Multiple phytochrome-interacting bHLH transcription factors repress premature seedling photomorphogenesis in darkness

Abstract

Background: An important contributing factor to the success of terrestrial flowering plants in colonizing the land was the evolution of a developmental strategy, termed skotomorphogenesis, whereby postgerminative seedlings emerging from buried seed grow vigorously upward in the subterranean darkness toward the soil surface.

Results: Here we provide genetic evidence that a central component of the mechanism underlying this strategy is the collective repression of premature photomorphogenic development in dark-grown seedlings by several members of the phytochrome (phy)-interacting factor (PIF) subfamily of bHLH transcription factors (PIF1, PIF3, PIF4, and PIF5). Conversely, evidence presented here and elsewhere collectively indicates that a significant component of the mechanism by which light initiates photomorphogenesis upon first exposure of dark-grown seedlings to irradiation involves reversal of this repression by rapid reduction in the abundance of these PIF proteins, through degradation induced by direct interaction of the photoactivated phy molecule with the transcription factors.

Conclusions: We conclude that bHLH transcription factors PIF1, PIF3, PIF4, and PIF5 act as constitutive repressors of photomorphogenesis in the dark, action that is rapidly abrogated upon light exposure by phy-induced proteolytic degradation of these PIFs, allowing the initiation of photomorphogenesis to occur.

Figure 1. PIF1 and PIF3 act redundantly as constitutive and light-modulated repressors of seedling deetiolation in the dark

A) Schematic representation of standard and modified protocols for seedling growth. Under standard conditions, seeds were exposed to 1.5h of white light (WL) during sterilization and plating (pre-stratification) and placed for 5d at 4°C in darkness (stratification). Three hours of WL were used to synchronize germination (post-stratification) before incubation for 45h at 21°C in the dark. A 5-min pulse of red light (Rp) was used as an alternative post-stratification treatment. Two-day-old dark-grown seedlings (D48h) were then transferred to Rc (7.2 μmol/m²/s) for 24h (R24h). Under the modified protocol conditions, a terminal 5-min pulse of far-red light (FRp) was provided after either pre- or post-stratification light treatments as indicated. Green light (G) was used as an alternative to WL during the pre-stratification treatment in one configuration of the modified conditions. B) Visible phenotypes of 2d-old (D48h) dark-grown seedlings under standard protocol conditions (pre-WL/post-WL) (Top) and modified conditions (pre-WL/post-WL+FRp) (Bottom). Photos of representative wild-type (Col-0) and pif mutant seedlings are shown. C) Time-course quantification of cotyledon separation in the dark and during the dark-tored light transition under standard protocol conditions (pre-WL/post-WL). D36h and D42h data-points were from an independent experiment. D) Northern blot analysis of CAB3 gene expression under standard protocol conditions (pre-WL/post-WL), except that after the post-stratification WL, seeds were placed in the dark for 39h at 21°C. Dark-grown seedlings (D42h) were exposed to Rc (9 μmol/m²/s) for 1h (D42h+R1h). 18S was used as normalization control. A representative blot is shown. E) Quantification of cotyledon separation in 2d-old (D48h) wild-type (Col-0) and pif mutant seedlings grown in darkness under the standard and the modified protocol conditions indicated in panel A. A more extensive analysis with additional treatments and measurement of hook angle and hypocotyl responses is shown in Figure S4. F) phyB mediates most of the cotyledon separation pseudo-dark effects observed in pif3 and pif1pif3 mutants. The indicated genotypes were grown for 2days in the dark (D48h) under standard pseudo-dark conditions (pre-WL/post-WL) or under modified true-dark conditions (pre-WL+FRp/post-none). ND: Not determined. pif3-3 and pif3phyB data from an independent experiment are included for comparison. G) True- and pseudo-dark regulation of LHCB1.4 marker gene expression by PIF1 and PIF3 in 2d-old dark-grown seedlings (D48h). Q-PCR analysis in wild-type (Col-0) and pif mutants was used to measure LHCB1.4 gene expression under the indicated pre- and post-stratification treatments, and PP2A was used as a normalization control. The LHCB1.4/PP2A ratio is represented as the mean and standard error of 3 independent biological replicates. Data are presented relative to the mean of Col-0 pre-WL/post-Rp set at unity. Data represent the mean and standard error of at least 25 (C) or 30 (E, F) seedlings. Asterisks in E, F and G indicate low germination precluding reliable measurements.

Figure 2. pif1pif3pif4pif5 quadruple mutant displays a robust pleiotropic constitutive-photomorphogenic phenotype in the dark

A) Visible phenotypes of 2d-old (2dD or D48h) and 4d-old (4dD or D96h) dark-grown seedlings grown under the standard conditions indicated in Figure 1A (pre-WL/post-WL). Photos of representative wild-type (Col-0) and pif mutant seedlings are shown. B) Visible phenotypes of 2d-old (D48h) seedlings grown in darkness under the modified schedules indicated in Figure 1A. Photos of representative seedlings for each genotype are shown. C) Quantification of cotyledon separation (Top) and hypocotyl elongation (Bottom) phenotypes under the indicated modified schedules. Data represent the mean and standard error of at least 20 seedlings. Asterisk indicates low germination precluding reliable measurements.

Figure 3. Pre-germinative-light-potentiated pseudo-dark responses are mediated by phy-regulated changes in PIF protein levels

A) Schematic representation of the modified growth protocol used. After the post-stratification WL treatment (3hWL) seeds were placed in darkness (3hWL+D(h)). Alternatively, a 5-min FRp was provided before the dark incubation (3hWL+FRp+D(h)). B) Immunoblot analysis of H:PIF3:MYC fusion protein. Protein extracts were prepared from H:PIF3:MYC transgenic lines [11] and immunoblotted with an anti-MYC antibody (upper and middle panels). Tubulin was used as loading control. H:PIF3:MYC signal normalized to tubulin was quantified from the blots shown in the middle panel using Image J software as described [24], and the data are presented relative to D0 time point (lower panel). C) Immunoblot analysis of endogenous PIF3 protein in wild-type (Col-0) and phyB-9 mutant seeds and seedlings. Protein extracts were immunoblotted with an anti-PIF3 antibody [11] (upper panel). Tubulin was used as loading control. PIF3 signal normalized to tubulin was quantified as in B), and the data are presented relative to Col-0 D0 time point (lower panel). D) Immunoblot analysis of endogenous PIF3 protein in wild-type (Col-0) and phyB-9 mutant seeds grown under the indicated conditions. Tubulin was used as loading control. E) Quantification of cotyledon separation phenotype of 2d-old dark-grown spa1spa2spa3 mutant seedlings grown under the standard protocol (3hWL+D45h). Data points represent the mean and standard error of at least 30 seedlings. F) Immunoblot analysis of endogenous PIF3 protein in wild-type (Col-0) and spa1spa2spa3 mutant seedlings grown as in Figure 3E (3hWL+D45h). Tubulin was used as loading control. Where indicated, protein extracts from pif3 and pif1pif3 mutants were used as controls. n.s.: non-specific band.

Figure 4. Model of phy-induced initiation of photomorphogenesis by direct removal of repressors PIF1, PIF3, PIF4 and PIF5

A) PIFs 1, 3, 4 and 5 collectively promote skotomorphogenesis during early post-germinative seedling development in darkness. Experiments under true-dark conditions show that PIF1 has a dominant role in this process, whereas PIF3, PIF4 and PIF5 act redundantly with PIF1. Light-induced activation of the phy molecule triggers rapid degradation of these PIFs, as a consequence of direct physical interaction of the photoreceptor with the PIFs. The phy-induced removal of PIF repression, revealed here under the pseudo-dark experimental dark conditions, is proposed to initiate the developmental transition from skotomorphogenesis to photomorphogenesis. B) Proteosome-pathway photomorphogenesis repressors COP1/SPA regulate the abundance of PIF3, and possibly other PIFs. It is proposed that part of COP1 and SPA action in repressing photomorphogenesis in the dark may be through promotion of PIF-protein accumulation, in addition to their established role in promoting the degradation of positive factors, such as HY5 [37, 38]. Light activates phy to remove the repressive action of the PIFs through direct molecular interaction (solid line), and to remove COP1/SPA activity through a possibly indirect mechanism (dotted line), thereby initiating seedling photomorphogenesis.