Conditional synergism between cryptochrome 1 and phytochrome B is shown by the analysis of phyA, phyB, and hy4 simple, double, and triple mutants in Arabidopsis

Abstract

Wild-type or phyA, phyB, or hy4 mutant Arabidopsis seedlings lacking phytochrome A (phyA), phytochrome B (phyB), or cryptochrome 1 (cry1), respectively, and the double and triple mutants were used in combination with blue-light treatments given simultaneously with red or far-red light. We investigated the interaction between phytochromes and cry1 in the control of hypocotyl growth and cotyledon unfolding. Under conditions deficient for cry1 (short exposures to blue light) or phyB (far-red background), these photoreceptors acted synergistically: Under short exposures to blue light (3 h/d) added to a red-light background, cry1 activity required phyB (e.g. the hy4 mutant was taller than the wild type but the phyBhy4 mutant was not taller than the phyB mutant). Under prolonged exposures to blue light (24 h/d) added to a far-red light background, phyB activity required cry1 (e.g. the phyAphyB mutant was taller than the phyA mutant but the phyAphyBhy4 mutant was not taller than the phyAhy4 mutant). Under more favorable light inputs, i.e. prolonged exposures to blue light added to a red-light background, the effects of cry1 and phyB were independent. Thus, the synergism between phyB and cry1 is conditional. The effect of cry1 was not reduced by the phyA mutation under any tested light condition. Under continuous blue light the triple mutant phyAphyBhy4 showed reduced hypocotyl growth inhibition and cotyledon unfolding compared with the phyAphyB mutant. The action of cry1 in the phyAphyB double mutant was higher under the red-light than the far-red-light background, indicating a synergistic interaction between cry1 and phytochromes C, D, or E; however, a residual action of cry1 independent of any phytochrome is likely to occur.

Figure 1

A, Experimental setting to modify independently the status of phytochromes and cryptochrome. B, Spectral photon distribution of light provided by red, far-red, and blue light sources.

Figure 2

cry1 requires phyB when exposures to blue light are short (3 h/d) but becomes independent of phyB when exposures are more extended (24 h/d). The seedlings were grown under continuous red light, continuous red light plus 3 h of blue light per day, or continuous red light plus blue light. A, Hypocotyl length relative to dark controls. B and C, Effects on hypocotyl growth of PHYB versus phyB and HY4 versus hy4 as affected by different genetic backgrounds in seedlings exposed to blue light for only 3 h/d (B) and in seedlings continuously exposed to supplementary blue light (C).

Figure 3

Under a background of far-red light (FR) the effects of phyB are significant only if cry1 is active. The seedlings were grown under continuous far-red light or far-red light plus blue light. A, Hypocotyl length relative to dark controls. B, Effects of PHYB versus phyB on hypocotyl growth in the HY4 compared with the hy4 background (all of the seedlings are phyA to avoid a high-irradiance reaction under far-red light). C, Effects of HY4 versus hy4 on hypocotyl growth in the PHYB compared with the phyB background. D, Angle between the cotyledons. E, Effects of PHYB versus phyB on cotyledon unfolding in the HY4 compared with the hy4 background. F, Effects of HY4 versus hy4 on cotyledon unfolding in the PHYB compared with the phyB background.

Figure 4

The effect of cry1 under continuous blue light is not affected by the presence or absence of phyA and phyB. Hypocotyl growth relative to dark controls (A) and cotyledon unfolding (B) in wild-type (WT), hy4, phyAphyB, and phyAphyBhy4 seedlings exposed to blue light (without a red- or a far-red-light background). In dark-grown seedlings the angle between the cotyledons was 0 for all genotypes. The arrows indicate the effect of cry1 in the PHYAPHYB and phyAphyB backgrounds.

Figure 5

Evidence for synergism between cry1 and novel phytochrome(s). Seedlings of phyAphyB double mutants were grown under continuous red (R) or far-red light (FR) in factorial combination with or without the simultaneous addition of continuous blue light. A, Hypocotyl length relative to dark controls. B, The effects of blue light on hypocotyl length (i.e. the difference between the seedlings receiving supplementary blue light and those not receiving supplementary blue light) are larger when red light was used as a background than when far-red light was used (the two double-mutant alleles were pooled). C, Angle between the cotyledons. D, Effects of blue light added to red or far-red light on cotyledon unfolding.