Ultraviolet B radiation enhances a phytochrome-B-mediated photomorphogenic response in Arabidopsis

Abstract

Ultraviolet B radiation (UV-B, 290-315 nm) can cause damage and induce photomorphogenic responses in plants. The mechanisms that mediate the photomorphogenic effects of UV-B are unclear. In etiolated Arabidopsis seedlings, a daily exposure to 2.5 h of UV-B enhanced the cotyledon opening response induced by a subsequent red light (R) pulse. An R pulse alone, 2.5 h of UV-B terminated with a far-red pulse, or 2.5 h of continuous R caused very little cotyledon opening. The enhancing effect of UV-B increased with fluence rate up to approximately 7.58 micromol m(-2) s(-1); at higher fluence rates the response to UV-B was greatly reduced. The phyA, phyA cry1, and cry1 cry2 mutants behaved like the wild type when exposed to UV-B followed by an R pulse. In contrast, phyB, phyB cry1, and phyB phyA mutants failed to open the cotyledons. Thus, phytochrome B was required for the cotyledon opening response to UV-B --> R treatments, whereas phytochrome A and cryptochromes 1 and 2 were not necessary under the conditions of our experiments. The enhancing effect of low doses of UV-B on cotyledon opening in uvr1 uvr2 and uvr1 uvr3 mutants, deficient in DNA repair, was similar to that found in the wild type, suggesting that this effect of UV-B was not elicited by signals derived from UV-B-induced DNA lesions (cyclobutane pyrimidine dimers and 6-4 photoproducts). We conclude that low doses of UV-B, perceived by a receptor system different from phytochromes, cryptochromes, or DNA, enhance a de-etiolation response that is induced by active phytochrome B.

Figure 1

Fluence-rate response curves for UV-B-induced cotyledon opening (A) and hypocotyl growth inhibition (B). One-day-old WT seedlings (24 h after the R pulse used to induce germination) were exposed for 3 d to a daily period of 2.5 h of UV-B (of the indicated fluence rate), followed by a saturating R pulse (see daily protocol at the top of the figure). Each point represents the mean ± se of at least six replicate boxes.

Figure 2

Angle between cotyledons (A) and hypocotyl length (B) of WT seedlings after 3 d of exposure to daily periods of 2.5 h of UV-B (6.6 μmol m⁻² s⁻¹) or darkness (D), followed by a saturating R or FR pulse (see daily protocol at the top of the figure). C, Phenotype of WT seedlings grown under the different irradiation treatments. Bar data are means and se of at least 16 replicate boxes. Different letters indicate significant differences between treatment means.

Figure 3

Angle between cotyledons (A) and hypocotyl length (B) of photoreceptor-deficient single and double mutants. The seedlings were exposed for 3 d to a daily period of 2.5 h of UV-B (6.6 μmol m⁻² s⁻¹) or darkness, followed by a saturating R (striped bars) or FR (solid bars) pulse. Bar data are means and se of at least six replicate boxes.

Figure 4

A, Angle between cotyledons of WT seedlings after 3 d of exposure to daily periods of 2.5 h of UV-B (6.6 μmol m⁻² s⁻¹) followed by darkness or saturating R or FR pulses (UV-B → D, UV-B → R, or UV-B → FR, respectively), or to 2.5 h of R (30 μmol m⁻² s⁻¹) followed by a saturating R pulse (R → R). Bar data are means and se of at least six replicate boxes; different letters denote significant differences between treatment means. B, PHYB level of Ler seedlings exposed for 3 d to daily periods of the indicated irradiation treatments (UV-B → R, 2.5 h UV-B followed by an R pulse; R → R, 2.5 h R followed by an R pulse; D → R, one R pulse a day). A phyB overexpressor line (ABO) was included as a positive control along with its corresponding WT (No-O).

Figure 5

Fluence-rate response curves for UV-B-induced cotyledon opening and hypocotyl growth inhibition in WT and DNA repair mutants. Two-day-old seedlings (48 h after the R pulse used to induce germination; compare with Fig. 1) were exposed for 3 d to a daily period of 2.5 h of UV-B (of the indicated fluence rate), followed by a saturating R pulse. Each datum point represents the mean ± se of at least six replicate boxes. The uvr1 uvr2 (A and B) and uvr1 uvr3 mutants (C and D) were tested in two different series of experiments; therefore, separate WT controls are shown for each mutant.