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First published online November 26, 2003; 10.1104/pp.103.034397 Plant Physiology 133:1617-1629 (2003) © 2003 American Society of Plant Biologists A Genomic Analysis of the Shade Avoidance Response in Arabidopsis1,[w]The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037
Plants respond to the proximity of neighboring vegetation by elongating to prevent shading. Red-depleted light reflected from neighboring vegetation triggers a shade avoidance response leading to a dramatic change in plant architecture. These changes in light quality are detected by the phytochrome family of photoreceptors. We analyzed global changes in gene expression over time in wild-type, phyB mutant, and phyA phyB double mutant seedlings of Arabidopsis in response to simulated shade. Using pattern fitting software, we identified 301 genes as shade responsive with patterns of expression corresponding to one of various physiological response modes. A requirement for a consistent pattern of expression across 12 chips in this way allowed more subtle changes in gene expression to be considered meaningful. A number of previously characterized genes involved in light and hormone signaling were identified as shade responsive, as well as several putative, novel shade-specific signal transduction factors. In addition, changes in expression of genes in a range of pathways associated with elongation growth and stress responses were observed. The majority of shade-responsive genes demonstrated antagonistic regulation by phyA and phyB in response to shade following the pattern of many physiological responses. An analysis of promoter elements of genes regulated in this way identified conserved promoter motifs potentially important in shade regulation.
Competition for sunlight is one of the most important aspects regulating plant development. Plants can detect the presence of neighboring vegetation by sensing changes in the quality of the light reflected from these neighbors. In shade-avoiding species, a pronounced elongation response is triggered, altering the whole architecture of the plant, and in response to prolonged vegetative shading, flowering is dramatically accelerated (Smith and Whitelam, 1997  ). In seedlings established in an open canopy, the shade avoidance response is vital to prevent overtopping by competitors growing alongside. However, this response can be detrimental in agriculture because it limits planting density. If a shade avoidance response is induced, this reallocation of resources into elongation growth often results in a decrease in crop yield from seeds or storage organs (Ballaré et al., 1997).
Light reflected from neighboring vegetation is depleted in red (R) wavelengths because of absorption of these wavelengths by chlorophyll but remains rich in far-red (FR) light. This change in light quality is detected by the phytochrome photoreceptors: R/FR reversible photochromic pigments (Quail, 1998
The phytochrome photoreceptors mediating the shade avoidance response have been well characterized (Quail, 1998
PhyB predominates in light-grown seedlings. phyB mutant seedlings of Arabidopsis seedlings fail to properly de-etiolate in white light and display a constitutive shade-avoiding phenotype characterized by an elongated hypocotyl, elongated petioles, and accelerated flowering (Reed et al., 1993
Therefore, in seedling establishment, phyA and phyB have contrasting effects in response to FR-rich light. PhyA interprets FR as a de-etiolation signal, whereas phyB interprets FR as a signal of shade. This antagonism between phyA and phyB can be seen clearly in etiolated seedlings that have been transferred to low R:FR light, where white light has been enriched with supplementary FR. Although phyA mutant seedlings of Arabidopsis show a clear promotion of hypocotyl elongation, in response to 24-h low R:FR treatment, wild-type seedlings show no response in this time period. The high levels of functional phyA in etiolated wild-type seedlings acts to completely counteract the shade avoidance response under these conditions (Johnson et al., 1994
PhyD and phyE have also been shown to act in shade avoidance. Double and triple mutant studies have revealed roles for both phyD and phyE acting redundantly with phyB (Devlin et al., 1998
Although the photoreceptors involved in shade avoidance have been well characterized, little is known of the signaling events downstream or of the extent of changes occurring at the molecular level as a result of shade avoidance. A number of phytochrome-interacting factors, including one transcription factor, have been isolated recently, but no clear role for these factors has been demonstrated in shade avoidance (Ni et al., 1998 Here, we have used DNA microarrays to analyze global changes in gene expression in response to simulated vegetative shade in wild-type and phytochrome mutant seedlings. One-week-old light-grown seedlings of Arabidopsis show a pronounced hypocotyl elongation response after just 24 h of simulated shade. These conditions formed the basis for our analysis. We demonstrate that under these conditions, phyB plays a major role as the photoreceptor mediating this promotion of hypocotyl elongation in response to shade, whereas phyA acts to moderate this response. We analyzed changes in gene expression in wild-type, phyB mutant, and phyA phyB double mutant seedlings under these conditions. We collected tissue at both 1- and 24-h time points to examine both early and late changes in gene expression in response to simulated shade.
Elongation Growth Response of 7-d-Old Seedlings of Arabidopsis to 24 h of Simulated Shade Wild-type, phyB mutant, and phyA phyB double mutant seedlings of Arabidopsis were grown for 7 d in constant white light (at which point they were producing their first true leaves) before being transferred to experimental treatments. Seedlings were then either maintained in white light (high R:FR) or transferred to white light supplemented with FR (low R:FR). Hypocotyl length was measured after 24 h in these treatments. Wild-type seedlings of Arabidopsis transferred to 24-h low R:FR conditions displayed elongated hypocotyls relative to those maintained in high R:FR, evidence of a strong shade avoidance response (Fig. 1). Under normal, high R:FR conditions, phyB mutant seedlings show an elongated hypocotyl phenotype relative to wild-type seedlings. Transfer of phyB mutant seedlings to low R:FR conditions resulted in a clear inhibition of hypocotyl elongation, the reverse of the normal shade avoidance response (Fig. 1). Although phyA phyB double mutant seedlings maintained in high R:FR also displayed an elongated hypocotyl phenotype relative to wild type in seedlings under the same high R:FR conditions, the phyA phyB double mutant seedlings demonstrated no response to low R:FR (Fig. 1).
These data are consistent with previous finding of phyB being the major photoreceptor for the shade avoidance response, with phyA acting to moderate the degree of response.
To analyze changes in gene expression in response to vegetative shade in the seedlings used above, the global gene expression profile of each of these genotypes grown in high R:FR was compared with that of seedlings grown in low R:FR. Seven-day-old wild-type, phyB mutant and phyA phyB double mutant seedlings, grown in constant light of high R:FR, were maintained in high R:FR or transferred to low R:FR conditions for 24 h. RNA was harvested after 1 and 24 h in each treatment and used to challenge Affymetrix 8K gene chips to monitor early and delayed responses to shade. Classification of genes as shade responsive was based upon two criteria. Genes showing at least 1.5-fold change in expression level in wild-type seedlings in response to shade and showing a consistent fit across the 12 chips to a predicted pattern based upon physiological observations in the wild-type and mutant seedlings used in the experiment were categorized as shade responsive. Ten distinct patterns of expression were searched for among the genes represented on the chip.
The Pearson correlation was used to assign a P value for closeness of fit for each gene to each predicted pattern (Pavlidis and Noble, 2001
Genes rapidly up-regulated by shade under phyB control would be expected to show an increase in expression in response to both 1- and 24-h low R:FR treatments in wild-type seedlings. They would be constitutively up-regulated relative to wild type grown in white light in the phyB and phyA phyB double mutants. Conversely, genes rapidly down-regulated by shade under phyB control would show the inverse of this pattern. Genes for which the transcript levels fit to one or other of these two patterns of expression were identified by closeness of fit, across the 12 chips used in the experiment, to patterns predicted for a hypothetical gene showing such regulation (for details, see "Materials and Methods"). Genes fitting with a P value < 0.01 and showing a minimum 1.5-fold change in expression level in response to shade in wild-type seedlings were selected. Seventeen genes were classified as rapidly up-regulated by shade under phyB control using this method. Figure 2A shows those genes for which the transcript levels fit to this pattern with a P value < 0.001. Among them are the genes encoding a putative CCAAT-binding transcription factor subunit (At1g08970), and the 20S proteasome beta subunit C, PBC1 (At1g21720). Full details of all 16 genes in this list can be seen in Supplemental Table I. Nine genes were classified as rapidly down-regulated by shade under phyB control. Full details of genes in this list can also be seen in Supplemental Table II.
Genes showing delayed up-regulation by shade under phyB control would be expected to show no increase in expression in response to 1-h low R:FR treatment in wild-type seedlings but to display an increase in expression in response to 24-h low R:FR treatment. They would be constitutively up-regulated relative to wild type grown in white light in the phyB and phyA phyB double mutants. Figure 2B shows those genes for which the transcript levels fit to this pattern with a P value < 0.001. Among them are the genes encoding a putative Pro-rich protein (At2g21140), the RING-H2 finger protein, RHA2b (At2g01150), an RNA helicase (At3g26560), a SAICAR synthetase (At3g21110), the potassium channel protein, AKT3 (At4g22200), the protease I (pfpI)-like protein (At2g38860). Full details of all 30 genes in this list can be seen in Supplemental Table III. Thirty-four genes were classified as showing delayed down regulation by shade under phyB control. Full details of all genes in this list can also be seen in Supplemental Table IV.
Responses mediated by phyA in FR-rich light are most pronounced after prolonged irradiation (Johnson et al., 1994
Genes rapidly up-regulated by shade under phyC, D, or E control would be expected to show an increase in expression in response to both 1 and 24 h of low R:FR treatments in wild-type, phyB mutant, and phyA phyB mutant seedlings. Eleven genes were classified as rapidly up-regulated by shade under phyC, D, or E control using this method. Figure 3A shows those genes for which the transcript levels fit to this pattern with a P value < 0.001. Among them are the genes encoding homeobox-Leu zipper, HAT4 (At4g16780), PIF3-like protein, PIL1 (At2g46970), pantothenate kinase (At1g60440), a putative bHLH transcription factor (At3g57795), and a putative centrin (At4g27280). Notably, the gene encoding GA 20-oxidase 3 (At5g07200) was found to fit to this pattern with P < 0.005. Full details of all 11 genes in this list can be seen in Supplemental Table VII. Seven genes were classified as rapidly down-regulated by shade under phyC, D, or E control. Full details of all genes in this list also can be seen in Supplemental Table VIII.
Many physiological responses to shade are regulated primarily by phyB but are moderated by the action of phyA under prolonged low R:FR irradiation. Genes showing such regulation by shade would demonstrate a differential regulation by shade under phyB control after 1 h of low R:FR treatment, but under prolonged irradiation, an antagonistic action of phyA would be revealed, reducing this differential regulation. Genes up-regulated by shade in this way would demonstrate an up-regulation after 1 h of low R:FR treatment in wild-type seedlings but would display a reduced change in expression in response to 24 h of low R:FR treatment. In phyB mutants grown in white light, these genes would be constitutively up-regulated relative to wild type grown in white light. In the absence of phyB, these genes would show no response to 1 h of low R:FR treatment, but the moderating effect of phyA would be made apparent in response to 24 h of low R:FR treatment in that these genes would display a down-regulation after 24 h of low R:FR treatment. In phyA phyB double mutants grown in white light, these genes would be, again, constitutively up-regulated relative to wild type grown in white light. However, in the absence of both phyA and phyB, these genes would show no response to either 1 or 24 h of low R:FR treatment. Conversely, genes down-regulated by shade in this way would demonstrate the inverse of this pattern. Genes for which the transcript levels fit to one or the other of these two patterns of expression were selected as previously described. These groupings formed by far the largest groupings of shade-responsive genes. Ninety-nine genes were classified as showing up-regulation by shade under the control phyB, moderated by phyA using this method. Figure 3B shows those genes whose transcript levels fit to this pattern with a P value < 0.001. Among them are the genes encoding HAT2 (At5g47370), a putative dimethylaniline monooxygenase (At4g28720), the auxin transport protein, PIN3 (At1g70940), IAA3 (At1g04240), a putative polyphosphoinositide-binding protein (At3g51670), a putative beta coatomer (At4g31480), BRI1 (At4g39400), cyclin A2 (At1g15570), GAI (At1g14920), a putative phosphatidylinositol phophatidylcholine transfer protein (At2g21540), a putative auxin-induced protein (At4g36110), a putative protein kinase (At2g47060), IAA1 (At4g14560), a protein similar to NPH3/RPT2 (At4g37590), alpha-xylosidase (At1g68560), PIN7 (At1g23080), a dof zinc finger transcription factor (At3g61850), a protein disulfide-isomerase-like protein (At3g54960), lactate dehydrogenase, LDH1 (At4g17260), spermine synthase, ACL5 (At5g19530), an AUX1 homolog (At5g49630), and a protein initiation factor (At4g18300). Notably, the gene encoding phototropin 1, PHOT1 (At3g45780) was found to fit to this pattern with P < 0.006. Full details of all 99 genes in this list can be seen in Supplemental Table IX. Forty-one genes were classified as showing down-regulation by shade under the control phyB, moderated by phyA. Full details of all genes in this list can also be seen in Supplemental Table X. Notably, this list includes the gene encoding transcription factor HY5 (At5g11260), previously demonstrated to be involved in light signaling.
The transcript encoding phytochrome A showed a rapid up-regulation in response to shade after 1 h of low R:FR treatment in both wild-type and phyB mutant seedlings (Fig. 3C). The expression of PHYA, however, showed no increase relative to white light levels after 24 h of low R:FR treatment. The absence of any PHYA transcript in the phyA phyB double mutant meant that no PHYA expression was observed in this line. The retention of a response to shade in the phyB mutant suggests the involvement of phytochromes C, D, or E. The response to 1 but not 24 h of low R:FR treatment, however, suggests that phyA acts antagonistically to phyC, D, or E in prolonged supplementary FR. The transcript encoding phytochrome B showed a rapid up-regulation in response to both 1 and 24 h shade in wild-type seedlings (Fig. 3C). The absence of any PHYB transcript in the phyB and phyA phyB double mutants meant that no PHYB expression was observed in this line. The two other phytochrome genes represented on the chip, PHYC and PHYD, showed no response to shade (Fig. 3C).
Northern blotting was used to confirm the expression profiles of a selection of genes showing a fit to one of several of the predicted patterns above. Transcript levels of the following genes were measured across the same 12 genotypes and treatments that were used in generating the chip data (Fig. 4). The transcripts encoding the putative CCAAT transcription factor (At1g08970), the homeodomain-Leu zipper transcription factor, HAT4 (At4g16780), the PIF3-like basic helix loop helix transcription factor, PIL1 (At2g46970), the homeodomain-Leu zipper transcription factor, HAT2 (At5g47370), and the Leu-rich repeat receptor kinase brassinosteroid insensitive 1, BRI1 (At4g39400), the auxin transport protein, PIN3 (At1g70940), and the transcript-encoding
A functional analysis of genes in each of the patterns of gene expression revealed a number of noteworthy trends. Seven functional categories were used to catalogue gene function: "cell metabolism related," "signaling molecules," "transcription factors," "cell wall related," "auxin related," "cell elongation/division related," and "others." Function was assigned based on the Affymetrix annotation provided by the Salk Institute Genomic Analysis Library. Those genes categorized as, simply, "expressed protein," "hypothetical protein," or "putative protein" were omitted from this functional analysis. For each of the patterns of gene expression, the percentage of the total number of differentially regulated genes that were allotted to each of seven functional categories was calculated. Results for patterns showing differential regulation by shade either under the control of phyB alone or where the action of phyB is moderated by phyA are shown in Figure 5.
A significantly greater percentage of the total number of genes down-regulated by shade, as opposed to up-regulated, were associated with cell metabolism. This was true for both early and late shade-regulated patterns and for those patterns representing regulation by phyB alone or by both phyB and phyA. In particular genes associated with photosynthesis and fatty acid metabolism and redox metabolism were highly represented among those down-regulated. A significant percentage of the total number of genes displaying rapid up-regulation in response to shade were transcription factors. Again, this was true both for patterns representing regulation by phyB alone or by both phyB and phyA. A number of cell wall-associated genes displayed late up-regulation in response to shade under phyB control. Of these, the majority were pectinesterases and pectate lyases, associated with cell wall loosening. It is also noteworthy that a substantial number of genes associated with cell elongation or division were up-regulated by shade. Again, this was true both for patterns representing regulation by phyB alone or by where phyA moderates the action of phyB. A large number or auxin-related genes were found to fit to the pattern corresponding to genes up-regulated by shade under the control of phyB but where the action of phyA moderates the response under prolonged low R:FR irradiation. In particular, a number of the indole-3-acetic acid (IAA) family of auxin-regulated transcription factors were regulated in this manner. This forms the largest functional grouping of genes fitting to this pattern. Coupled with the fact that this pattern represents the majority of shade-regulated genes, this represents a significant finding.
The first 500 bp of the promoters of all of the genes fitting with a P value < 0.001 to the largest shade-regulated grouping (those up-regulated by shade under the control of phyB but where the action of phyA moderates the response under prolonged low R:FR irradiation) was examined for overrepresented hexamer sequences. A number of hexamers corresponding to recognized eukaryotic transcription factor-binding motifs were identified as overrepresented, that is present at a significantly higher frequency among these sequences than among the promoter sequences of the Arabidopsis genome as a whole (for full list of overrepresented hexamers, see Supplemental Table XI). A P value was assigned to represent the significance of this overrepresentation (For details see "Materials and Methods"). The palindromic sequence ACTAGT, shown to be involved in the control of stromelysin gene expression in mammals, was present with a frequency of 40% among this group as opposed to a frequency of 11% throughout the whole genome (P = 0.00015). The sequence TTATTA, a classical T-box, involved in regulation of genes involved in development in a range of species, though not in plants, was present with a frequency of 88% among this group as opposed to a frequency of 60% throughout the whole genome (P = 0.00206). The sequence TAATTA representing the recognized core-binding motif for homeodomain proteins was present with a frequency of 68% among this group as opposed to a frequency of 40% throughout the whole genome (P = 0.00306). Notably, this frequency rises to 94% among this group if 1,000 bp of promoter sequence is considered. The sequence TGTGGT, the mammalian AML/CB-Falpha transcription factor family recognition site, was present with a frequency of 44% among this group as opposed to a frequency of 21% throughout the whole genome (P = 0.00542). Finally, the canonical E-box cis-acting sequence CATGTG was present with a frequency of 40% among this group as opposed to a frequency of 19% throughout the whole genome (P = 0.00792). This frequency rises to 64% among this group if 1,000 bp of promoter sequence is considered.
The shade avoidance response is a major determinant in planting density in many economically important crops. Crop yield severely reduces as planting density increases with increasingly more resources being directed toward elongation growth (Ballaré et al., 1997 ). Likewise, in many crop species such as cabbage (Brassica oleraceo var. capitata), where a tight rosette habit is desirable, or root crops, where allocation of reserves to storage is required, acceleration of flowering due to shade avoidance is detrimental. We analyzed changes in gene expression in response to shade in young seedlings previously established for 7 d in constant white light. Seedlings were exposed to either 1 or 24 h of constant low R:FR light before being harvested. Changes in gene expression in response to shade under these conditions were examined in seedlings of the phyB mutant and the phyA phyB double mutant and wild-type seedlings. PhyA and phyB represent the most abundant phytochromes in established seedlings, and both have been demonstrated to be involved to some degree in the shade avoidance response. A clear elongation response was seen in 7-d-old wild-type seedlings transferred to low R:FR for 24 h. Analysis of 7-d-old phyB mutant and phyA phyB double mutant seedlings confirmed that phyB acts as the major photoreceptor for this shade avoidance response, whereas phyA acts as a moderator in this response. One- and 24-h time points were chosen in an attempt to identify both early and late shade-regulated genes in these seedlings. Seven-day-old seedlings were chosen for this assay because seedlings of this age are not yet competent to flower. In more mature seedlings grown under such conditions, prolonged low R:FR light would cause an acceleration of flowering, meaning that changes in gene expression involved in the flowering process would be detected in addition to those directly involved in the shade avoidance response. Furthermore, this made cultivation on petri dishes possible, generating very uniform growth conditions and allowing harvest of a large number of whole seedlings from which RNA was pooled. We used pattern fitting software to search for genes responding to shade in a manner consistent with the physiological changes observed in response to shade. It was predicted that some genes would show a response mediated by phyB alone, some by phyA alone, and others by the antagonistic actions of both. One further category was searched for: those genes still showing a clear response to shade in the absence of both phyA and phyB. Genes showing at least 1.5-fold change in expression level in wild-type seedlings in response to shade and showing a consistent fit across the 12 chips to a predicted pattern based upon physiological observations (P < 0.01); were categorized as shade responsive.
Shade-responsive genes (301) were identified using these criteria fitting to one of 10 distinct, predicted patterns. This method for analysis of chip data allows more subtle changes in gene expression to be considered meaningful. Only 92 genes among the 301 fitting to these patterns of expression showed a greater than 2-fold change in expression level. The majority showed only modest changes in expression level but showed such tight fits to physiological patterns that we are confident that they are genuinely shade regulated. This is consistent with the difficulty, up until now, of identifying shade-responsive genes despite numerous efforts. The only previously identified shade-responsive gene is that encoding the HAT4 homeodomain transcription factor (Carabelli et al., 1993
Of particular interest were genes with a putative role in signaling or transcriptional regulation that could form part of a shade-specific signal transduction pathway as opposed to those genes that are end point targets of shade avoidance in affecting plant architecture. Manipulation of such end point genes is likely to have a constitutive effect on plant architecture that could be detrimental in terms of agricultural yield. Conversely, a shade-specific signal transduction pathway component could be manipulated as a method of engineering out the shade avoidance response without affecting plant growth under non-shade conditions. Two possible approaches could be used. The most obvious approach would involve manipulation of genes that show a change in expression in response to shade, specifically under the control of phyB, C, D, or E. Reducing the response of key genes regulated in this way would be predicted to reduce shade avoidance. Alternatively, genes showing change in expression in shade specifically under the control of phyA could be manipulated to enhance the phyA-mediated moderation of shade avoidance (Robson et al., 1996
A number of interesting genes encoding potential shade signal transduction components were identified, including a number of transcription factors, some of which already have been implicated as having a role in light signaling. The gene encoding homeobox-Leu zipper, HAT4 (Schena et al., 1993
The transcript encoding PIF3-like protein, PIL1, also fit closely to the predicted pattern for genes showing a rapid up-regulation of expression in shade specifically under the control of phyC, D, or E. PIL1 has been demonstrated to interact with TOC1, a protein playing a role both in the circadian clock and in the regulation of elongation growth by light (Makino et al., 2002
The transcript encoding the DNA-binding protein, LHY (Schaffer et al., 1998
Finally, the transcript encoding the bZIP protein HY5 (Oyama et al., 1997
A number of recognized components involved in protein degradation also form potential shade signal transduction components. The genes encoding 20S proteasome beta subunit C (PBC1) and the RING-H2 finger proteins (RHA2b and RMA1, both E3 ubiquitin-protein ligases) all show strong shade responsiveness. This suggests that protein degradation may play a role in the shade avoidance response. Protein degradation has been demonstrated to play a key role in regulation of light responses during seedling establishment. In particular, a key step in maintaining skotomorphogenesis in darkness involves the degradation of HY5 via the COP9 signalosome. The COP9 signalosome has similarity to the lid of the 19S regulatory particle of the 26S proteasome (which also contains a 20S proteolytic core) and has been shown to interact with E3 ubiquitin ligases (Schwechheimer and Deng, 2001
Two other intracellular signaling components, showing a response to shade, have been implicated previously in light signaling. The transcript encoding gigantea, a nucleoplasmically localized protein with a proposed role in phyB signaling (Huq et al., 2000
One further group of interesting genes encoding potential shade-specific signal transduction components include a number of hormone-related factors. In particular, a number of other auxin-related factors also fit to the predicted pattern for genes showing antagonistic regulation by phyA and phyB in response to shade. These include the transcripts encoding two of the PIN family of auxin transport proteins (Friml et al., 2002
Links between light and auxin signaling have been proposed on several occasions. The shy2/iaa3 mutant was isolated a suppressor of the hy2 phytochrome chromophore-deficient mutant (Kim et al., 1996
The genes encoding brassinosteoid insensitive 1 (BRI1; Nam and Li, 2002 It is most likely, however, that moderation of hormone signaling by light is an downstream event in light signaling, rather than an early step in shade-specific signal transduction.
It is significant that the pattern representing the largest grouping of shade-responsive genes was indicative of genes showing antagonistic regulation by phyA and phyB in response to shade. Thus, the majority of shade-responsive genes respond in the same manner as that seen for the whole-plant gross physiological response to shade. It remains to be seen whether any of the genes in this grouping are primary transcription-regulated targets of shade signaling, in which case this antagonistic action of phyA and phyB must concur at the promoters of these genes. It would seem quite likely that phyA and phyB action can converge on the same promoter. Evidence for the convergence of phyA and phyB on a single promoter element is provided by the fact that both phyA and phyB can bind the bHLH transcription factor, PIF3, known to bind to and activate transcription via G-box elements in the promoters light-regulated genes (Martinez-Garcia et al., 2000
The overrepresentation of certain recognized transcription factor-binding sites in the promoters of genes rapidly up-regulated by shade under the control of phyB, where phyA moderates this response in prolonged shade, could provide clues to key transcription factors acting specifically in early shade signaling. Two potential cis-acting sequences are of particular interest. The prevalence of the sequence, TAATTA, representing the recognized core-binding motif for homeodomain proteins (Ades and Sauer, 1994
The canonical E-box CIS-acting sequence CATGTG, which represents a binding site for a number of bHLH transcription factors in mammals (Berberich et al., 1992
Another interesting finding was the rapid up-regulation of both PHYA and PHYB transcripts in response to shade. It is potentially very significant that the transcripts encoding the major photoreceptors involved in the shade avoidance response are, themselves, shade up-regulated. In particular, the shade up-regulation of PHYB transcript is intriguing because it is suggestive of a negative feedback mechanism. Such changes in PHYB transcript levels in response to shade have been observed elsewhere. Hall et al. (2002
It must be made clear, however, that changes in transcript level are not necessarily indicative of changes in protein level. Circadian regulation of PHYB::LUC expression does not appear to be backed up by changes in the level of total phyB protein. Bognar et al. (1999 If the changes in phyB transcript level observed in response to shade are translated into equally significant changes at the protein level, this would lead a suppression of the shade avoidance response. Although increased levels of phyB protein would not change the ratio of Pr to Pfr established under shade, it would lead to an increase in the absolute amount of both forms. The increase in amount of the active PfrB form would lead to an increased inhibition of elongation growth relative to that seen initially in response to shade. In conclusion, we have identified a number of potential shade-specific signaling components. A key step, now, will be to test the effect of mutation in the genes encoding these factors. It would be expected that mutants deficient in key up-regulated signaling components would fail to show aspects of the shade avoidance response. As such, this work offers a number of potential targets that could be used to beneficially manipulate both crop plants and ornamentals to allow an increase in planting density without affecting plant architecture and flowering time.
Plant Material and Growth Conditions Seeds of wild type, the phyB-1 mutants, and the phyA-201 phyB-1 double mutant Arabidopsis of the Landsberg erecta ecotype were sown on Murashige and Skoog agar plates covered with discs of Whatman No. 1 filter paper (Whatman, Clifton, NJ). Seeds were stratified in darkness at 4°C for 3 d and then placed in continuous white light (photosynthetically active radiation, 60 µmol m-2 s-1; R:FR, 8.99) at 22°C for exactly 7 d. Seedlings were then either maintained in these conditions or transferred to white light supplemented with FR light (photosynthetically active radiation, 60 µmol m-2 s-1; R:FR, 0.05). Approximately 200 seedlings of each genotype were harvested after 1 and 24 h from each treatment by scraping whole seedlings into aluminum foil packets and flash freezing in LN2.
White light was provided by Philips (New York) cool-white 20-W F20T12/CW tubes. Supplementary FR light was provided by two QB1310CS-670-735, LED hybrid lamps (Quantum Devices Inc., Barneveld, WI). All light measurements were made using an LI-1800 portable spectroradiometer (LI-COR, Lincoln, NE).
Hypocotyl lengths were measured using Scion Image software (Scion, Frederick, MD) to analyze digital images of seedlings laid out flat on agar plates. Data represent the mean (±SE) of at least 29 seedlings for each treatment.
Total RNA was prepared from the staged tissue samples using the Qiagen RNeasy Plant Mini Kit (Qiagen, Valencia, CA). Double-stranded cDNA was synthesized from 5 µg of total RNA using Superscript II reverse transcriptase (Life Technologies/Gibco-BRL, Cleveland). This cDNA was used as a template to synthesize biotin-labeled cRNA by in vitro transcription using an ENZO BioArray High Yield RNA transcript labeling kit (ENZO, Farmingdale, NY). Amplified cRNA was fragmented and hybridized to GeneChip microarrays (Affymetrix, Santa Clara, CA) overnight at 45°C. The hybridized arrays were washed and stained with biotinylated anti-streptavidin antibody and a phycoerythrin streptavidin conjugate and then scanned using a Hewlett-Packard GeneArray Scanner (Hewlett-Packard, Palo Alto, CA). Affymetrix GeneChip software was used to determine the average difference values between perfectly matched oligonucleotide probes and 1-bp mismatches for each probe set. Data were then scaled globally such that the average intensity of each microarray equaled a target intensity of 200 (Wodicka et al., 1997
Pattern fitting was performed by a feature selection method known as template matching (Pavlidis and Noble, 2001
Total RNA was extracted using RNeasy (Qiagen) according to the manufacturer's instructions, and 10 µg was analyzed by northern blot as previously described (Alabadí et al., 2001
Stretches of sequence 500 bp upstream of the start codon were obtained for each gene encoding the group of 25 transcripts fitting with a P value < 0.001 to the pattern: rapid up-regulation by shade under the control of phyB, where phyA moderates this response in prolonged shade. Analysis for overrepresented hexamer "words" was performed using the statistical motif analysis tool available at The Arabidopsis Information Resource Web site (http://www.arabidopsis.org). The motif analysis program (originally written by Dr. Robert Ewing, Arabidopsis Functional Genomics Consortium, Carnegie Institute, Stanford, CA) compares the frequencies of 6-mer words in groups of sequences (on both strands) with the frequencies of these words in the current genomic sequence set of 27,117 sequences, also consisting of 500 bp upstream of the start codon of each gene (The Institute for Genomic Research, July 2002 release).
Upon request, all novel materials described in this publication will be made available in a timely manner for noncommercial research purposes, subject to the requisite permission from any third party owners of all or parts of the material. Obtaining any permissions will be the responsibility of the requestor.
The authors are grateful to Dr. Marty Straume (University of Virginia Center for Biomathematical Technology, Charlottesville) for advice on pattern fitting analysis and to Dr. John Hogenesch (Genomics Institute of the Novartis Research Foundation, San Diego, CA) for help with regulatory element analysis. Received October 6, 2003; returned for revision October 13, 2003; accepted October 15, 2003.
Article, publication date, and citation information can be found at http://www.plantphysiol.org/cgi/doi/10.1104/pp.103.034397.
1 This work was supported by the National Institutes of Health (grant no. GM56006), by the Torrey Mesa Research Institute (funding to S.A.K.), by EMBO (long-term fellowship to P.F.D.), and by Pew Charitable Trusts (fellowship to M.J.Y.). This is manuscript no. 15909–CB of The Scripps Research Institute.
[w] The online version of this article contains Web-only data.
2 These authors contributed equally to the paper.
3 Present address: Department of Life Sciences, King's College London, 150 Stamford Street, London SE1 9NN, UK.
4 Present address: Catedra Fisiología Vegetal, Facultad de Agronomia, Universidad de Buenos Aires, Av. San Martin 4453, C1417DSQ, Buenos Aires, Argentina. * Corresponding author; e-mail stevek{at}scripps.edu; fax 858–784–2973.
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ABSTRACT
RESULTS
-amylase (At4g17090) were tested. In each case, the transcript levels showed a close qualitative correlation to the to the microarray data (
