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.2003 Feb 18;100(4):2140-5.
doi: 10.1073/pnas.0437826100. Epub 2003 Feb 10.

Regulation of photoperiodic flowering by Arabidopsis photoreceptors

Affiliations

Regulation of photoperiodic flowering by Arabidopsis photoreceptors

Todd Mockler et al. Proc Natl Acad Sci U S A..

Abstract

Photoperiodism is a day-length-dependent seasonal change of physiological or developmental activities that is widely found in plants and animals. Photoperiodic flowering in plants is regulated by photosensory receptors including the red/far-red light-receptor phytochromes and the blue/UV-A light-receptor cryptochromes. However, the molecular mechanisms underlying the specific roles of individual photoreceptors have remained poorly understood. Here, we report a study of the day-length-dependent response of cryptochrome 2 (cry2) and phytochrome A (phyA) and their role as day-length sensors in Arabidopsis. The protein abundance of cry2 and phyA showed a diurnal rhythm in plants grown in short-day but not in plants grown in long-day. The short-day-specific diurnal rhythm of cry2 is determined primarily by blue light-dependent cry2 turnover. Consistent with a proposition that cry2 and phyA are the major day-length sensors in Arabidopsis, we show that phyA mediates far-red light promotion of flowering with modes of action similar to that of cry2. Based on these results and a finding that the photoperiodic responsiveness of plants depends on light quality, a model is proposed to explain how individual phytochromes and cryptochromes work together to confer photoperiodic responsiveness in Arabidopsis.

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Figures

Figure 1
Figure 1
Spectra of experimental and natural light. The spectra of experimental light (Cool White, Blue, Red, FR) and direct sunlight (Sun, University of California, Los Angeles campus on June 10, 2000, at 15:30 PST) are shown as the relative spectral irradiance in the wavelength range of 300–900 nm.
Figure 2
Figure 2
Immunoblots showing photoperiod-dependent daily oscillation of cry2 and phyA protein abundance. Seven-day-old wild-type (Col-4) seedlings grown in SD (A; ≈130 μmol·s−1·m−2) or LD (B; ≈65 μmol·s−1·m−2) photoperiod were sampled at the times indicated, starting 1 h after dawn. Each immunoblot was probed with anti-CRY2, stripped, and reprobed with anti-CRY1 and then with anti-PHYA, as described (18). The ribulose-1, 5-biphosphate carboxylase/oxygenase (RUBISCO) large subunit shown as the Ponceau S-stained band is included as a loading control. White and black bars depict the light and dark period, respectively. Hatched bars represent the subjective dark period illuminated with light. Levels of CRY2 (●), CRY1 (▴), and PHYA (■) were first normalized against the total protein measured from the Ponceau S-stained membrane, and the normalized values were divided by the lowest value of same immunoblot (Upper) and shown as the relative change of photoreceptor abundance (Lower).
Figure 3
Figure 3
Effects of different wavelengths of light on cry2 protein expression. (A) Seven-day-old wild-type (Col-4) seedlings were grown in 12 hBL/12 hD photoperiod illuminated with blue light (≈40 μmol·s−1·m−2) and then transferred to continuous red light (≈40 μmol·s−1·m−2). (B) Seedlings were grown in 12 hBL/12 hD photoperiod illuminated with blue light (≈35 μmol·s−1·m−2) and then transferred to continuous blue light with the same fluence rate. (C) Seedlings were grown in 12-h red light/12-h dark photoperiod illuminated with red light (≈60 μmol·s−1·m−2) and then transferred to continuous red light with the same fluence rate. Samples were collected at the times indicated. Immunoblots (Upper) and the relative change of the photoreceptor abundance (Lower) are as described in Fig. 2. The colored bars depict the light periods with respective light spectra, the black bar depicts the dark period, and the hatched bar depicts the subjective dark period illuminated with continuous light.
Figure 4
Figure 4
Effects of different wavelengths of light on the flowering time of variousArabidopsis photoreceptor mutants. (A) The flowering times of the indicated genotypes grown on nutrient medium (Murashige and Skoog + 1% sucrose, 1% phytoagar) under continuous FR light (≈56 μmol·s−1·m−2) were measured as described. The means of three independent experiments with individual samples containing 17–101 plants and the corresponding standard errors are shown. (B) Twenty-four-day-old plants of the indicated genotypes grown under continuous FR light as described inA. (C andD) Plants of indicated genotypes were grown on soil under continuous R+FR (C; LD; ≈75 μmol·s−1·m−2 with an R:FR ratio of 2.5:1.0) or continuous blue light (D; ≈50 μmol·s−1·m−2). The means of three independent experiments with individual samples containing 13–64 plants and the corresponding standard errors are shown.
Figure 5
Figure 5
Effects of continuous light onArabidopsis flowering time. (A) A model depicting functions of photoreceptors regulating floral initiation inArabidopsis grown in continuous lights. The arrows represent a stimulatory effect, and the lines terminated with a bar represent an inhibitory effect on flowering. (B) Thirty-six-day-old plants of the indicated genotypes grown under continuous cool white-plus-FR light (CW+FR; ≈93 μmol·s−1·m−2 with an R:B:FR ratio of ≈2.8:1.1:1.0). (C) Flowering time of the genotypes indicated grown on soil under CW+FR as described inB. The means of three independent experiments with individual samples containing 20–50 plants and the corresponding standard errors are shown.
Figure 6
Figure 6
Effects of different wavelengths of light on photoperiodic flowering ofArabidopsis. (A) Flowering time of wild-typeArabidopsis plants of the Wassilewskija (Ws), Columbia-4 (Col), and Landsbergerecta (Ler) ecotypes grown under LD, SD photoperiods illuminated with cool white light photoperiods (White, LD: 72 μmol·s−1·m−2; SD: 145 μmol·s−1·m−2), red light photoperiods (Red, LD: 55 μmol·s−1·m−2; SD: 113 μmol·s−1·m−2), red-plus-FR light photoperiods (R+FR, LD: 34 μmol·s−1·m−2; SD: 70 μmol·s−1·m−2; R:FR ratio of ≈2.5:1.0), blue light photoperiods (Blue, LD: 38 μmol·s−1·m−2; SD: 80 μmol·s−1·m−2), red-plus-blue light photoperiods (R+B, LD: 66 μmol·s−1·m−2; SD: 137 μmol·s−1·m−2; R:B ratio of ≈2.7:1.0), or FR-plus-blue light photoperiods (FR+B, LD: 38 μmol·s−1·m−2; SD: 78 μmol·s−1·m−2; FR:B ratio of ≈1.2:1.0). Fluence rates were adjusted so that plants grown in LD or SD received a similar amount of total irradiance per 24 h. The means of three independent experiments with individual samples containing 21–58 plants and the corresponding standard errors are shown. (B) An external coincidence model depicting photoreceptor functions in photoperiodic flowering ofArabidopsis. The arrows and the lines terminated with a bar represent a positive effect and a negative effect, respectively. The white bars depict the light (day) period and the black bars depict the dark (night) period. The waveform enclosed in a circle depicts the circadian oscillator that regulates the PRR depicted by the bars colored red-green-red. The green portion of the PRR represents the hypothesized “sensitive” phase that coincides with signaling of the B and FR pathways during LD photoperiods.
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