Elementary processes of photoperception by phytochrome A for high-irradiance response of hypocotyl elongation in Arabidopsis

Abstract

Elementary processes of photoperception by phytochrome A (PhyA) for the high-irradiance response (HIR) of hypocotyl elongation in Arabidopsis were examined using a newly designed irradiator with LED. The effect of continuous irradiation with far-red (FR) light could be replaced by intermittent irradiation with FR light pulses if given at intervals of 3 min or less for 24 h. In this response, the Bunsen-Roscoe law of reciprocity held in each FR light pulse. Therefore, we determined the action spectrum for the response by intermittent irradiation using phyB and phyAphyB double mutants. The resultant action spectrum correlated well with the absorption spectrum of PhyA in far-red-absorbing phytochrome (Pfr). Intermittent irradiation with 550 to 667 nm of light alone had no significant effect on the response. In contrast, intermittent irradiation with red light immediately after each FR light pulse completely reversed the effect of FR light in each cycle. The results indicate that neither red-absorbing phytochrome synthesized in darkness nor photoconverted Pfr are physiologically active, and that a short-lived signal is induced during photoconversion from Pfr to red-absorbing phytochrome. The mode of photoperception by PhyA for HIR is essentially different from that by PhyA for very-low-fluence responses and phytochrome B for low-fluence responses.

Figure 1

Effect of intermittent irradiation with FR light on PhyA-mediated inhibition of hypocotyl elongation in Arabidopsis. A, Light regime. Two-day-old etiolated seedlings were transferred to intermittent FR light irradiation of various cycles, continuous FR light, or darkness and grown for 5 d, and their hypocotyl lengths were measured. Long white bar, Continuous irradiation with FR light (30 μmol m⁻² s⁻¹); long black bars, continuous darkness; small white boxes, intermittent irradiation, FR light pulses (180 μmol m⁻² s⁻¹ for 10 s each) with LEDs. B, Responses to intermittent irradiation with FR light in wild-type (WT, ▵), phyA (○), phyB (□), and phyAphyB (⋄) mutants. Values at a dark interval of 0 min correspond to continuous irradiation. Error bars represent se.

Figure 2

Relationship between the hypocotyl length and the photon fluence of FR light per pulse. A, Light regime. Two-day-old etiolated seedlings of the wild type were transferred and grown in the intermittent irradiation with different intensities of FR light for different durations of exposure time in 3-min cycles for 5 d, and their hypocotyl lengths were measured. White bars, FR light pulses with LEDs; black bars, darkness. B, Responses to intermittent irradiation with FR light pulses for 1 s (⋄), 3 s (▵), 10 s (□), and 60 s (○) in the wild type (top graph) and the phyB mutant (bottom graph). Error bars represent se.

Figure 3

Effect of intermittent irradiation with FR light pulses for 1 d on further growth of the hypocotyl in darkness. A, Light regime. Two-day-old etiolated seedlings were treated as follows: one group (□) was kept under intermittent irradiation with FR light pulses (100 μmol m⁻² s⁻¹ for 10 s each) in 3-min cycles throughout the experiment, the second group (▴) was exposed to the same intermittent FR light irradiation for 1 d, then grown in the dark, the third group (●) was kept in the dark, and the fourth group (○) was kept under continuous irradiation with FR light (5.6 μmol m⁻² s⁻¹) for 9 d. White lines in black bars, FR light pulses with LEDs; black bars, darkness; white bars, continuous irradiation with FR light. B, Time courses of the hypocotyl growth in the wild type (top graph) and the phyB mutant (bottom graph). Error bars represent se.

Figure 4

Effect of intermittent irradiation with 320 to 780 nm of light on inhibition of hypocotyl elongation in the phyB and the phyAphyB double mutants. Two-day-old etiolated seedlings of the phyB mutant (○) and the phyAphyB double mutant (▵) were exposed to intermittent irradiation with monochromatic light in 3-min cycles for 1 d, then grown in the dark for 5 d, and their hypocotyl lengths were measured. Error bars represent se.

Figure 5

Action spectra for inhibition of hypocotyl elongation in the phyB and the phyAphyB double mutants by intermittent light treatment. Photon effectiveness for the response was calculated as reciprocal of the fluence required for 20% inhibition of hypocotyl elongation based upon the data in Figure 4. The area shaded gray in the figure corresponds to the differences between the spectrum of the phyB mutant (●) and the phyAphyB double mutant (▵), and is due to the involvement of PhyA for the response.