.06.01 to R.P.) and Utrecht University (D.K., R.S.).
Previously published online: www.landesbioscience.com/journals/psb/article/11401