A dynamic balance between gene activation and repression regulates the shade avoidance response.. Europe PMC Article. Abstract. Plants grown under dense canopies perceive through the phytochrome system a reduction in the ratio of red to far- red light as a warning of competition, and this triggers a series of morphological changes to avoid shade. Several phytochrome signaling intermediates acting as positive regulators of accelerated elongation growth and induction of flowering in shade avoidance have been identified. Here we report that a negative regulatory mechanism ensures that in the presence of far- red- rich light an exaggerated plant response does not occur. Strikingly, this unpredicted negative regulatory mechanism is centrally involved in the attenuation of virtually all plant responses to canopy shade. Keywords: Neighbor detection, phytochrome signaling, HFR1/SICS1 transcription factor.
Dynamic Balance Between Gene Activation And Expression
A dynamic balance between gene activation and repression regulates the shade avoidance response in Arabidopsis. A dynamic balance between gene activation and repression regulates the. response acting through a dynamic balance of positive and negative. A dynamic balance between gene activation and repression. gene expression are very rapid and reversible (Carabelli et al. 1996; Salter et al. 2003).
Results and Discussion. We found that persistency of a low R/FR signal causes a decrease in ATHB2 transcript level (Supplementary Fig. S1) suggesting the existence of a negative regulatory mechanism operating in low R/FR light to reduce the strength of the elongation response.
To identify negative regulators of the shade avoidance response, we devised a two- step screen consisting of (1) the identification of regulatory genes rapidly induced by FR- rich light and still up- regulated after prolonged exposure to low R/FR, and (2) the selection of the corresponding mutants with an altered shade avoidance response but a normal phenotype in high R/FR. Affymetrix Arabidopsis Genome Gene. Chip array (ATH1) analyses on young seedlings exposed to low R/FR for 1 h and 4 d identified seven regulatory genes (Supplementary Table S1). The greatest increase in transcript level in response to both brief and prolonged exposure to low R/FR was observed for HFR1, a gene encoding a b. HLH protein, previously identified as a component of phytochrome A (Fairchild et al.
Fankhauser and Chory 2. Soh et al. 2. 00. Duek and Fankhauser 2. The HFR1 transcript increased in abundance by 1.
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Dynamic Balance Between Gene Activation By A Growth
Supplementary Table S1). Recent work demonstrated that light, irrespective of its quality, enhances HFR1 protein stability (Duek et al. Jang et al. 2. 00.
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Dynamic regulation of FoxP3 expression controls the balance between CD4+ T cell activation and cell death. A dynamic balance between gene activation and repression regulates the shade avoidance response in Arabidopsis. Sessa G(1), Carabelli M, Sassi M, Ciolfi A. Dynamic balance between activation and repression regulates pre-mRNA alternative splicing during heart development. Gene Expression Regulation. Sessa G, Carabelli M, Sassi M, Ciolfi A, Possenti M, et al. (2005) A dynamic balance between gene activation and repression regulates the shade avoidance response in. A dynamic balance between gene activation and repression regulates the shade avoidance response in Arabidopsis. 10.1101/gad.364005 Genes & Dev. 2005. 19. Wiley Online Library will be unavailable on Saturday 7th November 2015 from 10:00-16:00 GMT / 05:00-11:00 EST / 18:00-00:00 SGT for essential maintenance.
Yang et al. 2. 00. Thus, it seems likely that the increase in HFR1 transcript provoked by prolonged low R/FR may result in a consequential increase in HFR1 protein level. To determine whether HFR1 is functionally involved in the regulation of shade avoidance responses, we isolated homozygous hfr. T- DNA insertional lines (Salk_0.
Salk_0. 49. 49. 7, which we designated hfr. Sequence analyses showed that the T- DNA is inserted in the third intron in hfr. ATG), causing deletion of the b. HLH domain, and 2. ATG in hfr. 1- 5. Northern blot analysis showed a stable band of ~1. To analyze the phenotype of these lines, seedlings were grown for 3 d under light–dark cycles, and then either maintained in high R/FR or transferred to low R/FR.
Hypocotyl length was measured after 4 d in these treatments. No phenotypic alteration was observed in hfr. R/FR (Supplementary Fig.
S2). By contrast, hfr. R/FR displayed elongated hypocotyls relative to wild type in low R/FR, evidence of an exaggerated shade avoidance response (Supplementary Fig. S2). Based on the hfr. R/FR, we propose to refer to this mutant as hfr. To investigate whether HFR1/SICS1 affects shade avoidance responses other than hypocotyl elongation, wild type, hfr.
R/FR, and then transferred to low R/FR. Phenotypes were analyzed after 6 and 1. Cotyledon and leaf area were significantly smaller in hfr. Fig. 1. A,C; Table 1). The complexity of the leaf vascular system was reduced in hfr.
Fig. 1. B,D). Moreover, petioles were extremely elongated in hfr. Fig. 1. C; Table 1). Finally, prolonged exposure to low R/FR resulted in early flowering in both hfr. Fig. 1. E; Table 1).
These results establish HFR1/SICS1 as a general regulator of the shade avoidance response. Exaggerated shade avoidance responses in hfr. A) Col- 0 and hfr.
R/FR and subsequently for 6 d in low R/FR. B) Dark- field image of leaves from seedlings grown as in A showing vascular strands.
Phenotypes of hfr. R/FRTo gain insights into the mechanism through which HFR1/SICS1 prevents an exaggerated shade avoidance response, we analyzed the expression of ATHB2 in hfr.
R/FR. In wild- type seedlings, ATHB2 transcript reached its maximum at 3. Fig. 2. A). Remarkably, the kinetic of ATHB2 induction by low R/FR is not significantly affected in seedlings lacking functional HFR1/SICS1 whereas the decrease of the ATHB2 m. RNA provoked by prolonged exposure to low R/FR in wild- type seedlings is essentially abolished in hfr. Fig. 2. A). On the other hand, returning the seedlings to high R/FR after 1 h exposure to low R/FR resulted in a rapid decrease of the ATHB2 m. RNA both in wild type and hfr.
Fig. 2. B), indicating that HFR1/SICS1 is not required for the down- regulation of ATHB2 by high R/FR. The HFR1/SICS1 gene is also rapidly down- regulated by high R/FR (Fig. B). HFR1/SICS1 is required to down- regulate ATHB2 expression in low R/FR. A) Northern analysis of ATHB2 in Col- 0 and hfr. R/FR (0) and then exposed to low R/FR for the indicated times. B) Northern analysis of ATHB2 in Col- 0..
These data strongly suggest that HFR1/SICS1 may down- regulate key regulators of the shade avoidance response in low R/FR to prevent an exaggerated reaction to changes in R/FR. To test this hypothesis we carried out Affymetrix Arabidopsis Genome Gene. Chip array (ATH1) analyses on young hfr. R/FR for 1 d. No significant difference was found in hfr. R/FR relative to wild type. By contrast, 3. 1 genes showed a differential regulation in hfr. R/FR relative to wild type (fold- change ≥ 1.
Supplementary Table S2). Notably, all of the 3. Fig. 3. A). A functional analysis of the 3. Among them are ATHB2; PIL2, a gene up- regulated by low R/FR in wild- type seedlings encoding a b. HLH protein similar to PIL1 (Salter et al. BIM1, a gene encoding a b. HLH transcription factor recently implicated in brassinosteroid signaling (Yin et al.
HLH proteins (Supplementary Table S2; Bailey et al. Devlin et al. 2. 00. Interestingly, the gene encoding PHYA, rapidly up- regulated by low R/FR in wild- type seedlings (Devlin et al. PAT3- like/FHL encoding a protein similar to FHY1/PAT3, a positive regulator of PHYA signal transduction (Desnos et al. Zeidler et al. 2. R/FR relative to wild type.
Together, the data indicate that several regulatory genes, some of which already implicated in shade avoidance responses, are differentially regulated by low R/FR in wild type and hfr. Among the 3. 1 genes, one further group encodes hormone- related factors (Supplementary Table S2). Significantly, several genes implicated in auxin and gibberellin pathways are up- regulated in hfr.
At. CKX5, a gene encoding a cytokinin oxidase/dehydrogenase involved in cytokinin breakdown (Werner et al. Links between the shade avoidance response and the auxin and gibberellin pathways have been established (Morelli and Ruberti 2.
Devlin et al. 2. 00. Vandenbussche and Van Der Straeten 2. Therefore, the up- regulation of the auxin and gibberellin signaling pathways and the down- regulation of the cytokinin pathway in hfr. R/FR relative to wild type is consistent with the enhanced shade avoidance response of these mutants. HFR1/SICS1 acts as a negative regulator in the signaling cascade induced by low R/FR. A) Col- 0, hfr. 1- 4/sics. R/FR and then either maintained in high R/FR or transferred to low R/FR for 1 d.
Global.. We next examined the kinetic of induction by low R/FR of seven genes selected among the 3. PIL1, absent in the Affymetrix Arabidopsis Genome Gene. Chip array (ATH1), in wild- type and hfr. The lack of functional HFR1/SICS1 does not significantly affect the kinetic of induction of these genes by low R/FR (Fig. B). By contrast, the transcript level of all of the eight genes was significantly higher in hfr. R/FR (Fig. 3. B).
FT, encoding a key integrator of different floral induction signals including low R/FR (Cerdan and Chory 2. Casal et al. 2. 00. Supplementary Table S2).
However, consistent with the early- flowering phenotype of the mutants in low R/FR, quantitative PCR analysis showed that FT expression is substantially higher in hfr. Fig. 3. C). The results described here uncover a gas- and- brake mechanism for the control of the shade avoidance response acting through a dynamic balance of positive and negative transcriptional regulators. Positive regulators of the shade avoidance response are induced within a few minutes of a change in R/FR (Carabelli et al. Salter et al. 2. 00. These rapid changes in gene expression ensure a fast reshaping of the plant body toward a light environment optimal for growth. Low R/FR also provokes a rapid induction of the HFR1/SICS1 gene to negatively regulate the downstream signaling initiated by the same signal. HFR1/SICS1 acting as a negative controller of the shade avoidance response ensures that an exaggerated reaction does not occur when the plant is unsuccessful in escaping canopy shade.
In this unfavorable environmental light condition, HFR1/SICS1 is likely to play a fundamental role in the acclimation of the plant both morphologically and biochemically, and by delaying flowering, to ensure a better seed production needed for long term survival. HFR1/SICS1, already implicated in phytochrome A and cryptochrome 1 signaling during the de- etiolation process (Fairchild et al. Fankhauser and Chory 2. Soh et al. 2. 00. Duek and Fankhauser 2. While it is established that low R/FR light is the primary cue for the detection of neighbors, and several phytochromes contribute to the perception of this signal (Franklin et al. Chen et al. 2. 00.
Vandenbussche et al. A major challenge in the future is to understand whether light signals perceived by different photoreceptors are integrated through HFR1/SICS1 to prevent exaggerated responses under canopy shade. Materials and methods.
Plant growth conditions and phenotypic analyses.